FUNCTIONAL PLA BASED SYSTEMS a Dissertation Presented to the Graduate Faculty of the University of Akron in Partial Fulfillment

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FUNCTIONAL PLA BASED SYSTEMS

A Dissertation

Presented to

The Graduate Faculty of The University of Akron

In Partial Fulfillment

of the Requirements for the Degree

Doctor of Philosophy

Colin Wright

December, 2015

FUNCTIONAL PLA BASED SYSTEMS

Colin Wright

Dissertation

Approved: Accepted:

______Advisor Department Chair Dr. Coleen Pugh Dr. Coleen Pugh

______Committee Chair Dean of the College Dr. Robert Weiss Dr. Eric Amis

______Committee Member Dean of the Graduate School Dr. Mathew Becker Dr. Chand Midha

______Committee Member Date Dr. William Landis

______Committee Member Dr. Yang Yun ii

ABSTRACT

Poly(lactic acid) (PLA), is used in a wide variety of applications. It is a well studied polymer and offers many advantages, such as being derived from renewable resources, being biodegradable, FDA approved for biomedical applications, and commercially available. The main synthetic drawback is that the only sites for post-polymerization functionalization are at the two end groups. By incorporating 3-hydroxy-2- bromopropionic acid as a co-monomer with lactic acid, a site for post-polymerization functionalization can be added. Since the halogen is alpha to a carbonyl, it is activated toward nucleophlic substitution, radical formation, and enolate chemistry. The spacing on the backbone of our polymer allows for additional functionalization including rearrangement, electrophilic aromatic substitution, and cationic ring-opening polymerization.

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DEDICATION

I would like to dedicate this dissertation to my parents for encouraging me to attend

graduate school.

ACKNOWLEDGMENTS

I would like to thank my mother and father for their unfailing support of me during my time in academia. I would like to thank the entire Pugh research group for their help.

Specifically, William Storms-Miller and Abishek Banerjee for starting this project and for all their help and advice. I would like to thank Dr. Matthew Becker, Dr. Robert

Weiss, Dr. Yang Yun, and Dr. William Landis for sitting on my thesis committee. Thank you for reading this thesis, listening to my presentation and your thoughtful comments.

TABLE OF CONTENTS

Page

LIST OF FIGURES ...... xii

LIST OF SCHEMES ...... xvii

LIST OF TABLES ...... xix

LIST OF EQUATIONS ...... xx

CHAPTER

I. THE PROBLEM ...... 1

II. LITERATURE REVIEW ...... 4

2.1 Poly(caprolactone) ...... 4

2.2 Poly(glycolic acid) ...... 5

2.3 Poly(lactic acid) ...... 5

2.4 Step Growth Polymerization ...... 7

2.4.1 Acid-catalyzed Condensation of Lactic Acid ...... 7

2.4.2 Carbodiimide Coupling of Lactic Acid ...... 8

2.4.3 Limitations of PLA Made by Step-Growth Polymerizations ...... 9

2.4.4 Lactides ...... 10

2.4.5 Formation of Lactides ...... 10

2.4.6 Ring-Opening Polymerization of Lactides ...... 11

2.4.7 Limits of ROP ...... 12

2.5 Enzymatic Polymerization of Lactic Acid ...... 13 vi

2.6 Modification of PLA ...... 14

2.6.1 Blending ...... 15

2.6.2 Nanofillers ...... 15

2.6.3 Copolymers ...... 16

2.6.4 Chemical Reactions on PLA ...... 18

2.6.5 Chain End Modification ...... 19

2.7 Introduction of Functional Groups onto Polyesters ...... 20

2.7.1 Alkyl Groups ...... 20

2.7.2 Alkyne Group ...... 21

2.7.3 Azide Group ...... 21

2.7.4 Amino Group ...... 23

2.7.5 Carboxylic Acid ...... 24

2.7.6 Cyano Group ...... 24

2.7.7 Hydroxyl Group ...... 26

2.7.8 Vinyl Group ...... 27

2.7.9 Mercapto Group ...... 29

2.8 Radical Grafting ...... 29

2.9 Motivation and Scope ...... 31

2.10 Routes for Post-polymerization Functionalization ...... 33

2.10.1 Nucleophilic Substitution ...... 33

2.10.2 Radical Chemistry ...... 34

2.10.3 Enolate Chemistry ...... 36

2.10.4 Cationic ROP using an Oxocarbenium Ion ...... 37

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2.10.5 Electrophilic Aromatic Substitution of an Oxocarbenium Ion ...... 38

2.11 Goal of the Project ...... 39

2.12 Summary ...... 39

III. EXPERIMENTAL ...... 40

3.1 Materials...... 40

3.2 Techniques...... 41

3.3 Reactions ...... 42

3.3.1 Synthesis of Poly[(lactic acid)-co-(2-bromo-3-hydroxypropionic acid)]...... 42

3.3.2 Large Scale Synthesis (10 g) of Poly[(lactic acid)-co-(2-bromo-3- hydroxypropionic acid)]...... 43

3.3.3 Synthesis of Poly[(lactic acid)-co-(2-iodo-3-hydroxypropionic acid)]...... 44

3.3.4 Synthesis of Poly[(lactic acid)-co-(2-azido-3-hydroxypropionic acid)-co-(2-bromo-3-hydroxypropionic acid)] by Reaction of Poly [(lactic acid)-co-(2-bromo-3-hydroxypropionic acid)] with Sodium Azide. ..45

3.3.5 Attempted Copper-Catalyzed Huisgen Alkyne-Azide Cycloaddition of Poly[(lactic acid)-co-(2-azido-3-hydroxypropionic acid)-co-(2-bromo- 3-hydroxypropionic acid)] with Propargyl Alcohol...... 47

3.3.6 Huisgen Alkyne-Azide Cycloaddition of Poly[(lactic acid)-co-(2- azido-3-hydroxypropionic acid)-co-(2-bromo-3-hydroxypropionic acid)] with Dimethyl Acetylenedicarboxylate...... 48

3.3.7 Synthesis of Methyl 2-Bromo-3-hydroxypropionate...... 49

3.3.8 Synthesis of Methyl 3-Acetoxy-2-bromopropionate...... 50

3.3.9 Test of the Stability of Methyl 3-Acetoxy-2-bromopropionate in the Presence of PMDETA...... 50

3.3.10 Synthesis of Poly[(lactic acid)-co-(2-bromo-3-hydroxypropionic acid)]...... 51

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3.3.11 Synthesis of Poly[(lactic acid)-co-(2-iodo-3-hydroxypropionic acid)]...... 52

3.3.12 Synthesis of poly[(lactic acid)-co-(3-hydroxy-2- azidopropionic acid)] Starting from the Brominated Copolymer at Room Temperature...... 52

3.3.13 Synthesis of poly[(lactic acid)-co-(3-hydroxy-2-azidopropionic acid)] Starting from the Brominated Copolymer at 0 ˚C...... 53

3.3.14 Synthesis of poly[(lactic acid)-co-(3-hydroxy-2-azidopropionic acid)] Starting from the Iodated Copolymer at Room Temperature...... 53

3.3.15 Synthesis of poly[(lactic acid)-co-(3-hydroxy-2-azidopropionic acid)] Starting from the Iodated Copolymer at 0 ˚C...... 54

3.3.16 Synthesis of poly[(lactic acid)-co-(3-hydroxy-2-azidopropionic acid)] Starting from the Brominated Copolymer with Triflic acid...... 54

3.3.17 Synthesis of poly[(lactic acid)-co-(3-hydroxy-2-azidopropionic acid)] Starting from the Brominated Copolymer with Thionyl Chloride...... 55

3.3.18 Synthesis of methyl-3-acetoxy-2-azido propionate with Various Amounts of Sodium Azide...... 57

3.3.19 Synthesis of methyl-3-acetoxy-2-azido propionate Starting from (L) methyl-3-acetoxy-2-azidopropionate...... 57

3.3.20 Determining pKa of 2-bromo-3-hydroxy propionic acid: ...... 57

3.3.21 Determining pKa of Lactic Acid: ...... 58

3.3.22 Synthesis of 4,4'-Diheptyl-2,2′-dipyridyl...... 58

3.3.23 Synthesis of Poly[(lactic acid)-co-(2-bromo-3-hydroxypropionic acid)]...... 60

3.3.24 Kinetic Study of the Polymerization of MMA Initiated from Methyl 3-Acetoxy-2-bromopropionate using HB...... 61

3.3.25 Kinetic Study of the Polymerization of MMA Initiated from the Brominated Polyester using HB...... 62

3.3.26 Determination of Dn/Dc v Percent Conversion Graph...... 63

IV. SYNTHESIS OF FUNCTIONALIZED POLY(LACTIC ACID) USING 2- BROMO-3-HYDROXYPROPIONIC ACID ...... 65

4.1 Introduction ...... 65

4.2 Synthesis of Brominated Polyesters ...... 66

4.3 Comparison of GPCLS to GPCPSt ...... 67

4.4 Iodinated Polyester ...... 69

4.5 Substitution with Sodium Azide ...... 70

4.6 Attempted Azide-Alkyne Cycloaddition using Copper ...... 71

4.7 Small Molecule “Click” Reaction ...... 73

4.8 Microwave-assisted Azide-Alkyne Cycloaddition ...... 74

4.9 Conclusion ...... 75

V. AZIDE EXPLORATORY FUNCTIONALIZATION OF PLA BASED POLYESTERS ...... 76

5.1 Introduction: ...... 76

5.2 Results and Discussion: ...... 77

5.3 Determination of Mechanism ...... 78

5.3.1 E2 Elimination ...... 81

5.3.2 Brominated Polyester at Room Temperature ...... 82

5.3.3 Brominated Polyester at 0 ˚C ...... 83

5.3.4 Iodated Polyester at Room Temperature ...... 83

5.3.5 Iodated Polyester at 0 ˚C ...... 84

5.3.6 Reason for Elimination ...... 84

5.3.7 Determination of pKa ...... 86

5.3.8 Model Compound with Various Acids ...... 88

5.3.9 Chain End Suppression ...... 90

5.3.10 Eliminating the Chain Ends ...... 91

5.4 Conclusions: ...... 94

VI. RADICAL GRAFTING OF PLA BASED POLYESTERS ...... 95

6.1 Introduction ...... 95

6.2 Grafting using Cu(I)Cl and PMDETA ...... 95

6.3 Kinetic Study using HB and the Model Compound: ...... 100

6.4 Graft Copolymers using Heptyl Bipyridine and Cu(I)Cl ...... 102

6.4.1 Chain Transfer using GPCPSt ...... 104

6.4.2 Chain Transfer using GPCLS ...... 105

6.5 Graft Copolymers using Heptyl Bipyridine and Cu(I)Br ...... 106

6.6 Graft Copolymers using Bipyridine and Cu(I)Br ...... 107

6.7 Graft Copolymers using Bipyridine and Cu(I)Cl ...... 109

VII. CONCLUDING THOUGHTS ...... 111

7.1 Future Directions ...... 111

REFERENCES ...... 113

LIST OF FIGURES

Figure Page

1.1 Functionalized PLGA, poly (lactic acid-co-glycolic acid-co-2-bromo-3- hydroxypropionic acid)...... 1

1.2 Various routes used to functionalize our polyester...... 2

2.1 Two stereoisomers of lactic acid and stereoppure PLA...... 5

2.2 Acid-catalyzed condensation of LA ...... 8

2.3 Three configurations of lactides ...... 10

2.4 Structure of CAL-B (NOVOZYME 435) 42,43 ...... 14

2.5 Half-life of various ratios of LA and GA in PLGA.56 ...... 17

2.6 End-capping PLA with itaconic anhydride ...... 17

2.7 “Click” reactions of various functional alkyenes on azide functional polyesters 75 ...... 22

2.8 Introduction of the amine functional group. 76 ...... 23

2.9 Introduction of the cyano group directly from the polymer backbone.81 ...... 25

2.10 Cyano-bearing polyesters made by Weder's group. 82 ...... 26

2.11 Cyano-functional aromatic polyesters. 83 ...... 26

2.12 Introduction of hydroxyl group without protection chemistry. 85 ...... 27

2.13 Pendant-vinyl group from the polyester backbone. 89 ...... 28

2.14 Introduction of a free mecapto group via enzymatic polymerization. 90 ...... 29

2.15. The three general approaches to preparing graft copolymers: grafting-onto (black), grafting-through (blue), and grafting-from (green)...... 29

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2.16 Routes to functionalize the brominated polymer...... 33

2.17 Polymerization of THF using an oxonium iniatior. 120 ...... 38

3.1 1H NMR spectrum of Poly[(lactic acid)-co-(2-bromo-3-hydroxypropionic acid)]. ..44

3.2 13C NMR spectrum of Poly[(lactic acid)-co-(2-bromo-3-hydroxypropionic acid)]. .44

3.3 1H NMR spectrum of Poly[(lactic acid)-co-(2-azido-3-hydroxypropionic acid)-co- (2-bromo-3-hydroxypropionic acid)]...... 46

3.4 13C NMR spectrum of Poly[(lactic acid)-co-(2-azido-3-hydroxypropionic acid)-co-(2-bromo-3-hydroxypropionic acid)]...... 47

3.5 1H NMR spectum of the attempted "click" reaction using CuBr and PMDETA ...... 48

