Thesis Is Divided Into Two Parts

The Pennsylvania State University

The Graduate School

Department of Chemistry

BRUSH-SHAPED HYBRID POLYPHOSPHAZENES FOR ADVANCED

BIOMATERIAL APPLICATIONS AND SUBSTITUENT EXCHANGE REACTIONS

BASED ON CYCLIC, OLIGOMERIC AND POLYMERIC PHOSPHAZENES

A Dissertation in

Chemistry

Xiao Liu

Submitted in Partial Fulfillment of the Requirements for the Degree of

Doctor of Philosophy

May 2013

The dissertation of Xiao Liu was reviewed and approved* by the following:

Harry R. Allcock Evan Pugh Professor of Chemistry Dissertation Advisor Chair of Committee

John V. Badding Professor of Chemistry

Ben Lear Assistant Professor of Chemistry

James P. Runt Professor of Polymer Science Associate Head of Graduate Studies

Barbara J. Garrison Shapiro Professor of Chemistry Head of the Chemistry Department

*Signatures are on file in the Graduate School

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ABSTRACT

The work described in this thesis is divided into two parts. The first part focuses on the synthesis and characterization of brush-shaped polyphosphazene hybrid materials for antibacterial surface coating and photocontrolled drug release applications. The second part describes the detailed study of substituent exchange reactions based on cyclic trimeric or tetrameric, and linear oligomeric and polymeric phospaphazenes. The reaction mechanism of substituent exchange is proposed and verified. The themes that ties the two parts of this thesis are solutions to some of the challenges of polyphosphazene synthesis, and the utilization of synthesis pathways to develop new phosphazene polymer architecture.

Chapter 1 outlines the fundamental concepts for polymeric materials used in biomedical applications. The properties of the polymers that are used as antibacterial coatings and controlled drug release are described. The chemistry and applications of phosphazenes is also outlined.

Chapter 2 deals with the synthesis of a series of densely grafted star- and comb- shaped molecular brushes composed of polystyrene, poly(tert-butyl acrylate) and poly(N-isopropylacrylamide) prepared by atom transfer radical polymerization (ATRP) using either cyclotriphosphazenes or polyphosphazenes as initiators. The grafting conditions were optimized for various monomers. The kinetics of the reaction were first-order with respect to the monomer concentration in both the cyclotriphosphazene and polyphosphazene systems. The lower critical solution temperature (LCST) of poly(N-isopropylacrylamide) brush polymers was measured by both dynamic light scattering (DLS) and differential scanning calorimetry (DSC), showed a sharp phase transition at 33 °C. Furthermore, star- and comb-block copolymers with a hard polystyrene core and a soft poly(tert-butyl acrylate) shell were also synthesized.

Chapter 3 outlines the preparation of densely grafted star- and comb-shaped molecular brushes composed of poly[2-(dimethylamino)ethyl methacrylate] produced via atom transfer radical polymerization (ATRP), species that were quaternized with iodomethane, iodobutane, iodoheptane, iododecane and iodododecane. Electrospinning of the quaternized brushes gave rise to microfibers with diameters in the range of 700 nm to 1.1 μm. The antibacterial activity of the quaternized brush species in both aqueous solution and as fibrous solids against Escherichia coli

(E. coli) has been evaluated. In aqueous solution, star-shaped brushes quaternized with iodoheptane showed the best antibacterial effect, with a minimum inhibitory concentration (MIC) as low as 250 μg mL-1. In the fibrous solid state, more than 99% of E. coli were killed within 2 hr after contacting 100 mg of microfibers electrospun from the star-shaped brush polymers quaternized with either iododecane or iodododecane.

Chapter 4 covers the synthesis and characterization of UV-cleavable star polymers composed of amphiphilic block copolymer arms and a UV-cleavable core. The inner lipophilic poly(methyl methacrylate) (PMMA) and the outer hydrophilic poly[poly(ethylene glycol) methyl ether methacrylate] (PPEGMA) were grafted by atom transfer radical polymerization (ATRP).

The effects of various factors, such as molecular weight, solution concentration, solvent and monomer on the photodegradation rate of the star polymers were studied in details. Significant aggregation of the polymer micelles in aqueous solution was detected by DLS with hydrodynamic radii of 86 and 111 nm for the two star-PMMA-PPEGMA micelles. The critical aggregation concentration (CAC) of star-PMMA179-PPEGMA89-2 was 0.0026 g/L and 0.022 g/L before and after UV-irradiation, indicating the reduced stability of the polymer micellar structures after UV-irradiation. As a result, spontaneous dissociation of cleaved micelles can be induced by the dilution effect in the human body for stimulus-controlled drug release.

Chapter 5 deals with substituent exchange reactions of sodium 2,2,2-trifluoroethoxide with trimeric and tetrameric aryloxycyclophosphazenes. The ease of displacement of OAr in

- cyclic trimeric and tetrameric molecules by CF3CH2O increased significantly with the presence of electron-withdrawing substituents in the polyphosphazene in the order, phenoxy <<

4-formylphenoxy < 4-cyanophenoxy ≈ 4-nitrophenoxy. Fully substituted

2,2,2-trifluoroethoxyphosphazene trimer and tetramer were formed by side group exchange, but these reactions were followed by an attack by the nucleophile on the α-carbon of the

2,2,2-trifluoroethoxy groups linked to phosphorus to give a species in which one trifluoroethoxy group had been replaced by an ONa unit, and bis(trifluoroethyl) ether was formed as a side product.

Chapter 6 describes the study of side group exchange reactions for short chain linear oligomeric phosphazenes, (RO)4P[N=P(OR2)]nOR (n = 6, 10, 20, and 40) as models for the corresponding linear high polymers (n ~ 15,000). Specifically, the exchange behavior of oligomers where OR = OCH2CF3, OC6H5, OC6H4CHO-p, OC6H4CN-p, and OC6H4NO2-p with sodium trifluoroethoxide was examined. The ease of aryloxy group replacement by trifluoroethoxy increased with the electron-withdrawing character in the order OR = OC6H5 <<

OC6H4CHO-p < OC6H4CN-p < OC6H4NO2-p, but the reaction was efficient only if the phosphazene contained no more than 20 repeating units. However, attempts to force the slower reactions by the use of excess sodium trifluoroethoxide induced secondary reactions at the

- + trifluoroethoxy units already introduced to produce CF3CH2OCH2CF3 and insert -O Na units in their place. The longest chain model underwent side group exchange reactions preferentially at the end units.

Chapter 7 discusses similar side group exchange reactions for linear high polymeric

organophosphazenes, [N=P(OR2)]n (n ~ 15,000). Specifically, the exchange behavior of polymers where OR = OCH2CF3, OCH2CF2CF2CF2CF2H, OCH2Cl3, OC6H4CHO-p, OC6H4CN-p, and OC6H4NO2-p with sodium trifluoroethoxide was examined. No aryloxy group replacement by trifluoroethoxy was detected, due to the well-protected backbone of polyphosphazenes by aryloxy side groups. For the exchange behavior of [N=P(OCH2CF3)2]n and

[N=P(OCH2CF2CF2CF2CF2H)2]n with NaOCH2CF2CF2CF2CF2H and NaOCH2CF3, partial substituent exchange could be achieved for both reactions. Furthermore, these side group exchange reactions are followed by reactions that introduce –O-Na+ groups attached to phosphorus in place of organic substituents, and this is a mechanism for subsequent hydrolysis and molecular weight decline. Substituent exchange reactions do appear to be an alternative synthetic approach in the synthesis of many polyphosphazenes, including the recently discovered trichloroethoxy/trifluoroethoxy containing polyphosphazenes.

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TABLE OF CONTENTS

LIST OF FIGURES…………………………………………………………………………...... xii

LIST OF SCHEMES…………………………………………………………………………….xv

LIST OF TABLES…………………………………………………………………………….. xvii

PREFACE…………………………………………………………………………………….. xviii

ACKNOWLEDGEMENTS……………………………………………………………………. xix

Chapter 1. Introduction to Polymer Chemistry…………………………………………………...1

1.1 History of Polymers……………………………………………………………………..1 1.2 Polymer Architecture and Composition………………………………………………....4 1.2.1 Polymer Architecture…………………………………………………………….4 1.2.2 Polymer Composition…………………………………………………………….7 1.3 Controlled Radical Polymerization (CRP)………………………………………………9 1.3.1 Nitroxide Mediated Polymerization (NMP)……………………………………..9 1.3.2 Atom Transfer Radical Polymerizaton (ATRP)………………………………...12 1.3.3 Reversible Addition-fragmentation Chain Transfer (RAFT)…………………...15 1.4 Polyphosphazenes………………………………………………………………………15 1.4.1 History of Polyphosphazenes……………………………………………………15 1.4.2 Synthetic Routs of Polyphosphazenes…………………………………………..16 1.4.2.1 Thermal Ring-opening Polymerization…………………………………16 1.4.2.2 Living Cationic Condensation Polymerization…………………………19 1.4.2.3 Substitutent Exchange Reaction………………………………………..19 1.4.3 Applications……………………………………………………………………..22 1.4.3.1 Polyphosphazenes as Elastomers……………………………………….22 1.4.3.2 Polyphosphazenes as Biomaterials……………………………………..25 1.5 References………………………………………………………………………………27

Chapter 2. Synthesis and Characterization of Brush-Shaped Hybrid Inorganic-Organic Polymers Based on Polyphosphazenes………………………………………………………………..33

2.1 Introduction……………………………………………………………………………..33 2.2 Experimental Section…………………………………………………………………...38 2.2.1 Materials………………………………………………………………………...38 2.2.2 Equipment……………………………………………………………………….38 2.2.3 Synthesis of Macroinitiators…………………………………………………….39 2.2.3.1 2-[2-(Tetrahydropyranyloxy)ethoxy]ethanol (1)……………………….39 2.2.3.2 Hexakis[2-[2-(tetrahydropyranyloxy)ethoxy]ethoxy] cyclotriphosphazene (T1)…………………………………………….39 2.2.3.3 Hexakis[2-(2-hydroxyethoxy)ethoxy] cyclotriphosphazene (T2).….....40 2.2.3.4 Hexakis[2-[2-(2-bromoisobutyryloxy)ethoxy]ethoxy] cyclotriphosphazene (T3)…………………………………………….40

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2.2.3.5 Poly[bis[2-[2-(tetrahydropyranyloxy)ethoxy]ethoxy] phosphazene] (P1)…………………………………………………………………...41 2.2.3.6 Poly[bis[2-(2-hydroxyethoxy)ethoxy]phosphazene] (P2)…….……...41 2.2.3.7 Poly[bis[2-[2-(2-bromoisobutyryloxy)ethoxy]ethoxy] phosphazene] (P3)…………………………………………………………………...41 2.2.3 Polymerization…………………………………………………………...……...42 2.2.4 Star- and Comb-polystyrene-block-poly(tert-butyl acrylate (sPS-b-PBA and cPS-b-PBA)……………………………………………………………………..43 2.2.5 Solvolysis of Brush Polymers…………………………………………………...43 2.2.6 Deprotection of Star-poly(tert-butyl acrylate) (sPBA) and Comb-poly(tert-butyl acrylate) (cPBA)………………………………………………………………...44 2.2.7 Determination of Lower Critical Solution Temperature of Comb-poly(N-isopropylacrylamide) (cPNPA)…………………………………44 2.3 Results and Discussion…………………………………………………………………46 2.3.1 Initiator Syntheses………………………………………………………………46 2.3.2 Polymerization…………………………………………………………………..46 2.3.2.1 Synthesis of Star-Shaped Brush Polymers……………………………...49 2.3.2.2 Synthesis of Comb-Shaped Brush Polymers…………………………...55 2.3.3 Analysis of the Grafted Side Chains…………………………………………….58 2.3.4 Hydrolysis of sPBA/cPBA……………………………………………………...60 2.3.5 Determination of Lower Critical Solution Temperature (LCST) of cPNPA…...61 2.3.6 Synthesis of Star- and Comb-Block Copolymers……………………………….64 2.4 Conclusions……………………………………………………………………………..66 2.5 References………………………………………………………………………………67

Chapter 3. Preparation of quaternized organic-inorganic hybrid brush polyphosphazene-co-poly[2-(dimethylamino)ethyl methacrylate] electrospun fibers and their antibacterial properties………………………………………………………………..72

3.1 Introduction……………………………………………………………………………..72 3.2 Experimental section……………………………………………………………………77 3.2.1 Materials………………………………………………………………………...77 3.2.2 Equipments……………………………………………………………………...77 3.2.3 Synthesis of Macroinitiators…………………………………………………….78 3.2.4 ATRP Polymerization…………………………………………………………...78 3.2.5 Quaternization of Star-poly[2-(dimethylamino)ethyl methacrylate] (S1) and Comb-poly[2-(dimethylamino)ethyl methacrylate] (C1)………………………..79 3.2.6 Electrospinning of Quaternized S1 and C1……………………………………..79 3.2.7 Antibacterial Assay……………………………………………………………...80 3.3 Results and discussion………………………………………………………………….83 3.3.1 Synthesis of S1 and C1………………………………………………………….83 3.3.2 Quaternization of S1 and C1 with different alkyl iodides………………………88 3.3.3 Electrospinning of QS10, QS12, QC10 and QC12 Brush Polymers…………...89 3.3.4 Antibacterial assay of quaternized S1 and C1…………………………………..96 3.4 Conclusion……………………………………………………………………………...97 3.5 References………………………………………………………………………………99

Chapter 4. UV-cleavable Unimolecular Micelles Synthesis and Characterization Toward Photocontrolled Drug Release Carriers…………………………………………………...105

4.1 Introduction……………………………………………………………………………105 4.2 Experimental Section………………………………………………………………….109 4.2.1 Materials……………………………………………………………………….109 4.2.2 Equipment……………………………………………………………………...109 4.2.3 Synthesis of Photo-cleavable Initiator…………………………………………110 4.2.3.1 Hexakis[(4-formyl-2-methoxy-5-nitro)phenoxy] cyclotriphosphazene (1)…………………………………………………………………...110 4.2.3.2 Hexakis[[4-(hydroxymethyl)-2-methoxy-5-nitro]phenoxy] cyclotriphosphazene (2)…………………………………………….110 4.2.3.3 Hexakis[[[4-(2-bromoisobutyryloxy)methyl]- 2-methoxy-5-nitro]phenoxy] cyclotriphosphazene (3)……………..110 4.2.4 Polymerization…………………………………………………………………111 4.2.5 Star-Poly(methyl methacrylate)-b-Poly[poly(ethylene glycol) methyl ether methacrylate] (star-PMMA-PPEGMA)………………..…………………….111 4.2.6 Micelle Preparation…………………………………………………………….112 4.2.7 Photocleavage of o-Nitrobenzyl Group………………………………………..112 4.2.8 Fluorescence Measurements…………………………………………………...113 4.2.9 Light Scattering Measurements………………………………………………..113 4.2.10 Transmission Electron Microscopy…………………………………………..113 4.3 Results and Discussion………………………………………………………………..115 4.3.1 Initiator Synthesis……………………………………………………………...115 4.3.2 Polymerization…………………………………………………………………117 4.3.3 Photoinduced Cleavage of Homogenous Star Polymers………………………120 4.3.3.1 Effects of Molecular Weight and Concentration…………………………….120 4.3.3.2 Effects of Monomers and Solvents…………………………………………..123 4.3.4 Micellar Properties and Dye Encapsulation of Unimolecular Micelles………..125 4.3.4.1 The Formation of Micellar Structures…………………………………125 4.3.4.2 Dye Encapsulation…………………………………………………….129 4.3.4.3 Photoinduced Dissociation of star-PMMA-PPEGMA………………131 4.4 Conclusions……………………………………………………………………………133 4.5 References……………………………………………………………………………..134

Chapter 5. Substituent exchange reactions of trimeric and tetrameric aryloxycyclophosphazenes with sodium 2,2,2-trifluoroethoxide………………………………………………………139

5.1 Introduction……………………………………………………………………………139 5.2 Experimental section…………………………………………………………………..142 5.2.1 Materials and Equipment………………………………………………………142 5.2.2 Synthesis of Cyclic Trimeric Compounds……………………………………..142 5.2.2.1 Hexakis(2,2,2-trifluoroethoxy)cyclotriphosphazene (1)………………142 5.2.2.2 Hexaphenoxycyclotriphosphazene (2)………………………………...142 5.2.2.3 Hexakis(4-formylphenoxy)cyclotriphosphazene (3)………………….143 5.2.2.4 Hexakis(4-cyanophenoxy)cyclotriphosphazene (4)…………………...143 5.2.2.5 Hexakis(4-nitrophenoxy)cyclotriphosphazene (5)…………………….143

5.2.3 Synthesis of Cyclic Tetrameric Compounds…………………………………...143 5.2.3.1 Octakis(2,2,2-trifluoroethoxy)cyclotetraphosphazene (6)…………….143 5.2.3.2 Octaphenoxycyclotetraphosphazene (7)………………………………143 5.2.3.3 Octakis(4-formylphenoxy)cyclotetraphosphazene (8)………………...143 5.2.3.4 Octakis(4-cyanophenoxy)cyclotetraphosphazene (9)…………………144 5.2.3.5 Octakis(4-nitrophenoxy)cyclotetraphosphazene (10)…………………144 5.2.4 Purification of N3P3(ONa)(OCH2CF3)5 (11)…………………………………...144 5.2.5 Substituent Exchange Reactions for Both Cyclic Trimer and Tetramer Derivatives……………………………………………………………………...144 5.3 Results and Discussion………………………………………………………………..147 5.3.1 Reactions of Hexaphenoxycyclophosphazene (2) with Sodium Trifluoroethoxide…………………………………..…………………………...147 5.3.2 Side Product 11………………………………………………………………...152 5.3.3 Reactions of Hexakis(2,2,2-trifluoroethoxy)phosphazene (1) with Sodium Phenoxide………………………………………………………………………154 5.3.4 Reactions at the Cyclic Tetrameric Level: Octaphenoxycyclophosphzene (7) with Sodium Trifluoroethoxide……………………………………………………...158 5.3.5 Reactions of Octakis(2,2,2-trifluoroethoxy)cyclophosphazene (6) with Sodium Phenoxide………………………………………………………………………160 5.3.6 Reactions of 3, 4, 5 and 8, 9, 10 with Sodium Trifluoroethoxide……………..160 5.3.7 Stability of 1, 2, 6 and 7 in the Presence of Nucleophiles……………………..163 5.4 Conclusions……………………………………………………………………………166 5.5 References……………………………………………………………………………..168

Chapter 6. Substituent Exchange Reactions of Linear Oligomeric Aryloxy Phosphazenes with Sodium 2,2,2-Trifluoroethoxide…………………………………………………………..171

6.1 Introduction……………………………………………………………………………171 6.2 Experimental Section………………………………………………………………….174 6.2.1 Materials……………………………………………………………………….174 6.2.2 Equipment……………………………………………………………………...174 6.2.3 Synthesis of Chlorophosphoranimine, Cl3P=NSiMe3…………………………174 6.2.4 Synthesis of Linear Oligomeric Compounds…………………………………..175 6.2.5 Substituent Exchange Reactions by Sodium 2,2,2-Trifluoroethoxide…………176 6.3 Results and Discussion………………………………………………………………..178 6.3.1 Synthesis of Starting Oligomers (1-6)…………………………………..……..178 6.3.2 The role of solubility and crystallinity in the synthesis of oligomeric aryloxyphosphazenes…………………………………………………………..180 6.3.3 Thermal Transitions……………………………………………………………181 6.3.4 Exchange reactions of linear aryloxyphosphazene oligomers (1a-1d) with sodium trifluoroethoxide………………………………………………………………..184 6.3.5 Influence of the end units………………………………………………………187 6.3.6 Reactions of substituted aryloxyphosphazene oligomers (2a-2d, 3a-3d and 4a-4d) with sodium trifluoroethoxide………………………………………………….190 6.4 Conclusions……………………………………………………………………………191 6.5 References……………………………………………………………………………..193

Chapter 7. Substituent Exchange Reactions on Polymeric Organophosphazenes, and Their Utility……………………………………………………………………………………...196

7.1 Introduction……………………………………………………………………………196 7.2 Experimental Section………………………………………………………………….199 7.2.1 Materials………………………………………………………………………..199 7.2.2 Equipment……………………………………………………………………...200 7.2.3 Synthesis of High Polymers 1-7………………………………………………..200 7.2.4 Substituent Exchange Reactions for Polymers 1-7 with nucleophiles…………201 7.3 Results and Discussion………………………………………………………………..203 7.3.1 Exchange reactions of 1-4 with NaOCH2CF3………………………………….203 7.3.2 Exchange reactions of 5-6 with NaOCH2(CF2)4H and NaOCH2CF3…………..203 7.3.2.1 Exchange reaction of 5 with NaOCH2(CF2)4H……………………...204 7.3.2.2 Exchange reaction of 6 with NaOCH2CF3…………………………..207 7.3.3 Stability of poly[bis(2,2,2-trifluorophosphazene)] in the presence of nucleophiles…………………………………………………………………….209 7.3.4 Application of the exchange reaction to the synthesis of polyphosphazenes containing trichloroethoxy units………………………………………………..214 7.4 Conclusions………………………………………………………………………….219 7.5 References…………………………………………………………………………...221

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LIST OF FIGURES

Figure 1-1. Common organic polymers…………………………………………………………..3

Figure 1-2. Various polymer architectures……………………………………………………….5

Figure 1-3. Different types of block copolymer structures……………………………………….8

Figure 1-4. Structures of three exemplary nitroxides commonly employed in NMP…………...11

Figure 1-5. Structures of poly(fluoroalkoxyphosphazenes) as elastomers……………………...24

Figure 2-1. Dependence of a) ln([M]0/[M]); b) Mn ; c) polydispersity on time in the polymerization of different monomers from T3…………………………………………………51

Figure 2-2. GPC traces of graft polymerization of a) sPS; b) sPBA from T3…………………..54

Figure 2-3. Dependence of ln([M]0/[M]) on time in the polymerization of different monomers from P3…………………………………………………………………………………………..57

Figure 2-4. Comparison of dependence of ln([M]0/[M]) on time in the polymerization of Sty and BA from T3 and P3……………………………………………………………………………...57

Figure 2-5. Dependence of Mn and PDI on time in the polymerization of BA from P3………...57

Figure 2-6. GPC traces of solvolysis of cPBA in n-butanol…………………………………….59

Figure 2-7. LCST of cPNAP determined by a) DSC; b) DLS………………………………….63

Figure 2-8. GPC traces of the subsequent synthesis of sPS-b-PBA…………………………….65

Figure 3-1. Dependence of ln([M]0/[M]) on time in the polymerization from T3 and P3……...85

Figure 3-2. GPC traces of graft polymerization from T3……………………………………….85

Figure 3-3. Dependence of Mn and polydispersity on time in the polymerization from a) T3 and b) P3……………………………………………………………………………………………...86

Figure 3-4. 1H NMR spectra of i) macroinitiator P3, ii) C1 and iii) QC1……………………...87

Figure 3-5. Influence of polymer concentration on morphology of QS10 with Mn of 25,600: a) 5% w/v; b) 10% w/v; c) 15% w/v……………………………………………………………………91

Figure 3-6. Influence of molecular weight on morphology of SQ10: a) Mn = 25,600, 15% w/v; b) Mn = 21,800, 25% w/v; c) Mn = 19,500, 30% w/v………………………………………………93

Figure 3-7. Influence of quaternized alkyl chain length on morphology with molecular weight of

a) b) c)

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21,800: a) QS10, 25% w/v; b) QS10, 27.5% w/v; c) QS10, 30% w/v; d) QS12, 40% w/v; e) QS12, 45% w/v; f) QS12, 50% w/v……………………………………………………………..93

Figure 3-8. FESEM image of electrospun QC10 (Mn = 240,300, 20% w/v, chloroform)……...95

Figure 4-1. GPC traces of star-PMMA with varied Mn and star-PMMA84-PPEGMA80-1….119

Figure 4-2. a) GPC traces of photodegradation of star-PMMA24; b) photodegradation of star-PMMA (10 mg/ml) with different Mn; c) photodegradation of star-PMMA61 with different concentration……………………………………………………………………………………122

Figure 4-3. a) Photodegradation of star polymers with different monomers; b) photodegradation of star-PPEGMA8 (10 mg/ml) with different solvents………………………………………..124

1 Figure 4-4. H NMR spectra of the star-PMMA179-PPEGMA89-2 amphiphilic copolymer: a) in CDCl3; b) in D2O……………………………………………………………………………….126

Figure 4-5. TEM micrograph of star-PMMA179-PPEGMA89-2 micelles…………………….126

Figure 4-6. DLS graph of star-PMMA179-PPEGMA89 micelle size distribution with concentration 1 g/L: a) in H2O; b) in THF……………………………………………………...128

Figure 4-7. Fluorescence study of star-PMMA-PPEGMA polymeric micelles: a) excitation -7 spectra of pyrene (6 × 10 M) in star-PMMA179-PPEGMA89-2; b) plot of I337/I333 vs log C for star-PMMA179-PPEGMA89-2 before and after UV irradiation; c) emission spectra of star-PMMA84-PPEGMA80-1 and star-PMMA179-PPEGMA89-2 before and after UV irradiation with concentration of 0.025 g/L polymer solution………………………………….130

Figure 4-8. Comparison of photodegradation rate of star-PMMA84-PPEGMA80, star-PMMA84 and star-PPEGMA8 with polymer concentration of 10 mg/ml………………………………..132

Figure 5-1. Structures of Cyclic Trimeric and Tetrameric Phosphazenes……………………..146

Figure 5-2. 31P NMR spectra for the reaction between 2 and sodium trifluoroethoxide (molar ratio 1:12) a) 2 hr; b) 2 days; c) 16 days; d) 50 days…………………………………………...150

Figure 5-3. 31P NMR spectra for reaction between 2 and sodium trifluoroethoixde (molar ratio 1:24) a) 2 hr; b) 1 day; c) 30 days; d) 50 days…………………………………………..……...150

Figure 5-4. Reactions of hexaphenoxycyclotriphosphazene (2) with sodium trifluoroethoxide: molar ratio a) 1:12; b) 1:24……………………………………………………………………..151

Figure 5-5. 31P NMR spectra for substituent exchange reaction between 1 and sodium phenoxide for 2 days: molar ratio a) 1:12; b) 1:24……………..…………………………………………..155

Figure 5-6. Reactions of hexakis(2,2,2-trifluoroethoxy)cyclotriphosphazene (1) with sodium phenoxide: molar ratio a) 1:12; b) 1:24………………………………………………………...155

Figure 5-7. 31P NMR spectra for substituent exchange reaction between 7 and sodium

xiv trifluoroethoxide for 3 days: molar ratio a) 1:16; b) 1:32………………………………………159

Figure 5-8. 31P NMR spectrum for substituent exchange reaction between 4 and sodium trifluoroethoxide for: a) 0 hr; b) 0.5 hr; c) 4 hr; d) 1 day……………………………………….162

Figure 5-9. 31P NMR spectra for substituent exchange reaction between 9 and sodium trifluoroethoxide for: a) 0 hr; b) 0.5 hr; c) 4 hr; d) 1 day…………………………………….…162

Figure 5-10. Formation of species 11 from the reaction of 1 with sodium trifluoroethoxide: molar ratio 1:12…………………………………………………………………………………165

Figure 6-1. i) 31P NMR of oligo(dichlorophosphazene) with n = 10; ii) 31P NMR spectrum of 1b……………………………………………………………………………………………….179

Figure 6-2. GPC trace of 1b, 2b, 3b and 4b…………………………………………………...182

Figure 6-3. DSC analysis of 2a, 2b, 2c, and 2d………………………………………………..183

Figure 6-4. 31P NMR spectra for the reaction between 1a and sodium trifluoroethoxide (molar ratio 1 : 4) i) 0 day; ii) 2 days; iii) 15 days……………………………………………………..186

Figure 6-5. 31P NMR spectra for the reaction between 1d and sodium trifluoroethoxide (molar ratio 1 : 4) i) 0 day; ii) 5 days; iii) 24 days; iv) 32 days………………………………………..189

31 Figure 7-1. P NMR spectra for substituent exchange reaction between 5 and NaOCH2(CF2)4H (1: 4, reflux in THF) for: a) 0 day; b) 1 day; c) 3 days; d) 6 days……………………………...206

Figure 7-2. 31P NMR spectra for substituent exchange reaction between polymer 5 and sodium trifluoroethoxide (1: 4, reflux in THF) for: a) 0 h; b) 1 day; c) 7 days; d) 18 days; e) 48 days; f) 58 days…………………………………………………………………………………………212

Figure 7-3. 31P NMR spectra for substituent exchange reaction between 5 and sodium trifluoroethoxide (1: 15, reflux in THF) for: a) 0 h; b) 7 day…………………………………..213

Figure 7-4. 31P NMR spectra for substituent exchange reaction 1 day reflux in THF between 6 and NaOCH2CF3 in the ratio of a) 1 : 0; b) 1:2; c) 1:3; d) 1:4………………………………….217

Figure 7-5. 31P NMR spectra for substituent exchange reaction 4 days reflux in THF between 5 and sodium trichloroethoxide in the ratio of a) 1 : 0; b) 1:4; c) 1:8; d) 1:12…………………...218

LIST OF SCHEMES

Scheme 1-1. Nitroxide Medicated Polymerization……………………………………………...11

Scheme 1-2. Atom Transfer Radical Polymerization…………………………………………....14

Scheme 1-3. Reversible Addition-fragmentation Chain Transfer Polymerization……………...14

Scheme 1-4. Synthesis of Poly(organophosphazenes) by Thermal Ring-opening Polymerization…………………………………………………………………………………...18

Scheme 1-5. Cationic Polymerization of Trichlorophosphoranimines………………………….18

Scheme 1-6. Substituent Exchange Reaction……………………………………………………21

Scheme 2-1. Synthesis of Macroinitiators T3 and P3…………………………………………...37

Scheme 2-2. Graft of Different Monomers from Initiators T3 and P3………………………….48

Scheme 3-1. Synthesis of Quaternized S1 and C1………………………………………………76

Scheme 4-1. Illustration of UV-cleavable Unimolecular Micelle……………………………...108

Scheme 4-2. Synthesis of Photo-cleavable Initiator 3………………………………………….116

Scheme 4-3. Grafting of Different Monomers from Initiator 3………………………………...119

Scheme 5-1. Synthesis of Mixed-substituent Poly(organophosphazenes) by Sequential or Simultaneous Addition of Nucleophiles to (NPCl2)n or by Side Group Exchange Reaction…..141

Scheme 5-2. Two step pathway involved in the formation of compound 11…………………..153

Scheme 5-3. Two separate processes leading to the formation of 11 and 12 during the reaction of sodium phenoxide with hexakis(2,2,2-trifluoroethoxy)cyclotriphosphazene (1)………………156

Scheme 5-4. Products from the reactions between aryloxy tetramers (7-10) and sodium trifluoroethoxide………………………………………………………………………………..159

Scheme 6-1. Structures of cyclic (i and ii), linear oligomeric (iii), and high polymeric (iv) systems………………………………………………………………………………………….173

Scheme 6-2. Synthesis and characterization of oligomeric aryloxyphosphazenes…………….177

Scheme 6-3. Substituent exchange reaction process of 1d……………………………………..189

Scheme 7-1. Synthesis of Cosubstituted Poly(organophosphazenes) by Sequential or Simultaneous Addition of Nucleophiles to (NPCl2)n or by Side Group Exchange Reactions….197

xvi

Scheme 7-2. Synthesis and Characterization of Polymeric Aryloxy and Alkoxyphosphazenes…………………………………………………………………………...202

Scheme 7-3. Substituent exchange reaction process of 5 with sodium trifluoroethoxide……...212

Scheme 7-4. Reversible substituent exchange reaction between 5 and 6………………………218

xvii

LIST OF TABLES

Table 2-1. Characterization Data for Initiators………………………………………………….45

Table 2-2. Reaction Conditions for Grafting of Different Monomers from Initiator T3 and P3………………………………………………………………………………………………...50

Table 2-3. GPC Characterization of Cleaved Brush Side Chanis……………………………….59

Table 2-4. Molecular Weight and Polydispersity of Block Brush Copolymers…………………65

Table 3-1. Reaction Conditions for Grafting of DMAEMA from Initiator T3 and P3…………82

Table 3-2. Kinetic Results for Star- and Comb-brush Synthesis………………………………..82

Table 3-3. Antibacterial Effect of Water Soluble Quaternized S1 and C1……………………...95

Table 4-1. Conditions and Results for Grafting of Different Monomer on Initiator 3………...114

Table 4-2. Conditions and Results for star-PMMA-PPEGMA………………………………114

Table 4-3. Properties of star-PMMA-PPEGMA Micelles……………………………………128

Table 5-1. Reaction Conditions and Major Products…………………………………………..149

Table 5-2. Reaction time of aryloxy trimer and tetramer derivatives with sodium trifluoroethoixde………………………………………………………………………………..162

Table 6-1. Characterization data of oligomeric organophosphazenes…………………………177

Table 6-2. Solubility of different aryloxyphosphazenes with varied chain length…………….182

Table 7-1. Characterization data for polymeric organophosphazenes…………………………202

Table 7-2. Exchange reaction of polymer 5 with NaOCH2(CF2)4H…………………………...206

Table 7-3. Exchange reaction of 6 with NaOCH2CF3………………………………………….208

Table 7-4. Molecular weight change with exchange reaction time…………………………….212

Table 7-5. Substituent exchange between 6 and NaOCH2CF3…………………………………217

Table 7-6. Substituent exchange between 5 and NaOCH2CCl3………………………………..217

xviii

PREFACE

Portions of this dissertation have been adapted for publication. Chapter 2 was adapted for publication in Macromolecules and was coauthored by Harry R. Allcock, Zhicheng Tian and

Chen Chen. Chapter 3 was adapted for publication in Polymer Chemistry and was coauthored by

Harry R. Allcock, Ayusman Sen, Hua Zhang, Zhicheng Tian. Chapter 4 was adapted for publication in Polymer Chemistry and was coauthored by Harry R. Allcock, Zhicheng Tian and

Chen Chen. Chapter 5 was adapted for publication in Dalton Transactions and coauthored by

Harry R. Allcock, Jonathan P. Breon and Chen Chen. Chapter 6 was adapted for publication in

Inorganic Chemistry and coauthored by Harry R. Allcock, Jonathan P. Breon and Chen Chen.

