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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 By Xiao Liu © 2013 Xiao Liu Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy May 2013 ii 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 iii 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. iv 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. v 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 vi 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. vii 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]
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