3.6 1H NMR spectrum of an aliquot taken from the CuBr/ PMDETA-catalyzed reaction of methyl 3-acetoxy-2-bromopropionate with propargyl alcohol...... 51

3.7 1H NMR spectrum of poly[(lactic acid)-co-(3-hydroxy-2-azidopropionic acid)] made with thionyl chloride...... 56

3.8 13C NMR spectrum of poly[(lactic acid)-co-(3-hydroxy-2-azidopropionic acid)] made with thionyl chloride...... 56

3.9 1H NMR spectrum of 4,4′-diheptyl-2,2′-dipyridyl...... 59

3.10 13C NMR spectrum of 4,4′-diheptyl-2,2′-dipyridyl...... 60

4.1 Plots of the refractive index increments (dn/dc) of poly[(lactic acid)-co-(2-bromo-3- hydroxypropionic acid)] (PLB) in THF (A), and the percent difference in the absolute molecular weights (measured by GPC using a light scattering detector in THF) vs. polystyrene-equivalent molecular weights of PLB (B), as a function of the molar composition of brominated repeat units...... 68

4.2 1H NMR spectra of poly[(lactic acid)-co-(2-bromo-3-hydroxypropionic acid)] (PLB; 3 MnPSt = 5.32 x 10 Da; Ð = 2.64) and functionalized derivatives: iodinated PLA 3 (PLI; MnPSt = 4.62 x 10 Da; Ð = 1.96), azide-functionalized PLA (PLBN3; MnPSt = 3 4.15 x 10 Da; Ð = 2.33), and the “clicked” product of PLBN3 with dimethyl 3 13 acetylenedicarboxylate (MnPSt = 5.05 x 10 Da; Ð = 2.51). Inset: C NMR spectra of PLI (red) and PLB (blue)...... 69

4.3 Comparison of the gel permeation chromatograms of poly[(lactic acid)-co-(2- 3 bromo-3-hydroxypropionic acid)] (PLB; MnPSt = 5.32 x 10 Da; Ð = 2.64) before (blue) and after (red) reaction with sodium iodide to produce iodinated PLA (PLI; 3 MnPSt = 4.62 x 10 Da; Ð = 1.96)...... 70 xiii

1 4.4 Comparison of the H NMR spectra of azide-functionalized PLA (PLBN3; MnPSt = 4.15 x 103 Da; Ð = 2.33) (green), the microwave-assisted “clicked” product of 3 PLBN3 with dimethylacetylenedicarboxylate (MnPSt = 5.05 x 10 Da; Ð = 2.51) (black), and the attemped CuBr/PMDETA-catalyzed “clicked” product of PLBN3 3 with propargyl alcohol (MnPSt = 4.45 x 10 Da, Ð = 2.58) (orange)...... 72

4.5 Comparison of the gel permeation chromatograms of azide-functionalized PLA 3 (PLBN3; MnPSt = 4.15 x 10 Da; Ð = 2.33) (green), the microwave-assisted “clicked” product of PLBN3 with dimethylacetylenedicarboxylate (MnPSt = 5.05 x 103 Da; Ð = 2.51) (black), and the attemped CuBr/PMDETA-catalyzed “clicked” 3 product of PLBN3 with propargyl alcohol (MnPSt = 4.45 x 10 Da, Ð = 2.58) 3 (orange), as well as the original PLB (MnPSt = 5.32 x 10 Da; Ð = 2.64) (blue)...... 73

5.1 General scheme of how a CSR works, with the dashed portions of the arch representing the different electronic environments of the CSR, showing the chiral proton (blue) in the two different electronic environments...... 77

5.2 Comparison of the gel permeation chromatograms of poly[(lactic acid)-co-(2- 3 bromo-3-hydroxypropionic acid)] (PLB; Mn, PSt = 6.00 x 10 DA) before (blue) and after (light blue) reaction with sodium azide to produce azide functionalized PLA 3 (PLN3; Mn, PSt = 1.20 x 10 DA)...... 78

5.3 Chiral shift reagent Europium tris[3-(heptafluoropropylhydroxymethylene)-(-)- camphorate ...... 79

5.4 Stacked proton NMR spectra of model compound with CSR added. Top (black): (R) model compound with 1.5 eq CSR. Bottom (blue): Azide substituted model compound with 1.5 eq CSR. Insert: Methyl ester resonance showing the splitting from different isomers...... 80

5.5 Crude aliquots of the model compound after reacting with various eq NaN3 0.5 (blue), 0.88 (red), 1.0 (green), and 1.6 (black)...... 81

5.6 Acid-base equilibrium...... 84

5.7 Titration of lactic acid with KOH as a control experiment ...... 86

5.8 Titration of 2-bromo-3-hydroxypropionic acid with NaOH done in triplicate. The black box highlights the buffer region and the red box is from the presumed reaction shown in red in which the second equivalence of hydroxide acts as a nucleophile to displace the bromine...... 87

5.9 Crude aliquots of the reaction of model compound with one equivalence of sodium azide. in the presence of various acids. Insert: Enhancement of the vinylic region showing the presence of elimination...... 88

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3 5.10 Stacked GPC traces of the starting brominated polyester (blue) (Mn, PSt = 9.7 x 10 2 DA) with no TFA (red) (Mn, PSt = 8.0 x 10 DA), 10% TFA (green) (Mn, PSt = 1.0 x 3 3 10 DA), 1% TFA (purple) (Mn, PSt = 1.8 x 10 DA), 0.1% TFA (light blue) (Mn, PSt = 1.8 x 103 DA)...... 90

5.11 gHSQC of poly[(lactic acid)-co-(3-hydroxy-2-azidopropionic acid)] (MnLS: 3.39 x 104, Đ =1.73) showing the overlap of the methyene and methylene of the azide repeat unit from 4.4-4.7 ppm...... 92

4 5.12 Stacked GPCLS traces of the starting brominated polyester (blue) (Mn,LS = 2.34 x 10 DA, Đ =1.97) (blue) and poly[(lactic acid)-co-(3-hydroxy-2-azidopropionic acid)] 4 (MnLS: 3.39 x 10 , Đ =1.73) (red)...... 93

6.1 Crude aliquot of the PLB-g-PMMA using PMDETA showing 20% conversion. ....96

6.2 1H spectrum of the precipitated graft copolymer showing the methyene of lactic acid at 5.16 ppm...... 97

6.3 Staked GPCPSt traces of the macroinitiator (blue) (Mn: 13,400 Da), and the attempted grafting using PMDETA (red) stopped at 20% conversion (Mn: 19,200 Da)...... 98

6.4 Staked GPCLS traces of the macroinitiator (red) (Mn: 35,800 Da, Mp: 47,000 DA), and the attempted grafting using PMDETA (blue) stopped at 20% conversion (Mn: 36,000 Da, Mp: 40,900 DA)...... 99

6.5 Conversion and first order monomer conversion in the atom transfer radical polymerizations of methyl methacrylate in toluene at 90 °C using CuCl, as the catalyst and 2,2'-diheptyl-4,4'-bypyridine as the ligand; [MMA]:[I]:[CuCl]:[Heptyl Bipy] = 200:1:1:1...... 100

6.6 Gel permeation chromatograms and the corresponding monomer conversions of aliquots taken from the atom transfer radical polymerizations of methyl methacrylate presented in Figure 6.5 using CuCl, as the catalyst and 2,2'-diheptyl-4,4'-bypyridine as the ligand in toluene at 90 °C; [MMA]:[I]:[CuX]:[Heptyl bipy] = 200:1:1:1. ...101

6.7 Number-average molecular weights (Mn) and polydispersity (Đ) corresponding to the atom transfer radical polymerizations of methyl methacrylate presented in Figure B using CuCl, as the catalyst and 2,2'-diheptyl-4,4'-bypyridine as the ligand in toluene at 90; [MMA]:[I]:[CuX]:[PMDETA] = 200:1:1:1...... 102

6.8 Conversion and first order monomer conversion in the atom transfer radical grafting of methyl methacrylate in toluene at 90 °C using CuCl, as the catalyst and 2,2'- diheptyl-4,4'-bypyridine as the ligand; from the brominated copolymer [MMA]:[I]:[CuCl]:[Heptyl bipy] = 200:1:2:2...... 103

6.9 Number-average molecular weights (Mn) and polydispersity (Đ) determined by GPCPSt corresponding to the atom transfer radical polymerizations of methyl methacrylate presented in Figure F using CuCl, as the catalyst and 2,2'-diheptyl- 4,4'-bypyridine as the ligand in toluene at 90; [MMA]:[I]:[CuX]:[PMDETA] = 200:1:1:1...... 104

6.10 Number-average molecular weights (Mn) and polydispersity (Đ) determined by GPCLS corresponding to the atom transfer radical polymerizations of methyl methacrylate presented in Figure E using CuCl, as the catalyst and 2,2'-diheptyl-4,4'- bypyridine as the ligand in toluene at 90; [MMA]:[I]:[CuX]:[PMDETA] = 200:1:1:1...... 105

4 6.11 GPCLS of the graft copolymer (Mn 4.30 x 10 ) stopped at 48 h (61% conversion) and precipitated 3x [MMA]:[I]:[CuBr]:[HB] = 200:1:2:2...... 106

6.12 1H NMR spectrum of MMA grafted from the brominated polyester stopped after 48 h, and precipitated 3x [MMA]:[I]:[CuBr]:[Bipy] = 200:1:1:1...... 108

4 6.13 Stacked GPCLS of the starting brominated repeat unit (red) (Mn 2.34 x 10 ), and the 4 graft-copolymer (blue) (Mn 8.46 x 10 ) stopped at 40% conversion and precipitated 3x [MMA]:[I]:[CuBr]:[Bipy] = 200:1:1:1...... 109

6.14 Crude 1H NMR spectrum of 1H NMR spectrum of MMA grafted from the brominated polyester stopped after 48 hours [MMA]:[I]:[CuCl]:[Bipy] = 200:1:1:1...... 110

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LIST OF SCHEMES

Scheme Page

1.1 Mechanism for rearrangement...... 3

2.1 Structure of PCL and monomers that it is derived from.13: ...... 5

2.2 Coupling of LA via Diisopropylcarbodiimide (DiPC) ...... 9

2.3 Formation of lactide rings ...... 11

2.4 General mechanism of transition metal catalyzed ROP, with the stereogenic carbon of each LA unit shown in parentheses ...... 12

2.5 Introduction of a methyl group onto PCL.68 ...... 21

2.6 Synthesis of alkyne functional lactone. 69 ...... 21

2.7 Synthesis of azide functional PCL. 74 ...... 22

2.8 Introduction of carboxylic acid group onto PLA. 57 ...... 23

2.9 Introduction of carboxylic acid directly from the polyester backbone.78 ...... 24

2.10 Introduction of the cyano group. 79 ...... 24

2.11 Introduction of the hydroxy group to a PCL-based copolymer. 86 ...... 27

2.12 Introduction of the vinyl group and subsequent reaction with a thiol. 88 ...... 28

2.13 General scheme showing radical grafting (ATRP) of methyl methacrylate from our brominated polyester...... 31

2.14 Example of nucleophlic substitution onto a polyester, where DSDOP is 2,2- dibutyl-2-stanna-1-dioxepane. 112 ...... 34

2.15 Synthesis of mPEG-TEMPO.116 ...... 36

2.16 Synthesis of four-armed star PCL.118 ...... 37

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2.17 Polymerization of δ-valerolactone starting from γ-burtyrolactone121 ...... 37

2.18 Mechanism of electrophilic aromatic substitution. Insert: Formation of a strong electrophile...... 39

4.1 Synthesis and functionalization of the statistical copolymer poly[(lactic acid)-co-(2- bromo-3-hydroxypropionic acid)] ...... 66

5.1 Formation of the thionyl chloride/DMF adduct and reaction with a carboxcylic acid ...... 91

6.1 Attempted grafting using the brominated copolymer with PMDETA as the ligand ...... 95

6.2 Establishing grafting kinetics using the model compound as an initiator with MMA ...... 100

6.3 Establishing grafting kinetics using the model compound as an initiator with MMA ...... 102

6.4 Grafting of MMA using the brominated copolymer with two equivalents of Cu(I)Br and HB...... 106

6.5 Grafting of MMA using the brominated copolymer with one equivalent of Cu(I)Br and Bipy...... 107

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LIST OF TABLES

Table Page

2.1 Physical Properties of Pure PLLA.23 ...... 6

5.1 Various halogenated polyesters reacting with sodium azide. - means that the integration was not resolved enough to determine the degree of substitution...... 82

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LIST OF EQUATIONS

Equation Page

1 Carothers Relationship, where Xn is the degree of polymerization, and p is the extent of reaction ...... 7

CHAPTER I

INTRODUCTION

The present work describes the functionalization of biodegradable polymers similar to poly (lactic acid) (PLA) or poly (Lactic acid-co-glycolic acid) (PLGA) (Figure

1.1). Our group has worked on hyperbranched polymers1,2 that were derived from 2- bromo-3-hydroxypropionic acid (BrH). A previous student from our group, Dr.

Abhishek Banerjee, has done extensive work with making high molecular weight polymers with structures similar to that of PLA and PLGA that incorporate BrH into the polymer (Figure 1.1). By having the halogen attached to the polymer backbone we are afforded a site for post polymerization functionalization.