Chapter 7 was adapted for publication in Macromolecules and coauthored by Harry R. Allcock,

Jonathan P. Breon and Chen Chen.

xix

ACKNOWLEDGEMENTS

I would like to thank Professor Harry R. Allcock for his guidance and support throughout my graduate career. The tutelage and encouragement he has provided helped develop my skills that will prove invaluable to me throughout my scientific career. I would also like to thank Prof. John V. Badding, Prof. Scott Phillips, Prof. James P. Runt for serving on my graduate committee and for their guidance during my doctoral work. I also want to thank Noreen

Allcock for her continual service and support during my time at Penn State. I would also like to thank The Pennsylvania State University and The National Institute of Health for funding my research.

I want to acknowledge several group members, past and present, who have greatly influenced my studies at Penn State with hard work, scientific discussion, guidance, and friendship. These people include Dr. Arlin Weikel, Dr. Mark Hindenlang, Dr. Nicholas Krogman,

Dr. Song Yun Cho, Dr. David Lee, Nicole Morozowich, Jonathan Breon, Zhicheng Tian, Chen

Chen, and Andrew Hess. I would also like to express my gratitude towards collaborators who proved to be valuable during my graduate career: Hua Zhang and Prof. Ayusman Sen in the

Department of Chemistry, at the Pennsylvania State University.

Finally, I would like to thank my family for all the love and support that they have shown me throughout my studies. I want to thank my mother, Xiaoping Zhang for always believing in me and pushing me to always achieve my goals. I also want to thank Xilin Ye for her never ending support and love. And finally, I would like to thank Yiqing Feng, Puhui Li and

Wei Luo, for all their care and support. Without you, this would not have been possible.

Dedicated to Hengjie Liu and Xiaoping Zhang

Chapter 1

Introduction to Polymer Chemistry

1.1 History of Polymers

Naturally occurring polymers such as cotton, cellulose, and proteins have been used by mankinds for centuries. However, it was not until the 19th century that the existence of macromolecules was demonstrated, and scientists began to acquire the knowledge to alter natural polymers for use as commercial products. In 1844, Charles Goodyear received a U.S. patent for vulcanizing rubber with sulfur and heat, which became the first successfully commercialized polymer in 1893.1,2 In 1884 Hilaire de Chardonnet started the first artificial fiber plant based on regenerated cellulose, or viscose rayon, as a substitute for silk, but it was very flammable.2 In

1907, the first totally synthetic polymer, a thermosetting phenol-formaldehyde resin called

Bakelite, was invented by Leo Baekeland.3 However until that time, scientists believed that polymers were clusters of small molecules, which were held together by an unknown force without definite molecular weights. Then in the 1920s, a new concept, referred as

“macromolecules” by Nobel Laureate Hermann Staudinger was proposed to suggest that polymers were composed of long chain-like molecules held together by covalent bonds.4 The macromolecular theory was further supported by Carothers when he synthesized the first synthetic rubber called neoprene in 1931, then the first polyester, and went on to invent nylon, a true silk replacement, in 1935.5-8 On December 10, 1953, Staudinger received his award for the concept of macromolecules and his prolonged effort to establish the science of large molecules when he was awarded the Nobel Prize for chemistry. Paul Flory was awarded the Nobel Prize in

Chemistry in 1974 for his work on polymer random coil configurations in solution in the 1950s.

Stephanie Kwolek developed an aramid, or aromatic nylon named Kevlar, patented in 1966.

These principles haeve been expanded to what we call polymer chemistry today. Several commercialized polymers such as polyethylene, polypropylene, poly(tetrafluoroethylene), poly(vinyl chloride), polystyrene and poly(methyl methacrylate) are listed in Figure 1-1.

Figure 1-1. Common organic polymers.

1.2 Polymer Definition and Architecture

Polymers are defined as large macromolecules composed of many repeat units, also

known as monomers, linked together by covalent bonds. The chemical and structural nature of

the repeat unit has significant influence over the chemical and physical properties of the resultant

polymer. In addition, the molecular weight is another important characteristics of a polymer.

Polymers of the same chemical formula have distinctly different properties when a high

molecular weight polymer is compared to a low molecular weight polymer. There is a critical

point where properties become independent of molecular weight and occurs at roughly 1000 to

2000 repeat units.

1.2.1 Polymer Architectures

Polymers are typically depicted as linear structures. In reality, polymers exist as random

coils with many conformations or are organized into crystalline domains. A polymer is called a

homopolymer if the polymer backbone consists of a single type of monomer. When a polymer

backbone is composed of two or more different monomers, this polymer is called a copolymer.

Thus, polymer skeletons can consist of linear, straight-chain macromolecules, as well as more

complicated architectures such as comb, star, dendrimer, and cross-linked systems. (Figure 1-2)

Figure 1-2. Various polymer architectures.

Linear homopolymers are the most commonly synthesized and studied polymeric architecture in polymer science. Different from the more complex architectures, linear polymers are generally more soluble in organic solvents. The chemical and physical properties of linear polymers, like viscosity and glass transition temperature, are dependent on the chemical nature of the repeat unit and the length of the skeleton.2 Some linear polymers also favor specific backbone orientations, which lead to the formation of crystalline domains.

Molecular brushes, which consis of multiple polymer chains grafted onto a linear polymer, are among the most intriguing macromolecular structures because they have unique chemical and physical properties.9-11 Due to competing forces between the backbone and side chains, brush polymers usually adopt a cylindrical conformation. The densely grafted side chains repel each other, but their ability to move apart is hindered by the backbone, which confines the side chains to a cylindrical volume.12 This leads to numerous distinctive properties of molecular brushes.

Star and dendrimer polymers are featured by a core with arms that extend outward. These two unique structures can be constructed by two main techniques: chain growth from a functional core (“graft from”) or attachment of the arms to the core (“graft onto”).13-15 The difference between star and dendrimer architectures is that a dendrimer has uniform branching from the core, whereas a star polymer may have arms of different lengths. Increased solubility and lower viscosity lead to less chain entanglement of star and dendrimer polymers.2

Cross-linked polymers are markedly different from the previously mentioned architectures, because the covalent linkages between chains make the polymers insoluble in all solvents and only allow the material to swell with the uptake of a solvent. The average number of monomers between the intermolecular cross-links is referred to as the cross-link density.

Elastomeric properties or hydrogels can be caused by as little as 1-2% cross-link density. An increased cross-link density, like 30% can result in a rigid or brittle polymer.

1.2.2 Polymer Composition

In order to obtain the desired properties, the use of combinations of different monomers for the synthesis of copolymers is also employed to overcome the limitations of some homopolymers. Many different copolymers have been synthesized such as random and alternating copolymers, block copolymers, and graft copolymers as shown in Figure 1-3.

Random copolymers are synthesized by the random addition of two monomers during a synthesis process. They are usually used to control the melting temperature (Tm) of a copolymer, because the polymer structure is largely irregular. The glass transition temperature (Tg) of the copolymer is expected to fall somewhere in between the values of the two homopolymers. Block copolymers can consist of two (diblock) or three (triblock) blocks. If phase separation occurs in polymer matrix, the Tm and Tg values are found in the range of the respective values of each homopolymers.1,2,16 Graft copolymers are another class of copolymers, where the main chain is composed of one polymer and the side chains are composed of different polymers. Crystallinity and melting points will be decreased with graft structures.

Figure 1-3. Different types of block copolymer structures.

1.3 Controlled Radical Polymerization (CRP)

The discovery of living anionic polymerization by Michael Szwarc had a tremendous effect on polymer science.17,18 Both synthetic polymer chemistry and polymer physics benefited from his pioneering work as it opened an avenue to the generation of well-defined polymers with precisely designed molecular architectures and nano-structured morphologies.19-22 However, ionic processes are very sensitive to moisture, carbon dioxide, and traces of acidic or basic compounds or other impurities. On the other hand, free radical polymerization is applicable to a large number of monomers, and tolerant of various functional groups. However, unlike living anionic polymerization, classical free radical polymerization (RP) essentially could not control molecular weight or polydispersity and could not yield block copolymers due to fast propagation and inevitable radical termination reactions.23-26 Therefore, elimination of termination reactions and minimization of concentrations of propagating radicals are essential to realize the controlled manner.

Over the past 20 years, the controlled radical polymerization (CPR) methods have been broadly developed and allow the preparation of a multitude of previously unattainable well-defined polymeric materials.27 The most widely used CPR methods are nitroxide mediated polymerization (NMP)28, atom transfer radical polymerization (ATRP)29, and reversible addition-fragmentation chain transfer (RAFT)30 polymerization. The controlled manner of these

CPRs is mainly the result of equilibria between growing radicals and dormant species, and minimization of the proportion of terminated chains in free radical polymerization.

1.3.1 Nitroxide Mediated Polymerization (NMP)

The first example of a successful CRP utilizing a nitroxide based system was reported in

1993 by Georges to describe a controlled polymerization of styrene in the presence of benzoyl

10 peroxide and the mediating stable free radical 2,2,6,6-tetramethyl-1-piperidynyl-N-oxy

(TEMPO).31 Polystyrene molecular weights increased linearly with conversion conducted at 120 oC and polydispersities below 1.3.

Control of molecular weight and polydispersity in NMP is achieved due to a dynamic equilibration between dormant alkoxyamines and actively propagating radicals. (Scheme 1-1) In this system, nitroxide radical (eg. TEMPO) serves as stable persistent radical, which neither react with itself nor with monomer to initiate the growth of new chains, and also not participate in side reactions such as the abstraction of β-H atoms.32 This persistent radical effect leads to the highly favored formation of one product to the near exclusion of other radical couplings due to one of the radical species being particularly stable and reacting quickly with the transient radical (eg. actively propagating radicals) to form a desired product.33,34 Therefore, in nitroxide mediated polymerization, the nitroxide (persistent radical) will repeatedly couple to the growing end of the polymer chain (transient radical), which would ordinarily be considered a termination step, but is, in this case, reversible (Scheme 1-1). As a result, the coupling of two active growing chains as in free radical polymerization will be minimized to protect it from termination steps, because of the high rate of coupling of the nitroxide to the growing chain end. Because of the persistent radical effect, it can be assumed that at any given time, almost all of the growing chains are “capped” by a mediating nitroxide, meaning that they dissociate and grow at very similar rates, creating a largely uniform chain length and structure.34 However, sometimes the equilibrium becomes so strongly shifted toward the dormant species and significantly reduces the polymerization rate, such that they fail to mediate polymerization of acrylates and several other monomers for this reason. The structures of three commonly employed nitroxides in NMP are shown in Figure

1-4.35-37

Scheme 1-1. Nitroxide Medicated Polymerization

Figure 1-4. Structures of three exemplary nitroxides commonly employed in NMP

1.3.2 Atom Transfer Radical Polymerization (ATRP)

As in NMP, polymerization in ATRP is also controlled by an equilibrium between propagating radicals and dormant species, predominately in the form of initiating alkyl halides/macromolecular species (Pn-X). (Scheme 1-2) The dormant species periodically react with transition metal complexes in their lower oxidation state, Mtm/L, acting as activators, where

Mtm represents the transition metal species in oxidation state m and L is a ligand. Then, the active propagating radicals (Pn•), and deactivating transition metal complexes in their higher oxidation state, X-Mtm+1/L are intermittently generated (Scheme 1-2). Afterward, a reversible reaction between deactivator X-Mtm+1/L and propagating radical occurs to reform the dormant species and the activator. Radical termination is diminished in ATRP as a result of the persistent radical effect (PRE), and the ATRP equilibrium becomes strongly shifted towards the dormant species preserving a concentration of growing radicals on the order of ppm. The molecular weight and polydispersity can be well controlled, because only few monomers are added on each activation-deactivation cycle.33,38

ATRP processes have evolved significantly during the past 15 years. By far the largest effort to develop ATRP has been reported by Matyjaszewski and coworkers.39-42 However, the original “normal” ATRP suffers some limitations such as less stability to oxidation. Reverse

ATRP is a convenient method for circumventing such oxidation problems, where the ATRP initiator and lower oxidation state transition metal activator are generated in situ from conventional radical initiators and the higher oxidation state deactivator.39,40 However, this method is featured in the inability to produce clean block copolymers. To address this problem, activator regenerated by electron transfer (ARGET) ATRP41 and initiators for continuous activator regeneration (ICAR) ATRP42 were invented. In ARGET ATRP, small amount of

13 catalyst is continuously regenerated by a reducing agent to account for unavoidable levels of radical termination41, while in ICAR ATRP, a source of organic free radicals is employed to continuously regenerate the activator, which would otherwise be consumed in termination reactions, when catalysts are used at very low concentrations.42 Both ARGET ATRP and ICAR

ATRP are considered as “green” procedures that only ppm amount of the catalyst is used.41,42

Recently, a new concept of electrochemically mediated ATRP (eATRP) has been invented, in which the ratio of the concentration of activator to deactivator is precisely controlled by electrochemistry.43 In this way, the reducing agents used en ARGET ATRP can be avoided to eliminate the additional side reactions.

Scheme 1-2. Atom Transfer Radical Polymerization.

Scheme 1-3. Reversible Addition-fragmentation Chain Transfer Polymerization.

1.3.3 Reversible Addition-fragmentation Chain Transfer (RAFT)

The control mechanism based on RAFT operates under very different principles than either NMP or ATRP. In RAFT, a steady state concentration of radicals is established via initiation and termination processes as in conventional RP.27 However, these processes rely on a thermodynamically neutral transfer reaction in which a minute amount of growing radicals undergo degenerative exchange with dormant species via a bimolecular transfer process.27 The exchange can proceed by addition-fragmentation chemistry with dithioesters. The exchange process usually proceeds via a short lived intermediate that in some cases can be considered a transition state as shown in Scheme 1-3.

1.4 Polyphosphazenes

1.4.1 History of Polyphosphazenes

High molecular weight polyphosphazenes are inorganic backbone polymers with an essentially linear backbone of alternating phosphorus and nitrogen atoms and two organic or organometallic side groups linked to each phosphorus atom.44-46 A distinctive feature of polyphosphazenes is the ease with which the polymer properties can be precisely tailored through changes in the side groups to optimize properties for specific applications, such as in bone regeneration scaffolds47,48, fire retardants49,50, low-temperature elastomers51, fuel cell membranes52,53 and solid or gel polymer lithium ion conductors.54,55 This unique synthetic approach has led to the development of over 700 different polymer structures with many different chemical and physical properties that are influenced by the different side groups.

Polyphosphazene chemistry dates back to the discovery and characterization of the small molecule ring compound hexachlorocyclotriphosphazene, (NPCl3)2, during the 1830s and

1870s.56,57 The first polyphosphazene was isolated by Stokes in the late 1890s, who

16 demonstrated that hexachlorocyclotriphosphazene could undergo thermal ring opening polymerization to form an insoluble inorganic rubber that decomposed under atmospheric condition.58,59 The major breakthrough in phosphazene chemistry occurred in 1964 when Allcock and Kugel discovered a technique to prepare soluble polyphosphazenes.44-46 Allcock and Kugel found that the Stoke’s “inorganic rubber” was a crosslinked poly(dichlorophosphazene) and was formed during the thermal polymerization of hexachlorocyclotriphosphazene. This crosslinking can be eliminated by terminating the polymerization before the crosslinking stage to yield soluble, high molecular weight poly(dichlorophosphazene), (NPCl2)n. After treatment with organic nucleophiles, hydrolytically stable phosphazene polymers were obtained. This discovery set the foundation for polyphosphazene chemistry as we know it today.

1.4.2 Synthetic Routes of Polyphosphazenes

1.4.2.1 Thermal Ring-opening Polymerization

The most widely used synthetic route to phosphazene high polymers is thermal ring-opening polymerization as shown in Scheme 1-4. This involves a thermal polymerization of

o (NPCl2)3 to (NPCl2)n in an evacuated glass tube with heating to 250 C for 6 to 30 hours. The polymerization is initiated by a cyclic cation, which is generated by the ionization of a phosphorus-halogen bond during the polymerization progress, as suggested by Allcock and

Best.60 The resultant cation and another phosphazene ring can interact to generate ring-opened cationic species and transfer the cationic charge to the phosphorus termini to yield the propagation cationic species.

The polymerization is terminated when the melt has become highly viscous. The melt polymerization must be monitored carefully because cross-linking can occur if the melt becomes too viscous. A high conversion from cyclic trimer to polymer (> 70%) can result in crosslinking.

The soluble reactive intermediate, (NPCl2)n, may then be subjected to nucleophilic substitution reactions to yield a variety of stable poly(organophosphzenes). The thermal ring-opening polymerization route can produce polyphosphazenes with molecular weights in excess of 1,000 kDa, but there is minimal control of the molecular weight and polydispersity.

Scheme 1-4. Synthesis of Poly(organophosphazenes) by Thermal Ring-opening Polymerization

Scheme 1-5. Cationic Polymerization of Trichlorophosphoranimines

1.4.2.2 Living Cationic Condensation Polymerization

Unlike thermal ring-opening polymerization, molecular weight and polydispersity of poly(organophosphazenes) can be well-controlled by living cationic polymerization of

N-silyl-phosphoranimines, first developed by Allcock, Manners and coworkers in the early

1990s.61,62 In this process, phosphorus pentachloride is utilized as a cationic initiator to polymerize a trichlorophosphoranimine (Cl3P=SiMe3) to give poly(dichlorophosphazene) as shown in Scheme 1-5. Kinetic plots of the polymerization rate (ln[M]0/[M] vs time) showed linear relationship to indicate the living character of this polymerization.

The polydispersity is usually narrow (Mw/Mn < 1.2) compared to higher values (~ 2) for the polymers from the ring-opening polymerization. More significantly, this synthetic route allows the formation of a series of block copolymers with other phosphazenes, organic polymers, or silicones.63,64 This new route to polyphosphazene chemistry has been widely explored and leads to numerous possible applications such as block copolymer micelles, dendrimers, or biocompatible block copolymers.

1.4.2.3 Substituent Exchange Reaction

An alternative synthesis approach for cosubstituted polyphosphazenes is to replace one organic substituent in the polymer by another using a second organic nucleophile (Scheme

1-6).65-67 This second approach is an appealing alternative, especially for the high polymers since it offers the prospect that single-substitutent poly(organophosphazene)s can be converted readily to mixed-substituent materials which are of broad technological interest. It also raises the possibility that a poly(organophosphazene) that is stable for long periods of time in the atmosphere might be employed as a general macromolecular intermediate for the preparation of other poly(organophosphazene)s. The chloro-derivative intermediate is sensitive to moisture and

20 must be stored under carefully controlled inert conditions. Also, the substitution reactions of the chloro intermediate can only be conducted in a limited number of organic solvents, such as benzene, tetrahydrofuran or dioxane.

Scheme 1-6. Substituent Exchange Reaction

1.4.3 Application of Polyphosphazenes

Generally, the materials properties of polyphosphazenes are determined by the skeleton and the side groups. Polyphosphazenes have inherently flexible backbones due to the bonding nature of the phosphorus-nitrogen atoms in the backbone.68 This flexibility can be altered by the use of different side groups, depending on the requirements of an application. Polyphosphazenes also have increased thermo-oxidative stability, and resistance to visible, ultraviolet, and high energy radiation, compared to most conventional organic polymers. These characteristics are useful for many target applications.

1.4.3.1 Polyphosphazenes as Elastomers

The most widely identified applications for polypohsphazenes are as high-performance elastomers with fluoroalkoxy or aryloxy side groups. The first preparation of a polyphosphazene elastomer was described by Rose in 1968, based on the similar structure of poly[bis(trifluoroethoxy)phosphazene] as shown in Figure 1-5b.69 This was a cosubstituted polyphosphazene with trifluoroethoxy and heptafluorobutoxy side groups. The presence of two different fluoroalkoxy side groups significantly reduced the crystallinity found in poly[bis(trifluoroethoxy)phosphazene] and allowed the elastomeric character to become manifest.

The second class of polyphosphazene elastomers consists of at least one of the two different side groups having a terminal –CF2H unit (Figure 1-5c), not –CF3 unit as in previous one.69,70 These polyphosphazenes have better solubility in a wide range of organic solvents and therefore have good processibility. In addition, they are considerably less expensive to produce.

Thus, most of the commercial development work has been focused on one particular type of polymer. In practice, more than two different fluoroalkoxy groups may be present in the same

23 polymer. The properties will be finely tuned with variations in the ratios of the side groups in order to satisfy the demanding needs of different applications.

Figure 1-5. Structures of poly(fluoroalkoxyphosphazenes) as elastomers

The particularly valuable properties of polyphosphazene elastomers come from their low glass transition temperatures (-66 oC, -87 oF), which allow the elastomeric character to be retained in low temperature environments. This property is markedly important, particularly in the aerospace, military, petrochemical, and gas pipeline areas. Polyphosphazene elastomer seals show critical sealing characteristic for air-filter assemblies in robust land vehicles. Furthermore, some specified polyphosphazene elastomer O-rings and diaphragms were shown to have favorable properties for use with JP-4 fuel at service temperatures from -55 to + 160 oC.70 In addition to good mechanical properties at low temperature, polyphosphazene elastomers also provide oil-, fuel- and hydraulic fluid-resistance character as well as their resistance to themo-oxidative decomposition and their nonflammability.71 These unique properties assures them an important role in the specialty polymer industry.

1.4.3.2 Polyphosphazenes as Biomaterials

Polyphosphazenes have been gaining significant attention in the biomedical field, because they can offer a number of crucial advantages for biological research and biomedical applications.72-74 As mentioned above, the side groups can be changed easily by macromolecular substitution methods to target specific combinations of properties. For example, hydrophobic polyphosphazenes with fluoroalkoxy or aryloxy side groups have been evaluated as bioinert biomaterials.75 The tested polyphosphazenes elicited only minimal biological response, which is similar to that of silicone rubber as an implantation standard. In addition, polyphosphazenes with

OCH2CF3/OCH2(CF2)xCF2H, OC6H5/OC6H5C2H5, and OCH2CF3/organosilicon, have also been considered for use in prosthetic blood vessels, artificial heart membranes, and some types of artificial heart valves, due to their elasticity and hydrophobicity that match the tissue being replaced.76 Besides, polyphosphazenes with crosslinkable composition are also used as dental

26 filling materials and soft denture liners.77 Advantages of these compositions are their resistance to bacterial and fungal colonization, and their impact-absorbing characteristics, and ease of fabrication in a dental office using simple equipment.

Other potential applications of polyphosphazenes as biomaterials are where bioerodible properties are needed, because there is always a pressing need for newly developed polymers for use as implantable materials that hydrolyze to benign small molecules. Unlike the current organic polymer implants based on poly(lactic-co-glycolic acid) (PLGA) or other polyesters, which hydrolyze to acidic products and cause tissue necrosis or irritation around the implant site, biomaterials based on polyphosphazenes have the advantage over polyesters, because they hydrolyze into non-toxic products, like phosphate, ammonium ion and side groups, with near-neutral pH values.78 Polyphosphazenes that contain amino acid ester79,80, glucosyl81, imidazole82, lactide and glycolide79 side groups hydrolyze in aqueous media. The major applications of these bioerodible polyphosphazenes focus on tissue reinforcement83, controlled delivery of drugs and vaccines84 and bone tissue regeneration.85

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Chapter 2

Synthesis and Characterization of Brush-shaped Hybrid Inorganic/Organic Polymers

Based on Polyphosphazenes

2.1 Introduction

Molecular brushes consisting of multiple polymer chains grafted onto a linear polymer are among the most intriguing macromolecular structures because they have unique chemical and physical properties.1-3 Due to competing forces between the backbone and side chains, brush polymers usually adopt a cylindrical conformation. The densely grafted side chains repel each other, but their ability to move apart is hindered by the backbone, which confines the side chains to a cylindrical volume.4 This leads to numerous distinctive properties of molecular brushes.

The most important attribute of molecular brushes is their molecular segregation. For example, unlike linear polymers, the reversible conformational changes in response to external stimuli can be limited to the single molecule identified by atomic force microscopy.5,6 In addition, steric repulsion between the side chains generates significant mechanical tension in the backbone which can be tuned by varying the grafting density, solvent quality, and the side chain length.7,8

Furthermore, a stable unimolecular micelle of cylindrical shape formed from amphiphilic molecular brush copolymers cannot dissociate in aqueous solution, which is one of the major disadvantages associated with polymer micelles formed from amphiphilic linear polymers.9,10

Because of their nonspherical macromolecular geometries and lengths up to a few hundred nanometers, brush polymers have afforded numerous potential applications in nanoscience, such as molecular actuators,11 templates for inorganic particles,9 and as precursors for nanocapsules12, nanotubes,13 and other carbon nanostructures.14 Another prominent application of molecular brushes is in the biological field, due to the similar molecular structures

34 of their natural counterparts known as proteoglycans,15,16 which are brush-like polyelectrolytes that consist of a protein backbone with carbohydrate side chains. Proteoglycans are found in a variety of places within the body and perform multiple biological functions such as cell signaling and cell surface protection,17 shock absorption, lubrication,18,19 and lung clearance.20 Therefore, molecular brushes have been widely studied as synthetic counterparts for natural proteoglycans, in order to better understand the architecture-property relationships, which could potentially lead to advances in biomedical applications.

However, even though a considerable number of molecular brushes have been synthesized and studied, most of these brush polymers are built up from backbones based on carbon-carbon polymers, such as poly(styrene),21 poly(methacrylate) derivatives,22,23 polynorbornenes,24 and poly(thiophene).25,26 A further study of molecular brushes based on novel polymer backbones offers the opportunity to extend this field in novel ways. The construction of molecular brushes based on non-carbon backbones provides a means to elucidate the effect of the backbone on molecular conformation as well as the resulting chemical and physical properties. Also, the lack of biodegradability of most molecular brushlike materials limits their applications in biological areas, some of which require degradation after fulfilling a function in the human body and allowing renal excretion of the small molecule products.27 The lengths of molecular brushes are usually up to hundreds of nanometers,9,11 which is too large for direct renal excretion, since it has been shown that only linear polymers with molecular weights below

40 kDa, or approximately 5 nm in diameter, are cleared readily through the renal system.27 A possible solution to this specific problem, is to design the backbone of molecular brush polymers to be biodegradable to release short side chain products, which are small enough to allow kidney-based excretion.

Polyphosphazenes provide a way to construct molecular brushes with a hydrolytically degradable backbone. Polyphosphazenes are hybrid organic-inorganic polymers that contain a flexible phosphorus-nitrogen backbone with various side groups, such as organic,16,28 organometallic,29 or inorganic units.30 Thus, polyphosphazenes potentially offer advantages over other polymers for biomedical applications since the side groups can be changed easily by macromolecular substitution methods to target specific combinations of properties. In addition, polyphosphazenes can be designed to be hydrolyzable, in a way that liberates the side groups and converts the backbone to a pH-buffered mixture of phosphate and ammonia which can neutralize the acid degradation products from polyester segments.31,32 The side groups can be selected to be biocompatible, the phosphate may be metabolized, and the ammonia, which is innocuous at low concentrations, can be excreted. 31,32

The preparation of some molecular brushes based on polyphosphazenes with organic polymer grafts has been reported in earlier publications.29,33 There are three main strategies for preparing such species. These are “grafting through” (the polymerization of macromonomers),34,35 “grafting onto” (the addition of previously prepared side chains to a backbone),36,37 and “grafting from” (the polymerization of side chains from a macroinitiator backbone).29,33,38 By far the largest effort to construct brush-like polyphosphazenes has been reported by Gleria and coworkers using the “grafting from” technique.29,39,40 Their strategy was to use a poly(organophosphazene) with organic side groups that would generate free radical sites when treated with peroxides or when exposed to high-energy radiation. These radical sites then served as initiation species for the free radical polymerization of vinyl-type monomers. However, one of the major drawbacks of this classical free radical polymerization process is the difficulty of controlling different structural parameters, including chemical composition, grafting density,

36 degree of polymerization of side chains and sequential grafting of second segments. In addition, a high concentration of radical species during free radical polymerization may cause intramolecular termination resulting in pendant macrocyles and even forming intermolecular coupling and macroscopic gelation.41

Controlled/living radical polymerization (CRP), especially atom transfer radical polymerization (ATRP), is a versatile route for the synthesis of well-defined polymers with predetermined molecular weights, narrow molecular weight distributions, various architectures, and useful end-functionalities.9,10,23,41 Thus, ATRP has been widely used for complete control/design of the molecular architecture of brushes, producing unique and novel molecules.4,10,22 More importantly, ATRP can maintain a low instantaneous concentration of radical species which necessarily limits termination events and avoids macroscopic gelation from intermolecular coupling. In fact, several researchers have explored the filed of synthesis of hybrid materials based on polyphosphazenes by ATRP technique.42-44

In this study, we report the preparation of nonlinear brushes with six-armed star architecture and comb structures through ATRP by using the starlike cyclotriphosphazene or a linear macroinitiator based on a polyphosphazene, with subsequent grafting-from various monomers as shown in Scheme 2-1. A series of six-armed star and comb brush polymers were synthesized and their compositions were analyzed. Confirmation of the functionality of the resultant polymers has also been demonstrated. Star- and comb-block copolymers from multifunctional polymeric macroinitiators were also synthesized.