O O O O H HO O O n m l Br

Figure 1.1. Functionalized PLGA, poly (lactic acid-co-glycolic acid-co-2-bromo-3- hydroxypropionic acid).

There are various routes to functionalize our polymer (Figure 1.2). Since the halogen is a good leaving group we can use various nucleophiles to introduce various new functionalities (Chapter III, IV). The halogen is located α to a carbonyl in the

presence of copper(I) species we can generate a radical that can be used to make graft copolymers (Chapter V).

O H HO O n X

Nuc:- 1)Silver Salt Radical 2) THF

O O O H H HO O HO O n O n HO H Nuc Radical n O

Figure 1.2. Various routes used to functionalize our polyester.

Our polymer can also be rearranged with heating to give a primary halogen

(Scheme 1.1). The addition of a silver salt traps the oxocarbenium intermediate and can be used for electrophilic substitution and cationic polymerizations.

O O

O O Br

O O Br O O

O O O O O O O Br O Br

Scheme 1.1. Mechanism for rearrangement.

CHAPTER II

LITERATURE REVIEW

Bio-based polyesters are polyesters made from renewable resources such as corn and other plants.3-5 They are useful for a wide range of applications such as drug delivery,6 tissue engineering,7,8 and are working to replace commodity plastics. The past few decades have brought about a large increase in research and use of bio-based polyesters. The main reason for their increased popularity is that they are made from renewable resources and degrade in far less time than traditional petroleum-based polymers. There are many types of polymers such as glycopolymers, sugars and DNA.

The three most popular polyesters are poly(caprolactone) (PLC), poly(glycolic acid)

(PGA), and poly(lactic acid) (PLA).

2.1 Poly(caprolactone)

Carothers discovered PCL in the 1930’s while working at DuPont.9,10 PCL can be made either by condensation polymerization of 6-hydroxyheptaonic acid or by the ring- opening polymerization (ROP) of ε-caprolactone (see scheme 2.1). PCL gained a lot of attention during the 1970’s and 1980’s fell out of favor for PLA and PGA, since the

1990’s has seen a rise in popularity.11 Of the three main polyesters, PCL has the longest degradation time, because of greater hydrophobicity from the long alkyl spacer between the hydroxyl and carboxylic acid groups. PCL is the ideal polyester for long term

applications, such as tissue engineering, but it does not have sufficient mechanical properties to be applied in load-bearing applications. PCL can be used for drug delivery, but because of its long degradation time, PGA and PLA are more commonly used.

Another issue with PCL is that it is achiral; therefore physical properties cannot be changed by altering the configuration of chiral centers within the polymer chains.12

O O + O ROP O n HO H3O OH HO H n O n Scheme 2.1. Structure of PCL and monomers that it is derived from.13:

2.2 Poly(glycolic acid)

PGA is structurally very similar to PLA, with a hydrogen atom in place of its methyl group. PGA does not have a sterocenter and is one of the simplest biological molecules. PGA is made either from the condensation polymerization of glycolic acid or by ROP of the glycolide. Like PCL and PLA, PGA is a bulk-eroding material, which means that the rate of degradation is related to the rate of water intake. PGA degrades in as little as 6 months, which is much quicker than either PCL or PLA.14 As a consequence of the decreased number of methylene units in PGA it is significantly more hydrophilic than the other hydroxy acids.

2.3 Poly(lactic acid)

The most commonly used bio-based polymer is PLA which was discovered along with PCL in 1932 by Carothers while working at DuPont.15 Lactic acid has two isomers,

L-lactic acid and D-lactic acid, and can be polymerized to give steropure polymers:

5 poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) (Figure 2.1). PLA (a mixture of both isomers) has found a wide variety of uses ranging from biomedical applications

(as scaffolds,14 sutures,16 bone screws,17,18 and wound dressings18,19), to more environmentally friendly products20 such as cups, apparel, containers, durable goods, and even coffins.21

O O O O n HO HO OH HO H n HO H O n OH O n

L-Lactic Acid PLLA D-Lactic Acid PDLA

Figure 2.1. Two stereoisomers of lactic acid and stereoppure PLA.

There are three different routes that can be used to polymerize lactic acid (LA): step growth, ring-opening, and enzymatic polymerizations. Each of these methods has advantages and disadvantages; therefore the method chosen to make PLA is application- dependent.22 Selected physical properties of pure PLLA are shown in table 2.1. These properties can be modified by a variety of methods that are discussed in later sections of this chapter.

Table 2.1 Physical Properties of Pure PLLA23

Physical Properties of pure PLLA Typical Value Density 1.25-1.30 g/cm3 Melting Temperature 170-190 °C Glass Transition Temperature 50-65 °C Heat of Fusion 93-203 J/g Tensile Modulus 6.9-9.8 GPa Tensile Strength 0.12-2.26 GPa Elongation at Break 12-26% 6 2.4 Step Growth Polymerization

Hydroxy-acids can be polymerized by condensation polymerization in bulk, catalyzed by strong protonic acids, such as p-toluenesulfonic acid (pTSA), or by using carbodiimide coupling (DCC). In order to drive the reaction to high conversion, the byproduct that is formed must be removed from the system following Le Chatlier's principle. The reason that high conversion is a necessity for condensation polymerization can be shown using Carothers relationship24 (eq. 2.1). From this equation at 70% conversion the number average degree of polymerization (Xn) is equal to 4, and even increasing the conversion to 95% only gives a polymer a Xn equal to 20. To achieve a polymer of Xn = 200, the extent of reaction must be = 99.5%.

Equation 1. Carothers Relationship, where Xn is the degree of polymerization, and p is the extent of reaction.

Despite the challenges of polycondensation, many groups have been able to make high molecular weight PLA using various methods.

2.4.1 Acid-catalyzed Condensation of Lactic Acid

Acid-catalyzed condensation polymerization of hydroxy acids is an equilibrium reaction, with water formed as a byproduct (Figure 2.2). Since LA is a hydroxy acid containing both a hydroxyl group and a carboxylic acid group on the same molecule, LA can undergo self-condensation to form a polymer. In order to drive the reaction to high

conversion, the water that is formed must be removed. There are multiple ways to remove the water, the most common being a combination of high heat and vacuum. Ikada et al. also synthesized PLLA (Mw= 1.6 x 104 Da) in three hours at 200 °C at 5 mm Hg.25 From our group Abhishek Banerjee was able to make PLA (Mn = 3.16 x 104 Da) using pTSA

95 ˚C, and a minimum amount of diphenyl ether as the high boiling point plastizser.

O -(n-1) H2O O n HO HO H OH 95 oC, pTSA, O n DPE, ~1-2 mmHg

Figure 2.2. Acid-catalyzed condensation of LA.

2.4.2 Carbodiimide Coupling of Lactic Acid

As shown in Scheme 2.2, a carbodiimide can react with a carboxylic acid and an alcohol to form an ester, with a urea formed as a side product. Since the urea that is formed is insoluble in common organic solvents, it precipitates as soon as formed, which driving the reaction to high conversion. Sunahara et al. azeotropically distilled water from the LA monomer and then reacted the dry monomer with dicyclocarbodiimide (DCC) and dimethylamino pyridine (DMAP). They were able to achieve Mn of up to 1.58 x 104

Da.26 DCC coupling can also be used for chain extension. Abhishek Banerjee was able to couple the chain ends of two PLA-based polyesters increasing the molecular weight from

Mn= 8.81x 103 to 1.53 x 104.27

Scheme 2.2. Coupling of LA via Diisopropylcarbodiimide (DiPC).

2.4.3 Limitations of PLA Made by Step-Growth Polymerizations

The main drawback of step-growth polymerizations is that they tend to give low molecular weight polymers (less than 1.6 x 104 Da).25 As a consequence, the extent of reaction must be high. However, high conversion leads to high viscosity, which makes

water removal very difficult. Polyesters made via condensation have polydispersity index (Đ) of ~2. For applications in which high molecular weight and narrow PDI are necessary, ROP is used.

2.4.4 Lactides

ROP of lactides is the most common industrial way to make PLA. The advantage of ROP over step-growth polymerization is that ROP is a chain process that can be living, thereby producing polymers with lower Đ and higher molecular weight. These features make it easier to control the properties of polymers from batch to batch. Lactide is a six- membered ring made up of two units of lactic acid. There are three stereo combinations:

LLA, DLA, and meso-LA (Figure 2.3). The first step of ROP is to make the lactide.

O O O

O O O O O O

O O O

rac-Lactide L-Lactide D-Lactide

Figure 2.3. Three configurations of lactides.

2.4.5 Formation of Lactides

Lactides are formed by depolymerization of PLA oligomers. These oligomers are formed by traditional condensation polymerization but with short reaction times to limit the molecular weight. These pre-polymers are then heated above 180 °C at 2 mm Hg in the presence of an inorganic salt (Sb2O3) and the lactide that is formed is distilled. The lactides can then be recrystallized from ethyl acetate.25

O H+/-OH O O R HO H O n HO O n/2 OH -(n-1) H O O 2 R n= 20-30 R R O R= Me, H

Scheme 2.3. Formation of lactide rings.

2.4.6 Ring-Opening Polymerization of Lactides

The ROP of lactides can be performed by a variety of polymerization mechanisms including, cationic, anionic, or coordination-insertion methods.6 The polymerization can also be performed in bulk or in solution. A popular initiator is stannous octoate (SnOct2)

(A tin(II) complexed with two 2-ethylhexanoate ligands), because it gives high molecular weight polymers under relatively mild conditions.5,6,25 As shown in Scheme 2.4, an alcohol coordinates to and displaces one of the ligands on the metal. The metal then coordinates with the carbonyl of the lactide ring before it opens and propagates. Through

4 25 this method, polymers with an Mv of 2.5 -3.0 x 10 Da can be obtained. Nanavati’s group28 uses ROP to make PLLA polymers that are drawn into fibers and heated in molten PLLA to further polymerize via condensation polymerization. This process

4 5 increases the molecular weight nine-fold from Mn 1.8 x 10 Da to 1.53 x 10 Da.

M RO O O

(R) O M OR (R) O O (S) O (S) O O

(R) RO O M O O O O H O O (R) HO O R O M O (S) O O (S) O

Scheme 2.4. General mechanism of transition metal catalyzed ROP, with the stereogenic carbon of each LA unit shown in parentheses.

2.4.7 Limits of ROP

There are a few drawbacks to ROP, most notably that the lactide must first be synthesized. This extra step adds time and cost to the synthesis of PLA. Another drawback of using transition metal (Mt) catalyzed ROP is the stereo purity.29 Through extensive testing of catalytic systems, Okuda’s group30 was able to make polymers with stereocontrol of up to 96 percent using scandium complexes. A second problem with using transition metal-based catalysts is that they are toxic and must be removed after polymerization. Recently, different groups have begun to explore the use of organic catalysts such as N-heterocyclic carbenes,31 amino acids,32 phosphazene bases,33 and the use of non toxic metals like calcium,34 magnesium,35 and iron.36

2.5 Enzymatic Polymerization of Lactic Acid

The third method for making PLA involves using enzymes to form the polymer.

Considered the “greenest” route, with a focus on getting rid of toxic chemicals and harsh conditions. Martinelle’s group37 modified lipase B to polymerize D,L-lactide and were able to increase both the percent conversion from 11% to 89% and the molecular weight from Mn= 220 to 740, but still could not make high molecular weight polymers.

Rikukawa’s group38 used the enzyme CALB shown in Figure 2.4 in various ionic liquids

3 to make L-lactide. They were able to make polymers with Mn ranging from 3.6 x 10 to

5.03 x 104 Da with yields around 35%. Bárzana’s group39 was able to polymerize L- lactide, in supercritical 1,1,1,2-tetrafluroethane using the enzyme Burkholderia cepacia.

The molecular weights ranged from 1.0 x 103 to 1.4 x 103 Da with yields slightly above

50%. They were also able to make PLA (Mn = 3.73 x104 Da) using lipase B in 1-hexyl-3- methylimidazolium hexafluorophosphate.40 Sobczak41 studied a number of enzymes to polymerize L-lactide from a block of poly(ethylene glycol) (PEG). Using PEG400, was

3 3 able to produce block polymers with a Mn between 1.9 x 10 and 4.2 x 10 Da (PLA Mn between 1.5 x 103 and 3.8 x 103 Da) in yields above 50%.

Figure 2.4. Structure of CAL-B (NOVOZYME 435). 42,43

2.6 Modification of PLA

While PLA has advantages such as being environmentally friendly, biocompatible, easier to process than other biopolymers, and is cheaper to process than petroleum-based polymers,44 it also has some disadvantages. PLA is very brittle and breaks before 10% elongation.45,46 Other potential weaknesses are the slow degradation rate and high hydrophobicity. Since PLA is a bulk-eroding material, the rate of degradation is related to the rate of water intake. The high hydrophobicity also results in low cell affinity47,48 and upon degradation, the resulting lactic acid and carboxylic acid end groups can cause inflammation in vivo. The main drawback is that PLA is chemically inert with no reactive side groups, which makes functionalization very challenging.44

2.6.1 Blending

One way to overcome the limitations of PLA is by blending it with other

46 polymers. Hirt’s group took PLA (Mn =110 kDa) and grafted poly(acrylic acid) off of

3 the polymer backbone. This mixture was then blended with PEG (Mn = 1.5 x10 Da) to increase the toughness of the polymer from less by as much as 43%. Hirt’s group49 also grafted poly([(3-hydroxy-butyrate)-co-(3-hydroxyhexanoate)] to PLA and found a large increase in toughness that decreased dramatically as the polymer aged . They attributed the loss of toughness to both physical aging and UV-assisted solvent-induced crystallization that occurs when they grafted from PLA.

Törmälä’s group45 blended PLLA with various ratios of PCL to study the mechanical and hydrolytic behavior of the blends and to try to limit the brittleness of

PLLA. The results showed that the initial properties of PLLA depend on how the polymer is processed with the tensile modulus of the non-extruded PLLA being 500 MPa higher than the same PLLA after extrusion. The addition on 20 wt.-% of PCL improved the strain-at-break from1.6% to over 100%.

2.6.2 Nanofillers

Nanofillers are fillers with at least one dimension less that 100 nm. They are sorted into four classes based on how many dimensions are less than 100 mn, with each class having different properties.50 Fillers can be blended into PLA by a variety of methods to give bionanocomposites that can be used as green plastics or scaffolds in the biomedical field. Okamoto’s group51 melt-extruded PLA with montmorillonite and found that it increased the overall rate of crystallization of the samples. Ueda’s group23,52-54 15

blended modified montmorillonite with PLA and found that the rate of biodegradation increased dramatically. They also found that other properties could be tuned depending on the choice of nanofiller used.

Ko's group blended PLA (ρ = 1.25 g/cm3), with polystryene (PS) (ρ = 1.05 g/cm3) in a twin-screw extruder and pressed into molds for tensile testing. Their results suggest that PLA is relatively compatible with PS based on the stress transfer calculations.

2.6.3 Copolymers

The most common example of a copolymer containing PLA is poly(Lactic Acid)- co-(Glycolic Acid) (PLGA). LA and GA are copolymerized to tune the degradation time of the polymers. Reed’s group55 found that by changing the molar ratio of LA and GA in the polymer, the crystallinity and rate of water intake can be tuned to effectively control the rate of degradation. Cutright et al. 56 found that the half-life of PLGA (T1/2) was 5 months, 2 weeks, and 7 months for pure PGA, a 50:50 blend of PLGA, and pure PLA

(respectively)( Figure 2.5). LA can also be copolymerized with other monomers to impart new functional groups along the polymer backbone.57

PLGA

6.5 5.5 4.5 3.5

months 2.5 1/2

T 1.5 0.5

-0.5 0 0.2 0.4 0.6 0.8 1 Ratio of LA in PLGA

Figure 2.5. Half-life of various ratios of LA and GA in PLGA.56

Figure 2.6. End-capping PLA with itaconic anhydride.

Weiss’ group took commercial PLA and modified it through a chemical recycling process followed by end-capping with itaconic anhydride to introduce carboxylic acid

58,59 groups. They found an increase in Tg when carboxylic acid groups are added, and that the Tg increased as the ion-pair became stronger. Another effect of having the ionic groups is that the crystallization of PLA was suppressed.

2.6.4 Chemical Reactions on PLA

Because of the lack of functionality along the polymer backbone, it remains a challenge to modify PLA. Chen's group60 coated the surface of PLA with apatite/collagen composite to increase the cell affinity for PLA. Yang's group61 hydrolyzed the surface of

PLA with a dilute alkali solution (0.25 M NaOH) to increase cell affinity. Care must be taken with this approach to limit degradation to just the surface and not the bulk of the polymer.

Photo grafting is attractive since it offers a low cost modification method using mild reaction conditions.44 There are three approaches to grafting: grafting-to, grafting- from, and grafting-through. Grafting-through involves the polymerization of a monomer, and a macromolecule with a radically polyermizable chain end. Since PLA is not polymerized radically, this method will not be discussed. As an example of grafting- from, Gaona-Lozano's group62 grafted N-vinylpyrrolidone from PLA and were able to decrease the water contact angle from 74˚ to 38˚ with a grafting efficacy of almost 80%.62

Fan's group63 used triallyl isocyanitrate to crosslink PLLA. They found that these crosslinks were bio-inert and the Young's Modulus increased from 2.10 to 33.93 MPa as the radiation dose was increased. The samples while becoming stronger also became more brittle as the amount of crosslinking was increased.

The proton alpha to the carbonyl on PLA is acidic and can be removed with a strong base like lithium N,N-disporpylamide (LDA), and the resulting enolate anion can then couple with electrophiles to give functionality along the polymer backbone. As mentioned before, low concentrations of base must be used (less than 0.25 equivalents

per monomeric unit).57 This same concept can be applied to PCL, with the rate of degradation decreasing dramatically.64

2.6.5 Chain End Modification

In order to preserve the “green” aspect of PLA, “click” chemistry is desired to modify PLA. As defined by Sharpless et al., click chemistry is defined as a modular reaction, quantitative, and is either solventless or uses benign solvents. 65 As such, “click” chemistry falls under the umbrella of green or environmentally friendly chemistry. One reaction that has seen a recent surge in the literature since its rediscovery is the copper catalyzed azide-alkyne cycloaddition. Other reactions that are so-called “click” reactions include the Diels-Alder reaction, reactions on strained epoxide rings, thiol-ene and thiol- yne additions, and amine-isocyante reactions.

Important features of PLA are the two chain ends, typically an alcohol and a carboxylic acid. Different polymerization methods can be used to modify the chain ends.

If PLA is made by condensation, then the chain ends will be an alcohol and a carboxylic acid. If ROP is used to make PLA, then the chain ends can be tailored based on the initiator that is chosen, as well as the terminating agent. These chain ends can be either small molecules or polymers and are used to help tailor the properties of PLA. As an example, PLA can be used to make micelles when encapped with hydrophilic polymers such as poly(ethylene glycol) (PEG). Long's group made 4-arm star PLA with alcohol chain ends that were further reacted via a Michael-like addition to add hydrogen bonding

DNA base pairs (adenine-thymine) to create PLA supramolecular structures.66 Similarly,

Biela's group attached the ends of PLA with hydrogen-bonding groups to form reversible

supramolecular structures, that also took advantage of the coordination of PDLA and

PLLA with each other to provide a secondary driving force for self-assembly.67

While PLA is an important polyester, it is difficult to work with because of its higher susceptibility to degradation compared to more hydrophobic polyesters like poly(caprolactone) (PCL). 68,69 This increase in hydrophobicity leads to a longer degradation time and a greater tolerance to chemical modifications involving harsh conditions. Much of the same chemistry that applies to PLA also applies to PCL.

2.7 Introduction of Functional Groups onto Polyesters

The main goal of this project is to add new functional groups to LA-based polyesters. The following are examples of methods that have been used to add new functional groups to polyesters.

2.7.1 Alkyl Groups

Alkyl groups can be added to increase the hydrophobicity of polyesters and to

70 4 decrease the rate of degradation. Vert's group stirred PCL (Mn: 5.0 x 10 Da; Đ: 1.7) with lithium diisopropylamide (LDA) and then added methyl iodide to the reaction

(Scheme 2.5). They found that they only achieved 10% methylation and a large drop

4 (50%) in molecular weight (Mn: 2.8 x 10 Da; Đ: 1.6). They also tried to graft monomethyl PEG (mPEG), where one of the chain ends is a methyl ether and the other end is a free alcohol, from the polymer backbone but also found a large drop (52%) in

4 molecular weight (Mn: 2.6 x 10 Da; Đ: 1.6).

Scheme 2.5. Introduction of a methyl group onto PCL.70

2.7.2 Alkyne Group

The alkyne group is a very important functional group since it is part of the Cu(I)- catalyzed Huisgen 1,3-dipolar cycloaddition of azides and alkynes reaction. This is a

“green” reaction, meaning that mild conditions are used and high yields are obtained.

Emrick's group71 modified caprolactone by stirring with LDA and then adding propargyl bromide (Scheme 2.6). They were able to synthesize the lactone in 75% yield and polymerized it by ROP using a tin catalyst to obtain the homopolymer (Mn: 6.0 x 103 Da;

Đ: 1.11). They also blended their modified lactone with lactide and found that they could control the final composition of the polymer.

Scheme 2.6. Synthesis of alkyne functional lactone. 71

2.7.3 Azide Group

The azide group is the second part of the Huisgen 1,3-dipolar cycloaddition that has been the focus of a great deal of research lately72-75. Zi-Chen Li's group76 mixed 2- bromo-cyclohexanone and with meta-bromoperoxybenzoic acid to produce 2-bromo- ε- caprolactone, which was then polymerized by tin-catalyzed ROP to produce a functional

polymer (Mn 6.8 x 103 Da; Đ: 1.26) (Scheme 2.7). They then stirred the polymer with

3 76 sodium azide to generate poly(2-azido-e-caprolactone) (Mn 6.9 x 10 Da; Đ: 1.24).

Scheme 2.7. Synthesis of azide functional PCL. 76

Jérôme's group77 also made azide functional caprolactones and reacted them with various alkynes (see Figure 2.7) to demonstrate the wide range of reaction onto polyesters. They found no loss in Mn (1.0 x 104 Da) after “clicking” but with the PDMS they reported a low extent or reaction (~5%) because the high molecular weight PDMS was not able to diffuse as rapidly as smaller molecules.

Figure 2.7. “Click” reactions of various functional alkyenes on azide functional polyesters. 77

2.7.4 Amino Group

Watterson’s group78 enzymatically copolymerized PEG with an aromatic diacid containing a free amine in the para position (Figure 2.8). They also varied the group in the para position of the aromatic ring to form a large library of functional polyesters.

This work is important because they did not see any crosslinking occurring if the amine was reacting instead of the hydroxyl groups.

Figure 2.8. Introduction of the amine functional group. 78

2.7.5 Carboxylic Acid

The carboxylic acid group can be used to couple alcohols or amines using DCC.

Vert’s group57 successfully reduced a benzyl ester side chain from the polyester backbone to give a free carboxylic acid. They stirred PLA with LDA to make the carbanion and then reacted that with benzyl bromoacetate to produce a protected polyester (See Scheme

2.8). They saw a large drop in molecular weight when they used LDA in solution, but by working on the surface of the PLA they were able to minimize the loss.

Scheme 2.8. Introduction of carboxylic acid group onto PLA. 57 23

79 Vert’s group also stirred PCL with LDA and then added CO2 to add the carboxylic acid group directly to the polymer backbone (see Scheme 2.9). As with lactic acid when 1 equivalent of LDA is added, the Mn decreases from 5.4 x 104 to 1.8 x 104,

4 but, when 0.5 equivalents of LDA are added, there is a slight increase in Mn to 2.0 x 10 , with 11% and 7% of the monomeric units being converted to carboxylic acid groups, respectively.

O O O LDA CO2 O O O n n n O OH

Scheme 2.9. Introduction of carboxylic acid directly from the polyester backbone.80

2.7.6 Cyano Group

Mikroyannidis and coworkers81 prepared polyesters with bulky cyano side groups

(see Scheme 2.10) to improve their solubility in organic solvents. They also saw an increase in thermal stability in both N2 and air compared to a non-cyano substituted analog. Based on their low inherent viscosities (ηinh 0.21-0.25 dl/g), these polyester were not high molecular weight.

NC CN O O O O n Cl Cl H O O Cl n NC CN n HO OH NC CN O n O C N R N C O H H O O C N R N C O n

R = , CH2 CH3

Scheme 2.10. Introduction of the cyano group. 81 24

Mikroyannidis’s group82 also prepared unsaturated cyano-substituted polyesters and cross-linked polymers (see Figure 2.9). These polyesters showed increased hydrophobicity and solubility in organic solvents compared to a non-cyano substituted reference (poly(4-hydroxy benzoic acid)). The polyesters were thermally crosslinked through both the olefinic and the cyano groups to produce thermally stable polyesters with initial decomposition temperatures ranging from 294 ˚C to 347 °C, as the cyano content was increased.

Figure 2.9. Introduction of the cyano group directly from the polymer backbone.83

Weder’s group84 added cyano pendant groups from the polyester backbone (see Figure

2.10). When deformed, the polyester changed color from orange to yellow, because of the dispersion of dye aggregates. Upon annealing under vacuum at 100 °C for two days, the polyester returned to its original orange color. The molecular weights ranged from Mn

3 3 5.5 x 10 to 7.8 x 10 Da, with Đ from 2.17 to 2.49. The maximum absorbance (PLλmax ) of that the polymer ranged from 540-595 nm.

Figure 2.10. Cyano-bearing polyesters made by Weder's group. 84

Wang’s group85 made cyano-functionalized aromatic polyesters by reacting a cyano diacid chloride with various aromatic diols (see Figure 2.11). They found that the addition of the cyano group made the polymers more soluble in organic solvents and increased the polymers thermal-stability (5% weight loss occurring between 391-406 °C).

These polymers had inherent viscosities (ηinh) varied between 0.63-0.68 dl/g.

CN CN O O DCE O O n Cl O O Cl n HO Ar OH Cl O O O Ar O H Pyridine n

Ar = CH2

C O O

Figure 2.11. Cyano-functional aromatic polyesters. 85

2.7.7 Hydroxyl Group

The hydroxyl group is an important chemical handle for further transformations.