Scheme 2-1. Synthesis of Macroinitiators T3 and P3

2.2 Experimental section

2.2.1 Materials

All reactions were carried out under a dry argon atmosphere using standard Schlenk line techniques. Tetrahydrofuran (EMD) and triethylamine (EMD) were dried using solvent purification columns.45 3,4-Dihydro-2H-pyran (Acros), diethylene glycol (Sigma), p-toluenesulfonic acid monohydrate (Alfa Aesar), pyridinium-p-toluenesulfonate (PPTS)

(Aldrich), sodium hydride (Aldrich), 2-bromopropionyl bromide (Sigma), pentamethyldiethylene triamine (PMDETA) (TCI), and copper (I) bromide (Sigma) were used as received. Styrene

(Aldrich) and t-butyl methacrylate (Aldrich) were stirred over calcium hydride for 2 days and distilled under vacuum. The distillates were stored at –54 oC before use.

N-isopropylacrylamide was purified by recrystallization from hexane to remove the inhibitor and dried under vacuum. Tris[2-(dimethylamino)ethyl]amine (Me6-TREN) was synthesized according to a literature procedure.46 Hexachlorocyclotriphosphazene (HCCTP) (Fushimi

Pharmaceutical Co., Japan or a Ningbo Chemical, China) was purified by recrystallization from hexanes and vacuum sublimation at 50 °C . Poly(dichlorophosphazene) was prepared by the thermal ring-opening polymerization of recrystallized and sublimed hexachlorocyclotriphosphazene in evacuated Pyrex tubes at 250 °C .47

2.2.2 Equipment.

1H and 31P NMR spectra were obtained using a Bruker AMX-360 NMR spectrometer, operated at 360 and 146 MHz respectively. 1H NMR spectra were referenced to tetramethylsilane signals while 31P NMR chemical shifts were referenced to 85% phosphoric acid as an external reference, with positive shift values downfield from the reference. All chemical shifts are reported in ppm. Molecular weight distribution data were obtained using a Hewlett-Packard HP

1090 gel permeation chromatograph equipped with two Phenomenex Phenogel linaer 10 columns and a Hewlett-Packard 1047A refractive index detector. The samples were eluted at 1.0 mL/min with a 10 mM solution of tetra-n-butyl ammonium nitrate in THF. The elution times were calibrated with polystyrene standards.

2.2.3 Synthesis of Macroinitiators

2.2.3.1 2-[2-(Tetrahydropyranyloxy)ethoxy]ethanol (1).

At -10 °C, 3,4-dihydro-2H-pyran (21.3 g, 0.23 mol) was added over a period of 45 min to a mixture of 46 mg of p-toluenesulfonic acid (PTS) in 180 ml (1.89 mol) of diethylene glycol.

The reaction mixture was stirred for 1 h at -10 °C and then for 2 h at room temperature. The mixture was poured into 500 ml of 1 M NaOH(aq) and extracted with dichloromethane (5 × 200 mL). The combined organic layers were dried over MgSO4 and concentrated under vacuum. The crude product was distilled at reduced pressure. The colorless liquid product was obtained with

1 yield of 30.7 g (70.0 % ). H NMR (CDCl3), δ: 4.55 (t, OCHO, 1 H, J = 3.54 Hz), 3.80-3.43 (m,

OCH2, 10 H), 1.72-1.44 (m, CH2, 6 H).

2.2.3.2 Hexakis[2-[2-(tetrahydropyranyloxy)ethoxy]ethoxy] cyclotriphosphazene (T1).

A THF solution (50 ml) of hexachlorocyclotriphosphazene (2.0 g, 5.75 mmol) was added dropwise to a THF (50 ml) suspension of the sodium salt of 1, prepared from

2-[2-(tetrahydropyranyloxy)ethoxy]ethanol (7.66 g, 40.3 mmol) and sodium hydride (1.73 g,

43.1 mmol). The solution was stirred for 48 h at reflux. THF was removed by rotary evaporation and the mixture was redissolved in dichloromethane. The mixture was transferred to a separatory funnel and extracted consecutively with deionized water (100 ml × 3), NaHCO3(aq) (100 ml × 3) and deionized water (100 ml × 3). The organic phase was dried over MgSO4 overnight then filtered, and the solvent removed by rotary evaporation. The crude product was purified by silica

40 column chromatography with an eluent of hexane/ethyl acetate (4:6). After evaporation of the

1 eluent, a colorless oil was obtained with a yield of 6.2 g (84.9%). H NMR (CDCl3), δ: 4.60 (t,

31 OCHO, 1 H, J = 3.44 Hz), 4.05-3.50 (m, OCH2, 10 H), 1.83-1.51 (m, CH2, 6 H). P NMR

(CDCl3), δ: 18.5 (s).

2.2.2.3 Hexakis[2-(2-hydroxyethoxy)ethoxy] cyclotriphosphazene (T2).

Hexakis[2-[2-(tetrahydropyranyloxy)ethoxy]ethoxy] cyclotriphosphazene (T1) (1 g,

0.79 mmol) was first dissolved in dichloromethane (5 mL) and a solution of pyridinium-p-toluenesulfonate (PPTS) (0.06 g, 0.24 mmol) in absolute ethanol (50 mL) was added slowly. The solution was heated at 50 °C for 1 day. The ethanol was removed by rotary evaporation and redissolved in deionized water. The crude product was purified by means of a

1 LH 20 column. Yield: 0.2 g, 33.1%. H NMR (D2O), δ: 4.17 (t, POCH2CH2O, 2 H, J = 4.43),

31 3.80-3.66 (m, OCH2, 6 H). P NMR (D2O), δ: 18.4 (s).

2.2.2.4 Hexakis[2-[2-(2-bromoisobutyryloxy)ethoxy]ethoxy] cyclotriphosphazene (T3).

Hexakis[2-(2-hydroxyethoxy)ethoxy] cyclotriphosphazene (T2) (1 g, 1.30 mmol) was placed in a 100 ml Schlenk flask with THF (50 ml), triethylamine (1.2 g, 11.8 mmol) and

4-dimethylaminopyridine (DMAP) (0.48 g, 3.90 mmol). A solution of 2-bromopropionyl bromide (2.7 g, 11.8 mmol) in 20 ml THF was added dropwise to the reaction mixture at 0 °C in an ice bath. The mixture was stirred for 2 days and allowed to warm to room temperature. The solvent was removed by rotary evaporation and the residue was redissolved in dichloromethane.

The mixture was transferred to a 200 ml separatory funnel and extracted consecutively with deionized water (100 ml × 3), NaHCO3(aq) (100 ml × 3) and deionized water (100 ml × 3). The organic phase was dried over MgSO4 then filtered, and the solvent removed by rotary evaporation. The crude product was passed through a silica gel column with an eluent of

41 dichloromethane and ethyl acetate (5:3). The solvent was removed, and the resulting light yellow

1 oil was dried under vacuum at room temperature. Yield: 0.41 g (27.0%). H NMR (CDCl3), δ:

4.17 (t, POCH2CH2O, 2 H), 3.96 (t, CH2OC(O), 2 H, J = 4.86 Hz) 3.67-3.61 (m, OCH2, 4 H),

31 1.85 (s, C(Br)CH3, 6 H). P NMR (CDCl3), δ: 18.4 (s).

2.2.2.5 Poly[bis[2-[2-(tetrahydropyranyloxy)ethoxy]ethoxy]phosphazene] (P1).

A THF solution (150 ml) of poly(dichlorophosphazene) (3.0 g, 25.9 mmol) was added to a THF (150 ml) suspension of the sodium salt of 1, prepared from

2-[2-(tetrahydropyranyloxy)ethoxy]ethanol (12.3 g, 64.7 mmol) and sodium hydride (2.90 g,

72.5 mmol). The reaction solution was stirred for 48 h at reflux. The polymer solution was concentrated by rotary evaporation and the residue was poured into water to obtain the precipitate of the polymeric product, which was further purified by repeated precipitation three times into water and n-hexane. The pure product was dried under vacuum to yield a yellow

1 adhesive solid: 8.7 g (74.5%). H NMR (CDCl3), δ: 4.60 (br, s, OCHO, 1 H), 4.04-3.49 (m,

31 OCH2, 10 H), 1.84-1.50 (m, CH2, 6 H). P NMR (CDCl3), δ: -7.85 (s). Mn=165,200, PDI = 3.8

2.2.2.6 Poly[bis[2-(2-hydroxyethoxy)ethoxy]phosphazene] (P2).

Poly[bis[2-[2-(tetrahydropyranyloxy)ethoxy]ethoxy]phosphazene] (P1) (9.61 g, 22.7 mmol) was first dissolved in dichloromethane (50 mL), and absolute ethanol (200 mL) was added slowly with pyridinium-p-toluenesulfonate (0.57 g, 2.27 mmol). The solution was strired at 50 °C for three days. The polymeric product was purified by dialysis against methanol for 4

1 days. An adhesive yellow product was obtained with yield of 4.86 g (50.2%). H NMR (D2O), δ:

31 4.19 (br, s, POCH2CH2O, 2 H), 3.78-3.66 (m, OCH2, 6 H). P NMR (D2O), δ: -5.65 (s).

2.2.2.7 Poly[bis[2-[2-(2-bromoisobutyryloxy)ethoxy]ethoxy]phosphazene] (P3).

Poly[bis[2-(2-hydroxyethoxy)ethoxy]phosphazene] (P2) (4.86 g, 19 mmol) was placed

42 into a 100 ml Schlenk flask with DMF (30 ml), triethylamine (5.78 g, 57.1 mmol) and

4-dimethylaminopyridine (2.32 g, 19.0 mmol). A solution of 2-bromopropionyl bromide (13.1 g,

57.1 mmol) in 10 ml DMF was added dropwise to the reaction mixture at 0 °C in an ice bath.

The reaction mixture was stirred overnight and allowed to warm to room temperature. The solvent was removed by rotary evaporation and the residue was redissolved in methanol and dialyzed against methanol for 3 days to remove the impurities. The solvent was removed, and the resulting adhesive yellow product was dried under vacuum at room temperature. Yield: 7.46 g

1 (71.0%). H NMR (CDCl3), δ: 4.17 (br, s, POCH2CH2O, 2 H), 4.06 (br, s, CH2OC(O), 2 H)

31 3.74-3.67 (m, OCH2, 4 H), 1.92 (s, C(Br)CH3, 6 H). P NMR (CDCl3), δ: -7.80 (s). Mn = 88,500,

PDI = 2.06

2.2.3 Polymerization.

A typical polymerization was as follows: Polymeric macroinitiator P3 (0.216 g, 0.78 mmol initiator centers), t-butyl acrylate (20 g, 0.156 mol) and pentamethyldiethylene triamine

(PMDETA) (0.135 g, 0.78 mmol) were placed in a 50 ml Schlenk flask and sparged with nitrogen for 30 min. Anisole (0.5 ml) was used as an internal standard. Afterward, deoxygenated copper (I) bromide (0.056 g, 0.39 mmol) and copper (II) bromide (4.4 mg, 0.02 mmol) were added. Approximately 0.2 ml of solution were removed, and the nitrogen-filled flask was heated at 90 °C under nitrogen. Periodically, additional 0.2 ml aliquots were removed to analyze the conversion and molecular weight by 1H NMR and GPC. The polymerization was terminated after 34.5 h at conversion 19.9% and was quenched by liquid nitrogen. The reaction mixture was then dissolved in dichloromethane and passed through a short alumina column to remove the copper catalyst. The polymer was purified by precipitation into cold methanol/water (4:1). Yield:

1.73 g (15.5%) of isolated polymer. The polymerization conditions of all other monomers are

43 listed in Table 2-1.

2.2.4 Star- and Comb-polystyrene-block-poly(tert-butyl acrylate) (sPS-b-PBA and cPS-b-PBA).

A typical polymerization procedure is as follows: star-polystyrene (sPS) (0.2 g, 0.066 mmol initiator centers), t-butyl acrylate (5.93 g, 46.3 mmol) and PMDETA (0.023 g, 0.132 mmol) were placed into a 20 ml Schlenk flask and sparged with nitrogen for 30 min. Deoxygenated copper (I) bromide (9 mg, 0.066 mmol) and copper (II) bromide (0.74 mg, 0.0033 mmol) were then added. The flask was heated at 90 °C under nitrogen. The polymerization was terminated after 24 h at a conversion of 61.6% and was quenched by liquid nitrogen. The reaction mixture was dissolved in dichloromethane and was passed through a short alumina column to remove the copper catalyst. The polymer was purified by precipitation into cold methanol/water (4:1) Yield:

2.5 g of isolated polymer.

2.2.5 Solvolysis of Brush Polymers.

The side chains of star-poly(t-butyl acrylate) (sPBA) and comb-poly(t-butyl acrylate)

(cPBA) were cleaved in the similar manner. Typically, 0.1 g of polymer (sPBA or cPBA) was dissolved in 5 ml THF in a 50 ml Schlenk flask. n-Butanol (18 ml) was added. After the addition of 8 drops of concentrated sulfuric acid, the mixture was heated to 90 °C for 19 days. The solvent was removed under vacuum, and the remaining polymer was dissolved in dichloromethane and precipitated into cold methanol/water (4:1). The resultant polymer was dried under vacuum and analyzed by GPC.

The side chains of star-polystyrene (sPS) and comb-polystyrene (cPS) were cleaved in the similar manner. In a typical reaction, 0.1 g of polymer (sPS or cPS) was dissolved in 20 ml THF in a 50 ml Schlenk flask, followed by the addition of 5 ml 1 M KOH ethanol solution. The

44 mixture was heated to 90 °C for 12 days. The solvent was removed by rotary evaporation. The crude product was redissolved in dichloromethane, extracted with deionized water, and dried over MgSO4. After precipitation in methanol, the resultant polymer was analyzed by GPC.

2.2.6 Deprotection of Star-poly(tert-butyl acrylate) (sPBA) and Comb-poly(tert-butyl acrylate) (cPBA).

Star-poly(acrylic acid) (sPAA) and comb-poly(acrylic acid) (cPAA) were prepared by hydrolysis of the tert-butyl esters of sPBA and cPBA following a literature method.48 A typical procedure is as follows: sPBA (0.2 g) and iodotrimethylsilane (0.62 g, 3.12 mmol) were allowed to react in 15 ml dry dichloromethane under nitrogen for 1 day. The volatiles were then evaporated. After redissolving the mixture in methanol, the crude product was dialyzed against methanol for 2 days. The solvent was removed by rotary evaporation and dried at room temperature under vacuum overnight. Yield: 0.06g

2.2.7 Determination of Lower Critical Solution Temperature of

Comb-poly(N-isopropylacrylamide) (cPNPA).

The lower critical solution temperature (LCST) of comb-poly(N-isopropylarylamide)

(cPNPA) was evaluated by dynamic light scattering (DLS) using a particle size analyzer

(Zetasizer Nano S, Malvern Instruments Ltd.) with a scattering angle of 90 ° and a thermostatically controlled cell having a heating rate of 1 °C min-1. Aqueous samples with a concentration of 2 mg/ml were filtered through a 0.45 μm syringe filter before measurement of particle size for each sample. Also, the LCST was determined by differential scanning calorimetry (DSC) with a TA Instruments Q10 and a heating rate of 10 oC/min and a sample size of ca. 10 mg.

Table 2-1. Characterization Data for Initiators

31 Entry P NMR Mn PDI RU T1 18.5 ppm 765.6 ------T2 18.4 ppm 1270.3 ------T3 18.4 ppm 1659.5 ------P1 -7.8 ppm 165,200 3.8 390 P2 -5.6 ppm --- a ------P3 -7.8 ppm 88,500 2.06 177 a P2 did not dissolve in THF.

2.3 Results and Discussion

2.3.1 Initiator Syntheses

In this work, the “graft-from” approach was used to construct both star-shaped and comb-shaped brush polymers. For this purpose, initiators T3 and P3 were synthesized as illustrated in Scheme 2-1 using hexachlorocyclotriphosphazene (T1) or poly(dichlorophosphazene) (P1) as starting materials. As a representative example, the following pathways were involved in the preparation of initiator P3. Diethylene glycol was monoprotected by dihydropyran to yield compound 1. The sodium salt of 1 was allowed to react with poly(dichlorophosphazene) to produce polymer P1. The singlet resonance at -7.85 ppm in the 31P

NMR spectrum suggested complete chlorine replacement. In the deprotection of the pyranyl moiety, the polymer P1 solution in THF/ethanol was treated at 50 °C in the presence of pyridinium p-toluenesulfonate to yield polymer P2. The completion of deprotection was confirmed by the 1H NMR spectrum with the total disappearance of the resonances at 4.60 ppm and between 1.84 and 1.50 ppm, which were due to pyranyl groups.

The macroinitiator P3 was synthesized by esterification of polymer P2 with 2-bromopropionyl bromide. The 1H NMR spectrum indicated 100% esterification of hydroxyl groups from the ratio between the methylene group at 4.17 ppm (2H) and the methyl groups at 1.92 ppm (6H). The initiator T3 was synthesized by a similar approach. Characterization data for the initiators are summarized in Table 2-1. Both of the initiators were used to induce polymerization of various monomers.

2.3.2 Polymerization

A series of star- and comb-shaped brushes were synthesized by grafting styrene, tert-butyl acrylate, and N-isopropylacrylamide from the aforementioned initiators (T3 and P3) by

47 controlled atom transfer radical polymerization (ATRP) as illustrated in Scheme 2-1. Because the initiating groups remain at the ends of the grafted side chains, it was possible to extend the side chains in a well-defined manner.

Scheme 2-2. Graft of Different Monomers from Initiators T3 and P3

2.3.2.1 Synthesis of Star-Shaped Brush Polymers

For each of the monomers studied, conditions were developed using T3 as an initiator, which provided linear first-order kinetic plots typical of a controlled living polymerization

(Table 2). Figure 2-1a shows the linear relationships of ln([M]0/[M]) vs time for these three monomers. This means that the concentration of growing radicals is constant during the polymerization in all systems, confirming the first-order in monomer concentration kinetics.

However, the molecular weight vs. time plot shown in Figure 2-1b illustrates marked deviations from the theoretical value calculated from the conversion of monomer in Table 2-2 , which is approximately twice as high as the molecular weight measured by GPC (Table 2-2). For a controlled living polymerization the observed molecular weight should coincide with the theoretical value. The deviation of molecular weight from the theoretical value may be due to the highly compact nature of the polymers, which results in lower hydrodynamic volumes of star brush polymers and does not correspond well to the linear standards.49

Table 2-2. Reaction Conditions for Grafting of Different Monomers from Initiator T3 and P3

[M]:[I]: Mn (PDI) temp time conv Entry M [CuBr]:[CuBr2]: (°C) (h) (%) GPCa Convb Cleavagec [Ligand] sPS Sty 1200:1:3:0.15:6 105 31.5 38.3 27,600 (1.22) 49,200 19,700 sPBA BA 1200:1:3:0.15:6 90 13.5 61.7 45,700 (1.10) 96,600 53,000 sPNPA NPA 1200:1:3:0.15:6 55 25 15.4 20,400 (1.35) 22,800 - cPS Sty 400:1:2:0.1:1 105 40 22.4 200,300 (1.94) 1,903,700 862,100 cPBA BA 400:1:2:0.2:1 90 34.5 19.9 125,700 (1.38) 1,749,600 2,086,300 cPNPA NPA 400:1:2:0.1:1d 50 96 5.2 173,900 (1.79) 1,349,000 - a Measured by GPC calibrated by linear polystyrene standards. b Calculated from conversion measured by NMR. c Calculated from cleaved side chains. d Reaction was conducted in methanol/DMF (10ml/7ml) and the ligand is Me6TREN.

a) 0.65 0.60 sPS 0.55 sPBA sPNPA 0.50 0.45 0.40

0.35

]/[M]

0 0.30 0.25

ln[M 0.20 0.15 0.10 0.05 0.00 0 5 10 15 20 25 30 35 Time (hr)

60000 b) sPS 55000 sPBA 50000 sPNPA 45000 40000

n 35000 M

30000 25000 20000 15000 10000 5000 0 0 5 10 15 20 25 30 35 Time (hr)

c) 1.35

1.30 sPS sPBA 1.25 sPNPA

PDI 1.20

1.15

1.10

1.05

0 5 10 15 20 25 30 35 Time (hr)

Figure 2-1. Dependence of a) ln([M]0/[M]); b) Mn; c) polydispersity on time in the polymerization of different monomers from T3.

During optimization of the ATRP of tert-butyl acrylate at 90 °C , crosslinking occurred when using higher concentrations of catalyst relative to initiator or lower monomer-to-initiator ratios. Even when the ratio of monomer-to-initiator increased to 150 to 1, crosslinking was still detected. One consequence of using radical polymerization to grow the side chains from the backbone is that radical-radical coupling must be significantly suppressed. When the concentration of the active species is too high, radical coupling resulted in aggregates of stars, the appearance of a high molecular weight shoulder on the GPC traces, and ultimately crosslinking.50,51 The results showed that crosslinking can be significantly suppressed when the monomer-to-initiator ratio is increased to 200 to 1.

The temperature should also be carefully controlled in order to obtain well-defined molecular structures. For example, it has been reported that, in some cases, ATRP of

N-isopropylacrylamide in grafting reactions when heated or at room temperature may result in gel-like products due to crosslinking.25 However, no crosslinking was detected in the present reactions either at room temperature or when heated. In contrast, increased temperature (50 oC) is necessary to improve the grafting efficiency of N-isopropylacrylamide.

Also, a sufficiently low active species concentration, which is 50 mol % relative to initiator, was used in order to avoid crosslinking and obtain well-defined molecular brushes with monomodal and narrow molecular weight distributions even at high monomer conversions. In addition, the deactivation species CuBr2 (2.5 mol %) was added to avoid its spontaneous formation in situ by radical termination. This established better control by anticipation of the persistent radical effect.52 Therefore, in every case, the graft polymerization of each monomer was well controlled, and this resulted in the synthesis of polymers with low polydispersity

(Figure 2-1c). The absence of termination reactions from recombination reactions is indicated

53 by the absence of a small shoulder in the high molecular weight portion of the GPC trace for both sPS and sPBA (Figure 2-2).

a) 1.0 3 hr 6.5 hr 10 hr 0.8 20 hr 29 hr

0.6

0.4

0.2 rel.RI-detector intensity 0.0

17.0 17.5 18.0 18.5 19.0 19.5 20.0 20.5 21.0 21.5 22.0 22.5 RT (min)

b) 1.0 1 hr 3 hr 5 hr 0.8 7 hr 11 hr

0.6

0.4

0.2 rel.RI-detector intensity 0.0

17.0 17.5 18.0 18.5 19.0 19.5 20.0 20.5 21.0 21.5 22.0 22.5 RT (min)

Figure 2-2. GPC traces of graft polymerization of a) sPS; b) sPBA from T3.

2.3.2.2 Synthesis of Comb-Shaped Brush Polymers.

Similarly, comb-shaped brush polymers were synthesized by grafting styrene, tert-butyl acrylate, and N-isopropylacrylamide from the P3 macroinitiator as illustrated in Scheme 2-2. As in the synthesis of the star-shaped brush polymers, Figure 3 shows linear first-order kinetic plots typical of a controlled living polymerization, which indicates conservation of radicals throughout the reaction of each monomer. However, a significant difference in polymerization rate was found when compared with the star-shaped brush system. These differences are apparent from the comparison in Figure 4, especially between the polymerization of styrene and tert-butyl acrylate from T3 and P3 initiators. In general, the polymerization rate in the comb-brush system is much slower than for the star-brush system. This is probably due to the more sterically hindered conformation of macroinitiator P3. Thus, the presence of adjacent polymer chains in comb-brush polymers may hinder polymer chain growth from the beginning. The star-brush system may also suffer from the same problem, but the relatively wide-open core structure will allow a much faster polymerization rate than the comb-brush system. Another specific example to illustrate the steric hindrance effect is the polymerization rate of styrene and tert-butyl acrylate in the same system. For example, sPBA has higher polymerization rate than sPS in star-brush reactions in the cyclotrimeric system (T3). This is probably due to the difference in reactivity among the different monomers. However, nearly identical polymerization rates of these two monomers were found in the comb-brush system (Figure 2-4), probably because the reactivity difference between styrene and tert-butyl acrylate is dominated by the steric hindrance effect, leading to the similar polymerization rates in the comb-brush system.

No linear increase of molecular weight was detected during the synthesis of comb-brush polymers in contrast to the structure found in the star system. The tendency of molecular weight

56 change throughout the polymerization process is illustrated in Figure 2-5. During the grafting of tert-butyl acrylate from the macroinitiator P3, the GPC-derived molecular weight initially appeared to increase from 88,500 to 107,700 but was followed by a dramatic decrease down to

67,900. Later, the molecular weight increased again gradually as the polymerization progressed and reached a final value of 125,700. This abnormal result may be due to the difference in hydrodynamic radii between the macroinitiator P3 and the resultant cPBA. In the first hour of polymerization, only 0.76% of tert-butyl acrylate was grafted onto the backbone of polyphosphazene P3. Thus, the molecular weight was still defined by the hydrodynamic radius of the polyphosphazene backbone structure and an expected increase in molecular weight was detected by GPC. However, as more and more tert-butyl acrylate was grafted, the hydrodynamic radius became dominated by brush poly(tert-butyl acrylate) units instead of polyphosphazene, and this resulted in the initial significant decrease in measured molecular weight, followed by a gradual increase. Therefore, the actual molecular weight may be higher than the value measured by GPC. In fact, both the molecular weights calculated from conversion and from chain cleavage indicate a remarkably higher value than the one reflected by GPC (Table 2-2).

An interesting aspect is that the molecular weights of cPBA increased with polymerization time, while the molecular weight distributions decreased from 2.06 to 1.38. This decrease of the molecular weight distribution may be attributed to the formation of well defined poly(tert-butyl acrylate) side chains by ATRP.53 It should be pointed out that the molecular weight distribution of P3 was larger than 2 since the macroinitiators were prepared by thermal ring-opening polymerization, which usually provides no control over molecular weight distribution. In addition, all the GPC traces of the polymer brushes were found to be unimodal without any trace of a shoulder, indicating that almost all the macroinitiators were converted to the corresponding polymer brushes.

0.25 cPS cPBA 0.20 cPNPA

0.15

]/[M]

0 0.10 ln[M 0.05

0.00

0 5 10 15 20 25 30 35 40 45 Time (hr)

Figure 2-3. Dependence of ln([M]0/[M]) on time in the polymerization of different monomers from P3.

0.55 0.50 sPS 0.45 sPBA cPS 0.40 cPBA 0.35

/[M]) 0.30 0

0.25

0.20 ln([M] 0.15 0.10 0.05 0.00 0 5 10 15 20 25 30 35 40 45 Time (hr)

Figure 2-4. Comparison of dependence of ln([M]0/[M]) on time in the polymerization of Sty and BA from T3 and P3.

140000 2.2 120000 2.0 100000 1.8 80000

PDI

n 1.6

M 60000

1.4 40000

20000 1.2

0 1.0 0 5 10 15 20 25 30 35 Time (hr)

Figure 2-5. Dependence of Mn and PDI on time in the polymerization of BA from P3.

2.3.3 Analysis of the Grafted Side Chains

In order to determine the uniformity of the grafted side chains, both the star- and comb-shaped brush polymers were subjected to solvolysis to release the grafted side chains. sPS and cPS were cleaved with potassium hydroxide in THF and ethanol to release the polystyrene side chains. On the other hand, sPBA and cPBA were cleaved using acid-catalyzed transesterfication in n-butanol to ensure that the tert-butyl ester groups of sPBA and cPBA side chains remained either intact or were replaced with n-butyl without formation of free carboxylic acid groups.

The GPC traces of the starting brush cPBA and the hydrolyzed product are given in

Figure 2-6. The peak of the cPBA polymer disappeared after 10 days solvolysis. Instead, a low and a high molecular weight fraction appeared which confirmed the degradation of the polyphosphazene backbone and the release of the cleaved side chains respectively. Complete cleavage required 19 days of reaction. The low molecular weight fraction has a symmetrical

GPC trace, the number-average molecular weight is 5,600, and the polydispersity is 1.16.

Following a similar procedure, the side chains of sPBA, sPS and cPS brush polymers were cleaved from the polymer backbones and the low molecular weight fractions were analyzed by

GPC. The results were listed in Table 2-3.

Table 2-3. GPC Characterization of Cleaved Brush Side Chanis Star Comb

sPS sPBA cPS cPBA Reaction Time 12 d 12 d 12 d 19 d Side Chain Mn 3,001 8,549 2,159 5,617 PDI 1.16 1.19 1.21 1.16 Polymer Mn 19,773 53,061 862,198 2,086,330

10 d 1.0 0 day 0 d 19 d 10 days 19 days 0.8

0.6

0.4

0.2 relRI-detector intensity

0.0 12 13 14 15 16 17 18 19 20 21 22 RT (min)

Figure 2-6. GPC traces of solvolysis of cPBA in n-butanol

No significant tailing of the cleaved side chains from all four brushes was detected by

GPC and the narrow unimodal distribution of the detached poly(tert-butyl acrylate) and polystyrene substantiates the well-controlled ATRP reaction of tert-butyl acrylate and styrene initiated by T3 and P3. The potential reactions that could lead to a bimodal side chain distribution such as intra- or intermolecular coupling have clearly been effectively suppressed by controlling the amount of catalyst used.

It is interesting to compare the molecular weights calculated based on the cleaved side chains with those detected by GPC (Table 2-2). For star-like brush polymers, no significant difference of the molecular weights was found by these two methods which is probably due to the relatively less densely grafted polymer side chains on star-shaped initiator T3 as a consequence of its relatively open conformation. However, a significant difference in molecular weight calculated by these two methods was found for comb-like polymers (cPS and cPBA). As mentioned above, the probable reason is that the densely grafted and compact side chains make the hydrodynamic radius much smaller than that of the corresponding linear polymers, so the molecular weights of comb-like polymers appear to be lower when compared to the molecular weights calculated from either conversion or from the cleaved side chains.

Moreover, no remaining backbone polymer was detected in the GPC traces. This may be because of the low proportion of backbone with respect to side chains, or because of degradation of the polyphosphazene backbone into ammonium ion and phosphate.31,32

2.3.4 Hydrolysis of sPBA/cPBA

The brushes with PBA side chain were subjected to further functionalization in order to obtain a negatively charged polyelectrolyte.

sPBA and cPBA were treated with trimethylsilyl iodide to deprotect the tert-butyl ester

61 group and form a carboxylic acid group.48,54 These conditions avoid the possible cleavage of grafted side chains under harsh acidic or basic conditions. The reaction was completed within 24 hours. The resultant product was 100% deprotected, as determined by 1H NMR with complete disappearance of tert-butyl group at 1.4 ppm and was soluble in water but insoluble in tetrahydrofuran and chloroform, which is significantly different from the parent polymers, thus indicating the total cleavage of tert-butyl groups from the brush polymers.

2.3.5 Determination of Lower Critical Solution Temperature (LCST) of cPNPA

The lower critical solution temperature (LCST) of cPNPA was examined by both differential scanning calorimetry (DSC) and dynamic light scattering (DLS).

Figure 2-7a shows the enthalpy of transition of the grafted polymer cPNPA in water using DSC by repeatedly cycling the solution between 20 and 55 °C . An endothermic transition at 32.3 ± 0.3 °C was detected which is assigned to the enthalpy change associated with the breaking/making of hydrogen bonds between poly(isopropyl acrylamide) grafts and water.55 This result is consistant with previous studies of poly(isopropyl acrylamide) and confirms that the stimuli-responsive conformational change of cPNPA is reversible and sensitive to temperature variations.

In addition, the LCST and the corresponding molecular conformation transition of polymer cPNPA were also studied by DLS, which shows a change in molecular shape after passing through the LCST at 33 °C . In Figure 2-7b the variations of apparent hydrodynamic radius (Rh) and Mw with temperature are summarized. The hydrodynamic radius of brush polymer cPNPA was found to gradually decrease from 26 nm at 20 °C to 21 nm at 32.5 °C , followed by a large increase from 21 nm to 33 nm within a small temperature interval of 0.5 °C .

The first slow decrease of Rh from 26 to 21 nm reflects the shrinkage of the individual brush

62 polymer, where the repulsion of the densely grafted side chains represents the extension force and acts against the entropic contraction force from the phase transition due to increase of temperature. This result is similar to that reported earlier for a different system.56 Afterward, the significant increase of Rh was detected beyond the LCST of cPNPA up to 34 nm, which indicates the aggregation of grafted copolymer molecules due to the enhanced hydrophobicity of poly(n-isopropylacrylamide).

a) 12.0 1st cycle 2nd cycle 3rd cycle

11.5

11.0 HeatFlow (W/g)

10.5

20 30 40 50 60 o Temperature ( C)

b) 40 38 Size 36 34 32 30 28 26

Size(nm) 24 22 20 18 16 20 25 30 35 40 o Temperauture ( C)

Figure 2-7. LCST of cPNAP determined by a) DSC; b) DLS

2.3.6 Synthesis of Star- and Comb-Block Copolymers

One of the advantages of the ATRP polymerization is the preservation of chain end functionality, from which di-block or even tri-block copolymers can be synthesized in a well-controlled manner.57 Studies show that star polymers synthesized by ATRP also exhibit conservation of active species. Hence, it is possible to synthesize star-block copolymers by polymerization of another monomer from a preformed polymeric macroinitiator.58 Therefore, both star- and comb-bush polystyrene polymers, sPS and cPS were used as macroinitiators for the copper bromide/PMEDTA mediated polymerization of tert-butyl acrylate. Figure 8 illustrates

GPC traces for the polymerizations of the homo-copolymers (sPS) and block copolymers

(sPS-b-PBA). The absence of a high molecular weight shoulder indicates no detectable brush-coupling product and a controlled polymerization reaction. The reaction was terminated at

61.6% conversion after 20 h for sPS-b-PBA. The GPC chromatograms shifted cleanly to higher molecular weight from 27,600 (Mw/Mn = 1.10) to 107,600 (Mw/Mn = 1.16). Furthermore, block copolymers cPS-b-PBA were also synthesized using cPS as a macroinitiator. The characterization data are listed in Table 4 together with sPS-b-PBA. Thus, a star- or comb-block copolymer consisting of a hard, high Tg segment in the core and a soft, low Tg segment in the shell was confirmed, which is a promising architecture in the design of thermoplastic elastomeric materials.

1.2 Star-PS Star-PS-b-PBA Star-PS-b-PBA 1.0

0.8 Star-PS

0.6

0.4

0.2 rel.RI-detector intensity 0.0

15.5 16.0 16.5 17.0 17.5 18.0 18.5 19.0 19.5 20.0 20.5 RT (min)

Figure 2-8. GPC traces of the subsequent synthesis of sPS-b-PBA.

Table 2-4. Molecular Weight and Polydispersity of Block Brush Copolymers. Star Comb

sPS sPS-b-PBA cPS cPS-b-PBA Mn 27,600 107,600 200,300 435,400 PDI 1.10 1.16 1.94 1.85

2.4 Conclusions

A variety of well-defined, densely grafted molecular brushes based on polyphosphazenes were synthesized by ATRP polymerization. Three different monomers, styrene, tert-butyl acrylate and N-isopropylacrylamide have been grafted from cyclotriphosphazene or polyphosphazene initiators to form star- or comb-shaped brush polymers.

Both systems follow first-order reaction kinetics during polymerization, exhibiting living polymerization features. The resultant polymers show well-defined structures with controlled molecular weight and low polydispersity. Also, the side chains, when cleaved from the skeleton, have a relatively low polydispersity Mw/Mn ≤ 1.21 which demonstrates the controlled nature of the grafting procedure. Positively charged molecular brushes were obtained through hydrolysis of tert-butyl groups to provide free carboxylic acid functional groups. Also, the thermal sensitivity of poly(N-isopropylacrylamide) brush polymers remains intact and independent on the side chain length. The interesting change of hydrodynamic radius before and after its LCST exhibits the unique properties of single cylindrical brush molecule with stimuli responsive behavior. Furthermore, the resultant functionalized block brush polymers with a hard polystyrene core and a soft poly(tert-butyl acrylate) shell are promising candidates for a variety of applications in thermoplastic elastomeric materials and in the biomedical field, such as drug delivery and tissue engineering.

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Chapter 3

Antibacterial Properties of Quaternized Organic-Inorganic Hybrid Brush

Polyphosphazene-co-poly[2-(dimethylamino)ethyl methacrylate] Electrospun Fibers

3.1 Introduction

Contamination caused by bacterial adhesion and proliferation on synthetic surfaces such as food processing/packaging, food service, and the waste-water treatment industries, as well as personal households, is a major concern. 1,2 Patients are significantly threatened by infections associated with post implantation and catheterization procedures, which presents a serious challenge. For example, about 1.7 million healthcare associated infections cause 99,000 deaths annually in the United States as reported by the Centers of Disease Control and Prevention.3 The cost of hospital-acquired infections has become a major burden on health care, which added about $30-45 billion to health care cost every year.4 Therefore, the development of new antibacterial materials is urgently needed.

Various classes of antibacterial materials are currently being investigated or are commercially available, among which are biguanides,5 pyridinium compounds,6 phenol derivatives, and antibiotics.7 However, a major concern about the currently available materials is the loss of antibacterial activity by leaching of the biocide and the emergence of multi-resistant pathogens.8 Recently, the implementation of naturally occurring antimicrobial host-defense peptides generated in the innate immune system has attracted much attention from both the academic and clinical points of view.9-11 When compared with conventional bactericidal drugs, antibacterial peptides posses the advantage of acting at very low concentrations and exhibiting a broad spectrum of antibacterial activity. Moreover, they have a very low propensity to promote pathogen resistance.12 Although the antibacterial effect of the peptides may involve complex

73 mechanisms, the cationic peptides selectively bind to negatively charged bacterial cell surface and disrupt bacterial cell membranes through peptide-membrane interactions, leading to disruption of membrane integrity and cell death.13 However, the wide application of the peptides as promising antibacterial agents is limited due to their high manufacturing costs, susceptibility to proteolysis and the lack of the ability to use systemically poor pharmacokinetics.14

Alternatively, synthetic cationic polymers have been widely deployed as antibacterial materials showing high levels of sustained antibacterial activity without any apparent reduction in their effectiveness. This is not only because they mimic the structural features and functions of antibacterial peptides, but also because they are relatively inexpensive to synthesize and can be produced on a large scale.15 Several cationic polymers have shown a high degree of antibacterial activity, such as polymethacrylates,16,17 polydiallyammonium salts,18,19 polyarylamides,20,21, and polyethylenimines,22-24 among which quaternized poly[2-(dimethylamino)ethyl methacrylate]

(PDMAEMA) has been widely used as a potential antibacterial material to inhibit the growth of

Escherichia coli (E. coli), Bacillus subtilis and Staphylococcus aureus 25-27 In addition,

PDMAEMA has also been incorporated in an anti-adherent coating on poly(methyl methacrylate) disks to inhibit the binding of E. coli, macrophages, and fibroblasts.25 A hypothesis for the mode of action of quaternized PDMAEMA has been suggested as follows: (1) adsorption onto the bacterial cell surface, (2) diffusion through the cell wall, (3) binding to the cytoplasmic membrane, (4) disruption of the cytoplasmic membrane, (5) release of cell cytoplasmic constituents, and (6) bacterial cell death.25,28

Surface coating is one of the most effective methods to produce an antibacterial surface with long-term stability on food packaging or medical devices. This can be accomplished by surface grafting of polymer brushes on pre-modified surfaces via the “grafting from” method, in

74 which polymerization is initiated by active species. Various surfaces such as glass slides, silicon wafers, magnetic Fe3O4 nanoparticles and polypropylene films have been grafted successfully with quaternized PDMAEMA and have showed a killing efficiency of up to almost 100%.29-31

Also, the results indicate that surface charge density is a critical element in designing a surface for maximum kill efficiency. The most active biocidal surfaces had charge densities of more than

1-5×1015 accessible quaternary amine units/cm2.30 As a simpler alternative, antibacterial surface modification can also be achieved by coating microfibers with antibacterial activity by electrospinnning techniques without modification of the surface with initiation species.

Electrospinning is an attractive approach for preparing continuous fibers from micro-scale to sub-micrometer-scale due to its low cost, wide applicability of materials and high production rate.32 The resultant fibrous materials show morphological similarities to the natural extra-cellular matrix, characterized by ultrafine continuous fibers, high surface-to-volume ratio, high porosity and variable pore-size distribution. Quaternized PDMAEMA and its copolymer derivatives have been fabricated into microfibers by electrospinning.33,34 The microfibers exhibited a significant antibacterial effect on both gram-positive and gram-negative bacteria.35,36

In this work, we have constructed star- and comb-shaped quaternized PDMAEMA from either cyclotriphosphazenes or polyphosphazenes by ATRP polymerization (Scheme 3-1), followed by an examination of the corresponding antibacterial effects on E. coli using both solutions of the polymer and electrospun microfibers. The design rationale is that 1) a more condensed positive charge density can be obtained through the densely grafted brush structure, 2) polyphosphazenes have been widely used as a versatile platform for a potential replacement of currently used biomaterials in tissue engineering and drug delivery due to their excellent biodegradability and biocompatibility,35,37 3) porous microfibers will improve the accessibility of

75 surface charge to bacteria in solution. To our knowledge, this is the first time that quaternized brush PDMAEMA derivatives have been successfully electrospun into microfibers and their antibacterial activity examined.

Scheme 3-1. Synthesis of Quaternized S1 and C1a

a The abbreviation S and C indicate star and comb brush polymers respectively. The abbreviation Q indicates that the amine is quaternary. The number indicates a carbon chain length with n repeat units.

3.2 Experimental section

3.2.1 Materials

All synthetic reactions were carried out under a dry argon atmosphere using standard

Schlenk line techniques. Tetrahydrofuran (EMD) and triethylamine (EMD) were dried using solvent purification columns.38 1-Iodomethane, 1-iodobutane, 1-iodoheptane, 1-iododecane and

1-iodododecane were purchased from Sigma and used without further purification.

3,4-Dihydro-2H-pyran (Acros), diethylene glycol (Sigma), p-toluenesulfonic acid monohydrate

(Alfa Aesar), pyridinium-p-toluenesulfonate (PPTS) (Aldrich), sodium hydride (Aldrich),

2-bromopropionyl bromide (Sigma), pentamethyldiethylene triamine (PMDETA) (TCI), and copper (I) bromide (Sigma) were used as received. 2-(Dimethylamino)ethyl methacrylate

(DMAEMA) (Aldrich) was stirred over calcium hydride for 2 days and distilled under vacuum.

The distillates were stored at –54 oC before use. Hexachlorocyclotriphosphazene (HCCTP)

(Fushimi Pharmaceutical Co., Japan or a Ningbo Chemical, China) was purified by recrystallization from hexanes followed by vacuum sublimation at 50 °C .

Poly(dichlorophosphazene) was prepared by the thermal ring-opening polymerization of recrystallized and sublimed hexachlorocyclotriphosphazene in evacuated Pyrex tubes at

250 °C .39

3.2.2 Equipments

1H and 31P NMR spectra were obtained using a Bruker AMX-360 NMR spectrometer, operated at 360 and 146 MHz respectively. 1H NMR spectra were referenced to tetramethylsilane signals while 31P NMR chemical shifts were referenced to 85% phosphoric acid as an external standard, with positive shift values downfield from the reference. All chemical shifts are reported in ppm. Molecular weight distribution data were obtained using a Hewlett-Packard HP 1090 gel

78 permeation chromatograph equipped with two Phenomenex Phenogel linaer 10 columns and a

Hewlett-Packard 1047A refractive index detector. The samples were eluted at 1.0 mL/min with a

10 mM solution of tetra-n-butyl ammonium nitrate in THF. The elution times were calibrated with polystyrene standards. Glass transition temperatures were determined by differential scanning calorimetry (DSC) with a TA Instruments Q10 and a heating rate of 10 oC/min and a sample size of ca. 10 mg. Scanning electron microscopy (SEM) was accomplished using a

Philips FEI Quanta 200 Environmental Scanning Electron Microscope. The SEM samples were prepared by placement of a polymer sample onto carbon tape, followed by insertion into the

SEM equipment. The use of low vacuum mode was used for imaging under the following conditions: 20 KeV source voltage, pressure approx. 0.75 Torr, and a working distance of approx.

10 mm.

3.2.3 Synthesis of Macroinitiators

Both initiators T1 and P1 were synthesized according to a previous described method.40

3.2.4 ATRP Polymerization.

A typical polymerization was as follows: Polymeric macroinitiator P3 (0.078 g, 0.14 mmol initiator centers), 2-(dimethyamino)ethyl methacrylate (8.8 g, 0.056 mol) and pentamethyldiethylene triamine (PMDETA) (0.049 g, 0.28 mmol) were placed in a 50 mL

Schlenk flask and purged with nitrogen for 30 min. Anisole (0.5 mL) was used as an internal standard. Afterward, deoxygenated copper (I) bromide (0.02 g, 0.14 mmol) was added.

Approximately 0.2 mL of solution was removed. Periodically, additional 0.2 mL aliquots were removed to analyze conversion and molecular weight by 1H NMR and GPC. The polymerization was terminated after 45 min at conversion 19.5% and was quenched by liquid nitrogen. The reaction mixture was then dissolved in dichloromethane and passed through a short alumina

79 column to remove the copper catalyst. The polymer was purified by precipitating three times into hexane. The resultant light yellowish solid was dried under vacuum. Yield: 1.45 g (16.3%) of isolated polymer. The reaction conditions for different brush polymers are listed in Table 3-1.

3.2.5 Quaternization of Star-poly[2-(dimethylamino)ethyl methacrylate] (S1) and

Comb-poly[2-(dimethylamino)ethyl methacrylate] (C1).

The star- and comb-shaped poly[2-(dimethylamino)ethyl methacrylate] polymers were quaternized with alkyl iodides with varied alkyl chain length. In a typical reaction, S1 (0.31 g) was dissolved in 15 mL of acetone. Iodomethane (0.42 g, 2.96 mmol) was added at room temperature at a molar ratio of 1.5 relative to the amino groups. Although the solution became turbid after 10 min, the mixture was stirred overnight to ensure a quantitative conversion.

Acetone was removed by rotary evaporation, and the residue was redissolved in deionized water and dialyzed in deionized water for 4 days. After freeze-drying under vacuum, a yellow solid was obtained with a yield of 0.18 g (30.5%). A similar method was used for the quaternization of

S1 and C1 with various alkyl iodides.

3.2.6 Electrospinning of Quaternized S1 and C1.

Electrospinning was accomplished with the apparatus described previously.41

Parameters that were kept constant during spinning were a working distance of 12 cm, a flow rate of polymer solution of 1 mL/h, and an applied potential of 20 kV. A Gamma High Voltage

Supply ES40P-20W (0-40 KV, 20 W, Gamma High Voltage Research) with a low current output was used as the power source. A positive voltage was applied to the polymer solution in the syringe by attaching an alligator clip to the needle from the positive lead. The electrospinning was carried out at ambient temperature and pressure. The spun nanofiber mats were dried under vacuum at room temperature for 24 h.

Electrospinning conditions were optimized in order to obtain bead-free fibers from quaternized S1 and C1 by varying parameters such as the nature of the solvent and the concentration of the solution. The concentration of the solution was varied from 10% to 50%

(w/v). The effect of solvents on electrospinning was investigated using examples such as dichloromethane, chloroform, THF, DMF and a co-solvent system consisting of DMF and THF in a 1:1 ratio. The diameter, morphology, shape of the fibers were examined using scanning electron microscopy (SEM),

3.2.7 Antibacterial Assay

Different masses of polymers or fibers were transferred to sterile 14 mL polypropylene tubes (Falcon) and 3 mL of bacterial solutions which contain approximately 1×105 cfu/mL of

E.coli in LB broth were then added. The tubes were incubated at 37 °C with shaking at 230 rpm for 12-16 h. The visual turbidity of the tubes was noted both before and after incubation.

Aliquots from tubes (100 μL) that appeared to have little or no cell growth were streaked on LB agar plates to distinguish between bacteriostatic or bactericidal effects. Bacterial growth was studied by visually inspecting the LB broth for turbidity (bacterial growth causes clear LB broth to turn turbid). Lack of turbidity may correspond to either very low bacterial growth

(bacteriostatic effect) or complete killing of bacteria (bactericidal effect). To establish whether the samples were bacteriostatic or bactericidal, 100 μL aliquots were taken from the incubated

LB broth and were streaked on nutrient agar plates. The plates were then incubated at 37 °C

(E.coli) for 16-20 h, and colonies were counted. Bacterial colonies indicate the presence of live bacteria in the aliquots that were streaked. If the material being tested does not kill but inhibits the growth of bacteria (bacteriostatic), bacteria will grow when removed from the solution containing the material and the colonies will be observed following streaking the aliquot. If the

81 material being tested is bactericidal, no bacterial colonies would be observed after streaking. All experiments were carried out in triplicate. Negative control tubes contained only inoculated broth with no polymer.

Table 3-1. Reaction Conditions for Grafting of DMAEMA from Initiator T3 and P3 [M]:[I]: temp time Entry M PDI [CuBr]:[CuBr2]:[Ligand] (°C) (min) n S1 1200:1:3:0.15:6 0 8 19,500 1.14 1200:1:3:0.15:6 0 15 21,800 1.15 1200:1:3:0.15:6 0 20 25,600 1.27 C1 400:1:2:0.1:1 0 20 240,300 1.94

Table 3-2. Kinetic Results for Star- and Comb-brush Synthesis temp app -2 o kp , kP·, 10 L [P·], System ( C) 4 -1 10 s mol-1 s-1 106 mol L-1 S1 0 5.03 1.35 3.73 C1 0 0.79 1.35 0.58

3.3 Results and discussion

3.3.1 Synthesis of S1 and C1

Star- and comb-shaped poly[2-(dimetylamino)ethyl methacrylate] (S1 and C1) were synthesized via atom transfer radical polymerization (ATRP) using T3 and P3 as initiators, which follows similar synthetic procedures to those reported earlier.40

As illustrated in Figure 3-1, the linear relationships of ln([M]0/[M]) vs time indicated first-order in monomer concentration kinetics. However, a significant difference in polymerization rate was found by comparison of the star- and comb-shaped brush polymers. This is probably due to the more sterically hindered conformation of macroinitiator P3, retarding the further growth of polymer side chains.40 In addition, the different polymerization rates in these two systems were also demonstrated by the stationary concentration of radicals [P·], which was

app estimated from the ratio of the apparent propagation rate constant (kp ) and the rate constants of

app 42,43 app radical propagation (kP·), i.e., [P·] = kp / kP·. The apparent propagation rate constants (kp ) in these polymerizations were calculated from the slopes of the straight lines plotted in Figure 1.

43 The rate constants of radical propagation (kP·) were calculated from ln(kP·) = 14.685-2669/T.

The estimated concentrations of growing radicals are compiled in Table 3-2. The stationary concentration of growing radicals [P·] in the star system is two order of magnitude higher than in the comb system.

Moreover, in an earlier study, brush-like poly[2-(dimethylamino)ethyl methacrylate] materials were synthesized by ATRP at 25 °C or elevated temperature as reported in the literature.44,45 Also, reducing the monomer concentration and increasing the copper catalyst amount were taken into account in order to enhance the grafting efficiency.46 However, in the

84 present system, serious crosslinking was detected when the polymerization was conducted under previously reported reaction conditions. Crosslinking occurred even at room temperature within one half hour probably attributed to the large amount of heat generated during polymerization.

The extremely fast polymerization rate is probably due to the larger stationary concentration of radicals [P·] (Table 3-2) when compared to the values from other brush systems reported in the literature.47 For example, the [P·] calculated from a three-arm brush polymer consisting of poly(2-trimethylsilyloxyethyl methacrylate) gave the value of 2.21 × 10-8 mol/L, which is two orders of magnitude less than the value (3.73 × 10-6 mol/L) of the similar six-arm brush system of S1 in the present work. Therefore, an ice bath was applied during the ATRP of

2-(dimethyamino)ethyl methacrylate, and the polymerization was terminated within an hour to prevent crosslinking.

A quasi linear relationship of molecular weight vs. time was found for both S1 and C1 as shown in Figure 3-3. Also, S1 maintained a relatively low polydispersity during polymerization (1.34). However, the molecular weight distribution decreased significantly from

2.06 to 1.74 for C1. The decrease in PDI may result from the significant difference of hydrodynamic radius between the initial macroinitiator and the final product.48

0.7

0.6 sPDMA cPDMA

0.5

0.4

]/[M] 0

0.3 Ln[M 0.2

0.1

0.0

-5 0 5 10 15 20 25 30 35 40 45 50 Time (min)

Figure 3-1. Dependence of ln([M]0/[M]) on time in the polymerization from T3 and P3.

1.0 5 min 10 min 0.8 15 min 20 min 25 min

0.6

0.4

0.2 rel.RI-detector intensity 0.0

15.5 16.016.5 17.017.518.0 18.519.0 19.520.0 20.521.0 RT (min)

Figure 3-2. GPC traces of graft polymerization from T3.

a) 60000 1.3 50000 M n PDI

40000 1.2 n

PDI

M 30000

20000 1.1

10000

1.0 0

0 5 10 15 20 Time (min)

b) 350000 2.1 300000 M n PDI 250000 2.0

200000 PDI n

1.9 M 150000

1.8 100000

50000 1.7 0 10 20 30 40 50 Time (min)

Figure 3-3. Dependence of Mn and polydispersity on time in the polymerization from a) T3 and b) P3.

c’’

a’’ b’’ d’’,e’’ iii)

c’

b’ a’ d’,e’ ii)

e a c,d b i)

Figure 3-4. 1H NMR spectra of i) macroinitiator P3, ii) C1 and iii) QC1.

Figure 3-4 i) and ii) show the 1H NMR spectra of the macroinitiator P3 and the C1 brush. The spectra provide evidence of incorporation of 2-(dimethylamino)ethyl methacrylate into the side chains of the brush polymers. The peak at 1.9 ppm (e) represents the methyl protons of ester groups in P3. After chain extension with 2-(dimethylamino)ethyl methacrylate, the peak from the methyl protons (e) disappeared while new peaks, 3.9 ppm (a’), 2.4 ppm (b’), and 2.2 ppm (c’) from poly[2-(dimethylamino)ethyl methacrylate] appeared, as shown in Figure 3-4 ii), which indicated successful grafting of poly[2-(dimethylamino)ethylmethacrylate].

3.3.2 Quaternization of S1 and C1 with different alkyl iodides

To systematically examine the antibacterial effectiveness of brush S1 and C1 polymers, the two brush polymers were quaternized with alkyl iodides with varied chain lengths as shown in Scheme 1. The completeness of quaternization was verified by the 1H NMR spectra shown in

Figure 3-4. Taking the quaternization with iodomethane as an example, after quaternization, a significant chemical shift of methyl groups was observed from 2.2 ppm (c’) to 3.3 ppm (c’’) due to the positive charge from the quaternization. It was also found that there are no peaks remaining at the original peak position (2.2 ppm), which indicates quantitative quaternization of the C1 brushes. Thus, positively charged C1 brushes were confirmed. As mentioned earlier, in a solution of neutral brush polymers, because of steric repulsion between the closely packed side chains, the polymer backbone tends to adopt a cylindrically shaped conformation, due to the entropically unfavorable chain extension.40,49 This effect is enhanced by columbic repulsions when the side chains carry a positive or negative charge to give an even stiffer cylindrical molecular conformation and to dissociate the bridging between molecules.50 This may improve the antibacterial effect due to the heavy concentration of positive charge and the stiff cylindrical molecular conformation.30 The increase of stiffness of the brush polymers was demonstrated by

o the DSC traces. The glass transition temperature (Tg) of the macroinitiator P3 is around -47.7 C,

o while the Tg increased considerably up to 19.2 C after grafting of the poly[2-(dimethylamino)ethyl methacrylate] side chains (C1). Furthermore, C1 quaternized with

o iodomethane (QC1) has a Tg as high as 130.3 C which is a significant increase compared to C1.

This is probably due to the columbic repulsive force which results in an even stiffer molecular conformation.

As the length of the quaternized alkyl chain increased, the water solubility of the quaternized brush polymers decreased. QS10, QS12, QC10 and QC12 are completely insoluble in water. Therefore, the antibacterial tests were divided into two separate series. The first series included the water soluble brush polymers which were tested by dissolving the corresponding macromolecules in aqueous solution. On the other hand, the water insoluble brush polymers were subjected to electrospinning to form insoluble spun mats, for use in antibacterial testing.

3.3.3 Electrospinning of QS10, QS12, QC10 and QC12 Brush Polymers

Electrospinning has been widely used for fabrication of continuous fibers with diameters that range from micrometer to nanometer scales.51 Because various factors markedly influence the morphology of the resulting fibers, in this study only the effect of the nature of the solvents and the concentration of solution were examined in order to obtain bead-free fibers, while keeping other parameters constant.

Chloroform was found to be the best solvent for the electrospinning of QS10 and

QS12. For QS10 with a molecular weight of 25,600, continuous microfibers were obtained at a concentration of 15% (w/v) with a diameter of around 1.1 μm (Figure 3-5c). Lower concentrations resulted in electrospraying with a consequent beaded structure or beads-on-string structures typical of electrospinning at low concentrations (Figure 3-5a and b). Thus, a clear

90 trend of decreasing density of beads and fiber defects was illustrated with increasing concentration as shown in Figure 3-5. However, no fibrous structure could be obtained by using dichloromethane as a solvent within this concentration range. This is probably because dichloromethane failed to provide sufficient solution viscosity and surface tension to the polymer solution, which are key parameters in the electrospinning process.52 In addition, dichloromethane has a lower boiling point (39.6 oC) than chloroform (61.2 oC) and may evaporate too rapidly to facilitate sufficient polymer chain entanglement as the solution jet travels towards the collector, thus causing droplet formation.

a) b) c)

Increase in Polymer Concentration

Figure 3-5. Influence of polymer concentration on morphology of QS10 with Mn of 25,600: a) 5% w/v; b) 10% w/v; c) 15% w/v.

The effect of molecular weight on the electrospinning process and mesh morphology was also investigated. As shown in Figure 6, the minimum polymer concentration, necessary to obtain bead-free fibers, increased as the molecular weight of QS10 brush polymers decreased from 15% w/v (Mn = 25,600) to 30% w/v (Mn = 19,500). This occurs for the same reason mentioned above; namely, a higher viscosity solution is generated by the higher molecular weight brushes. In addition, the effect of quaternized alkyl chain length on the spun fibers was also investigated as illustrated in Figure 3-7. For the same molecular weight, QS10 showed a lower minimum polymer concentration (25% w/v) for bead-free fibers than the quaternized derivative with iodododecane (QS12) (40% w/v). Many researchers have shown that the increase in net charge density increases the charge repulsion in the solution jet, thereby leading to more plastic stretching during electrospinning. This favors the formation of thinner fibers instead of beads.34,53,54 Therefore, it seems to be reasonable that QS10 with short alkyl chains has a higher net charge density than QS12. Thus, a lower polymer concentration is needed for the formation of bead-free fibers.

Increase in Molecular Weight

a) b) c)

Increase in Polymer Concentration

Figure 3-6. Influence of molecular weight on morphology of SQ10: a) Mn = 25,600, 15% w/v; b) Mn = 21,800, 25% w/v; c) Mn = 19,500, 30% w/v.

a) b) c)

d) e) f)

Figure 3-7. Influence of quaternized alkyl chain length on morphology with molecular weight of 21,800: a) QS10, 25% w/v; b) QS10, 27.5% w/v; c) QS10, 30% w/v; d) QS12, 40% w/v; e) QS12, 45% w/v; f) QS12, 50% w/v.

However, by contrast, it was difficult to obtain bead-free fibers from the comb-shaped brush polymers, QC10 and QC12. Varied electrospinning conditions were attempted using different solvents, such as dichloromethane, chloroform, THF, DMF and a co-solvent system consisting of THF and DMF in a 1:1 ratio with solution concentration from 5% w/v to 40% w/v.

Unfortunately, only fibers with ill-defined morphology were obtained from QC10 using a polymer concentration of 20% w/v in chloroform. A possible reason of the failure to obtain well-defined fibers is that a lower degree of polymer entanglement resulted in solutions with a viscosity too low to favor fiber formation. As discussed above, the positively charged cylindrical brush molecules may become even stiffer than their neutral counterpart due to the columbic repulsion along the backbone. As a result, far less polymer chain interactions occur, although C1 has much higher molecular weight than that of S1.

Figure 3-8. FESEM image of electrospun QC10 (Mn = 240,300, 20% w/v, chloroform)

Table 3-3. Antibacterial Effect of Water Soluble Quaternized S1 and C1 Conc. (mg/mL) 6.7 13.3 26.7 33.3 66.7 133.3 QS1 -a - - - + + QS4 - - - Biostatic + + QS7 +b + + + + + QC1 - - - - + + QC4 - - - - + + QC7 - - + + + + a without antibacterial effect; b with antibacterial effect

3.3.4 Antibacterial assay of quaternized S1 and C1

The quaternized S1 and C1 were evaluated by either dissolving the polymers or suspending electrospun mats in an aqueous solution containing gram-negative bacteria (E. coli) with 1×105 cfu/mL. The relative antimicrobial activity of polymers and their electrospun fibers toward gram-negative E.coli, was studied using aqueous LB broth and a minimum inhibitory concentration (MIC) test. MIC is the lowest concentration at which a compound kills more than

99% of the bacteria. A lower MIC corresponds to a higher antimicrobial effectiveness.55-57 The results are summarized in Table 3-3.