When preforming condensation reactions, the pendant hydroxyl group can also participate in the reaction, resulting in a cross-linked network. The hydroxyl group can therefore be protected with either a silyl or an acetal group. Others have taken advantage of the reduced nucleophilicty of 2° hydroxyl groups and used lower temperatures and rare 26

earth metals as catalysts to prevent the 2° hydroxyl group from reacting.86 Gross’s group87 used Novazyeme-435 to polymerize only primary hydroxyl groups of a diol containing secondary hydroxyl group (see Figure 2.12).

OH OH O OH OH O O O n HO n Novazyeme-435 bulk OH HO n HO OH O OH o O O 6 OH OH 6 90 C, 42 h, vacuum n O OH OH

Figure 2.12. Introduction of hydroxyl group without protection chemistry. 87

Jérôme’s group88 polymerized 1,4,8-hioxaspiro-[4.6]-9- undecanone (TOSUO) with PCL by ring-opening polymerization using aluminum isopropyl oxide as the catalyst(see

Scheme 2.11). They then deacylated the copolymers to prepare a ketone, which was then reduced using sodium borohydride. Each of the reactions was quantitative and the copolymer experienced no loss in molecular weight.

Scheme 2.11. Introduction of the hydroxy group to a PCL-based copolymer. 88

2.7.8 Vinyl Group

Pendant vinyl groups can be used to make graft polymers and for a site for thiol- ene click reactions.89 Klok’s group90 has taken a different approach to condensation polymerization by using the Baylis-Hillman reaction (see Scheme 2.12). Unlike a typical

condensation between an alcohol and a carboxylic acid group, the Baylis-Hillman reaction is a base-catalyzed condensation between an aldehyde and an acrylate. This reaction allows the introduction of orthogonal vinyl and alcohol groups. Klock’s group was able to achieve molecular weight ranging from Mn 1.9 x 103 to 3.9 x 103 Da and Đ from 1.6-2.1. They then reacted the vinyl groups with methyl 3-mercaptopropionaote in

100% conversion.

O O DABCO, 3 H n n H H H O O O O N N N MeOH/DMF n O O O O O OH O O OH OH O O

O SH pyridine, THF, 12 H

O O O O

S S

H O O O O N N n O OH O O OH OH O O

Scheme 2.12. Introduction of the vinyl group and subsequent reaction with a thiol. 90

Harth’s group91 copolymerized α-allyl-δ-valerilactone with valerolactone to

3 produce a copolymer with a Mn 3.4 x 10 Da with a Đ of 1.16 (see Figure 2.13). They then used the resulting vinyl group to make cross-linked nanoparticles by reacting the pendant vinyl groups with 2,2’-(ethane-1,2-diylbis(oxy))bis(ethane-1-thiol) via thio-ene

“click” chemistry.

Figure 2.13. Pendant-vinyl group from the polyester backbone. 91

2.7.9 Mercapto Group

The reactivity of the thiol group is similar to that of the hydroxyl group and much of the same chemistry applies to both. Matsumora’s group92 synthesized polyesters containing free thiol groups enzymatically (see Figure 2.14). They used many different enzymes and found the highest molecular weight (Mn: 1.4 x 104 Da) was achieved using the lipase enzyme, Candida antartica (lipase CA).

O O n n O Lipase CA O HO OH O O OH 6 6 SH O SH O n

Figure 2.14. Introduction of a free mecapto group via enzymatic polymerization. 92

2.8 Radical Grafting

PLA is difficult to functionalize because it lacks specific reactive sites along the polymer backbone.93 One effective method for changing the properties of a given polymer is to graft it with a second monomer that can impart different properties (Figure

2.15).

Figure 2.15. The three general approaches to preparing graft copolymers: grafting-onto (black), grafting-through (blue), and grafting-from (green).

There are three general approaches to preparing graft copolymers: grafting-onto,

94,95 grafting-through96-98 and grafting-from. 99,100 The grafting-onto and grafting-through methods are limited by low coupling efficiencies because of steric hindrance and often

high polydispersities. Since a living or controlled polymerization mechanism, such as controlled radical polymerization,101 can be used, the grafting-from method is generally the most convenient technique to prepare graft copolymers with controlled graft lengths.

The synthesis of graft copolymers by a grafting-from approach requires a macroinitiator with multiple initiating sites along its polymer backbone.

Although PLA has been used to prepare block copolymers by initiation of vinyl monomers from one102 or both of its (functionalized) endgroups,103 there are few reports of graft copolymerization of vinyl monomers from the PLA backbone, primarily because of the lack of specific initiating sites along the polymer backbone. Nevertheless, the surface of PLA can be grafted non-specifically under photochemical conditions by activation of the substrate with benzophenone as a photosensitizer.93 For example, the surfaces of PLA particles and films were photochemically grafted with acylamide, acrylic acid and maleic anhydride;104 with N-vinylpyrrolidone; 62,105 and with N- vinylcaprolactam106 in order to increase the hydrophilicity of the surface of PLA. Other polyester films and fibers, such as those of poly(ε-caprolactone) (CL), have also been grafted with monomers such as acrylic acid107 and acrylamide108 by generating radicals on their surface, without an added initiator, using electron beam pre-irradiation.

In-situ reactive grafting offers another non-specific grafting procedure. For example, PLA was grafted with a low molecular weight PEG-acrylate by blending it with the acrylate at elevated temperature in the presence of a radical initiator;109 in this case, the grafting efficiency decreased as the amount of blended PEG-acrylate increased. The

PEG graft plasticized the PLA and made it more ductile as well as more hydrolytically degradable. PLA has also been grafted with acrylic acid110 and with glycidyl

methacrylate111 by reactive blending to improve the compatibility of blends of PLA and starch.

O Br O O O Cu / Ligand O HO H m n N Toluene O O O O O 2 HO H m n O

Scheme 2.13. General scheme showing radical grafting (ATRP) of methyl methacrylate from our brominated polyester.

The alpha carbonyl group activates the bromine atoms of PLB toward homolytic cleavage and radical formation, which is the basis of atom transfer radical polymerization

(ATRP) of (meth)acrylates, (meth)acrylamides, and similar monomers.112 Therefore, the bromine atoms of PLB provide specific initiating sites from which to graft vinyl monomers by a grafting-from approach. As outlined in Figure 2.15, this chapter explores the ability of methyl methacrylate to be grafted from brominated PLA under ATRP conditions. Somewhat similarly, MMA has been grafted from PCL by ATRP from a classic ATRP initiator attached pendant to the PCL backbone.113 The graft copolymerization of PLA with different monomers (Scheme 1) will provide PLA with unique properties and/or a high concentration of functional groups.

2.9 Motivation and Scope

As shown previously, PLA has its limitations, with the most important being that

PLA does not have a readily accessible site along the backbone for post-polymerization modification. In the absence of a site for chemical attachment of drugs or other molecules

of interest, these molecules must be physically entrapped into PLA, by forming micelles or by nano-encapsulation, both of which have limited loading capacity.

Our group1,2 works on hyperbranched polyacrylates that are made by self- condensing vinyl polymerization (SCVP) of various ester-substituted acrylate inimers.

These hyperbranched polymers are true analogues of linear polyacrylates since they have a free ester along the polymer backbone. The inimers are synthesized from 2-halo-3- hydroxypropionic acid, which is a halogenated constitutional isomer of lactic acid. It is an ideal comonomer to copolymerize with either LA GA to give functional PLA-based polymers.

The polymers that will be modified are copolymers of 2-bromo-3-hydroxy propionic acid (BrH) and D,L-Lactic acid. The present work will focus on using the halogen along the main chain of the PLA based copolymer as a site for post- polymerization modification. By having the bromine alpha to a carbonyl opens a wide range of synthetic pathways to functionalize (see Figure 2.16). The bromine can potentially undergo nucleophilic substitution, radical chemistry, eand nolate chemistry, and it can be used as an initiator for cationic ROP.

The goal of this project is to establish which of the routes shown in Figure 2.16 are viable with our polyester. By having mulitple routes of post-polymerization functionalization we are greatly expanding the potential of our polymers. As an example if a drug cannot tolerate the conditions required for radical coupling that drug may be able to be attached via nucleophilic substitution.

O O HO H H O HO O n O O E n E+

O H n HO O n Br Nuc:- R radical

O H HO O O H Nuc HO O R

Figure 2.16. Routes to functionalize the brominated polymer.

2.10 Routes for Post-polymerization Functionalization

Nearly any type of chemical transformation can be performed on this system and each will be discussed in the section below. However the ester linkage and the two methylene spacer between the hydroxyl and acid group in the polymer backbone impart limitations.

2.10.1 Nucleophilic Substitution

Since this polymer has a halogen alpha to a carbonyl, it is activated toward nucleophilic substitution. Lecomte’s group114 used a halogenated derivative of ε- caprolactone to make a functional monomer or polymer in which the chloride can be reacted with sodium azide (see Scheme 2.14). The azido group was then used for further modifications. However, there was always competition between nucleophilic substitution and elimination reactions, which will be discussed in more detail in Chapter III. 33

Scheme 2.14. Example of nucleophlic substitution onto a polyester, where DSDOP is 2,2- dibutyl-2-stanna-1-dioxepane. 114

2.10.2 Radical Chemistry

The halogen-carbon bond α to a carbonyl is weak and can undergo homolytic cleavage under redox conditions or by abstraction if other radicals are present,115 generating a carbon-centered radical that can add across double bonds. This radical can be used as a macroinitiator to make graft copolymers using atom transfer radical polymerization (ATRP). This aspect is the focus of Dr. Xiang Yang’s (a former graduate student in our group) graduate thesis. All monomers can be polymerized radically with the exceptions to the generality being olefins, 1,1-dialkyl alkenes, vinyl ethers, aldehydes, and ketones.116

2.10.2.1 Stable Free-Radical Polymerization

Stable Free-Radical Polymerization (SFRP) is an improvement over ATRP, in which the propagating radicals are mediated or temporally deactivated by stable radicals.

Radicals used in SFRP include triazolinyl, trityl, dithicarbamate, and nitroxides.116 When nitroxides are chosen for SFRP the reaction is called nitroxide-mediated polymerization

(NMP). Nitroxides will react with propagating radicals to lower their concentration so that bimolecular termination is negligible. Since the radical coupling is reversible it can produce living polymerizations.

2.10.2.2 Radical Quenching

4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO-OH) is a stable nitroxide radical that can function as a radical trap. This reaction is advantageous because it allows adding a hydroxyl group onto the polymer backbone without protection/deprotection chemistry. Chenal’s group has investigated the cytotoxicity of the nitroxide bond. They exposed three cell lines (mouse fibroblast cells, human umbilical vein endothelial cells, and murine macrophage cells) to the nitroxide and found that it did not affect the cell viability.117 TEMPO-OH can be used in biomedical applications to help modify physical properties of the brominated polymer system, or as a site to attach drugs.

Since TEMPO-H contains a stable radical, it can be modified before quenching radicals. Toy’s group118 coupled mono-methyl ether PEG (mPEG) Mw = 5.0 x104 DA to the free hydroxyl group of TEMPO-OH. The PEG can then be coupled to the polyester backbone via the radical site on the TEMPO-OH (see Scheme 2.15).

Scheme 2.15. Synthesis of mPEG-TEMPO.118

2.10.3 Enolate Chemistry

Zinc reacts with α-haloesters by insertion into the activated carbon-halogen bond and subsequent tautomerization to form the zinc enolate.119 This enolate will then react with an electrophile, traditionally aldehydes and ketones, to form a β-hydroxy ester in the case of aldehydes and ketones. The enolate can also be reacted with anhydrides, phosphonates and α,β-unsaturated carbonyls. Shen’s group120 has made four-arm star

PCL by reacting pentaerythritol with a samarium(II) enolate to produce PCL with Mn from 3.1 x 104 - 6.6 x 104 with Đ ranging from 1.19 -1.29 (Scheme 2.16).

Scheme 2.16. Synthesis of four-armed star PCL.120

Jedliński’s group121 converted γ-burtyrolactone into the enolate using potassium tert- butoxide and polymerized δ-valerolactone to produce poly(δ-valerolactone) with Mn from 2.4 x 103 -3.5 x 104 with Đ ranging from 1.51 -1.82 (Error! Reference source not

found.).

Scheme 2.17. Polymerization of δ-valerolactone starting from γ-burtyrolactone.121

2.10.4 Cationic ROP using an Oxocarbenium Ion

Cyclic ethers can be polymerized catatonically and in the case of oxoranes (three- membered rings) anionically. The cationic polymerization is initated by strong protic acids (CF3SO3H, H2SO4, HCl) that have non-nucleophilic counter anions. Tobolsky’s group122 polymerized THF using triethyloxonium tetrafluroborate in dichloroethane

3 (Figure 2.17). Mn’s ranged from 1.54 to 3.0 x 10 by osmotic measurements to 1.28 to

3.42 x 103 by 14C end-group analysis.

Figure 2.17. Polymerization of THF using an oxonium iniatior. 122

Silver salts with non-nucleophilic counter anions can be used to form oxocarbenium ions since the silver halide is insoluble in almost any solvent and drives the reaction forward.123 This polymer can react with a silver salt, such as silver hexafluorophoshpate, silver trifflate, and silver nitrate, to form an oxocarbenium ion that can be used to polymerize THF.