The antibacterial activity of water soluble quaternized S1 (Mn = 21,800) and C1 (Mn =

240,300) is summarized in Table 3-3. The star-shaped brush, QS7 showed the best antibacterial effect, with a minimum inhibitory concentration (MIC) as low as 250 μg mL-1. By contrast, QS1 and QS4 showed antibacterial activity only with an MIC between 3330 μg mL-1 and 6670μg mL-1. This is probably because the longer hydrophobic alkyl chains can facilitate the entry of these cationic polymers into bacterial cells. A similar trend has also been reported with a pendant quaternary amine-containing methacrylate polymer and synthetic polypeptides.58 By contrast, the quaternized C1 brush polymer did not have as great an antibacterial effect as quaternized S1. The best result came from QC7 with an MIC around 26.7 mg/mL. The difference in antibacterial effect between star and comb brush polymers may be the result of the dense polymer side chains in C1, from which the bacterial cells are far less accessible. Therefore, both the alkyl chain length and molecular conformation are the key factors to achieve the low MIC.

For the water insoluble electrospun fibers from QS10 and QS12 (Mn = 21,800), the antibacterial tests were also conducted with suspensions of gram-negative bacteria (E. coli), containing 1×105 cfu/mL in contact with 100 mg of each fibers. About 99.0% of E. coli cells

97 were killed after coming into contact with QS10 and QS12 fibers within 2 hours. It is well known that the cell surfaces of bacteria are negatively charged. Consequently, when cells come into contact with QS10 and QS12 microfibers with quaternized ammonium salts on the surface, the normal function of the cell membrane is disrupted because of the electrostatic interaction.36,59

Therefore, the QS10 and QS12 microfibers show high antibacterial efficiency due to the combination of the hydrophobic interaction of long alkyl chain and electrostatic interaction generated from quaternization.

3.4 Conclusion

Well-defined molecular brush polymers formed by grafting poly[2-(diemthylamino)ethyl methacrylate] from a cyclotriphosphazene ring or a polyphosphazene backbone have been obtained via atom transfer radical polymerization, followed by quaternization with varied alkyl chain length iodides to generate antibacterial properties. The two systems showed significant differences in polymerization rate, which is the consequence of the different stationary concentration of radicals [P·] due to the molecular conformation discrepancy between star and comb structures. The successful grafting and quaternization of poly[2-(dimethylamino)ethyl methacrylate] side chains was demonstrated by

1 o H NMR and DSC techniques. The Tg of quaternized brush polymers can be as high as 130.3 C

o compared with its neutral counterpart (Tg: 19.2 C) due to the strong columbic repulsions between adjacent side chains. The resultant quaternized brushes can be electrospun to form continuous microfibers with diameters in the range of 700 nm to 1.1 μm. In addition to the effect of different solvents and the concentration of polymer solution, the molecular conformation appears to be another crucial factor to obtain well-defined fibers. Among the water soluble quaternized bush polymers, QS7 showed the best antibacterial effect with MIC as low as 6

98 mg/mL, due to its well balanced positive charge and hydrophobic long alkyl chains. In addition, more than 99% E. coli were killed within 2 hr after contact with 100 mg of microfibers electrospun from QS10 and QS12. Thus, these polymers have excellent antibacterial behavior which may be valuable for coatings on food packages or medical devices.

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Chapter 4

UV-cleavable Unimolecular Micelles: Synthesis and Characterization Toward

Photocontrolled Drug Release Carriers

4.1 Introduction

Polymeric micelles self-assembled from amphiphilic block copolymers in an aqueous solution have attracted significant attention in diverse fields as excellent nanocarriers in applications such as drug delivery, sensing, and image enhancement.1-3 This carrier-mediated drug delivery method offers promise to improve the loading efficacy and/or provide significant protection of reagents with poor solubility and/or low stability in physiological environments.4,5

For example, most drugs administered orally, while convenient, will be exposed to the acidic, enzyme-rich environment of the gastrointestinal tract as well as to first-pass liver metabolism, and thus their bioavailability will be significantly reduced.6

Despite their advantages, the major concern of classical micelles in medical applications is their relative instability to infinitely dilute environments such as those encountered after oral or parenteral administration, because the formation of classical micelles is thermodynamically favorable only above a specific concentration of the amphiphilic molecules (critical micelle concentration, cmc). During severe dilution and under shear conditions in the bloodstream, micelles begin to disassemble, resulting in an undesired burst release and serious toxicity problems due to potentially large fluctuations in drug concentrations before the drugs reach the target sites.7,8

These problems associated with the classical micelle structure can be overcome by developing a shell/core cross-linked structure.9-12 Such covalently reinforced micelle-like carriers have the potential to circumvent these problems by eliminating the dependence on dynamic

106 equilibrium. An attractive alternative approach is to construct micelle-like macromolecules in which multiple amphiphilic components are synthesized and covalently attached to form amphiphilic star polymers, often called “unimolecular micelles”.13-15 These micelles are intrinsically stable during dilution since their formation is independent of polymer concentration.16 In addition, the size of these unimolecular containers can be easily tuned from less than 10 nm to hundreds of nanometers by changing the molecular weight of the polymer chains during synthesis.17

Polymeric unimolecular micelles with higher molecular weights are beneficial for a long circulation time in the bloodstream and for passive targeting on cancer tissues.18 However, studies showed that linear polymers below 40 kDa, or approximately 5 nm in diameter, are cleared readily through renal excretion.19 To balance these two aspects, biodegradable, hydrophobic segments, like poly(ε-caprolactone) and poly(L-lactide) have been utilized as the hydrophobic core in unimolecular micelles to give them the ability to hydrolyze into smaller linear polymer chains, which can be readily excreted out of the human body through the kidneys after drug delivery.20-22 Alternatively, stimuli-responsive functional groups can be incorporated into unimolecular micelles. As a result, the dissociation of unimolecular micelles can be easily controlled by applying external stimuli to the system to cleave the amphiphilic side chain from the inner core. Considering the dilution environment in vivo, the cleaved unimolecuar micelle should disassemble to small linear polymer chains spontaneously after the stimulus is applied.

Therefore, both the dissociation and drug release can be realized in a controlled manner, as shown in Scheme 4-1.

Cyclotriphosphazenes are useful for improving the properties of organic polymers. The versatile synthetic strategy developed for the preparation of cyclotriphosphazenes with various

107 substituents makes the synthesis of multifunctional initiators or terminators simple.23 Hybrid materials have been synthesized by using cyclotriphosphazenes as pendant, cross-linking agents, and additives in an effort to incorporate the best properties into the final materials.24-27 Recently, cyclotriphosphazenes were also used in biological applications.28 For example, azabisphosphonate dendrimers based on a cyclotriphosphazene core showed therapeutic potential in the treatment of an inflammatory disease, rheumatoid arthritis, and anti-osteoclastic activity on mouse and human cells.28 In addition, cyclotriphosphazenes are biocompatible and have the potential to be biodegradable to non-toxic phosphate and ammonium ion in human body.

In this paper, a 6-arm star photolabile/biocompatible amphiphilic block copolymer was described based on a cyclotriphosphazene core. The photolabile group, o-nitrobeznyl alcohol,29-31 was introduced to create the photocleavable inner core, followed by grafting hydrophobic poly(methyl methacrylate) (PMMA) as the inner arm, and hydrophilic poly[poly(ethylene glycol) methyl ether methacrylate] (PPEGMA) as the outer arm by atom transfer radical polymerization (ATRP).32,33 The photo-cleavage and micelle aggregation behavior of the resulting star polymers were characterized using gel permeation chromatography

(GPC), dynamic light scattering (DLS), transmission electron microscopy (TEM) and fluorescence.

108

Scheme 4-1. Illustration of UV-cleavable Unimolecular Micelle

109

4.2 Experimental Section

4.2.1 Materials

All reactions were carried out under a dry argon atmosphere using standard Schlenk line techniques. Tetrahydrofuran (EMD) and triethylamine (EMD) were dried using solvent purification columns.34 Vanillin (Alfa Aesar), benzyl bromide (Aldrich), 2-bromopropionyl bromide (Sigma), pentamethyldiethylene triamine (PMDETA) (TCI), cesium carbonate (Sigma), sodium borohydride (Aldrich), copper (I) bromide (Sigma), and copper (II) bromide (Sigma) were used as received. Methyl methacrylate (Sigma), poly(ethylene glycol) methyl ether methacrylate (475 g/mol) (Sigma), and 2-(dimethylamino)ethyl methacrylate (Aldrich) were purified by removing inhibitor using an alumina column. The products were stored at –54 oC before use. Hexachlorocyclotriphosphazene (HCCTP) (Fushimi Pharmaceutical Co., Japan or

Ningbo Chemical, China) was purified by recrystallization from hexanes and vacuum sublimation at 50 °C.

4.2.2 Equipment

1090 gel permeation chromatography equipped with two Phenomenex Phenogel linear 10 columns and a Hewlett-Packard 1047A refractive index detector. The samples were eluted at 1.0 mL/min with a 10 mM solution of tetra-n-butyl ammonium nitrate in THF. The elution times were calibrated with polystyrene standards.

110

4.2.3 Synthesis of Photo-cleavable Initiator

4.2.3.1 Hexakis[(4-formyl-2-methoxy-5-nitro)phenoxy] cyclotriphosphazene (1)

A THF solution (10 mL) of hexachlorocyclotriphosphazene (0.5 g, 1.44 mmol) was added dropwise to a THF (100 mL) suspension of cesium carbonate and

4-hydroxy-5-methoxy-2-nitrobeznaldehyde (2 g, 10.1 mmol), which was synthesized according to a previously reported procedure.35 The solution was stirred for 3 h at reflux. THF was removed by rotary evaporation and the mixture was washed with water, acetone and hexane. The product was dried under vacuum. A white solid was obtained with a yield of 1.6 g (85.5%). 1H

NMR (DMSO), δ: 10.2 (s, OCHO, 1 H), 7.97 (s, ArH, 1 H), 7.40 (s, ArH, 1 H), 3.86 (s, OCH3, 3

H). 31P NMR (DMSO), δ: 8.77 (s).

4.2.3.2 Hexakis[[4-(hydroxymethyl)-2-methoxy-5-nitro]phenoxy] cyclotriphosphazene (2)

To a suspension of 1 (2.9 g, 2.2 mmol) in methanol (250 mL) was added sodium borohydride (0.92 g, 26.5 mmol). The reaction mixture was stirred for 1 day at room temperature.

After evaporation of the solvents, the resulting solid was washed with methanol, acetone and hexane. The product was dried under vacuum. A white solid was obtained with a yield of 2.3 g

1 (78.9%). H NMR (DMSO), δ: 7.93 (s, ArH, 1 H), 7.43 (s, ArH, 1 H), 5.70 (s, CH2OH, 1 H),

31 4.83 (d, CH2OH, 2 H, J = 2.92 Hz), 3.80 (s, OCH3, 3 H). P NMR (DMSO), δ: 8.81 (s).

4.2.3.3 Hexakis[[[4-(2-bromoisobutyryloxy)methyl]-2-methoxy-5-nitro]phenoxy] cyclotriphosphazene (3).

Compound 2 (2.15 g, 1.60 mmol) was placed in a 100 mL Schlenk flask with THF (200 mL) with triethylamine (1.97 g, 19.5 mmol). A solution of 2-bromopropionyl bromide (4.48 g,

19.5 mmol) in 10 mL THF was added dropwise to the reaction mixture at 0 °C in an ice bath.

The mixture was allowed to warm to room temperature and was stirred for 2 days. The solvent

111 was removed by rotary evaporation and the residue was redissolved in dichloromethane. The mixture was transferred to a 200 mL separatory funnel and was extracted consecutively with deionized water (100 mL × 3), NaHCO3 (aq.) (100 mL × 3) and deionized water (100 mL × 3).

The organic phase was dried over MgSO4 then filtered, and the solvent was removed by rotary evaporation. The crude product was passed through a silica gel flash column with an eluent of dichloromethane and ethyl acetate (50:1). The solvent was removed, and the resulting light yellow solid was dried under vacuum at room temperature. Yield: 3.40 g (95.7 %). 1H NMR

(CDCl3), δ: 8.11 (s, ArH, 1 H), 7.18 (s, ArH, 1 H), 5.61 (s, CH2OC(O), 2 H), 3.82 (s, CH3O, 3 H),

31 1.98 (s, C(Br)CH3, 6 H). P NMR (CDCl3), δ: 8.91 (s).

4.2.4 Polymerization

A typical polymerization was as follows: photo-cleavable initiator 3 (0.1 g, 0.045 mmol initiator centers), methyl methacrylate (10 g, 99.0 mol) and pentamethyldiethylene triamine

(PMDETA) (0.062 g, 0.36 mmol) were placed in a 50 mL Schlenk flask and sparged with nitrogen for 30 min. Afterward, deoxygenated copper (I) bromide (0.026 g, 0.18 mmol) was added. The nitrogen-filled flask was heated at 90 °C under nitrogen. The polymerization was terminated after 18 h and was quenched by liquid nitrogen. The reaction mixture was then dissolved in dichloromethane and passed through a short alumina column to remove the copper catalyst. The polymer was purified by precipitation into methanol (× 3). Yield: 1.73 g of isolated polymer. The polymerization conditions of all other monomers are listed in Table 4-1.

4.2.5 Star-Poly(methyl methacrylate)-b-Poly[poly(ethylene glycol) methyl ether methacrylate] (star-PMMA-PPEGMA).

A typical polymerization procedure is as follows: star-PMMA84 (Mn = 52697, 0.2 g,

0.023 mmol initiator centers), poly(ethylene glycol) methyl ether methacrylate (PEGMA) (475

112 g/mol) (8.65g, 18.2 mmol), and PMDETA (0.016 g, 0.091 mmol) were dissolved in 2 ml toluene in a 20 mL Schlenk flask and sparged with nitrogen for 30 min. Deoxygenated copper(I) bromide

(6.7 mg, 0.046 mmol) and copper(II) bromide (0.5 mg, 0.0023 mmol) were then added. The reaction was conducted at room temperature for 30 min. The polymerization was then quenched by liquid nitrogen. The reaction mixture was dissolved in dichloromethane and passed through a short alumina column to remove the copper catalyst. The polymer was purified by precipitation into ethyl ether (× 3) Yield: 0.5 g of isolated polymer. The polymerization conditions of the other star polymers are listed in Table 4-2.

4.2.6 Micelle Preparation

Nanopure water (20 mL) with a resistivity of 18.2 MΩ /cm was added dropwise to a mildly stirred solution of star-PMMA84-PPEGMA80-1 or star-PMMA179-PPEGMA89-2 (200 mg) in THF (5 mL). Once the water was added, all the THF was removed under reduced pressure as monitored by 1H NMR spectroscopy. The resulting solution was then diluted to obtain a micelle concentration in the range of 5 to 1 × 10−4 g/L. For fluorescence measurements, a pyrene solution in THF (1.2 × 10−3 M) was added to nanopure water to give a final pyrene concentration of 12 × 10−7 M. Following dilution, the THF was removed under reduced pressure, and its removal was confirmed by 1H NMR spectroscopy. The pyrene solution was then mixed with the diblock copolymer solutions to obtain copolymer concentrations ranging from 2.5 to 5 ×

10−5 g/L, while the pyrene concentration of the samples was maintained at 6 × 10−7 M. All the samples were sonicated for 10 min and were allowed to stand for 1 day before further measurements.

4.2.7 Photocleavage of o-Nitrobenzyl Group

A long wavelength (300 nm) UV light from EFOS Ultracure 100ss Plus, UV spot lamp,

113

Mississauga, Ontario, Model E3000 equipped with 15 W long wavelength tubes filtered at 300 nm was used for photocleavage of the o-nitrobenzyl groups. The quartz cuvettes that contained polymer solutions were placed in the chamber and were irradiated with 300 nm UV light at a distance of 10 cm. GPC was used to monitor the photocleavage reaction by comparing the areas between the starting polymer and the cleaved side chain.

4.2.8 Fluorescence Measurements

Excitation spectra of pyrene were measured using a Photon Technology International

(PTI) fluorescence spectrometer using an 814 photomultiplier detection system. For the excitation spectra, the emission wavelength was set at 391 nm. All the samples were measured in a 1 × 1 cm quartz cuvette at room temperature.

4.2.9 Light Scattering Measurements

The sizes and size distributions of the unimolecular micelles were evaluated by dynamic light scattering using a particle size analyzer (Zetasizer Nano S, Malvern Instruments Ltd.) at room temperature (25 °C) with a scattering angle of 90°. Samples were filtered through a 0.45

μm syringe filter before measurement of particle size for each sample.

4.2.10 Transmission Electron Microscopy

Transmission electron microscopy (TEM) was performed using a KEOL 2010 unit, operated at an acceleration voltage of 200 kV. For observation of the size and distribution of the micellar particles, a drop of sample solution (concentration = 1 g/L) was placed onto a 400-mesh copper grid coated with carbon. About 1 min after deposition, the grid was tapped with a filter paper to remove surface water, followed by air-drying. Negative staining was performed by using a droplet of a 2.5 wt % uranyl acetate solution.36 The samples were air-dried at room temperature overnight.

114

Table 4-1. Conditions and Results for Grafting of Different Monomer on Initiator 3

[M]:[I]: temp time f f Entry M solvent Mn PDI [CuBr]:[CuBr2]:[Ligand] (°C) (h) a b star-PPEGMA8 PEGMA 1200:1:3:0.15:6 rt 0.42 toluene 26,237 1.15 c star-PDMA48 DMA 1200:1:3:0.15:6 rt 1 bulk 47,289 1.20 d e star-PMMA24 MMA 1600:1:3:0:6 90 18 DPE 16,843 1.14 star-PMMA33 MMA 1800:1:3:0:6 90 18 DPE 21,751 1.16 star-PMMA61 MMA 2000:1:3:0:6 90 18 DPE 38,573 1.20 star-PMMA84 MMA 2200:1:3:0:6 90 18 DPE 52,697 1.19 star-PMMA179 MMA 2400:1:3:0:6 90 18 DPE 109,617 1.11 aDegree of polymerization per arm. bPEGMA = poly(ethylene glycol) methyl ether methacrylate. cDMA = 2-(dimethylamino)ethyl methacrylate. dMMA = methyl methacrylate. eDPE = diphenyl ether. fMeasured by GPC calibrated by linear polystyrene standards.

Table 4-2. Conditions and Results for star-PMMA-PPEGMA b Entry star-PMMA Block copolymer DP PMMA/DPPPEGMA block ratio a a a a 1 Mn PDI Mn PDI GPC H NMR c star-PMMA84-PPEGMA80-1 52,697 1.19 151,022 1.29 2.4 0.95 star-PMMA179-PPEGMA89-2 109,617 1.11 146,573 1.28 13.8 2.0 aMeasured by GPC calibrated by linear polystyrene standards. bDP = degree of polymerization. cDegree of polymerization per arm.

115

4.3 Results and Discussion

The significance of this work is the development of a new class of UV-cleavable unimolecular micelles composed of well-defined 6-arm structure based on a cyclic phosphazene platform. The cyclophosphazene core is biocompatible and biodegradable to non-toxic phosphate and ammonium ion under physiological conditions. This core is connected via o-nitrobenzyl, photolabile functional groups, to amphiphilic organic polymer arms. The unimolecular micelles thus generated are appropriate for controlled drug release behavior following an external stimulus, such as UV irradiation. This is important in order to address the growing need for biomedically useful polymers which can undergo breakdown to small, excretable molecules.

This work illustrates the feasibility of synthesis of a broad range of new species with photolabile groups and polymer star structures.

4.3.1 Initiator Synthesis

The “graft-from” approach was used to construct amphiphilic star block copolymers.

For this purpose, initiator 3 was synthesized as illustrated in Scheme 4-2 using hexachlorocyclotriphosphazene as a starting material. The hexachlorocyclotriphosphzene was treated with 4-hydroxy-5-methoxy-2-nitrobeznaldehyde in the presence of cesium carbonate. The cesium carbonate synthetic route resulted in a significantly higher substitution efficiency than the use of a sodium aryloxide.37 The singlet resonance at 8.77 ppm in the 31P NMR spectrum suggested complete chlorine replacement. The aldehyde groups of compound 1 can easily be reduced by sodium borohydride23 to give the free hydroxyl groups in compound 2, which were further esterified with 2-bromopropionyl bromide. The 1H NMR spectrum indicated 100% esterification of hydroxyl groups from the ratio between the methyl group at 3.82 ppm (3 H) and the methyl groups at 1.98 ppm (6 H). The resultant compound 3 with photolabile o-nitrobenzyl was used to construct photo-cleavable star amphiphilic block copolymers.

116

Scheme 4-2. Synthesis of Photo-cleavable Initiator 3

117

4.3.2 Polymerization

A series of well-defined star-shaped polymers was synthesized by grafting methyl methacrylate, 2-(dimethylamino)ethyl methacrylate, or poly(ethylene glycol) methyl ether methacrylate from the aforementioned initiator 3 by controlled atom transfer radical polymerization (ATRP) as shown in Scheme 3. This provided compounds for the evaluation of different components for the photo-cleavability of the resulting star polymers. In addition, poly(methyl methacrylate) was selected as the inner hydrophobic block to provide the ability to encapsulate hydrophobic guest molecules as well as biocompatibility for the carriers in vivo.38-40

Thus, a number of star-PMMA polymers with varied molecular weights were synthesized by changing the ratio of monomer-to-initiator (Table 4-1). The GPC traces of star-PMMA indicated well-controlled molecular weights and low PDI values as shown in Figure 4-1.

Poly[poly(ethylene glycol) methyl ether methacrylate] was added as a second block in the star polymers to provide a polar corona that would compatibilize the stars with an aqueous environment in vivo.41 This component was selected for the corona because of its structural similarity to the FDA-approved poly(ethylene glycol). Furthermore, it shows excellent biocompatibility in vitro cell assays and provides easy access to high-molecular weight

PEG-based polymers using relatively mild synthetic conditions.42-44 Thus, the poly[poly(ethylene glycol) methyl ether methacrylate] blocks were polymerized from star-PMMA star homopolymers by ATRP. Two different star-PMMA-PPEGMA amphiphilic block copolymers, were synthesized with different hydrophobic PMMA (Mn = 52,697 and 109,617) segments

(Table 4-2) in order to investigate their ability to encapsulate hydrophobic molecules.

Based on GPC measurements, the block ratios between PMMA and PPEGMA were 2.4 and 13.8 for the two star-PMMA-PPEGMA block copolymers, respectively. However, the

118

GPC equipment was calibrated for polystyrene standards, and therefore the measured molecular weights are not necessarily accurate, making it difficult to directly compare the block copolymers.6 For this reason, 1H NMR spectroscopy was employed to provide more reliable block ratios of PMMA and PPEGMA, which were 0.95 and 2.0. Therefore, the compositions of two star block copolymers per arm are star-PMMA84-PPEGMA80-1 and star-PMMA179-PPEGMA89-2, which possess similar lengths of hydrophilic PPEGMA repeating units (80 and 89) but different hydrophobic PMMA repeating units (84 and 179).

119

Scheme 4-3. Grafting of Different Monomers from Initiator 3

1.4 Initiator 16843 21751 1.2 38573 52697 109617 star-PMMA -PPEGMA -1 84 80 1.0

0.8

0.6

0.4

rel.RI-detector intensity 0.2

0.0 800 900 1000 1100 1200 1300 RT (min)

Figure 4-1. GPC traces of star-PMMA with varied Mn and star-PMMA84-PPEGMA80-1.

120

4.3.3 Photoinduced Cleavage of Homogenous Star Polymers

The series of photo-cleavable homogenous star polymers, star-PMMA, star-PDMA and star-PPEGMA, were subjected to UV irradiation in order to investigate the effect of monomers, solvent, molecular weight, and concentration on their photoinduced dissociation behavior.

4.3.3.1 Effects of Molecular Weight and Concentration

Star-PMMA species with varied molecular weights were studied for their photoinduced dissociation in the solution state in THF. The results obtained by GPC are shown in Figure 4-2.

Initially, the solution was colorless. With increasing irradiation time, the solution gradually turned slightly yellow. To determine the time that was needed to achieve the maximum degree of cleavage of the o-nitrobenzyl groups, GPC was used to monitor the photolysis process. All the dissociation was completed within 3 hours of UV-irradiation. Figure 4-2a illustrates the process of dissociation of a star-PMMA24 solution. After irradiation with UV light for 3 hours, the GPC trace shows a dominant low molecular weight (Mn = 1,514) species, while almost no remaining starting star-PMMA24 was detected. This indicated a complete detachment of the PMMA chains from star-PMMA24.

The continuous photodegradation of star-PMMA at both varied Mn and concentration were also followed by GPC measurements. As shown in Figure 4-2b, the photodegradation rate of star-PMMA increased with increasing molecular weight, which may be due to the difference in the absolute number of photolabile o-nitrobenzyl groups in the three star-PMMA polymers.

Considering the same concentration (10 mg/ml) and volume of solvent (2 ml THF) used every time for the photodegradation, star-PMMA33 contains the most o-nitrobenzyl group, whereas star-PMMA84 contains the least. In addition, it is reported that the photolysis of o-nitrobenzyl is

121 an intramolecular process and does not require contact of the chromophore with solvent molecules. Thus, all of the o-nitrobenzyl groups have the same probability of absorbing UV light and undergoing the ester cleavage reaction regardless of the location of the chromophore

29 moieties. Therefore, star-PMMA84 with the least of o-nitrobenzyl groups showed the fastest photodegradation rate, in contrast to the slowest rate of star-PMMA33 which contained the largest amount of o-nitrobenzyl units.

The effect of solution concentration on the photodegradation rate of star-PMMA61 was also investigated as shown in Figure 2c. The degradation of star-PMMA61 at a concentration of

3 mg/ml was complete after 0.5 hours irradiation. By contrast, around 80% of the star’s arm-chains were cleaved after 1.25 hour irradiation of star-PMMA61 with 20 mg/ml, which is additional evidence for the conclusion drawn above.

122

a) star-PMMA star-PMMA24 PMMA 24 24 0 hr 0.5 hr Mn = 16,843 Mn = 1,514 1.25 hr

3 hr

rel.RI-detector intensity

15 16 17 18 19 20 21 22 23 RT (min)

b) 100 90 80 70 60

50 40 30 star-PMMA in THF 20 33 star-PMMA in THF 61 10 CleavedSide Chain (%) star-PMMA in THF 84 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Irradiation time (hr)

c) 100

star-PMMA , 3 mg/ml in THF 61 20 star-PMMA , 10 mg/ml in THF 61

CleavedSide Chain (%) star-PMMA , 20 mg/ml in THF 61 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Irradiation time (hr)

Figure 4-2. a) GPC traces of photodegradation of star-PMMA24; b) photodegradation of star-PMMA (10 mg/ml) with different Mn; c) photodegradation of star-PMMA61 with different concentration.

123

4.3.3.2 Effects of Monomers and Solvents

The influence of monomers and solvents on the photodegradation rate of the star polymers was also evaluated with GPC.

As discussed above, the photodegradation rate is dependent on the absolute number of photolabile o-nitrobenzyl groups present in solution, therefore a similar photodegradation rate is expected for star-PPEGMA8 (Mn = 26,237) and star-PMMA33 (Mn = 21,751) due to the similar

-6 -6 number of o-nitrobenyl groups, 4.57 × 10 mol for star-PPEGMA8 and 5.52 × 10 mol for star-PMMA33, which were calculated from mass (20 mg) and molecular weight of each star polymer. However, as shown in Figure 3a, star-PPEGMA8 showed the highest photodegradation rate of the four star polymers, much higher than that of star-PMMA33.

Therefore, the photodegradation rate was highly dependent on the type of monomers in this situation. In addition, similar results were also found between star-PDMA48 (Mn = 47,289) and star-PMMA84 (Mn = 52,697), where star-PMMA84 had a much higher degradation rate than star-PDMA48, even though both polymer solutions contained a similar number of o-nitrobenzyl groups. Thus, the photodegradation rate is highly dependent on the type of monomer grafted to the star polymers and increases in the order of star-PDMA < star-PMMA < star-PPEGMA.

In contrast to the effect of different monomer units, the type of solvent had little influence on the photodegradation rate of the star polymers. As illustrated in Figure 4-3b, photo-induced dissociation of star-PPEGMA8 was carried out in three solvents, toluene, THF and water. Almost complete cleavage was achieved within 0.5 hours for all three solvents.

124

a) 100

40 star-PEG in THF 8 star-PMMA in THF 84 20 star-PMMA in THF 33

CleavedSide Chain (%) star-PDMA in THF 48 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Irradiation time (hr)

b) 100

star-PEG in THF 8 20 star-PEG in Toluene 8

CleavedSide Chain (%) star-PEG in H O 8 2 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Irradiation time (hr)

Figure 4-3. a) Photodegradation of star polymers with different monomers; b) photodegradation of star-PPEGMA8 (10 mg/ml) with different solvents.

125

4.3.4 Micellar Properties and Dye Encapsulation of Unimolecular Micelles

4.3.4.1 The Formation of Micellar Structures

The formation of micellar structures from star-PMMA-PPEGMA in aqueous solution was evidenced by 1H NMR spectroscopy. Figures 4-4a, and b depict the NMR spectra recorded for the star-PMMA179-PPEGMA89-2 copolymer in different deuterated solvents. In CDCl3, the star copolymer chains are fully solvated and all the signals expected for each block are detectable.

In D2O, once the micellar structure was formed, the signal intensity of the methyl groups in the

PMMA block at 0.84 ppm was significantly reduced, while the signals from PPEGMA residues were still evident, indicating the formation of PMMA hydrophobic core micelles with an outer hydrophilic corona of PPEGMA chains. In addition, the formation of polymer micelles was also evidenced by TEM micrography as shown in Figure 4-5.

126

PEGMA block

No micelle formation

Methyl group of PMMA block

Micelle formation Reduced intensity b)

1 Figure 4-4. H NMR spectra of the star-PMMA179-PPEGMA89-2 amphiphilic copolymer: a) in CDCl3; b) in D2O

500 nm

Figure 4-5. TEM micrograph of star-PMMA179-PPEGMA89-2 micelles.