2.10.5 Electrophilic Aromatic Substitution of an Oxocarbenium Ion

Strong electrophiles can add to an aromatic ring via electrophilic aromatic substitution. As briefly mentioned in section 2.10.4 this polymer can form an oxocarbenium ion, which is a strong electrophile. In the presence of an aromatic ring, the oxocabenium ion will add to the ring via electrophilic aromatic substitution (Scheme

2.18). During this reaction a positive charge is delocalized over the aromatic ring, so electron rich aromatic rings (i.e. anisole) will react faster with electrophiles.

Scheme 2.18. Mechanism of electrophilic aromatic substitution. Insert: Formation of a strong electrophile.

2.11 Goal of the Project

The goal of this project is to establish multiple methods of functionalization for our brominated polyester. By having multiple methods of functionalization established, it gives us more leeway in the types of molecules that can be incorporated, i.e. if a drug cannot tolerate radical coupling conditions it may be attached via nucleophilic substitution.

1.1 Summary

PLA is a useful polymer with few routes to directly functionalize it from the backbone. In this dissertation a brominated constitutional isomer of lactic acid will be copolymerized with LA to prepare a new polyester with a site for post-polymerization functionalization along the backbone. The unique structure of this monomer allows for multiple methods of functionalization can be implemented. This thesis will focus on two of the routes: nucleophilic substitution and radical grafting.

39 CHAPTER III

EXPERIMENTAL

3.1 Materials

Aliquat 336 (A336, Arcos Organics, 88.2-90.3%), dimethyl acetylenedicarboxylate (Aldrich, 99%), diphenyl ether (Linden, 99%), Europium tris[3-

(heptafluoropropylhydroxymethylene)-(-)-camphorate (Aldrich), hydrobromic acid

(Sigma Aldrich, 48% w/v% aq), D,L-lactic acid (Arcos Organics, 85% w/v% aq),

N,N,N',N'-pentamethyldiethylenetriamine (PMDETA, Aldrich, 99%), potassium bromide

(Arcos Organics, 98%), propargyl alcohol (Arcos Organics, 99%), D,L-serine (Sigma,

≥98%), sodium azide (EMD, 99.9%), sodium iodide (Baker Analyzed, 99.7%), sodium nitrite (Alfa Aesar, 98%) and p-toluenesulfonic acid monohydrate (pTSA•H2O; Fischer

Scientific, >99%) were used as received. 2-bromo-3-hydroxypropionic acid was

synthesized as described previously.124 Cuprous bromide (Aldrich, 98%) was purified by stirring with glacial acetic acid overnight, washing several times with ethanol, and then drying in vacuo.125 Acetone (Fischer Scientific, 99.7%) was stirred over 4 Å molecular

sieves and then distilled under N2. Diethyl ether (EMD, 99%) was dried by distillation from purple sodium benzophenone ketyl under N2. N,N-Dimethylformamide (DMF,

Sigma, ≥99.8%) was stirred over CaH2 and then distilled under N2. Triethylamine

(Sigma Aldrich, >99%) was distilled from KOH under N2 and stored over KOH. All other reagents and solvents were commercially available and were used as received.

40 3.2 Techniques

All reactions (under N2 atmosphere) and polymerizations (under vacuum) were conducted on a Schlenk line unless noted otherwise. Microwave syntheses were performed using a Milestone START 60 Hz microwave. 1H (300 MHz) and 13C (75

MHz) NMR spectra (δ, ppm) were recorded on a Varian Mercury 300 spectrometer.

Unless noted otherwise, all spectra were recorded in CDCl3, and the resonances were

measured relative to residual solvent resonances and referenced to tetramethylsilane (0.00

ppm). Number- (Mn) and weight-average (Mw) molecular weights and molecular mass

distributions (Ð = Mw/Mn) were determined by size exclusion chromatography (SEC) relative to linear polystyrene from calibration curves of log Mn vs. elution volume at 35

˚C using THF as solvent (1.0 mL/min), a guard column and a set of 50, 100, 500, 104 Å and linear (50-104 Å) Styragel 5 mm columns, a Waters 486 tunable UV/Vis detector set at 254 nm, a Waters 410 differential refractometer, and Millenium Empower 3 software.

Absolute molecular weights were determined by SEC with a light scattering detector

(SECLS) at 35 ˚C using THF (distilled from LiAlH4 and filtered through a 0.45 µm PTFE

filter) as solvent (1.0 mL/min), a guard column and a set of 50 Å, 104 Å and linear (50-

106 Å) Phenogel 5 µm columns and a 500 Å American Polymer Standard 5 µm column,

and a Wyatt Technology miniDAWN TREOS three-angle (46.6˚, 90.0˚, 133.4˚) light

scattering detector equipped with a Ga-As laser (659 nm, 50 mW), with the concentration

at each elution volume determined using a Wyatt Optilab T-rEX differential

refractometer (658 nm). The molecular weight data were calculated using Astra 6.0.3.16

software (Wyatt Technology) and a Zimm fit. The refractive index (RI) increments

(dn/dc values) were determined using 0.6, 1.4, 1.9, 2.6 and 5.4 mg/mL solutions and were

measured off-line in THF (filtered through a 0.45 µm PTFE filter) at room temperature at

658 nm using the Optilab T-rEX differential refractometer calibrated with aq NaCl and a

New Era syringe pump at 0.3 mL/min. All samples (approximately 0.5 g/L) were dissolved and filtered through a 0.45 µm PTFE filter.

pKA was determined using a Vernier pH probe, calibrated with pH 4 and pH 10 standard buffer solutions by immersing the pH probe in the solutions and allowing the probe to equilibrate.

3.3 Reactions 3.3.1 Synthesis of Poly[(lactic acid)-co-(2-bromo-3-hydroxypropionic acid)]

A melted solution of 85% D,L-lactic acid (0.41 g, 3.8 mmol), 2-bromo-3- hydroxypropionic acid (0.66 g, 3.9 mmol), diphenyl ether (1 mL) and pTSA•H2O (5.0 mg, 26 µmol) in a Schlenk tube were stirred at 95 °C at atmospheric pressure for 1 h and at reduced pressure (1-3 mm Hg) for 48 h. After opening the Schlenk tube to the atmosphere, CH2Cl2 (10 mL) was added and the tube was cooled in an ice bath to precipitate diphenyl ether. The precipitate was removed by gravity filtration, and the polymer was precipitated in methanol (50 mL). The copolymer was collected in a fritted glass funnel and reprecipitated four times from CH2Cl2 (10 mL) into methanol (50 mL) to

4 yield 0.36 g (40%) of brominated PLA as a white solid; MnPSt = 2.03 x 10 Da, Ð = 1.93; composition 50% lactate units, 50% 2-bromo-3-hydroxypropionate units. 1H NMR: 1.4-

13 1.7 (CH3), 4.4–4.7 (CH2CHBr), 5.1-5.3 (CH(CH3)CO2). C NMR: 16.7 (CH3), 39.3

(CBr), 64.9 (CH2CHBr), 69.1 (CHCH3), 166.7 (CHBrCO2), 169.5 (C(CH3)CO2).

3.3.2 Large Scale Synthesis (10 g) of Poly[(lactic acid)-co-(2-bromo-3-

hydroxypropionic acid)]

A melted solution of D,L-lactic acid (4.1 g, 38 mmol), 2-bromo-3- hydroxypropionic acid (6.6 g, 39 mmol), pTSA•H2O (0.50 g, 2.6 mmol) and diphenyl ether (1 mL) was stirred with a mechanical stirrer at 90 °C at atmospheric pressure for 2 h, and at 90 °C under reduced pressure (1-3 mm Hg) for 118 h. After opening the system to the atmosphere, the polymerization mixture was dissolved in THF (30 mL) and precipitated in methanol (300 mL). The solvents were decanted, and the copolymer was reprecipitated from THF (30 mL) into methanol (300 mL) to yield 5.2 g (65%) of

3 halogenated PLA as a white solid; Mn = 5.32 x 10 Da, Ð = 2.64; composition 49% lactate units, 51% 2-bromo-3-hydroxypropionate units. 1H NMR (see Figure 3.1): 1.4-

13 1.7 (bm, CH3), 4.4–4.7 (m, CH2CHBr), 5.1-5.3 (m, CH(CH3)CO2). C NMR (see Figure

3.2): 16.8 (CH3), 39.5 (CBr), 64.8 (CH2CHBr), 65.5 (CH2CHBr), 69.1 (CHCH3), 166.6

(CHBrCO2), 169.3 (C(CH3)CO2).

Figure 3.1. 1H NMR spectrum of Poly[(lactic acid)-co-(2-bromo-3-hydroxypropionic acid)].

Figure 3.2. 13C NMR spectrum of Poly[(lactic acid)-co-(2-bromo-3-hydroxypropionic acid)].

3.3.3 Synthesis of Poly[(lactic acid)-co-(2-iodo-3-hydroxypropionic acid)]

A solution of poly[(lactic acid)-co-(2-bromo-3-hydroxypropionic acid)] (Mn =

5.32 x 103 Da, Ð = 2.64; 0.40 g, 1.8 mmol Br) and sodium iodide (0.30 g, 2.0 mmol) in 44

acetone (10 mL) was stirred at room temperature (23 °C) for 23 h. The solvent was removed using rotary evaporation. The residue was dissolved in CH2Cl2 (15 mL) and passed through celite. The solvent was removed using a rotary evaporator and then the vacuum on a Schlenk line to yield 0.49 g (92%) of copolymer as a yellow solid; Mn =

4.62 x 103 Da, Ð = 1.96; composition 49% lactate units, 51% 2-iodo-3-

1 hydroxypropionate units. H NMR (see Figure 4.2): 1.5-1.7 (b, CH3), 4.4-4.7

13 (CH2CHI), 5.1-5.3 (b, CHCH3). C NMR (see insert Figure 4.2): 12.6 (CHI), 16.7

(CH3), 66.0 (CH2), 69.0 (CHCH3), 169.0 (O2CCHI), 169.4 (O2CCHCH3).

3.3.4 Synthesis of Poly[(lactic acid)-co-(2-azido-3-hydroxypropionic acid)-co-(2-

bromo-3-hydroxypropionic acid)] by Reaction of Poly[(lactic acid)-co-(2-bromo-

3-hydroxypropionic acid)] with Sodium Azide

A solution of sodium azide (0.12 g, 1.8 mmol) and poly[(lactic acid)-co-(2-

3 bromo-3-hydroxypropionic acid)] (Mn = 5.32 x 10 Da, Ð = 2.64; 49:51 LA/Br units) (1.6 g, 7.2 mmol Br) in a 0.014 M solution of A336 (10 mL, 0.14 mmol) in DMF was stirred at 0 °C for 6 h. The system was warmed to room temperature and a vacuum was applied for 24 h to remove DMF. The residue was dissolved in CH2Cl2 (20 mL) and passed through Celite, and solvent was removed by rotary evaporation. The residue was precipitated twice from CH2Cl2 (5 mL) into 1:1 (v:v) hexanes / ethyl ether (200 mL) to

3 yield 0.77 g (50%) of azide-containing copolymer as a white solid; Mn = 4.15 x 10 Da,

Ð = 2.33; composition 47% lactate units, 42% 2-bromo-3-hydroxypropionate units, 11%

1 2-azido-3-hydroxypropionate units. H NMR (see Figure 3.3): 1.4-1.7 (CH3), 4.2-4.4

13 (m, CH2CHN3), 4.4-4.8 (m, CHN3, CH2CHBr), 5.1-5.3 (CHCH3). C NMR(see Figure

3.4): 16.6 (CH3), 39.3 (CHBr), 60.0 (CHN3), 64.5 (CH2), 65.3 (CH2), 69.1 (CHCH3),

70.1 (CHCH3), 166.4 (O2CCHBr), 169.1 (O2CCH(CH3), O2CCHN3).

Figure 3.3. 1H NMR spectrum of Poly[(lactic acid)-co-(2-azido-3-hydroxypropionic acid)-co-(2-bromo-3-hydroxypropionic acid)].

Figure 3.4. 13C NMR spectrum of Poly[(lactic acid)-co-(2-azido-3-hydroxypropionic acid)-co-(2-bromo-3-hydroxypropionic acid)].

3.3.5 Attempted Copper-Catalyzed Huisgen Alkyne-Azide Cycloaddition of Poly[(lactic

acid)-co-(2-azido-3-hydroxypropionic acid)-co-(2-bromo-3-hydroxypropionic

acid)] with Propargyl Alcohol

A solution of CuBr (3.2 mg, 22 µmol) and PMDETA (3.6 mg, 21 µmol) in THF

(2 mL) was stirred at room temperature for 10 min to complex the CuBr. A solution of poly[(lactic acid)-co-(2-bromo-3-hydroxypropionic acid)-co-(2-azido-3-

3 hydroxypropionic acid)] (Mn = 4.15 x 10 Da, Ð = 2.33; 47:42:12 LA/Br/N3 units) (0.20 g, 0.42 mmol N3) and propargyl alcohol (35 mg, 0.62 mmol) in THF (2.0 mL) was added.

The reaction mixture was degassed by four freeze-pump-thaw (5-15-5 min) cycles, and

backfilled with nitrogen. After stirring at 40 °C for 2 h, the solution was passed through basic activated alumina, and THF was removed by rotary evaporation. Only 42 mg of

3 copolymer was recovered; Mn = 4.45 x 10 Da, Ð = 2.58. No resonances for the azide or

1 triazole ring were detected in its H NMR spectrum: 1.5-1.7 (m, CH3), 4.4-4.8 (m,

13 CH2CHBr), 5.1-5.3 (m, CHCH3). C NMR: 16.0 (CH3), 38.8 (CBr), 63.8 (CH2CHBr),

69.3 (CHCH3), 168.4 (CHBrCO2, C(CH3)COO).

Figure 3.5. 1H NMR spectum of the attempted "click" reaction using CuBr and PMDETA.

3.3.6 Huisgen Alkyne-Azide Cycloaddition of Poly[(lactic acid)-co-(2-azido-3-

hydroxypropionic acid)-co-(2-bromo-3-hydroxypropionic acid)] with Dimethyl

Acetylenedicarboxylate

A solution of poly[(lactic acid)-co-(2-bromo-3-hydroxypropionic acid)-co-(2-

3 azido-3-hydroxypropionic acid)] (Mn = 4.15 x 10 Da, Ð = 2.33; 47:42:12 LA/Br/N3 units) (0.31g, 1.5 mmol N3) and dimethyl acetylenedicarboxylate (0.22 g, 1.5 mmol) in acetone (10 mL) was heated in a microwave in air (protected by a drying tube) for 4 min

(3 min ramp to 56 °C, 1 min hold at 56 °C, power 65 W). The solution was concentrated by rotary evaporation, CH2Cl2 (2 mL) was added, and the resulting solution was precipitated in 1:1 (v:v) hexanes / diethyl ether (80 mL) to yield 0.16 g (72%) of

3 copolymer as a white solid; Mn = 5.05 x 10 Da, Ð = 2.51; composition 56% lactate units, 32% 2-bromo-3-hydroxypropionate units, 6% 2-azido-3-hydroxypropionate units,

1 6% "clicked" [1,2,3]-triazole units. H NMR (see Figure 4.2): 1.5-1.7 (CH3), 3.9-4.0

13 (CH3O2C), 4.2-4.4 (CH2CHN3), 4.4-4.8 (CHN3, CH2CHBr), 5.0-5.3 (CHCH3). C NMR:

16.6 (CH3CH), 39.2 (CHBr, CH(N3C2)), 52.9 (CH3O2C), 53.6 (CH3O2C), 61.7 (CHN3),

64.6 (CH2), 65.3 (CH2), 69.0 (CHCH3), 70.1 (CHCH3), 127.9 (N-C=C-N), 160.1

(CO2CH3), 166.5 (O2CCHBr), 169.2 (O2CCH(CH3), O2CCHN3, CH(N3C2)).

3.3.7 Synthesis of Methyl 2-Bromo-3-hydroxypropionate

A solution of 2-bromo-3-hydroxypropionic acid (10 g, 60 mmol) and concentrated hydrobromic acid (10 drops) in methanol (80 mL, 2.0 mol) was stirred at reflux for 17 h. The solvent was removed by rotary evaporation. The resulting oil was dissolved in CH2Cl2 (150 mL), and washed twice with aq NaHCO3 (75 mL ea) and once with brine (100 mL). The organic layer was dried over MgSO4. After filtration, the solvent was removed by rotary evaporation, and the residue was distilled (105-110 °C / 4 mm Hg) to yield 8.7 g (79%) of methyl 2-bromo-3-hydroxypropionate as a slightly

1 2 3 yellow oil. H NMR: 2.22 (br s, OH), 3.83 (s, CH3), 3.95 (dd, CHHOH, J = 12.0 Hz, J

= 5.5 Hz), 4.06 (dd, CHHOH, 2J = 12.1 Hz, 3J = 7.4 Hz), 4.36 (dd, CHBr, 3J = 5.6 Hz, 3J

13 = 7.4 Hz). C NMR: 44.4 (CHBr), 53.4 (CH3), 63.8 (CH2OH), 169.6 (C= O).

3.3.8 Synthesis of Methyl 3-Acetoxy-2-bromopropionate

A solution of acetyl chloride (4.2 mL, 31 mmol) in dry diethyl ether (total solution volume 10 mL) was added dropwise to an ice-cooled solution of methyl 2- bromo-3-hydroxypropionate (4.6 g, 25 mmol) and triethylamine (4.2 mL, 30 mmol) in dry diethyl ether (5 mL). The reaction mixture was then stirred at room temperature (23

°C) for 16 h. The reaction mixture was poured into ice water (200 mL), the aqueous mixture was extracted four times with diethyl ether (50 mL ea), and the combined organic layers were dried over MgSO4. After filtration, the solvent was removed by rotary evaporation, and the yellow residue was distilled (84-85 °C / 3 mm Hg) to yield

3.6 g (63%) of methyl 3-acetoxy-2-bromopropionate as a colorless oil. 1H NMR: 2.08

2 3 (s, CH3CO2), 3.82 (s, CH3O2C), 4.37(m, CH2CHBr), 4.95 (dd, CHHO, J = 11.8 Hz, J =

6.5 Hz), 4.42 (dd, CHHO, 2J = 11.8 Hz, 3J = 6.3 Hz), 4.79 (dd, CHBr, 3J = 6.4 Hz, 3J =

13 6.4 Hz). C NMR: 20.8 (CH3CO2), 40.4 (CHBr), 53.4 (CH3O2C), 64.2 (CH2), 168.3

(CH3CO2), 170.2 (CO2CH3).

3.3.9 Test of the Stability of Methyl 3-Acetoxy-2-bromopropionate in the Presence of

PMDETA

A solution of methyl 3-acetoxy-2-bromopropionoate (0.20 g, 0.87 mmol) and

PMDETA (0.15 g, 0.88 mmol) in DMF (2 mL) was stirred at room temperature for 1 h.

An aliquot was removed for 1H NMR analysis (see Figure 3.6), which demonstrated that

methyl α-bromoacrylate was produced, based on the vinyl resonances at 6.19 and 6.81 ppm.

Figure 3.6. 1H NMR spectrum of an aliquot taken from the CuBr/ PMDETA-catalyzed reaction of methyl 3-acetoxy-2-bromopropionate with propargyl alcohol.

3.3.10 Synthesis of Poly[(lactic acid)-co-(2-bromo-3-hydroxypropionic acid)]

A melted solution of D,L-lactic acid (4.1 g, 39 mmol), 2-bromo-3- hydroxypropionic acid (6.8 g, 33 mmol), pTSA•H2O (0.54 g, 3.2 mmol) and diphenyl ether (DPE) (1.0 mL) was stirred at 90 °C at atmospheric pressure for 1 h and at 90 °C under reduced pressure (1-3 mm Hg) for 4 days. At 23 h, 46 h, 52.5 h, and 72 h DPE (1.0 mL each time) was added when the reaction system stopped stirring. The polymer was then dissolved in CH2Cl2 (50 mL) and stored in the freezer overnight to crystallize the

DPE. The solvent was then filtered and the solvent removed by rotary evaporation.

CH2Cl2 (10 mL) added and the polymer precipitated into methanol (200 mL) twice to

3 yield 6.24 g (78%) of halogenated PLA as a white solid; Mn = 5.90 x 10 , Đ = 3.27; composition 49% lactate units, 51% 2-bromo-3-hydroxypropionate units. 1H NMR: 1.4-

13 1.7 (bm, CH3), 4.4–4.7 (m, CH2CHBr), 5.1-5.3 (m, CH(CH3)CO2). C NMR: 16.6

(CH3), 39.3 (CBr), 65.3 (OCH2CHBr), 69.1 (CHCH3), 166.7 (CHBrCO2), 169.5

(C(CH3)CO2).

3.3.11 Synthesis of Poly[(lactic acid)-co-(2-iodo-3-hydroxypropionic acid)]

A solution of poly[(lactic acid)-co-(2-bromo-3-hydroxypropionic acid)] (Mn = 5.9 x 103, Đ = 3.27; 2.37 g, 10 mmol Br) and sodium iodide (2.36 g, 16 mmol) in acetone

(50 mL) was stirred at room temperature (23 °C) for 23 h. The reaction was then passed through celite and the solvent was removed using a rotary evaporator and then the vacuum on a Schlenk line to yield 2.59 g (85%) of copolymer as a yellow solid; Mn = 4.3 x 103, Đ = 2.46; composition 42% lactate units, 48% 2-iodo-3- hydroxypropionate units.

1 13 H NMR: 1.5-1.7 (b, CH3), 4.4-4.7 (CH2CHI), 5.1-5.3 (b, CHCH3). C NMR: 12.6

(CHI), 16.7 (CH3), 66.0 (CH2), 69.0 (CHCH3), 169.0 (O2CCHI), 169.4 (O2CCHCH3).

3.3.12 Synthesis of poly[(lactic acid)-co-(3-hydroxy-2-azidopropionic acid)] Starting

from the Brominated Copolymer at Room Temperature

As an example: A solution of poly[(lactic acid)-co-(2-bromo-3-hydroxypropionic

3 acid)] (Mn=5.9 x 10 , Đ = 3.27; 0.20 g, 8.8 mmol Br) and sodium azide 14 mg, 2.2 mmol) in dry DMF (2 mL) was stirred at room temperature (23°C) for 1 h. The solvent was then removed via the Schlenk line. The reaction mixture was then diluted with

CH2Cl2 (10 mL), passed through celite, concentrated on a rotovapor, and precipitated into a 1:1 (v:v) hexanes: ether (40 mL) to yield 0.17 g (62%) of copolymer as a white solid;

3 1 Mn=4.5 x 10 , Đ =2.64. H NMR: 1.4-1.7 (m, CH3), 4.2-4.3 (m, CHN3), 4.4-4.6 (m,

13 CH2), 5.3-5.1 (m, CHCH3). C NMR: 16.6 (CH3), 39.6 (CHBr), 60.0 (CHN3), 64.2

(CH2), 69.1 (CHCH3), 166.8 (CHBrCO2), 169.3 (C(CH3)COO), 173.2 (CHN3CO2).

3.3.13 Synthesis of poly[(lactic acid)-co-(3-hydroxy-2-azidopropionic acid)] Starting

from the Brominated Copolymer at 0 ˚C

As an example: A solution of poly[(lactic acid)-co-(2-bromo-3-hydroxypropionic

3 acid)] (Mn=5.9 x 10 , Đ = 3.27; 0.20 g, 8.8 mmol Br) and sodium azide 14 mg, 2.2 mmol) in 2 mL of a stock solution of DMF and aliquot 336 [3.65 M] was stirred at 0 °C for 6 h. The solvent was then removed via the Schlenk line. The reaction mixture was then diluted with CH2Cl2 (10 mL), passed through celite, concentrated on a rotovapor, and precipitated into a 1:1 (v:v) hexanes: ether (40 mL) to yield 0.046 g (62%) of

3 1 copolymer as a white solid; Mn=6.0 x 10 , Đ =2.96. H NMR: 1.4-1.7 (m, CH3), 4.2-4.3

13 (m, CHN3), 4.4-4.6 (m, CH2), 5.3-5.1 (m, CHCH3). C NMR: 16.6 (CH3), 39.6 (CHBr),

60.0 (CHN3), 64.2 (CH2), 69.1 (CHCH3), 166.8 (CHBrCO2), 169.3 (C(CH3)COO), 173.2

(CHN3CO2).

3.3.14 Synthesis of poly[(lactic acid)-co-(3-hydroxy-2-azidopropionic acid)] Starting

from the Iodated Copolymer at Room Temperature

As an example: A solution of poly[(lactic acid)-co-(2-iodo-3-hydroxypropionic

3 acid)] (Mn=4.30 x 10 , Đ =2.46; 0.20 g, 6.8 mmol I) and sodium azide 11 mg, 1.7 mmol) in dry DMF (2 mL) was stirred at room temperature (23°C) for 1 h. The solvent was then

removed via the Schlenk line. The reaction mixture was then diluted with CH2Cl2 (10 mL), passed through celite, concentrated on a rotovapor, and precipitated into a 1:1 (v:v) hexanes: ether (40 mL) to yield 0.11 g (47%) of copolymer as a white solid; Mn=3.10 x

3 1 10 , Đ =2.01. H NMR: 1.4-1.7 (m, CH3), 4.2-4.3 (m, CHN3), 4.4-4.6 (m, CH2), 5.3-5.1

13 (m, CHCH3). C NMR: 16.6 (CH3), 39.6 (CHBr), 60.0 (CHN3), 64.2 (CH2), 69.1

(CHCH3), 166.8 (CHBrCO2), 169.3 (C(CH3)COO), 173.2 (CHN3CO2).

3.3.15 Synthesis of poly[(lactic acid)-co-(3-hydroxy-2-azidopropionic acid)] Starting

from the Iodated Copolymer at 0˚C

As an example: A solution of poly[(lactic acid)-co-(2-iodo-3-hydroxypropionic

3 acid)] (Mn=4.30 x 10 , Đ =2.46; 0.20 g, 6.8 mmol I) and sodium azide 14 mg, 1.7 mmol) in 2 mL of a stock solution of DMF and aliquot 336 [2.82 M] was stirred at 0 °C for 6 h.