127

The hydrodynamic radii of the polymeric micelles was measured by dynamic light scattering (DLS). The measurements revealed the formation of a dispersion of star polymer particles in aqueous solution. The hydrodynamic radii of the polymer micelles are presented in

Table 4-3. The micelle size distribution was always a bimodal pattern with a smaller size component and a large size counterpart, which suggested the existence of aggregated species. As an example, a DLS graph of size distribution of star-PMMA179-PPEGMA89-2 micelles in a polymer solution with a concentration of 1 g/L is shown in Figure 6a. The majority of particles are located at ~111 nm (92.2%) with a minority of smaller sized particles at ~3.2 nm (7.8%). In order to verify that the larger particles (~ 111 nm) are the result of aggregation of unimolecular star polymeric micelles, DLS in THF was utilized for both star block polymers. Considering that both segments in the star polymers are well-solvated in THF, the star-PMMA-PPEGMA polymers should dissolve as unimers and exist as separate coils in THF. As illustrated in Figure

4-6b, the results reveal that the hydrodynamic radius of star-PMMA179-PPEGMA89-2 polymer is ~7.7 nm when the polymers exist as unimolecular micelles in THF solution. Therefore, the significantly larger particles (~111 nm) in aqueous solution are caused by the aggregation of star polymeric molecules, rather than the formation of real unimolecular micelles. This aggregation has been observed and reported in the previous literature, and is attributed to the aggregation of the hydrophobic inner core of star block copolymers.16,21,45,46 The effect of a bimodal size distribution should be minimized after oral or intravenous administration, because the micelles will be diluted in the stomach and the small intestine, where the dilution would cause the breakdown of the secondary aggregates into single micellar entities.16,47

128

Table 4-3. Properties of star-PMMA-PPEGMA Micelles entry Hydrodynamic radiusa CACb H2O THF Before UV After UV star-PMMA84-PPEGMA80-1 86 nm 5.5 nm 0.078 g/L 0.16 g/L star-PMMA179-PPEGMA89-2 111 nm 7.7 nm 0.0026 g/L 0.022 g/L a Polymer concentration is 1 g/L. bCritical aggregation concentration.

a) 1.0

0.8

0.6

0.4 Amplitude

0.2

0.0 1 10 100 1000 Size (nm)

b) 1.0

0.8

0.6

0.4 Amplitude

0.2

0.0 1 10 100 1000 Size (nm)

Figure 4-6. DLS graph of star-PMMA179-PPEGMA89 micelle size distribution with concentration 1 g/L: a) in H2O; b) in THF

129

4.3.4.2 Dye Encapsulation

The encapsulation ability of the polymeric micelles was studied by a fluorescence technique using pyrene as a hydrophobic dye probe. The method is based on the sensitivity of the pyrene probe to the hydrophobicity and polarity of its microenvironment. The spectroscopic properties of pyrene change significantly during transfer from an aqueous environment to the nonpolar environment of the micelle core.48 As the polymeric micelle

(star-PMMA179-PPEGMA89-2) concentration increased from 0.00005 g/L to 2.5 g/L, the (0,0) band in the excitation spectrum of pyrene shifted from 333 to 337 nm, the total fluorescence intensity (I337) increased, and the I337/I333 ratio decreased as shown in Figure 4-7a and b. In addition, Figure 4-7b also indicated a second aggregation of polymer micelles as mentioned above following the increased polymer concentration in aqueous solution, from which the critical aggregation concentration (CAC) can be calculated for both star-PMMA-PPEGMA polymeric micelles (Table 4-3). Due to the larger hydrophobic PMMA block in star-PMMA179-PPEGMA89-2, star-PMMA179-PPEGMA89-2 showed smaller CAC (0.0026 g/L) than the CAC (0.078 g/L) of star-PMMA84-PPEGMA80-1 in agreement with previous

48,49 studies. Furthermore, star-PMMA179-PPEGMA89-2 had a significantly higher encapsulation ability than star-PMMA84-PPEGMA80-1 as shown in Figure 4-7c. This was indicated by the much more intense fluorescence signal of star-PMMA179-PPEGMA89-2 at the same polymer concentration (0.025 g/L). This result also results from the larger hydrophobic inner core in star-PMMA179-PPEGMA89-2.

130

a) 0.5g/L 0.25g/L 0.05g/L 0.025g/L 0.005g/L 0.0025g/L 0.0005 g/l 0.00025 g/l

0.00005 g/l FluorescenceIntensitry

300 310 320 330 340 350 Wavelength (nm)

b) 1.1 Before UV After UV 1.0

0.9

0.8

333

/ 337

I 0.7

0.6

0.5

-5 -4 -3 -2 -1 0 Log C (g/L)

star-PMMA -PEGMA -2 c) 179 89 star-PMMA -PEGMA -2 after UV 179 89 star-PMMA -PEGMA -1

84 80

FluorescenceIntensitry

340 360 380 400 420 440 460 480 500 520 540 Wavelength (nm)

131

4.3.4.3 Photoinduced Dissociation of star-PMMA-PPEGMA

The star-PMMA84-PPEGMA80-1 has a comparable photodegradation rate to that of the single segmented arm, star-PMMA84 and star-PPEGMA8 as shown in Figure 4-8. In addition, the block structure of star-PMMA-PPEGMA seems to have no effect on photodegradation rate.

However, the CAC of star-PMMA-PPEGMA changed significantly before and after

UV-irradiation. For example, the CAC of star-PMMA179-PPEGMA89-2 increased dramatically from 0.0026 g/L to 0.022 g/L during UV-irradiation (Table 3 and Figure 7b). The fluorescence intensity of star-PMMA179-PPEGMA89-2 polymeric micelles decreased only slightly after the

UV-irradiation at a polymer concentration of 0.025 g/L above the post-irradiation CAC (0.022 g/L). This indicated that its micellar structure is maintained after irradiation, despite all the side chains having detached from the inner core. However, the fluorescence intensity decreased dramatically when the polymer concentration was reduced to 0.022 g/L, suggesting the loss of the micellar structure and the release of encapsulated pyrene due to dissociation. The result revealed that the stability of the polymer micelles decreased considerably once the covalently bonded side chains were detached from the core. This is the desired property for controlled drug release within the human body, because initially stable drug-loaded micellar carriers are necessary for delivery and protection of the drugs before reaching the targeted sites to avoid the premature drug release. Then, once the drug has been delivered to the targeted sites, the instability of the drug –loaded micelles can be triggered with UV light to induce drug release by dissociation of the micellar structure. Therefore, the star-PMMA-PPEGMA polymer micelles designed in this work showed several advantages over the traditional micelle systems based on linear block copolymers. The release mechanism of our system comes from the dissociation of

UV-cleaved polymer micelles on dilution to initiate controlled release. This is in contrast to the diffusion mechanism of the traditional micelle systems.

132

100

40 star-PMMA -PEG 84 80 20 star-PMMA 84

CleavedSide Chain (%) star-PEG 8 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Irradiation time (hr)

Figure 4-8. Comparison of photodegradation rate of star-PMMA84-PPEGMA80, star-PMMA84 and star-PPEGMA8 with polymer concentration of 10 mg/ml.

133

4.4 Conclusions

UV-cleavable unimolecular star amphiphilic block copolymers, star-PMMA-PPEGMA, were synthesized with photolabile o-nitrobenzyl groups at the cyclotriphosphazene core. The study showed that the photodegradation rate of the star polymers is independent of molecular weight, solution concentration, or solvent, but dependent on the various monomers used. Both DLS and fluorescence studies suggest that these unimolecuar micelles tend to aggregate into large micellar agglomerates with an average radius of ~86 and

~111 nm for star-PMMA89-PPEGMA80-1 and star-PMMA179-PPEGMA89-2 respectively.

However, the micelles showed stimulus-responsive properties during UV-irradiation. The stability of the polymeric micelles decreased significantly after UV-irradiation, as indicated by the CAC of star-PMMA179-PPEGMA89 changing from 0.0026 g/L to 0.022 g/L. The resultant micelles have a greater tendency to dissociate and release an encapsulated drug on dilution in physiological conditions. This unique property demonstrates that the polymer micelles formed from these UV-cleavable star polymers have potential applications as drug carriers for photo-controlled drug release systems.

134

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139

Chapter 5

Substituent Exchange Reactions of Trimeric and Tetrameric Aryloxycyclophosphazenes

with Sodium 2,2,2-Trifluoroethoxide

5.1 Introduction

Polyphosphazenes are hybrid macromolecules with an essentially linear backbone of alternating phosphorus and nitrogen atoms and with two organic side groups linked to each phosphorus.1 These polymers have been widely studied for various applications such as bone regeneration scaffolds2-4, fire retardants5, 6, low-temperature elastomers7, fuel cell membranes8-10 and solid or gel polymer lithium ion conductors11, 12. A distinctive feature of polyphosphazenes is the ease with which the polymer properties can be tuned through changes in the side groups.

The most widely explored method for the synthesis of poly(organophosphazenes) is based on the thermal ring-opening polymerization of hexachlorocyclotriphosphazene, (NPCl2)3, to high polymeric poly(dichlorophosphazene), (NPCl2)n, followed by the replacement of the labile chlorine atoms in this macromolecular intermediate by organic groups, such as alkoxy, aryloxy, or amino units13, 14 as shown in Scheme 5-1. In addition, cosubstituted poly(organophosphazenes) can be synthesized by sequential or simultaneous addition of different nucleophiles to conveniently obtain various materials with tunable properties.

However, as mentioned in earlier publicatons15-17, an alternative synthesis approach is to replace one organic substituent in the polymer by another using a second organic nucleophile

(Scheme 5-1). This second approach is an appealing alternative, especially for the high polymers since it offers the prospect that single-substitutent poly(organophosphazene)s can be converted readily to mixed-substituent materials which are of broad technological interest. It also raises the possibility that a poly(organophosphazene) that is stable for long periods of time in the

140 atmosphere might be employed as a general macromolecular intermediate for the preparation of other poly(organophosphazene)s. The chloro-derivative intermediate is sensitive to moisture and must be stored under carefully controlled inert conditions. Also, the substitution reactions of the chloro intermediate can only be conducted in a limited number of organic solvents, such as tetrahydrofuran or dioxane.

In addition to its utility for the preparation of new polymers, a systematic study of organic side group exchange reactions can also provide useful information as a practical guide to optimize the classical synthesis of cosubstituted poly(organophosphazenes) prepared by the

18-23 sequential or simultaneous reactions of two or more nucleophiles with (NPCl2)n. Earlier preliminary work showed that fluorinated alkoxy units24 and phenoxy side groups can be displaced from polyphosphazenes.15 For example, for polyphosphazenes that bear both trifluoroethoxy and phenoxy side groups, replacement of phenoxy side groups by

2,2,2-trifluoroethoxide occurs at non-geminally substituted phosphorus atoms. The reverse reaction, exchange of the trifluoroethoxy group by phenoxide ions, was not detected.15 Therefore, the order of the sequential addition of different side groups must be considered in order to obtain the desired materials with targeted compositions.

The work presented here is an attempt to examine these processes in more detail.

Because the reactions of small molecule cyclic phosphazenes often mimic the behavior of the high polymers, and are easier to analyze, this study is focused on the behavior of cyclic trimeric and tetrameric species. The comparable processes for high polymers will be addressed in a later study.

141

Scheme 5-1. Synthesis of Mixed-substituent Poly(organophosphazenes) by Sequential or Simultaneous Addition of Nucleophiles to (NPCl2)n or by Side Group Exchange Reaction

142

5.2 Experimental section

5.2.1 Materials and Equipment

All reactions were carried out under an atmosphere of dry argon using standard Schlenk line techniques. Tetrahydrofuran (EMD) was dried using solvent purification columns.25

2,2,2-Trifluoroethanol (Aldrich) was purified by vacuum distillation from CaH2 (Aldrich).

Phenol (Aldrich) was purified by sublimation. 4-Nitrophenol was recrystallized twice from toluene. Hexachlorocyclotriphosphazene, (NPCl2)3, (from various sources including Fushimi

Pharmaceutical Co., Japan, and Ningbo Chemical, China) was purified by recrystallization from hexane and by vacuum sublimation at 50 °C. Octachlorocyclotetraphosphazene, obtained from the sublimation residues after the trimer sublimation, was recrystallized twice from toluene and was then vacuum sublimed. Other reagents were used as received. 31P and 1H NMR spectra were obtained with a Bruker 360 WM instrument operated at 145 and 360MHz, respectively.

Spectrometric analysis data were collected using the turbospray ionization technique with use of an Applied Biosystems API 150EX LC/MS mass spectrometer.

5.2.2 Synthesis of Cyclic Trimeric Compounds

5.2.2.1 Hexakis(2,2,2-trifluoroethoxy)cyclotriphosphazene (1).

This compound was prepared from (NPCl2)3 and sodium trifluoroethoxide according to

1 31 1 a procedure described previously. Yield: 75.8%. P NMR (CDCl3), ppm: δ +17.45 (3P, s). H

+ NMR (CDCl3), ppm: δ 4.28 (2H, m, OCH2CF3) m/z=730 ([M+H] ) m/z was calculated for

C12H12F18N3O6P3.

5.2.2.2 Hexaphenoxycyclotriphosphazene (2)

This compound was prepared via a procedure reported earlier.1 Yield: 81.4%. 31P NMR

1 (CDCl3), ppm: δ +9.35 (3P, s). H NMR (CDCl3), ppm: δ 6.98, 7.16, 7.23 (5H, m, OC6H5)

143

+ m/z=694 ([M+H] ) m/z was calculated for C36H30N3O6P3.

5.2.2.3 Hexakis(4-formylphenoxy)cyclotriphosphazene (3)

26 31 The synthesis procedure was reported previously. Yield: 91.2%. P NMR (CDCl3),

1 ppm: δ +7.67 (3P, s). H NMR (CDCl3), ppm: δ 7.13, 7.72 (4H, d, OC6H4CHO), 9.92 (1H, s,

+ OC6H4CHO) m/z=862.4 ([M+H] ) m/z was calculated for C42H30N3O12P3.

5.2.2.4 Hexakis(4-cyanophenoxy)cyclotriphosphazene (4)

Prepared as described previously.27 Yield: 64.2%. 31P NMR (d-DMSO), ppm: δ +7.41

1 (3P, s). H NMR (d-DMSO), ppm: δ 7.17, 7.83 (4H, d, OC6H4CN).

5.2.2.5 Hexakis(4-nitrophenoxy)cyclotriphosphazene (5)

Synthesized via a process described earlier.28 Yield: 82.7%. 31P NMR (d-DMSO), ppm:

1 δ +7.30 (3P, s). H NMR (d-DMSO), ppm: δ 7.31,8.16 (4H, d, OC6H4NO2).

5.2.3 Synthesis of Cyclic Tetrameric Compounds

5.2.3.1 Octakis(2,2,2-trifluoroethoxy)cyclotetraphosphazene (6)

14 31 Prepared as described previously. Yield: 36.1%. P NMR (CDCl3), ppm: δ -1.44 (3P,

1 + s). H NMR (CDCl3), ppm: δ 4.27 (2H, t, OCH2CF3). m/z=973 ([M+H] ) m/z was calculated for

C16H16F24N4O8P4.

5.2.3.2 Octaphenoxycyclotetraphosphazene (7)29

31 1 Yield: 36.1%. P NMR (CDCl3), ppm: δ -11.79 (3P, s). H NMR (CDCl3), ppm: δ 6.96,

+ 7.10, 7.17 (5H, m, OC6H5). m/z=925 ([M+H] ) m/z was calculated for C48H40N4O8P4.

5.2.3.3 Octakis(4-formylphenoxy)cyclotetraphosphazene (8)30

Yield: 57.1%. 31P NMR (d-DMSO), ppm: δ -13.96 (3P, s). 1H NMR (d-DMSO), ppm: δ

+ 7.10, 7.71 (4H, d, OC6H4CHO), 9.92 (1H, s, OC6H4CHO). m/z=1149.3 ([M+H] ) m/z was calculated for C56H40N4O16P4.

144

5.2.3.4 Octakis(4-cyanophenoxy)cyclotetraphosphazene (9)

A mixture of (NPC12)4 (1.5 g, 3.2 mmol), 4-cyanophenol (3.6 g, 29.1 mmol) and

Cs2CO3 (9.5 g, 29.1 mmol) in THF (150 ml), was stirred at reflux for 1 day. The reaction mixture was filtered, and the filtrate was concentrated and precipitated into water. After filtration, the crude 4b was reprecipitated twice from DMSO into water and twice from DMSO into THF.

The product was dried under vacuum to yield 2.96 g of a white solid. Yield: 82.7%. 31P NMR

1 (d-DMSO), ppm: δ -14.26 (3P, s). H NMR (d-DMSO), ppm: δ 7.14, 7.83 (4H, d, OC6H4CN).

5.2.3.5 Octakis(4-nitrophenoxy)cyclotetraphosphazene (10)

Compound 10 was prepared via an earlier procedure.31 Yield: 70.5%. 31P NMR

1 (d-DMSO), ppm: δ -14.58 (3P, s). H NMR (d-DMSO), ppm: δ 8.13, 7.26 (4H, d, OC6H4NO2)

5.2.4 Purification of N3P3(ONa)(OCH2CF3)5 (11)

The crude products from the ligand exchange reaction of hexakis(2,2,2-trifluoroethoxy)cyclophosphazene with sodium trifluoroethoxide in THF were dried by rotary evaporation and were redissolved in dichloromethane. After extraction with ammonium chloride (10 wt%) (×3) and deionized water (×3), the organic layer was dried over

MgSO4 and filtered, and the solvent was removed by rotary evaporation. A solution of the crude products in a 2:1 mixture of dichloromethane and ethyl acetate was passed through a silica gel column prepared using the same solvent. The product was dried under vacuum to give 11, a

31 1 yellow liquid. P NMR (CDCl3), ppm: δ +18.99, 19.58 (3P, d), 11.71 (2P, t). H NMR (CDCl3), ppm: δ 4.28 (2H, m, OCH2CF3)

5.2.5 Substituent Exchange Reactions for Both Cyclic Trimer and Tetramer Derivatives.

All the substituent exchange reactions were carried out in a similar manner. The following is a typical procedure. A solution of 1 (1 g, 1.37 mmol) in THF (10 ml) was added

145 dropwise to a stirred solution of sodium phenoxide (1.91 g, 16.5 mmol) in THF (90 ml). The mixture was stirred at reflux in THF. Typically, reactions were allowed to proceed for up to 50 days. At timed intervals, starting after one day, samples were taken and the reaction progress was monitored by 31P NMR and mass spectrometry. The presence of the etheric side products was established by mass spectrometric analysis of the reaction mixtures.

146

Figure 5-1. Structures of Cyclic Trimeric and Tetrameric Phosphazenes.

147

5.3 Results and Discussion

The compounds to be discussed are shown in Figure 5-1. In the following sections the substituent exchange reactions of 1 and 2 with the nucleophiles, sodium phenoxide and sodium trifluoroethoxide will be discussed first, together with the reactant ratio effects in this system, followed by similar reactions at the cyclic tetramer level (6 and 7). In the following section, the use of sodium trifluoroethoxide as the exchange reagent for substituted aryloxycyclophosphazenes will then be described both for the cyclic trimeric (3-5) and tetrameric systems (8-10). Finally, the stability of 1, 2, 6 and 7 in the presence of excess nucleophile in phosphazene syntheses will be discussed. A summary of the results is given in

Table 1.

5.3.1 Reactions of Hexaphenoxycyclophosphazene (2) with Sodium Trifluoroethoxide.

Sodium trifluoroethoxide can induce substituent exchange reactions with various aryloxycyclotriphosphazenes, such as hexakis(p-nitrophenoxy)cyclotriphosphazene, hexakis(o-nitrophenoxy) cyclotriphosphazene, hexakis(p-chlorophenoxy)cyclotriphosphazene and hexaphenoxycyclotriphosphazene according to an earlier study.32 It was also reported that, when sodium trifluoroethoxide reacts with 2, significant amounts of 1 and a large amount of

N3P3(OC6H5)3(OCH2CF3)3 were formed. Very little N3P3(OC6H5)2(OCH2CF3)4 and almost no

N3P3(OC6H5)(OCH2CF3)5 were obtained. However, in this earlier work, the reaction was studied over a relatively short period of time (3 days), and the progress of the entire reaction and identification of the ultimate products were not monitored or examined in detail. Such information is crucial for fully understanding the processes involved in exchange reactions. This is the major issue addressed in this paper.

In this investigation, when hexaphenoxycyclotriphosphazene 2 reacted with sodium

148 trifluoroethoxide in a ratio of 1:12 at reflux temperature (66 °C) in THF, hexakis(2,2,2-trifluoroethoxy)cyclophosphazene (1) is generated by a substitutent exchange reaction. This product was detected by 31P NMR spectrometry after two days of reaction (Figure

5-2b) together with a second phosphazene product (11) (doublet-triplet, 19.9 ppm and 11.1 ppm).

This second product was also formed in other substituent exchange reactions and will be discussed later. The amounts of both products 1 and 11 increased as the reaction progressed and as the starting material, 2, was consumed. After 50 days reaction, no further change of the ratio in the components in the reaction was detected, which indicated that the interaction had reached a steady state. At this stage the percentage of each phosphazene component in the reaction mixture was 2 16.9%, 1 10.8% and 11 72.3% (Figure 5-2d).

The reaction rate and ratios of the products were significantly dependent on the concentration of the nucleophile. The rate of consumption of 2 under reaction conditions b (Table

5-1) was much faster than under conditions a as the amount of trifluoroethoxide was doubled.

Thus, after 50 days reaction, the starting material 2 was almost completely consumed (reaction conditions b and Figure 5-3c). Furthermore, the higher concentration of trifluoroethoxide caused a greater tendency for 1 to be transformed to side product 11 (Figure 5-4b).

The absence of partly exchanged products from reactions carried out under these conditions suggests that the reaction proceeds as a self-accelerating process due to the strong electron-withdrawing effect of trifluoroethoxy side groups. This electron-withdrawal renders the cyclophosphazene more susceptible to further attack by the nucleophile.

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Table 5-1. Reaction Conditions and Major Products

Molar ratio of Reaction Initial Nucleophile nucleohpile: Major products phosphazene phosphazene a b a 2 1:12 1, 11 b 2 1:24 1, 11 c 1 1:12 11, 12

d 1 1:24 11, 12

e 3 1:12 1, 11 f 4 1:12 1, 11 g 5 1:12 1, 11 h 7 1:12 6, 13, 14 i 7 1:24 6, 13, 14 j 6 1:12 13, 14, 15

k 6 1:24 13, 14, 15

l 8 1:12 6, 13, 14 m 9 1:12 6, 13, 14 n 10 1:12 6, 13, 14 o 1 1:12 11 p 6 1:12 13 q 2 1:12 -

r 7 1:12 - a 1 = N3P3(OCH2CF3)6; 2 = N3P3(OC6H5)6; 3 = N3P3(OC6H4CHO-p)6; 4 = N3P3(OC6H4CN-p)6; 5 = N3P3(OC6H4NO2-p)6; 6 = N4P4(OCH2CF3)8; 7 = N4P4(OC6H5)8; 8 = N4P4(OC6H4CHO-p)8; 9 = b - + N3P3(OC6H4CN-p)6; 10 = N3P3(OC6H4NO2-p)6; 11 = N3P3(OCH2CF3)5(O Na ); 12 = - + - + N3P3(OCH2CF3)5(OC6H5); 13 = N4P4(OCH2CF3)7(O Na ); 14 = N4P4(OCH2CF3)6(O Na )2; 15 = N4P4(OCH2CF3)8-x(OC6H5)x (x=1~4)

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Figure 5-2. 31P NMR spectra for the reaction between 2 and sodium trifluoroethoxide (molar ratio 1:12) a) 2 hr; b) 2 days; c) 16 days; d) 50 days.

Figure 5-3. 31P NMR spectra for reaction between 2 and sodium trifluoroethoixde (molar ratio 1:24) a) 2 hr; b) 1 day; c) 30 days; d) 50 days.

151

Figure 5-4. Reactions of hexaphenoxycyclotriphosphazene (2) with sodium trifluoroethoxide: molar ratio a) 1:12; b) 1:24

152

5.3.2 Side Product 11

Side product 11 was shown by 31P NMR and mass spectrometry to have the structure shown in Scheme 5-2. The mass spectral identification was accomplished using a negative model mass spectrometer under a neutral mobile phase (acetonitrile) to give a mass of 645.9, which matches the structure shown in Scheme 5-2.

153

Scheme 5-2. Two step pathway involved in the formation of compound 11.

154

This product is formed from 1, itself generated by side group exchange from 2.

Evidence in favor of this interpretation is as follows. (a) No partially exchanged phenoxy-trifluoroethoxy cyclic phosphazenes were detected by mass spectrometry for these reaction conditions: only the fully exchanged cyclophosphazene 1 was present. (b) There is a parallel increase in the formation of 1 and 11 as the reaction proceeds. (c) No products similar to

11 but containing phenoxy groups were detected in any of these reactions.

The mechanism shown in Scheme 5-2 supposes that trifluoroethoxide ion participates in a nucleophilic attack on the alpha-carbon atom of a side group in 1. Two factors favor the view that the main attacking species is trifluoroethoxide rather than phenoxide. First, in a separate reaction between 1 and sodium trifluoroethoxide, the etheric side product CF3CH2OCH2CF3 was identified by mass spectrometry (mass = 163.01). Second, in the reaction between 2 and sodium trifluoroethoxide, the concentration of trifluoroethoxide is always higher than phenoxide even at the end of the side group exchange process.

5.3.3 Reactions of Hexakis(2,2,2-trifluoroethoxy)phosphazene (1) with Sodium Phenoxide.

The reverse reaction is less facile than the one discussed above. No hexaphenoxycyclophosphazene (2) was detected in the substituent exchange reactions between 1 and sodium phenoxide at a ratio 1 to 12 (reaction conditions c) or 1 to 24 (conditions d). Only the mono-exchanged derivative, N3P3(OCH2CF3)5OPh (12), and side product 11 (Figure 5-5) were detected. Both reactions reached a steady state after 10 and 25 days respectively. (Figure 5-6)

From the mass spectra, the final products from reaction conditions c and d were compounds 11 and 12 (Scheme 5-3), which suggests that the two products were formed through different pathways.

155

Figure 5-5. 31P NMR spectra for substituent exchange reaction between 1 and sodium phenoxide for 2 days: molar ratio a) 1:12; b) 1:24

Figure 5-6. Reactions of hexakis(2,2,2-trifluoroethoxy)cyclotriphosphazene (1) with sodium phenoxide: molar ratio a) 1:12; b) 1:24

156

Scheme 5-3. Two separate processes leading to the formation of 11 and 12 during the reaction of sodium phenoxide with hexakis(2,2,2-trifluoroethoxy)cyclotriphosphazene (1)

157

The reason why no fully exchanged product, hexaphenoxycyclophosphazene, (2), was detected is probably due to the higher steric hindrance characteristic of phenoxide units and the lower nucleophilicity which make it difficult to achieve full replacement of trifluoroethoxy groups. Thus, the smaller and more nucleophilic trifluoroethoxide ion can replace larger phenoxy groups to lower the overall steric restrictions around phosphorus, but the reverse process is more restricted. Therefore, although this exchange reaction is reversible, it relies significantly on the preferred production of 1. Meanwhile, 1 is still liable to be attacked by a nucleophile (either phenoxide or trifluoroethoxide) at the α-carbon of the alkoxy side unit to generate side product

11. The mass spectrometric identification of trifluoroethylphenyl ether (mass = 176.02) as a reaction product from the interaction of 1 with sodium phenoxide is further evidence for the process shown.

On the other hand, the final products from reaction conditions c and d also depended on concentration as with conditions a and b. In the initial stage, condition d induced a much higher reactivity than did condition c. For example, 23.3% of the starting material (1) was consumed under conditions c after two days (Figure 5-6a). By contrast, 58.3% of 1 had reacted with sodium phenoxide under conditions d (Figure 5-5b). Furthermore, when the interactions reached their steady state, only small amounts of compounds 11 and 12 were generated under conditions c.

Most of the starting material 1 remained intact (Figure 5-6a). However, when twice the amount of sodium phenoxide was used (condition d), a significant amount of compound 11 was generated, with roughly 75% of 1 consumed in the end (Figure 5-6b). This showed that hexakis(2,2,2-trifluoroethoxy)cyclophosphazene is more prone to attack by the nucleophile as the concentration of the phenoxide increases, which follows the same tendency as the results obtained for conditions a and b.

158

5.3.4 Reactions at the Cyclic Tetrameric Level: Octaphenoxycyclophosphzene (7) with

Sodium Trifluoroethoxide.

As in the cyclic trimer reactions, in the tetrameric system, octakis(2,2,2-trifluoroethoxy)cyclophosphazene (6), was produced through substituent exchange reactions between octaphenoxycyclophosphazene (7) and sodium trifluoroethoxide, but at a much faster rate. The side products 13 and 14 were detected by 31P NMR and confirmed by mass spectra. Again, reactions took place through the pathways shown in Scheme 5-4. However, the faster reaction rate at the cyclic tetramer level was illustrated by the following observation: in the cyclic trimer system, the starting material 2 could still be detected by 31P NMR spectrometry after one month (reaction conditions a and b), but in the cyclic tetramer system, most of the starting material 7 had been converted to 6, together with compounds 13 and 14 within 3 days with only 7.7% and 1.8% of starting material 7 remaining (conditions h and i and Figure 5-7).

These effects may reflect the greater flexibility of the larger ring and the resultant greater exposure of the phosphorus atoms to nucleophilic attack, due to the more open structure and lower steric hindrance compared to the cyclic trimer.1 It also suggests that this effect could be even more evident at the high polymer level. In addition, recent calculations show that the formal positive charge on phosphorus is greater in the tetramer vs the trimer which is another likely source of the greater rate of reaction in the tetramer. Actually, the trimer/tetramer results presented here are consistent with the effects found for other related reactions. Thus, the rate of

33 exchange of chloride ion in (NPCl2)4, is faster than in (NPCl2)3. Hydrolysis of (NPCl2)4 or

34, 35 (NPF2)4, proceeds faster than the hydrolysis of (NPC12)3 or (NPF2)3. Also, aminolysis of

36 (NPCl2)4 takes place more rapidly than the reaction of (NPC12)3, and the degradation of cyclophosphazenes to phosphoranes in the presence of catechol or o-phenylenediamine occurs more readily with cyclic tetramers than with trimers.37, 38

159

Scheme 5-4. Products from the reactions between aryloxy tetramers (7-10) and sodium trifluoroethoxide.

Figure 5-7. 31P NMR spectra for substituent exchange reaction between 7 and sodium trifluoroethoxide for 3 days: molar ratio a) 1:16; b) 1:32

160

5.3.5 Reactions of Octakis(2,2,2-trifluoroethoxy)cyclophosphazene (6) with Sodium

Phenoxide.

The reverse exchange reaction at the cyclic tetramer level (reactions j and k) gave results similar to their cyclic trimeric counterparts. No octaphenoxycyclophosphazene (7) was generated in either reaction after 22 days, and only partially-substituted N4P4(OCH2CF3)8-x(OC6H5)x (15) species were identified by mass spectrometry. The value of x can be from 1 to 4. Furthermore, the side product 13 appeared after 5 days reaction, showing that the α-carbon attack on the side group by the nucleophile also took place in these reactions.

5.3.6 Reactions of 3, 4, 5 and 8, 9, 10 with Sodium Trifluoroethoxide.

In all of these side group exchange reactions fully-substituted hexakis(2,2,2-trifluoroethoxy)cyclophosphazene (1) or octakis(2,2,2-trifluoroethoxy)cyclophosphazene (6) were generated by side group exchange. This is similar to the results discussed above for reactions a and k. However, a much faster exchange reaction rate occurred with these substituted aryloxycyclotetraphosphazene than with the parent phenoxyphosphazenes. The replacement reaction proceeds much faster because of the electron-withdrawing substituent groups on the aryl rings. This renders the skeleton more electron-deficient and more liable to attack by nucleophiles. This assumption was confirmed by the reactions of cyclic trimers and tetramers with 4-formylphenoxy (3 and 8), 4-cyanophenoxy (4 and 9) and 4-nitrophenoxy (5 and 10) side groups with sodium trifluoroethoxide. As shown in

Table 5-2, within half an hour, hexakis(2,2,2-trifluoroethoxy)cyclophosphazene (1) or octakis(2,2,2-trifluoroethoxy)cyclophosphazene (6) were formed by substituent exchange reactions and were detected by 31P NMR spectrometry. This same process would require 1 or 2 days for hexaphenoxycyclophosphazene (2) and octaphenoxycyclophosphazene (7). Moreover,

161 all of the starting materials were consumed within half an hour, except in the case of

4-formylphenoxy trimer (3) (2 hr). Again, this is much faster the analogous reactions of hexaphenoxycyclophosphazene (2) and octaphenoxycyclophosphazene (7) with sodium trifluoroethoxide (reactions a and b). Taking 4-cyanophenoxy trimer as an example, as shown in

Figure 5-8, all of the starting material (hexakis(4-cyanophenoxy)cyclophosphazene, 4) was converted to 1 within 30 minutes without the appearance of any detectable side product 11.

However, after 4 hours, compound 11 was detected and increased in concentration as the reaction proceeded. Similar results at the cyclic tetramer level between octakis(4-cyanophenoxy)cyclophosphazene (4) and sodium trifluoroethoxide were also obtained, as seen in Figure 5-9. This process is quite similar to that of its cyclic trimeric counterpart (4).

Therefore, the assumption proposed at the beginning of this section was reinforced by the results obtained in these reactions. Thus, the replacement reactivity was dramatically increased due to the electron-deficiency within the cyclic trimeric or tetrameric rings.

162

Figure 5-8. 31P NMR spectrum for substituent exchange reaction between 4 and sodium trifluoroethoxide for: a) 0 hr; b) 0.5 hr; c) 4 hr; d) 1 day

Figure 5-9. 31P NMR spectra for substituent exchange reaction between 9 and sodium trifluoroethoxide for: a) 0 hr; b) 0.5 hr; c) 4 hr; d) 1 day.

Table 5-2. Reaction time of aryloxy trimer and tetramer derivatives with sodium trifluoroethoixde Trimers Tetramers 2 3 4 5 7 8 9 10 a d d t1 2 d 0.5 hr 0.5 hr 0.5 hr 1 d 0.5 hr 0.5 hr 0.5 hr b t2 - 2 hr 0.5 hr 0.5 hr 7 d 0.5 hr 0.5 hr 0.5 hr c t3 3 d 1 d 4 hr 1 hr 2 d 3 d 1 d 18 hr a time for the appearance of 1 or 6 b time for complete consumption of starting material c time for the appearance of 11 or 13 and 14 d d: day, hr: hour

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5.3.7 Stability of 1, 2, 6 and 7 in the Presence of Nucleophiles.

Compounds 11, 13 and 14 always appeared during different substituent exchange reactions, which indicated a relatively high sensitivity of both hexakis(2,2,2-trifluoroethoxy)cyclophosphazene (1) and octakis(2,2,2-trifluoroethoxy)cyclophosphazene (6) under these reaction conditions. Actually, it has been shown that fluoroalkoxycyclophosphazenes39, aryloxy- and spiroarylenedioxycyclophosphazenes28 undergo hydrolysis in basic aqueous media.

Hexakis(2,2,2-trifluoroethoxy)cyclophosphazene can be hydrolyzed in solution by aqueous alkali, undergoing nucleophilic attack by hydroxide ion at phosphorus via an SN2-type mechanism through a nongeminal pathway.39 Such a mechanism allows retention of the ~120o N-P-N ring angle during formation of a trigonal-bipyramidal transition state. Moreover, the initial product formed during the hydrolysis of 1 is believed to be 11. Cyclic tetramers behave similarly, but with two- to four-fold faster hydrolysis rates, which can be explained by the reasons discussed above. Care was taken to exclude water from all the reactions described here, and it can be assumed that the attacking species is trifluoethoxide rather than hydroxide. A similar cleavage mechanism and products have also been detected from the reaction of excess nucleophile with heterophosphazenes by Manners et al.40

Nevertheless, a key reaction to supplement the side group exchange studies is the interaction of hexakis(2,2,2-trifluoroethoxy)cyclophosphazene (1) with sodium trifluoroethoxide

(molar ratio 1:12) under the same reaction condition as in all the other reactions. As shown in

Figure 5-10, the side product 11, appeared after one day of reaction, even though initially it constituted less than 2% of the product mixture. As the reaction proceeded, 1 was gradually converted to compound 11 and no other products were detected. After 41 days, approximately 90%

164

of 1 had been converted into 11. Moreover, as mentioned earlier, chemical ionization mass spectrometric analysis of the reaction mixture revealed the presence of bis(trifluoroethyl) ether

(mass 163.01), which is the other side product consistent with the proposed mechanism. These experiments indicate the sensitivity of hexakis(2,2,2-trifluoroethoxy)cyclophosphazene (1) when exposed to a large excess of a nucleophile under normal synthetic conditions. Thus, attention should be paid to this aspect when synthesizing small molecules or even high polymers to limit the exposure time of the final products to an excess of the nucleophile to prevent the generation of the undesired side product.

165

Figure 5-10. Formation of species 11 from the reaction of 1 with sodium trifluoroethoxide: molar ratio 1:12

166

By contrast, no side product was detected from the reactions of hexaphenoxycyclophosphazene (2) or octaphenoxycyclophosphazene (7) with sodium phenoxide under exchange reaction conditions after one week. This illustrates the ability of 2 and 7 to resist nucleophilic attack, and may reflect the excellent protection of the backbone by bulky phenoxy groups especially to nucleophilic attack by sodium phenoxide. It may also be connected with the absence of a bridging CH2 group, which would enhance the conformational flexibility of both the polymer and the nucleophile and deprive the system of a site for alpha-carbon attack.

5.4 Conclusions

The side group exchange reactions of cyclic trimeric and tetrameric aryloxycyclophosphazenes with sodium 2,2,2-trifluoroethoxide are dependent on the electron-withdrawing effects of different side groups. The ease of displacement of OAr in cyclic

- trimeric and tetrameric molecules by CF3CH2O increased significantly in the order, OAr = phenoxy << 4-formylphenoxy < 4-cyanophenoxy ≈ 4-nitrophenoxy derivatives. In addition, the side product generated via the attack by the nucleophile on the α-carbon of the

2,2,2-trifluoroethoxy group was detected in all the exchange reactions after 2,2,2-trifluoroethoxy phosphazene trimer or tetramer had been formed. The cyclic phenoxyphosphazene trimer and tetramer showed significantly better stability in the presence of sodium phenoxide than did the

2,2,2-trifluoroethoxy trimer or tetramer.

The information obtained here is valuable as a model study for the synthesis of phosphazene high polymers. As in the cyclic trimer or tetramer, the α-carbon of the fluoroalkoxy side groups in the corresponding high polymeric phosphazenes may also be liable to attack by excess nucleophilic reagents in the reaction solution, ultimately generating products with low molecular weights. These results are also significant in view of the report by Ferrar that the

167

reaction of excess sodium trifluoroethoxide with (NPCl2)n leads to lower molecular weight trifluoroethoxyphosphazene polymers.41 Furthermore, reaction of poly[bis(trifluoroethoxy)phosphazene] with excess sodium trifluoroethoxide also resulted in the transformation of a crystalline polymer to an amorphous one. This result coincides with changes in DSC thermograms, optical micrographs, and 31P NMR spectra, that suggest a secondary reaction by sodium trifluoroethoxide on the polymer.

The work reported here is also important with respect to the order in which nucleophiles should be added to high polymeric (NPCl2)n to obtain specific mixed-substituent side group ratios and avoid both side group exchange and the formation of side products similar to 11. In order to understand these aspects in more detail, comparable reactions of short chain linear phosphazenes and high polymers are currently underway.

168

5.5 References

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Hoboken, NJ, 2003.

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25. A. B. Pangborn, M. A. Giardello, R. H. Grubbs, R. K. Rosen and F. J. Timmers,

Organometallics, 1996, 15, 1518.

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27. G. A. Carriedo, L. FernandezCatuxo, F. J. G. Alonso, P. G. Elipe, P. A. Gonzalez and

G. Sanchez, J. Appl. Polym. Sci., 1996, 59, 1879.

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Chapter 6

Substituent Exchange Reactions of Linear Oligomeric Aryloxy Phosphazenes with Sodium

2,2,2-Trifluoroethoxide

6.1 Introduction

1-4 The reactions of phosphazene linear high polymers, [N=PR2]n (where n ~ 15,000) , are crucial for the synthesis of a wide variety of materials that are of interest in biomedicine5-7, energy storage8-12, and aerospace applications.13-15 However, some of these reactions are complicated, and experimental conditions often have to be optimized by studies of small molecule model systems. Phosphazene cyclic trimers and tetramers, [N=PR2]3 or 4, have traditionally been used as models for the high polymers, but their rigid skeletal structures and their reactivities differ in important ways from the behavior of their linear high polymeric counterparts. Short chain linear phosphazenes, are alternative small molecule models because their general architecture is more reminiscent of the high polymers. Use of a living cationic condensation method has provided reliable access to linear phosphazene oligomers with precisely tailored chain lengths, and this presents new opportunities to examine model reactions.16,17 The only disadvantage of these linear models is the higher ratio of end to middle units in the oligomers than in the high polymers, but this is less serious for species that contain more than five or six middle units.

One of the potentially useful reactions under development for polyphosphazenes involves the exchange reactions of one organic side group by another.18-20 This is an alternative technique to the normal synthesis procedure in which the chlorine atoms in high molecular

3,18 weight poly(dichlorophosphazene), (NPCl2)n, are replaced by organic nucleophiles. Because the chlorophosphazene high polymer is hydrolytically sensitive it must be stored and handled

172 under anhydrous conditions. Most organophosphazenes are water-stable. Hence, the use of an organophosphazene high polymer as a starting point for the introduction of other side groups has considerable appeal, particularly for the synthesis of mixed-substituent derivatives. In earlier studies we examined the side group exchange reactions of organophosphazene cyclic trimers in some detail21,22, but the behavior of linear oligomers has not been reported. Here we describe the similarities and differences between the cyclic and linear oligomeric systems in the exchange process.

In the following text the codes used are as follows: side groups in the linear oligomers are denoted by numerals; 1 = phenoxy, 2 = formylphenoxy, 3 = cyanophenoxy, and 4 = nitrophenoxy, whereas the chain length repeating units are indicated by letters; a is n = 6, b is n =

10, c is n = 20, and d is n = 40. The number of repeating units, n, in the linear oligomers is defined as shown in Scheme 6-1. Note that, in the discussion of end group influence, both the

P(OR)4 and P(OR)3 sites are important.

173

Scheme 6-1. Structures of cyclic (i and ii), linear oligomeric (iii), and high polymeric (iv)

systems.

174

6.2 Experimental Section

6.2.1 Materials

All reactions were carried out under an atmosphere of dry argon using standard Schlenk line techniques. Tetrahydrofuran (THF) (EMD) was dried using solvent purification columns.23

2,2,2-Trifluroethanol (Aldrich) was purified by vacuum distillation from CaH2 (Aldrich). Phenol

(Aldrich) was purified by sublimation. 4-Nitrophenol was recrystallized twice from toluene.

Sulfuryl chloride (Aldrich) and phosphorus trichloride (Aldrich) were purified by distillation.

Phosphorus pentachloride (Aldrich) was purified by sublimation under vacuum before use.

Lithium bis(trimethylsilyl)amide (Aldrich) and other reagents were used as received.

6.2.2 Equipment

1H and 31P NMR spectra were obtained using a Bruker AMX-360 NMR spectrometer, operated at 360 and 146 MHz respectively. 1H NMR spectra were referenced to tetramethylsilane signals while 31P NMR chemical shifts were referenced to 85% phosphoric acid as an external reference with positive shift values downfield from the reference. All chemical shifts are reported in ppm. Molecular weight distribution data were obtained using a Hewlett-Packard HP

1090 gel permeation chromatograph equipped with two Phenomenex Phenogel linear 10 columns and a Hewlett-Packard 1047A refractive index detector. The samples were eluted at 1.0 mL/min with a 10 mM solution of tetra-n-butylammonium nitrate in THF. The elution times were calibrated with polystyrene standards. Glass transition temperatures were determined by differential scanning calorimetry (DSC) with a TA Instruments Q10 and a heating rate of 10 oC/min and a sample size of ca. 10 mg.

6.2.3 Synthesis of Chlorophosphoranimine, Cl3P=NSiMe3

The synthesis of the chlorophosphoranimine monomer followed a previously reported

175

16,24,25 procedures with some modifications. Briefly, PCl3 (46.25 g, 0.33 mol) was added dropwise to LiN(SiMe3)2 (56.93 g, 0.33 mol) in ether (500 mL) at 0 °C over 30 min. The mixture was allowed to remain at 0 °C and was stirred for another 1 h. Sulfuryl chloride (45.22 g, 0.33 mmol) was then added slowly over 30 min, and the reaction mixture was stirred at 0 °C for 2 h. After completion of the reaction, the insoluble salt was removed by filtration. The crude product was condensed to one-third of its original volume by removing the ether and was purified by vacuum

1 distillation at room temperature to yield a colorless liquid. Yield: 63%. H NMR (CDCl3): δ 0.16

31 (s, 9H). P NMR (CDCl3): δ −57.08.

6.2.4 Synthesis of Linear Oligomeric Compounds

A typical synthetic procedure for oligomeric aryloxyphosphazene derivatives was as follows: For the synthesis of the 20 repeating unit oligo(phenoxyphosphazene), a 25 mL portion of a methylene chloride solution of PCl5 (0.36 g, 1.72 mmol) was stirred at room temperature for

2 h to dissolve the PCl5. To this solution, was added Cl3P=NSiMe3 (4.00 g, 17.18 mmol), and the mixture was stirred at room temperature for 4 h. The progress of the reaction was monitored

31 using P NMR spectroscopy until complete conversion of the Cl3P=NSiMe3 to oligo-dichlorophosphazene had occurred. The solvent was removed under reduced pressure to yield a viscous liquid. The product was redissolved in THF (50 mL) and was treated with an excess of phenol (4.2 g, 44.67 mmol) and cesium carbonate (14.55 g, 44.67mmol). The reaction mixture was stirred at reflux for 1 day, followed by concentration of the solution under reduced pressure and then precipitation of the oligomer three times into water and hexanes to isolate a white product 1c. Yield: 60.5%. The other oligomeric aryloxyphosphazenes were synthesized in a similar manner by varying the ratio of PCl5 to Cl3P=NSiMe3 to control the number of repeating units, followed by substitution with aryloxy nucleophiles as shown in Scheme 6-2.

176

6.2.5 Substituent Exchange Reactions by Sodium 2,2,2-Trifluoroethoxide

All the substituent exchange reactions were carried out in a similar manner. Generally, one equivalent of aryloxy side groups reacted with two equivalents of sodium trifluoroethoxide under reflux in THF. The following is a typical procedure. A solution of 1a (1 g, 4.33 mmol) in

THF (10 ml) was added dropwise to a stirred solution of sodium 2,2,2-trifluoroethoxide, prepared from 2,2,2-trifluoroethanol (1.91 g, 17.3 mmol) and sodium hydride (0.42g, 17.3 mmol) in THF (90 ml). The mixture was stirred at reflux in THF. At timed intervals, starting after the first day, samples were taken and the reaction progress was monitored by 31P NMR, mass spectrometry and GPC. The presence of the etheric side products was established by mass spectrometric analysis of the reaction mixtures.

177

Scheme 6-2. Synthesis and characterization of oligomeric aryloxyphosphazenes.

Table 6-1. Characterization data of oligomeric organophosphazenes a o b o b c c Compounds Side group n Tg ( C) Tm ( C) Mn DP PDI Yield (%) 1a phenoxy 6 -15.8 - 828 3.4 1.11 15.1 1b phenoxy 10 -11.6 - 876 3.8 1.06 20.3 1c phenoxy 20 -7.68 29.6 8,975 38.8 1.05 60.5 1d phenoxy 40 -5.68 60.4 12,834 55.5 1.09 73.6 2a formylphenoxy 6 0.21 - 5,461 19.0 1.17 11.2 2b formylphenoxy 10 19.11 - 5,963 20.8 1.27 13.8 2c formylphenoxy 20 37.8 105.3 - - - 40.5 2d formylphenoxy 40 42.2 157.9 - - - 39.9 3a cyanophenoxy 6 1.35 - 6,379 22.7 1.17 25.5 3b cyanophenoxy 10 69.0 - 7,010 25.0 1.11 21.7 3c cyanophenoxy 20 76.8 185.8 - - - 33.2 3d cyanophenoxy 40 77.5 229.3 - - - 40.4 4a nitrophenoxy 6 47.48 - 4,095 12.7 1.24 12.9 4b nitrophenoxy 10 62.8 - 4,078 12.7 1.19 13.2 4c nitrophenoxy 20 75.1 210.7 - - - 44.5 4d nitrophenoxy 40 82.9 253.3 - - - 35.7 aRepeating units; bMeasured by differential scanning calorimetry; cMeasured by GPC calibrated by linear polystyrene standards.

178

6.3 Results and Discussion.

6.3.1 Synthesis of Starting Oligomers (1-6)

The starting materials with chlorine substituents were synthesized by a living cationic polymerization.16,24 The chlorine atoms were then replaced by nucleophilic substitution to yield oligo-organophosphazenes. The number of repeating units in the oligomeric species was controlled by the ratio of PCl5 to Cl3P=NSiMe3 in a solution of PCl5 in dichloromethane. Under these conditions the oligomerization proceeded quantitatively without side reactions.17 Each oligomeric dichlorophosphazene was then treated with various aromatic nucleophiles in the presence of cesium carbonate. The cesium carbonate synthetic route26 resulted a significantly higher substitution efficiency than the use of the sodium aryloxide. However, the DP (degree of polymerization) calculated from GPC data was different from the targeted value. The discrepancy may be the result of the differences of hydrodynamic radius between oligomers and polystyrene, which is used to calibrate GPC. Figure 6-1 shows typical 31P NMR spectra of the synthesis of a phenoxy oligomer with 10 repeating units (1b) as reported in previous literature.17

179

b’, c’

a’

ii)

c d

a b

Figure 6-1. i) 31P NMR of oligo(dichlorophosphazene) with n = 10; ii) 31P NMR spectrum of 1b.

180

6.3.2 The role of solubility and crystallinity in the synthesis of oligomeric aryloxyphosphazenes.

Aryloxyphosphazenes with electron-withdrawing functional groups like formyl, cyano, or nitro on the aromatic ring are only sparingly soluble in common organic solvents. For example, the cyclic trimers, hexakis(4-cyanophenoxy)cyclotriphosphazene and hexakis(4-nitrophenoxy)cyclotriphosphazene and their cyclic tetrameric counterparts dissolve only in hot DMSO or DMF,27,28 while high polymeric poly[bis(4-cyanophenoxy)phosphazene] and poly[bis(4-nitrophenoxy)phosphazene] are almost insoluble in any organic solvent.27 This insolubility is attributed to both the extended rigid structure and the high polarity of the side chains. Both factors may induce extensive side-group stacking in the solid state and thus lead to microcrystallinity and consequently to insolubility.29,30 Similar behavior was found for polyphosphazenes that possess a donor-acceptor substitution pattern within the mesogenic group in side-chain liquid crystalline polyphosphazenes.29 Microcrystallite formation and its role in causing precipitation of partly substituted products complicates the reaction chemistry by necessitating forcing reaction conditions to bring about complete chlorine replacement.

This study showed that the solubility of the aryloxy oligomers decreased as the number of repeating units increased. Thus, the oligomers with 6 and 10 repeating units were more soluble in common organic solvents than the oligomers with 20 and 40 repeating units. Table 6-2 illustrates the solubility of aryloxyphosphazenes with varied skeletal chain lengths. Specifically, most of the oligomers were soluble in DMSO at either room temperature or when heated, and all the oligomers with 6 or 10 repeating units were also soluble in common organic solvent such as

THF, DCM and acetone. For these species the molecular weights of oligomers with 6 and 10 repeating units could be estimated by GPC as shown in Figure 2, and the data are shown in Table

181

1. However, the solubility decreased significantly as the number of repeating units exceeded 10, as illustrated in Table 2, and this limited the characterization by GPC.

6.3.3 Thermal Transitions

The thermal properties of the oligomers were studied by DSC analysis. The appearance of glass-like transitions in the DSC curves of short chain linear oligomers is perhaps surprising.

However, these transitions were detected at progressively higher temperatures from 0.2 oC to

46.5 oC with the increase in the number of repeating units. probably due to lower mobility of oligomer chains as the chain length increased from 2a to 2d. Melting transitions (Tm) were also detected for 2c, 3c and 4c as the repeating units reached 20, which provided evidence for crystallinity. Typical examples are shown in Figure 3 for 2a to 2d. However, oligomers 2a and

2b with 6 and 10 repeating units respectively showed no Tm transitions and this is probably the reason for the good solubility in common organic solvents. By contrast, the solubility of 2c, 2d

o o and 2d decreased dramatically with the appearance of Tm transitions at 105.3 C, 157.9 C and

160.0 oC. Although microcrystallinity was also detected for the phenoxyphosphazene oligomers, this was easily disrupted by common organic solvents such as THF or dichloromethane, due to the lower relative polarity of the aryl ring, making phenoxyphosphazene oligomers more soluble than other aryloxy oligomers.

182

Table 6-2. Solubility of different aryloxyphosphazenes with varied chain length n THF DCM Acetone Ethyl acetate DMSO Nitrobenzene rta heat rt heat rt heat rt heat rt heat rt heat 2a 6 Sb S S S S S P P S S S S 2b 10 S S Pc P S S P P S S S S 2c 20 Id I I I P S I I S S P S 2d 40 I I I I I P I I S S I P 3a 6 S S I I S S P P S S P P 3b 10 S S I I S S I P P S I I 3c 20 I I I I P P I I S S I I 3d 40 I I I I I I I I P S I I 4a 6 S S I I S S I P S S S S 4b 10 I P I I S S I I P S P P 4c 20 I I I I P P I I P P I P 4d 40 I I I I I P I I I P I I a room temperature; b S: soluble; c P: partially soluble; d I: insoluble

1.0 2b 1b 4b 1b 2b 3b 0.8 3b 4b

0.6

0.4

0.2 rel. RI-detector intensity RI-detector rel.

0.0

14 16 18 20 22 24 RT (min)

Figure 6-2. GPC trace of 1b, 2b, 3b and 4b.

183

2a 2b 2c 2d 2d

Endo 2b 2a

0 50 100 150 o Temperature ( C)

Figure 6-3. DSC analysis of 2a, 2b, 2c, and 2d.

184

6.3.4 Exchange reactions of linear aryloxyphosphazene oligomers (1a-1d) with sodium trifluoroethoxide

Earlier preliminary studies demonstrated that phenoxy side groups on both cyclotriphosphazenes and cyclotetraphosphazenes can be replaced by sodium trifluoroethoxide to generate fully substituted trifluoroethoxy cyclophosphazenes.21,22,31 These reactions are followed by an attack by excess nucleophile on the α-carbon of the 2,2,2-triuoroethoxy groups linked to phosphorus to give a species in which one trifluoroethoxy group has been replaced by an -O- Na+ unit.22 However, no substituent exchange was detected with high molecular weight poly(diphenoxyphosphazene), and a large excess of sodium trifluoroethoxide is required to replace the phenoxy groups in non-geminal cosubsituted high polymeric

18 [NP(OCH2CF3)(OPh)]n. Side group steric hindrance by the phenoxy groups is one possible explanation for this phenomenon, in which the backbone is well protected by the shielding effect of the phenoxy side units. Furthermore, as shown in Table 1, the Tg’s of 1a-1d increase from -15 o o C to 0.95 C, and the appearance of a Tm when the number of repeating units reached 20 or more, is also consistent with considerable steric hindrance associated with a decrease of polymer backbone mobility and side chain stacking. Therefore, it is reasonable that, as the molecular architecture changes from cyclotriphosphazene, cyclotetraphosphazene, and linear oligophosphazene, to high molecular weight polyphosphazene, there may be a chain length, beyond which no substituent exchange occurs due to the sequential build-up of steric hindrance by the side groups. In order to test this hypothesis, a series of exchange reactions were conducted between linear oligomeric phenoxyphosphazenes and sodium trifluoroethoxide.

In this investigation, it was found that substituent exchange reactions occur when a ratio of 1:4 was used between oligomeric phenoxyphosphazenes (1a, 1b and 1c) and sodium

185 trifluoroethoxide at reflux temperature (66 oC) in THF. As shown in Figure 4, the exchange reaction with oligomer 1a, was fast, since more than 87.6% of the original phosphorus signals

(-13.1 ppm ~ -13.9 ppm, -18.1 ppm ~ -19.1 ppm and -22.3 ppm ~ -23.0 ppm) from 1a disappeared after two days of reaction. Concurrently, a new group of peaks appeared between

1.2 ppm and -7.1 ppm from the replacement of phenoxy units by trifluoroethoxy units (Figure

6-4ii). However, the 31P NMR spectra are too complicated to determine the exact percentage of replacement due to the multiple coupling of adjacent phosphorus atoms with different chemical environments and to their varying partially substituted patterns (geminal/non-geminal, cis- or trans-). After 15 days of reaction, the original diphenoxyphosphazene peaks had almost completely disappeared, but with the appearance of a new peak at +6.4 ppm (Figure 6-4iii).

186

b a c

ii)

6.4 ppm

iii)

Figure 6-4. 31P NMR spectra for the reaction between 1a and sodium trifluoroethoxide (molar ratio 1 : 4) i) 0 day; ii) 2 days; iii) 15 days.

187

Similar substituent exchange patterns were also detected from the reaction between 1b and sodium trifluoroethoxide under the same reaction conditions. However, the substituent exchange reaction of 1c was slightly different from 1a and 1b. For one thing the exchange rate was slower. The original phosphorus signal could still be detected by 31P NMR at -18.9 ppm after 15 days reaction, a point when none of the P(OC6H5) signals remained for 1a and 1b. This slower substitution reaction rate may result from the gradually increasing steric hindrance as the chain length increased. Note that 1c is the oligomer from which the first appearance of a Tm was detected by DSC at 29.6 oC. Meanwhile, as in 1a, a new peak near +6.4 ppm was also detected in the 31P NMR spectra from both 1b and 1c after 15 days of reaction.

6.3.5 Influence of the end units

In contrast to the substituent exchange reactions of 1a, 1b and 1c, an exchange reaction with 1d was detected only for the end units, as shown in Figure 6-5. As illustrated in Figure 6-5i and 6-5ii, the doublet peaks (-12.4 ppm and -12.8 ppm) originally assigned to the end units of 1d, had completely disappeared after 5 days reaction, and new doublets appeared at -6.4 ppm and

-6.8 ppm, assigned to the trifluoroethoxy units after the replacement of phenoxy units. This preferred end unit replacement process was not detected during the reactions of 1a, 1b and 1c, probably because the exchange rate at both the end units and the middle units occurs at similar rates due to the weaker shielding effect of the side chains. As mentioned above, significant steric

o o restriction is present in 1d as indicated by the elevated Tg (-5.68 C) and Tm (60.4 C), when compared with 1a, 1b and 1c, a phenomenon clearly associated with the increased length of the chain.

The replacement rate differences between the end units and the middle units can be clearly differentiated in oligomer 1d. In addition, with compounds 1a, 1b and 1c, as the

188 reaction continued from 5 days (Figure 6-5ii) to 24 days (Figure 6-5iii), a new broad peak appeared at +6.8 ppm associated with the loss of trifluoroethoxy groups at the end units. This is probably the result of an attack by excess sodium trifluoroethoxide on the α-carbon of the terminal trifluoroethoxide groups. In earlier studies this cleavage reaction was detected with both hexaphenoxycyclotriphosphazene and octaphenoxycyclotetraphosphazene.22 In fact, the etheric side product CF3CH2OCH2CF3 was identified by mass spectrometry (mass = 163.01) in those reaction mixtures, thus verifying that the proposed cleavage reaction also occurs in the exchange reaction of 1d. The similar peaks (+6.4 ppm) detected from substituent exchange reactions in 1a, 1b and 1c also originate from the same cleavage reactions. Scheme 6-3 provides a brief summary of the substituent exchange reaction of 1d and the sequential cleavage reaction.

189

Phenoxy ending units i)

Trifluoroethoxy ending units

ii)

+6.8 ppm from cleavage

iii)

iv)

Figure 6-5. 31P NMR spectra for the reaction between 1d and sodium trifluoroethoxide (molar ratio 1 : 4) i) 0 day; ii) 5 days; iii) 24 days; iv) 32 days Scheme 6-3. Substituent exchange reaction process of 1d.

190

These results suggest that the first step in the side group exchange reactions of the high molecular weight polymer may be at the end units, assuming that the end units are exposed and not buried in a convoluted conformation.

6.3.6 Reactions of substituted aryloxyphosphazene oligomers (2a-2d, 3a-3d and 4a-4d) with sodium trifluoroethoxide

All these substituent exchange reactions followed a similar pattern to the one found for the phenoxyphosphazene oligomers (1a-1d), but with faster rates. Side group replacement between sodium trifluoroethoxide and oligomers with 6 (2a, 3a, 4a), 10 (2b, 3b, 4b) and 20 (2c,

3c, 4c) repeating units were complete within one day. For example, the reaction of 3c with sodium trifluoroethoxide showed none of the original 31P NMR signals after one week. Instead only a major peak at -6.5 ppm from trifluoroethoxy-phosphorus units was present. These replacement reactions presumably proceed much faster because of the electron-withdrawing substituent groups on the aryl rings. This renders the skeleton more electron-deficient and more liable to attack by nucleophiles. This phenomenon also occurs with the cyclic trimeric and tetrameric counterparts.22 As with the phenoxy short chain species, the introduced trifluoroethoxy-phosphorus units on the oligomers were susceptible to nucleophilic attack by excess sodium trifluoroethoxide, leading to the formation of –PO- Na+ units, with the concurrent appearance of a new phosphrous signal near +6 ppm. However, unlike the phenoxyphosphazene oligomer 1d, separate end group replacement was not detected by 31P NMR. However, the end group replacement for 2d, 3d and 4d may occur but, due to the very low solubility of oligomers

2d, 3d and 4d in THF, it was hard to detect from the 31P NMR spectra. Therefore, a general conclusion can be drawn that the ease of exchange reactions depends mainly on how well the backbone is protected by the side chains, and not on physical factors such as solubility or

191 crystallinity. For example, complete side group replacement took place for oligomers 1a-1c,

2a-2c, 3a-3c and 4a-4c, even though 2c, 3c and 4c are insoluble in THF and show a strong tendency for crystallinity. By contrast, 1d shows excellent solubility in THF, but no middle unit exchange reactions were detected in the presence of sodium trifluoroethoxide. This is the same as the behavior of the high polymeric counterpart.18 In addition, from the earlier study of small cyclic trimer and tetramer model reactions, 4-cyanophenoxy and 4-nitrophenoxy derivatives have a very limited solubility in THF, but the complete replacement of the aryloxy by trifluoroethoxy groups was still detected, with even faster rates,22 probably because of their more open structure compared with the linear organophosphazenes, making the ring more liable to attack by nucleophiles. But the most significant conclusion is that exchange reactions at the high polymer level appear to be modeled better by the behavior of short chain linear counterparts than by small cyclic molecules, although these latter reactions are easier to perform.

6.4 Conclusions

A series of oligomeric aryloxyphosphazenes with phenoxy, 4-formylphenoxy,

4-cyanophenxy and 4-nitrophenoxy side groups undergo substituent exchange reactions with sodium 2,2,2-trifluoroethoxide. Exchange reactions were detected for phosphazene chain lengths of 6, 10 and 20 repeating units. In this sense the ease of displacement of OAr by CF3CH2O is similar to the situation for their cyclophosphazene counterparts. Fully substituted

2,2,2-trifluoroethoxyphosphazene linear oligomers were formed by side group exchange, but these reactions are followed by an attack by trifluoroethoxide on the α-carbon of the

2,2,2-trifluoroethoxy groups linked to phosphorus to give a species in which one trifluoroethoxy group has been replaced by an ONa unit, and bis(trifluoroethyl) ether is formed concurrently as a side product. However, when the number of repeating units exceeds 20, side group exchange

192 occurs only at the end groups, due to the side group steric hindrance in the middle units. The comparable reactions with phosphazene high polymers are currently underway.

193

6.5 References

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2. Allcock, H. R.; Kugel, R. L.; Valan, K. J. Inorg. Chem. 1966, 5, 1709-1715.

3. Allcock, H. R.; Kugel, R. L. Inorg. Chem. 1966, 5, 1716-1718.

4. Allcock, H. R. Chemistry and Applications of Polyphosphazenes; Wiley-Interscience:

Hoboken, NJ, 2003.

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H. R. Polym. Chem. 2010, 1, 1459-1466.

6. Deng, M.; Nair, L. S.; Nukavarapu, S. R.; Jiang, T.; Kanner, W. A.; Li, X. D.;

Kumbar, S. G.; Weikel, A. L.; Krogman, N. R.; Allcock, H. R.; Laurencin, C. T.

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7. Deng, M.; Nair, L. S.; Nukavarapu, S. P.; Kumbar, S. G.; Jiang, T.; Weikel, A. L.;

Krogman, N. R.; Allcock, H. R.; Laurencin, C. T. Adv. Funct. Mater. 2010, 20,

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8. Fei, S. T.; Wood, R. M.; Lee, D. K.; Stone, D. A.; Chang, H. L.; Allcock, H. R. J.

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2010, 46, 6563-6565.

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11. Lee, D. K.; Allcock, H. R. Solid State Ionics 2010, 181, 1721-1726.

12. Thielen, J.; Meyer, W. H.; Landfester, K. Chem. Mater. 2011, 23, 2120-2129.

13. Reed, C. S.; Taylor, J. P.; Guigley, K. S.; Coleman, M. M.; Allcock, H. R. Polym. Eng.

Sci. 2000, 40, 465-472.

14. Allcock, H. R.; Taylor, J. P. Polym. Eng. Sci. 2000, 40, 1177-1189.

15. Allcock, H. R. Adv. Mater. 1994, 6, 106-115.

16. Honeyman, C. H.; Manners, I.; Morrissey, C. T.; Allcock, H. R. J. Am. Chem. Soc.

1995, 117, 7035-7036.

17. Allcock, H. R.; Crane, C. A.; Morrissey, C. T.; Nelson, J. M.; Reeves, S. D.;

Honeyman, C. H.; Manners, I. Macromolecules 1996, 29, 7740-7747.

18. Allcock, H. R.; Kim, Y. B. Macromolecules 1994, 27, 3933-3942.

19. Allcock, H. R.; Maher, A. E.; Ambler, C. M. Macromolecules 2003, 36, 5566-5572.

20. Allcock, H. R.; Connolly, M. S.; Harris, P. J. J. Am. Chem. Soc. 1982, 104, 2482-2490.

21. Allcock, H. R.; Smeltz, L. A. J. Am. Chem. Soc. 1976, 98, 4143-4149.

22. Liu, X.; Breon, J. P.; Chen, C.; Allcock, H. R. J. Chem. Soc., Dalton Trans. 2012, 41,

2100-2109.

23. Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J.

Organometallics 1996, 15, 1518-1520.

24. Wang, B.; Rivard, E.; Manners, I. Inorg. Chem. 2002, 41, 1690-1691.

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25. Chang, Y.; Lee, S. C.; Kim, K. T.; Kim, C.; Reeves, S. D.; Allcock, H. R.

Macromolecules 2001, 34, 269-274.

26. Carriedo, G. A.; Alonso, F. J. G.; Gonzalez, P. A. Macromol. Rapid Commun. 1997, 18,

371-377.

27. Carriedo, G. A.; FernandezCatuxo, L.; Alonso, F. J. G.; Elipe, P. G.; Gonzalez, P. A.;

Sanchez, G. J. Appl. Polym. Sci. 1996, 59, 1879-1885.

28. Allcock, H. R.; Walsh, E. J. J. Am. Chem. Soc. 1972, 94, 4538-4545.

29. Allcock, H. R.; Kim, C. Macromolecules 1989, 22, 2596-2602.

30. Allcock, H. R.; Dembek, A. A.; Kim, C.; Devine, R. L. S.; Shi, Y. Q.; Steier, W. H.;

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196

Chapter 7

Substituent Exchange Reactions with High Polymeric Organophosphazenes

7.1 Introduction

High molecular weight polyphosphazenes are inorganic backbone polymers with an essentially linear backbone of alternating phosphorus and nitrogen atoms and two organic or organometallic side groups linked to each phosphorus atom.1-4 A distinctive feature of polyphosphazenes is the ease with which the polymer properties can be precisely tailored through changes in the side groups to optimize properties for specific uses, such as in bone regeneration scaffolds5-7, fire resistance and retardance8,9, low-temperature elastomers10, fuel cell membranes11-13 and solid or gel polymer lithium ion conductors.14,15

Most of the classical high molecular weight linear polyphosphazenes have been prepared via a ring-opening polymerization of hexachlorocyclotriphosphazene, (NPCl2)3, to high polymeric poly(dichlorophosphazene), (NPCl2)n, followed by replacement of the labile chlorine atoms in this macromolecular intermediate by organic groups, such as alkoxy, aryloxy, or amino units4,16 as shown in Scheme 7-1. In addition, cosubstituted poly(organophosphazenes) can be synthesized by the sequential or simultaneous reaction of different nucleophiles with poly(dichlorophosphazene) to obtain materials with tunable properties controlled by the side group ratios. (Scheme 7-1)

197

Scheme 7-1. Synthesis of Cosubstituted Poly(organophosphazenes) by Sequential or Simultaneous Addition of Nucleophiles to (NPCl2)n or by Side Group Exchange Reactions

198

Alternatively, cosubstituted polyphosphazenes could in principle be prepared through the replacement of organic side groups already linked to a phosphazene chain by other groups.16-18 (Scheme 1) This appealing alternative offers the prospect that single-substitutent poly(organophosphazenes) can be converted readily to mixed-substituent materials that are of broad interest. More important, from a practical point of view, most poly(organophosphazenes) are much more stable reaction intermediates than poly(dichlorophosphazene), and might be used as general macromolecular intermediates for the preparation of other poly(organophosphazenes) with adjustable properties.

Poly[bis(2,2,2-trifluoroethoxy)phosphazene], [NP(OCH2CF3)2]n, was one of the first stable phosphazene polymers synthesized.2,3,19-21 Since that time numerous studies have been conducted on this polymer. However, its synthesis by the reaction of poly(dichlorophosphazene) with sodium trifluoroethoxide in an etheric solvent such as THF, while straightforward to perform in the hands of most investigators, has nevertheless proved curiously challenging in some laboratories. Attempts to scale-up this process from the normal 100-200 g scale to, for example, a kilogram level occasionally yielded no substituted high polymer at all, even in laboratories where no problems had been encountered at a smaller scale.

A clue to the explanation was initially provided by Ferrar at Eastman Kodak, who reported22 that the properties of the fully substituted polymer varied with the presence of excess sodium trifluoroethoxide, a condition often employed to ensure the replacement of all the chlorine atoms in (NPCl2)n. Ferrar and co-workers found that excess nucleophile (NaOCH2CF3) can attack fully substituted poly[bis(2,2,2-trifluoroethoxy)phosphazene] and bring about changes in the polymer morphology, such as transformation of a crystalline polymer to an amorphous one.22 However, according to their observations a chemical analyses of the polymers did not

199

detect a difference in the materials before and after the treatment with NaOCH2CF3 despite large changes in the polymer physical properties.22 Later, Kolich and coworkers invented a surface treatment of mixed-substituent polyphosphazene fluoroelastomers with sodium trifluoroethoxide to produce a material having enhanced solvent resistance by a side group exchange reaction.23

Our earlier work with the organic side group exchange processes carried out on small molecule cyclic or linear phosphazene oligomers24-26 revealed a possible explanation. These studies showed that side group displacements by organic nucleophiles such as sodium

2,2,2-trifluoroethoxide are sometimes accompanied by side reactions such as α-carbon attack by excess of the nucleophile.3,22,24Hence, a part of this present study was devoted to examine the reaction of high molecular weight [NP(OCH2CF3)2]n with sodium trifluoroethoxide.

Two related issues are discussed in this paper. First, we consider the possibility that a useful means for diversifying the structure of poly(organophosphazenes) is by organic side group exchange reactions. Possible starting materials are either aryloxy or fluoroalkoxy-substituted polymers. Second, we provide an explanation for a few puzzling reports involving problems with the synthesis of poly[bis(trifluoroethoxy)phosphazene)] by the reactions of poly(dichlorophosphazene) with sodium trifluoroethoxide. Furthermore, as an example of the potential synthetic utility of the exchange process, the substituent exchange reaction has been applied to the synthesis of trichloroethoxy/trifluoroethoxy cosubtituent polyphosphazenes.

7.2 Experimental Section

7.2.1 Materials

All reactions were carried out under an atmosphere of dry argon using standard Schlenk line techniques. Tetrahydrofuran (EMD) was dried using solvent purification columns.27

2,2,2-Trifluroethanol (Aldrich) was purified by vacuum distillation from CaH2 (Aldrich). Phenol

200

(Aldrich) was purified by sublimation. 4-Nitrophenol was recrystallized twice from toluene.

Hexachlorocyclotriphosphazene, (NPCl2)3, (from various sources including Fushimi

Pharmaceutical Co., Japan, and Ningbo Chemical, China) was purified by recrystallization from hexanes and by vacuum sublimation at 50 °C. High molecular weight poly(dichlorophosphazene) was prepared by the thermal ring-opening polymerization of recrystallized and sublimed hexachlorocyclotriphosphazene in evacuated Pyrex tubes at 250 °C.3

7.2.2 Equipment

1090 gel permeation chromatograph equipped with two Phenomenex Phenogel linear 10 columns and a Hewlett-Packard 1047A refractive index detector. The samples were eluted at 1.0 mL/min with a 10 mM solution of tetra-n-butyl ammonium nitrate in THF. The elution times were calibrated with polystyrene standards. Glass transition temperatures were determined by differential scanning calorimetry (DSC) with a TA Instruments Q10 and a heating rate of 10 oC/min and a sample size of ca. 10 mg.

7.2.3 Synthesis of High Polymers 1-7

A typical synthetic procedure for polymeric aryloxyphosphazenes derivatives is as follows: A THF solution (150 mL) of poly(dichlorophosphazene) (3.0 g, 25.9 mmol) was added to a THF (150 mL) suspension of cesium carbonate (21.1 g, 64.7 mmol) and phenol (6.09 g, 64.7 mmol). The reaction solution was stirred for 48 h at reflux. The polymer solution was

201 concentrated by rotary evaporation, and the residue was poured into water to precipitate the polymeric product, which was further purified by repeated precipitation three times into both water and hexanes. The pure product was dried under vacuum to yield a white solid 1. Yield:

74.5%. The other poly(organophosphazene)s were synthesized in a similar manner by using different alkoxy or aryloxy nucleophiles as shown in Scheme 7-2. The characterization data of

1-7 are listed in Table 7-1.

7.2.4 Substituent Exchange Reactions for Polymers 1-7 with nucleophiles

All the substituent exchange reactions were carried out in a similar manner. Generally, one equivalent of polymer side group was exposed to two equivalents of nucleophiles under reflux in THF. The following is a typical procedure. A solution of 1 (1 g, 4.33 mmol) in THF (10 ml) was added dropwise to a stirred solution of sodium 2,2,2-trifluoroethoxide, prepared from

2,2,2-trifluoroethanol (1.91 g, 17.3 mmol) and sodium hydride (0.42g, 17.3 mmol) in THF (90 ml). The mixture was stirred at reflux in THF. At timed intervals, starting after the first day, samples were taken and the reaction progress was monitored by 31P NMR, mass spectrometry and GPC. The presence of etheric side products was established by mass spectrometric analysis of the reaction mixtures.

202

Scheme 7-2. Synthesis and Characterization of Polymeric Aryloxy and Alkoxyphosphazenes.

Table 7-1. Characterization data for polymeric organophosphazenes o a o a b Compounds Side group Repeating units Tg ( C) Tm ( C) Mn PDI Yield (%) 1 OC6H5 3800 0.95 143.4 890,470 1.78 88.3 c 2 OC6H4CHO-p - 46.5 160.0 - - 89.5 3 OC6H4CN-p - 79.7 245.5 - - 76.6 4 OC6H4NO2-p - 84.3 268.0 - - 85.6 5 OCH2CF3 2270 -64 230 551,860 1.4 84.3 6 OCH2(CF2)4H 4680 -20.52 - 2,383,458 2.33 80.3 7 OCH2CCl3 4200 12.4 132.5 1,450,000 2.37 51.2 aMeasured by differential scanning calorimetry; bMeasured by GPC calibrated by linear polystyrene standards; cPolymer 2-4 can not dissolve in THF

203

7.3 Results and Discussion

7.3.1 Exchange reactions of 1-4 with NaOCH2CF3

No substituent exchange was detected with high polymers 1-4 when exposed to

16 NaOCH2CF3. Similar results were obtained in an earlier study. A possible explanation is that side group steric hindrance by the aryloxy groups, shields access to the backbone and to the

P-O-C side linkages. Aryloxy group replacement can be achieved in cyclotriphosphazenes, cyclotetraphosphazenes, and linear oligomers with no more than 20 repeating units.24-26 With the linear short chain species side group exchange reactions occur preferentially at the end units once the number of repeating units reach 40. Considering the much more significant shielding effect at the high polymer level due to chain coiling, it is perhaps understandable that no substituent replacement of aryloxy groups was detected with 1-4. The resistance of the longest chain aryloxy-substituted linear oligomers and polymers to side group exchange suggests that these species may be more resistant to degradative side reactions than their alkoxy-substituted counterparts.

7.3.2 Exchange reactions of 5-6 with NaOCH2(CF2)4H and NaOCH2CF3

High-performance elastomers based on fluoroalkoxyphosphazenes are the basis of some of the most promising applications of polyphosphazenes. The idealized structure is

[N=P(OCH2CF3)(OCH2(CF2)4H)]n, where the presence of two or more different side groups markedly reduces the tendency for crystallinity.19,28,29 The elastomeric properties of the final products are highly dependent on both the ratio of the side groups and the distribution sequence along the polymer chain. Therefore, a study of possible substituent exchange reactions during synthesis is an important aspect of this field. In addition, the possibility of α-carbon attack to generate etheric side products as detected for cyclic or oligomeric phosphazenes25,26 raises the

204 possibility that hydrolytically sensitive sites may be introduced which would be detrimental to the properties of these high polymeric polyphosphazenes.

7.3.2.1 Exchange reaction of 5 with NaOCH2(CF2)4H

It was found that substituent exchange reactions occur when a ratio of 1:4 was used between the side groups in 5 and NaOCH2(CF2)4H at reflux temperature in THF. Roughly 61% of the OCH2CF3 groups were replaced by OCH2(CF2)4H units after one day reaction, as

1 determined by H NMR spectrometry. The theoretical value of the molecular weight Mn of the resulting polymer after one day of reaction should be 919,012, calculated by the molecular weight difference between OCH2CF3 and OCH2(CF2)4H side groups. However, this value is different from the one measured by GPC, which is 658,973 in Table 7-2. A possible explanation for the discrepancy may be the change of hydrodynamic radius of the resulting polymer after more than half of the side groups in 5 have been replaced. After 3 days of reaction, the molecular weight and replacement percentage remained almost the same as at day 1, Mn = 680,185 and the percent of replacement was 59.3%. However, after 6 days of reaction both Mn and the replacement percentage decreased. Mn declined from 680,185 to 287,124, while the first appearance of the etheric side product CF3CH2OCH2(CF2)4H was detected by mass spectrometry.

The results suggested that the significant decrease of Mn after 6 days is mainly the result of the introduction of hydrolytically sensitive P-O-Na+ sites, on the polymer backbone by α-carbon attack. Isolation and purification of the polymer involves exposure to aqueous media which brings about the chain cleavage process. This attack may also occur within the first three days reaction, although no etheric side product was detected and it was not significant enough to lead to an obvious molecular weight decline.

These results were also supported by 31P NMR spectrometry as shown in Figure 7-1. No

205 significant change was detected during the first 3 days (Figures 7-1a to 1c). In contrast, multiple peaks appeared and became broader after 6 days of reaction, which indicated the occurrence of

α-carbon attack on polymer backbone.

206

Table 7-2. Exchange reaction of polymer 5 with NaOCH2(CF2)4H. 0 day 1 day 3 days 6 days Mn 551,860 658,973 680,185 287,124 % of replacement 0 61.3 59.3 52.1

a) 0 day

b) 1 day

c) 3 days

d) 6 days

31 Figure 7-1. P NMR spectra for substituent exchange reaction between 5 and NaOCH2(CF2)4H (1: 4, reflux in THF) for: a) 0 day; b) 1 day; c) 3 days; d) 6 days.

207

7.3.2.2 Exchange reaction of 6 with NaOCH2CF3

The same exchange reaction between 5 and NaOCH2(CF2)4H, was detected throughout

6 days of reaction. The replacement percentage was 69.7% after 1 day, and did not show a significantly change within 6 days (Table 7-3). The apparent decline of molecular weight from

2,383,458 to 333,463 in the first day may be the result of a replacement of larger side groups,

OCH2(CF2)4H, by smaller side groups, OCH2CF3. After 6 days reaction, the α-carbon attack on polymer backbone became significant and led to the second molecular weight decline as in the exchange reaction between 5 and NaOCH2(CF2)4H.

208

Table 7-3. Exchange reaction of 6 with NaOCH2CF3. 0 day 1 day 3 days 6 days Mn 2,383,458 333,463 336,865 131,218 % of replacement 0 69.7 72.6 64.4

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7.3.3 Stability of poly[bis(2,2,2-trifluorophosphazene)] in the presence of nucleophiles

According to the exchange reactions discussed above, fluoroalkoxyphosphazenes showed some instability in the presence of excess nucleophiles in this reaction system, and this resulted in a molecular weight decline following purification. In this current study, poly[bis(2,2,2-trifluoroethoxyphosphazene)] (5) was treated with an excess of sodium trifluoroethoxide with a more extended reaction time (58 days) than in the literature (14 days)22 in order to detect possible changes by 31P NMR, GPC and mass spectrometry to study the reaction process in details.

In this investigation, poly[bis(2,2,2-trifluoroethoxyphosphazene)] (5) showed a long-term instability in the presence of excess nucleophiles. As illustrated in Figure 7-2, after 1 day of reaction, a single sharp 31P NMR peak at -6.6 ppm appeared beside the major peak at -7.1 ppm from 5. The relative intensity of these two peaks changed as the reaction proceeded. The major peak at -7.1 ppm became broader after 7 days, while small broad peaks at -2.5 ppm and

-10.1 ppm were detected in the 31P NMR spectrum. In addition, a decline in molecular weight was also detected within the first two weeks, as shown in Table 7-4. Indeed, the molecular weight declined from the original 700,000 to 160,000 after 1 day reaction. This was followed by a further decrease to 57,000 after one week. However, GPC data could not be obtained after two weeks because the polymer to precipitate could not be isolated by precipitation from he reaction solution during isolation. Furthermore, the major peak at -7.1 ppm broadened continuously with increasing reaction time. After 48 days, a new broad peak at +6.4 ppm was detected, the intensity of which increased during the following ten days. Bis(trifluoroethyl) ether was detected by mass spectrometry from the exchange reaction between 5 and sodium trifluoroethoxide, which indicated that the cleavage reaction from α-carbon attack by excess nucleophile also occurred in

210 this exchange reaction process.

Therefore, we propose the reaction mechanism shown in Scheme 7-3 between 5 and sodium trifluoroethoxide. Initially, the α-carbon attack by excess sodium trifluoroethoxide occurred, even during the first day of reaction, although neither bis(trifluoroethyl) ether nor a significant change in 31P NMR was detected. But some hydrolytically sensitive ONa sites had been introduced along the polymer backbone, which served as chain cleavage sites during the subsequent purification via aqueous media. This led to a serious decline in molecular weight

(from 700,000 to 160,000) following precipitation into acidic water. As the exchange reaction proceeded, more and more ONa sites were introduced until after 2 weeks 5 could not be isolated by precipitation. In the end, after 58 days most of the trifluoroethoxy side groups on the polymer had been replaced by ONa units and chain cleavage as indicated by the presence of a broad peak at +6.4 ppm an indication of the degradation. In order to confirm that the chain cleavage is the result of an attack by excess NaOCH2CF3, and not from trace amounts of water or free alcohol in the reaction system, two reference reactions were conducted by refluxing 5 in THF, either with

31 or without HOCH2CF3. Both experiments showed no change in either the P NMR spectrum or mass spectrum after one month of treatment. Consequently, the presence of NaOCH2CF3 is crucial for the α-carbon attack. Moreover, the effect of increasing the excess nucleophile,

NaOCH2CF3 on the cleavage reaction was also studied by using a 1:15 ratio of NaOCH2CF3, rather than the original ratio of 1:4. The presence of a larger excess of NaOCH2CF3 led to a faster cleavage reaction, as shown in Figure 7-3. After 7 days, almost no 31P peak at -7.1 ppm could be detedted. Instead, a prominent broad peak appeared at +6 ppm, attributed to a much faster cleavage reaction. Therefore, based on this work, in the synthesis of poly[bis(2,2,2-trifluoroethoxy)phosphazene], the amount of NaOCH2CF3 should be strictly

211 controlled to not only achieve complete substitution, but also to avoid the introduction of hydrolytically sensitive sites on the polymer backbone due to the α-carbon attack.

212

a) 0 day

b) 1 day

c) 7 days

d) 18 days

e) 48 days

f) 58 days

Figure 7-2. 31P NMR spectra for substituent exchange reaction between polymer 5 and sodium trifluoroethoxide (1: 4, reflux in THF) for: a) 0 h; b) 1 day; c) 7 days; d) 18 days; e) 48 days; f) 58 days. Table 7-4. Molecular weight change with exchange reaction time 0 day 1 day 1 week 2 weeks a Mn 700,000 160,000 57,900 - a Can not precipitate from acidic deioned water, no GPC data

Scheme 7-3. Substituent exchange reaction process of 5 with sodium trifluoroethoxide

213

a) 0 day

b) 7 days

Figure 7-3. 31P NMR spectra for substituent exchange reaction between 5 and sodium trifluoroethoxide (1: 15, reflux in THF) for: a) 0 h; b) 7 day.

214

7.3.4 Application of the exchange reaction to the synthesis of polyphosphazenes containing trichloroethoxy units

Recently, a new class of polyphosphazenes containing trichloroethoxy units was synthsized in our laboratory by the reactions of poly(dichlorophosphazene) with sodium trichloroethoxide.30 The homo-substituted poly[bis(2,2,2-trichloroethoxy)phosphazene] (6) had only limited solubility in common organic solvents, probably due to its high crystallinity.30 This resulted in poor processability and limited its prospective applications. However, the cosubstituted derivatives with trifluoroethoxy units showed much improved solubility in common organic solvents, together with excellent film-forming properties and potential fire-resistant application.30 As mentioned above, the substituent exchange reaction is an alternative synthetic approach that allows access to cosubstituted polyphosphazenes, starting from a more stable homo-substituted precursor rather than (NPCl2)n. Therefore, the feasibility of synthesis of cosubstituted polyphosphazenes containing trichloroethoxy/trifluoroethoxy units by substituent exchange reaction was examined.

In general, this study showed that the fully substituted poly[bis(2,2,2-trifluoroethoxy)phosphazene] can be obtained by an exchange reaction between 6 and NaOCH2CF3 within 1 day. By contrast, only a maximum of 25% replacement could be achieved between 5 and NaOCH2CCl3, even though a large excess nucleophile (12 eq.) was used.

The substituent exchange reaction data for the reaction of 6 with NaOCH2CF3 are listed in Table 7-5. Replacement percentages as high as 57.4% and 71.8 % can be achieved within 1 day by using a ratio of 1 to 2 or 1 to 3 of repeating units of 6 and NaOCH2CF3. No further change was detected by 31P NMR with longer reaction times. In addition, 100% replacement of

215 trichloroethoxy by trifluoroethoxy occurred, as the ratio was increased to 1:4 as shown in Figure

7-4. More important, the resultant products retained relatively high molecular weights with trichloroethoxy as the nucleophile, which indicated a greater resistance to α-carbon attack and molecular weight decline if the reaction time was controlled within 1 day.

On the other hand, complete replacement of trifluoroethoxy by trichloroethoxy could not be achieved via the exchange reaction between 5 and NaOCH2CCl3, even with 12 equivalents of nucleophile and an extended reaction time of 4 days. The results are shown in

Table 7-6. The maximum replacement was only 25% (Figure 7-5). Moreover, in this case, serious polymer breakdown occurred due to severe α-carbon attack by the excess nucleophile after an extended reaction time of 4 days.

Therefore, it is plausible to presume a reversible reaction equilibrium between 5 and 6 in the presence of nucleophiles, where this equilibrium lies strongly on the side of 6 as illustrated in Scheme 7-4. A possible driving force for this process may be the release of steric hindrance generated by the replacement of the more bulky trichloroethoxy side groups by the less bulky trifluoroethoxy unit. Furthermore, the electron-withdrawing ability of the trichloroethoxy side group may also play an important role in determining the degree of substitution. Unlike general alkoxy or fluoroalkoxy polyphosphazenes, the 31P NMR chemical shift of 6 is at -11.5 ppm rather than ~-7 ppm as in other alkoxy or fluoroalkoxy polyphosphazenes. This indicates the unusual electron-withdrawing effect of the trichloroethoxy group.30 This is the first time that a reversible substituent exchange equilibrium has been detected in polyphosphazene reactions.

The results from this study are important with respect to the synthesis of soluble and processible cosubstituted polyphosphazenes that bear both trichloroethoxy and trifluoroethoxy units. First, this study showed that the order of addition of the different nucleophiles is crucial for

216 determining the composition and resultant properties of the final products. This is because of the greater ease of replacement of trichloroethoxy units by sodium trifluoroethoxide. Second, a targeted product with a specific composition can also be synthesized by the exchange reaction between 6 and NaOCH2CF3. Thus, the more stable intermediate poly[bis(2,2,2-trichloroethoxy)phosphazene] (6) rather than the moisture-sensitive poly(dichlorophosphazene) can be starting point for the synthesis of mixed substituent polyphosphazenes. Moreover, the desired composition of the final products can be finely tuned by varying the amount of NaOCH2CF3.

217

Table 7-5. Substituent exchange between 6 and NaOCH2CF3 NaOCH2CF3 Reaction time Replacement % Mn 6 1:2 1 d 57.4% 734,455 6 1:3 1 d 71.8% 917,033 6 1:4 1 d 100% 826,686

Figure 7-4. 31P NMR spectra for substituent exchange reaction 1 day reflux in THF between 6 and NaOCH2CF3 in the ratio of a) 1 : 0; b) 1:2; c) 1:3; d) 1:4.

Table 7-6. Substituent exchange between 5 and NaOCH2CCl3 NaOCH2CCl3 Reaction time Replacement % Mn 5 1:4 4 d 20% 674,331 5 1:8 4 d 24% 128,311 5 1:12 4 d 25% -a a Can not precipitate from acidic deioned water, no GPC data

218

Figure 7-5. 31P NMR spectra for substituent exchange reaction 4 days reflux in THF between 5 and sodium trichloroethoxide in the ratio of a) 1 : 0; b) 1:4; c) 1:8; d) 1:12. Scheme 7-4. Reversible substituent exchange reaction between 5 and 6.

219

7.4 Conclusions

The most important conclusion from this work is that large excess amounts of fluoroalkoxide nucleophiles should be avoided in the synthesis of fluoroalkoxy phosphazene polymers from poly(dichlorophosphazene) or during the side group exchange processes. No exchange reactions were detected between polymeric aryloxyphosphazenes and NaOCH2CF3 probably due to protection of the reaction sites by the steric hindrance of the aryloxy side groups.

By contrast, the side group replacement between [N=P(OCH2CF3)2]n and

NaOCH2CF2CF2CF2CF2H or [N=P(OCH2CF2CF2CF2CF2H)2]n and NaOCH2CF3 revealed that only partial substituent exchange can be achieved for both reactions. Moreover, these side group exchange reactions are also followed by the formation of sodium-oxo groups at phosphorus in place of the organic substituents, especially when an excess of the attacking nucleophile is present. This leads to subsequent hydrolysis and molecular weight decline after exposure to water during isolation and purification. Thus, an explanation for the variation of properties of poly[bis(2,2,2-trifluoroethoxy)phosphazene] was formed, based on its exposure to excess sodium trifluoroethoxide by a situation that leads to the formation of bis(trifluoroethyl) ether from

α-carbon attack. The introduced P-ONa units then serve as hydrolytically sensitive sites and, in the presence of water, these can be sites for chain cleavage. Thus, in the synthesis reactions in which trifluoroethoxy groups are introduced the amount of sodium trifluoroethoxide nucleophile should be well controlled to ensure the complete replacement of chlorine atoms and to avoid the introduction of hydrolytically sensitive sites. Achieving this balance is the key to the synthesis of stable high polymers. Finally, the substituent exchange process provides a method for the synthesis of cosubstituted polyphosphazenes with both trichloroethoxy and trifluoroethoxy side units. By using poly[bis(2,2,2-trichloroethoxy)phosphazene] as a starting stable intermediate, the

220 ratio between the two different side groups can be finely tuned by controlling the amount of sodium trifluoroethoxide in the reaction mixture.

221

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224

VITA

Xiao Liu

Xiao Liu, son of Hengjie Liu and Xiaoping Zhang, was born on October 3, 1980 in

Shenyang, P. R. China. He earned his Bachelor of Science degree in Chemistry in July 2003 under the direction of Dr. Yuguang Ma. He earned an Mater of Science degree in Polymer

Chemistry from National University of Singapore under the direction of Dr. Hardy Chan in 2005.

He worked for National University of Singapore as a research engineer from 2005 to 2007. Xiao pursued his graduate education at The Pennsylvania State University, where he joined the research group under the tutelage of Evan Pugh Professor Harry R. Allcock in August in 2007.