The solvent was then removed via the Schlenk line. The reaction mixture was then diluted with CH2Cl2 (10 mL), passed through celite, concentrated on a rotovapor, and precipitated into a 1:1 (v:v) hexanes: ether (40 mL) to yield 0.15 g (67%) of copolymer as

3 1 a white solid; Mn=3.2 x 10 , Đ =2.87. H NMR: 1.4-1.7 (m, CH3), 4.2-4.3 (m, CHN3),

13 4.4-4.6 (m, CH2), 5.3-5.1 (m, CHCH3). C NMR: 16.6 (CH3), 39.6 (CHBr), 60.0

(CHN3), 64.2 (CH2), 69.1 (CHCH3), 166.8 (CHBrCO2), 169.3 (C(CH3)COO), 173.2

(CHN3CO2).

3.3.16 Synthesis of poly[(lactic acid)-co-(3-hydroxy-2-azidopropionic acid)] Starting

from the Brominated Copolymer with Triflic acid

As an example: A solution of poly[(lactic acid)-co-(2-bromo-3-hydroxypropionic

3 acid)] (Mn=9.70 x 10 , Đ =4.22; 0.11 g, 0. 51 mmol Br) and sodium azide 29 mg, 0.45

mmol) in 1 mL of a stock solution of DMF and TFA [0.05 M] was stirred at room temperature (23°C) for 1 h. The solvent was then removed via the Schlenk line. The reaction mixture was then diluted with CH2Cl2 (8 mL), passed through celite, concentrated on a rotovapor, and precipitated into a 1:1 (v:v) hexanes: ether (40 mL) to

3 1 yield 0.04g (47%) of copolymer as a yellow solid; Mn=1.0 x 10 , Đ =2.80. H NMR:

13 1.4-1.7 (m, CH3), 4.2-4.3 (m, CHN3), 4.4-4.6 (m, CH2), 5.3-5.1 (m, CHCH3). C NMR:

16.6 (CH3), 39.6 (CHBr), 60.0 (CHN3), 64.2 (CH2), 69.1 (CHCH3), 166.8 (CHBrCO2),

169.3 (C(CH3)COO), 173.2 (CHN3CO2).

3.3.17 Synthesis of poly[(lactic acid)-co-(3-hydroxy-2-azidopropionic acid)] Starting

from the Brominated Copolymer with Thionyl Chloride

As an example: Thionyl chloride was (0.8 mL, 11.0 mmol) was added to a 25 mL addition funnel containing a solution of DMF (1 mL, 12.8 mmol) in benzene (5 mL). The contents were left to settle for 20 minutes into two phases. The bottom phase was added to a solution of poly[(lactic acid)-co-(2-bromo-3-hydroxypropionic acid)] (MnLS=2.34 x

104, Đ =1.97; 0.40 g, 1.79 mmol Br) in DMF (2 mL), cooled to 0 ˚C in an ice bath, and stirred for 5 minutes. Sodium azide (0.18 g, 2.69 mmol) was added all at once and the reaction was stirred for 19 h. DMF was then removed via trap-to-trap distillation and

CH2Cl2 (25 mL) was added. The mixture was passed through celite and the solvent was removed by rotary evaporation. CH2Cl2 (1 mL) was added and precipitated into MeOH

4 (100 mL) twice to yield 0.14 g (42%) of copolymer as a white solid; MnLS=3.39 x 10 , Đ

1 =1.73. H NMR (see Figure 3.7): 1.4-1.7 (m, CH3), 4.2-4-4.6 (m, CH2CHN3), 5.3-5.1

13 (m, CHCH3). C NMR (see Figure 3.8): 16.6 (CH3), 53.0 (CHN3), 64.9-65.7 (CH2),

69.1-70.1 (CHCH3), 166.8 (CHN3CO2), 169.3 (C(CH3)COO). 55

Figure 3.7. 1H NMR spectrum of poly[(lactic acid)-co-(3-hydroxy-2-azidopropionic acid)] made with thionyl chloride.

Figure 3.8. 13C NMR spectrum of poly[(lactic acid)-co-(3-hydroxy-2-azidopropionic acid)] made with thionyl chloride.

3.3.18 Synthesis of methyl-3-acetoxy-2-azido propionate with Various Amounts of

Sodium Azide

In a typical example methyl-3-acetoxy-2-bromopropionate (0.10 g, 0.46 mmol) and sodium azide (0.015g 0.23 mmol) in DMF (1.0 mL) stirred at room temperature (23

˚C) for 1 h. After that an aliquot was removed and added to 0.7 mL chloroform for NMR analysis.

3.3.19 Synthesis of methyl-3-acetoxy-2-azido propionate Starting from (L) methyl-3-

acetoxy-2-azidopropionate

A solution of (L) methyl-3-acetoxy-2-bromopropionate (0.2g, 0.42 mmol) and sodium azide (6.1 mg, 0.94 mmol) was stirred for 1 h at room temperature. Brine (20 mL) was added and was extracted with CH2Cl2 (10 mL) three times. The organic layers were then combined, dried with MgSO4 filtered, and the solvent removed by rotary evaporation and dried on the Schlenk line to yield 96 mg (56%) 1H-NMR: 2.03 (s,

3 3 2 CH3CO2), 3.77 (s, CH3O2C), 4.06 (dd, CHN3, J = 5.6, J = 4.2 Hz), 4.32 (dd, CHH, J =

3 2 3 13 11.6 Hz, J = 5.8 Hz), 4.40 (dd, CHH, J = 11.6, J = 4.1 Hz). C NMR: 20.7 (CH3CO2),

53.2 (CH3O2C), 60.4 (CHN3), 63.6 (CH2), 168.3 (CO2CH3), 170.4 (CH3CO2).

3.3.20 Determining pKa of 2-bromo-3-hydroxy propionic acid

As an example: Solution of 2-bromo-3-hydroxypropionic (0.189 g, 1.1mmol,

[0.11 M]) was titrated with NaOH stock solution [0.12 M]. pKa readings were recorded when the value stabilized for 1 minute. The reaction was performed three times (see

Figure 5.8).

3.3.21 Determining pKa of Lactic Acid

As an example: Solution of lactic acid (0.128 g, 1.2 mmol, [0.12 M]) was titrated with NaOH stock solution [0.12 M]. pKa readings were recorded when the value stabilized for 1 minute. The reaction was performed three times (see Figure 5.8).

3.3.22 Synthesis of 4,4'-Diheptyl-2,2′-dipyridyl

A solution of lithium diisopropylamide (13.5 mL, 100 mmol) in dry THF (10 mL) was prepared under a N2 atmosphere in a Vacuum Atmospheres drybox. The closed reaction flask was removed from the drybox, attached to a Schlenk line under N2, and cooled to -78 °C in an acetone/ CO2 bath. A solution of 4,4′-dimethyl-2,2′-dipyridyl (2.1 g, 11.5 mmol) in dry THF (55 mL) was added dropwise over 10 min to the reaction mixture, and then 1-bromohexane (3.5 mL, 24 mmol) was added via a syringe. The reaction was allowed to warm to room temperature and was stirred for 20 h. The reaction was quenched with water (100 mL), the organic layer was separated, and the solvent was removed by rotary evaporation. The resulting solid was passed through a column of neutral alumina, using Et2O/hexanes (12:1) as the eluant, and recrystallized from acetonitrile (200 mL) to yield 2.2 g (53%) of 4,4′-diheptyl-2,2′-dipyridyl as white needles

(Mp 42 ˚C). 1H NMR (see Figure 3.9): δ 0.88 (t, CH3, 6 H), 1.28 (m, [CH2]4, 16 H),

1.69 (m, CH2CH2Ar, 4 H), 2.7 (t, CH2Ar, 4 H), 7.12 (d, 2 aromatic H para to Ar), 8.24

(s, 2 aromatic H ortho to Ar), 8.55 (d, 2 aromatic H meta to Ar). 13C NMR (see Figure

3.10): δ 14.0 (CH3), 22.6 ([CH2]3CH2CH3), 29.0 ([CH2]2 CH2CH2CH3), 29.2

([CH2CH2[CH2]2 CH3), 30.4 ([CH2CH2[CH2]2 CH3), 31.7 (CH2CH2Ar), 35.5

(CH2CH2Ar), 121.3 (aromatic C 5,5'), 123.8 (aromatic C 6, 6'), 148.9 (aromatic C 3, 3'),

152.8 (aromatic C 4, 4'), 156.2 (aromatic C 1, 1').

Figure 3.9. 1H NMR spectrum of 4,4′-diheptyl-2,2′-dipyridyl.

Figure 3.10. 13C NMR spectrum of 4,4′-diheptyl-2,2′-dipyridyl.

3.3.23 Synthesis of Poly[(lactic acid)-co-(2-bromo-3-hydroxypropionic acid)]

A melted solution of 85% D,L-lactic acid (2.3 g, 21.2 mmol), 2-bromo-3- hydroxypropionic acid (1.0 g, 5.6 mmol), diphenyl ether (3 mL) and pTSA•H2O (28 mg,

1.46 mmol) in a Schlenk tube were stirred at 80 °C at atmospheric pressure for 2 h and at reduced pressure (1-3 mm Hg) for 72 h. After opening the Schlenk tube to the atmosphere, CH2Cl2 (6 mL) and the polymer precipitated in diethyl ether (150 mL). The copolymer was collected in a fritted glass funnel and reprecipitated from CH2Cl2 (6 mL) into diethyl ether (150 mL) to yield 1.43 g (59%) of brominated PLA as a white solid;

4 4 MnPSt =1.34 x 10 Da, Ð = 2.78; MnLS =3.58 x 10 Da, Ð = 1.64; composition 77%

1 lactate units, 22% 2-bromo-3-hydroxypropionate units. H NMR: 1.4-1.7 (CH3), 4.4–4.7

13 (CH2CHBr), 5.1-5.3 (CH(CH3)CO2). C NMR: 16.7 (CH3), 39.3 (CBr), 64.9

(CH2CHBr), 69.1 (CHCH3), 166.7 (CHBrCO2), 169.5 (C(CH3)CO2).

3.3.24 Kinetic Study of the Polymerization of MMA Initiated from Methyl 3-Acetoxy-

2-bromopropionate using HB

In a typical procedure, a solution of MMA (2.0 g, 20 mmol) and 4,4'-diheptyl-

2,2'-bipryidine (71 mg, 0.20 mmol) in toluene (1.5 mL) in a Schlenk tube sealed with a rubber septum was degassed by three freeze-pump/10 min-thaw cycles. The solution was frozen in liquid N2, and CuCl (10 mg, 0.10 mmol) was added into the Schlenk tube under a positive flow of N2. The contents of the Schlenk tube were again degassed by one freeze-pump/10 min-thaw cycle, and the mixture was stirred for 10 min to allow the ligand to complex the catalyst; the color of the mixture changed from colorless to dark red. The solution was frozen in liquid N2 and put under positive flow of N2. A solution of methyl 3-acetoxy-2-bromopropionate (23 mg, 0.10 mmol) in toluene (0.5 mL) was added and two more cycles of freeze-pump/10 min-thaw were performed and then the tube was backfilled with N2. The polymerization mixture was stirred at 90 °C under N2. Aliquots were removed periodically using an N2-purged syringe while the Schlenk tube was under

1 a positive flow of N2. The monomer conversion of each aliquot was determined by H

NMR spectroscopy by comparing the vinyl resonances of the monomer at 5.62-6.22 ppm and the methyl resonances of the polymer at 0.96-1.64 ppm. The molecular weight distributions of each aliquot were determined by GPCPSt.

3.3.25 Kinetic Study of the Polymerization of MMA Initiated from the Brominated

Polyester using HB

The graft copolymers were synthesized in 29-39% yield as in the following example. A solution of MMA (2.0 g, 20 mmol) and 4,4'-diheptyl-2,2'-bipryidne (140 mg,

0.40 mmol) in toluene (1.5 mL) in a Schlenk tube sealed with a rubber septum was degassed by three freeze-pump/10 min-thaw cycles. The solution was frozen in liquid

N2, and CuCl (20 mg, 0.20 mmol) was added into the Schlenk tube under a positive flow of N2. The contents of the Schlenk tube were again degassed by one freeze-pump/10 min-thaw cycle, and the mixture was stirred for ten minutes to allow the ligand to complex the catalyst as indicated by a color change from clear to dark red. The contents of the Schlenk tube were frozen, and a solution of poly[(lactic acid)-co-(2-bromo-3-

4 hydroxypropionic acid)] (77:23 LA/Br; GPCPSt Mn = 1.34 x 10 , Đ = 2.78; GPCLS Mn =

3.58 x 104, Đ = 1.64; 0.39 mg, 0.10 mmol Br) in toluene (0.5 mL) was added into the

Schlenk tube under a positive flow of N2. After degassing the polymerization mixture by two additional freeze-pump/10 min-thaw cycle and then backfilling the Schlenk tube with

N2, the polymerization mixture was stirred at 90 °C for 66 h. Aliquots were removed periodically using an N2-purged syringe while the Schlenk tube was under a positive flow

1 of N2. The monomer conversion of each aliquot was determined by H NMR spectroscopy by comparing the vinyl resonances of the monomer at 5.62-6.22 ppm and the methylester resonances of the polymer at 3.43-3.72 ppm. The molecular weight distributions of each aliquot were determined by GPCPSt, LS. The polymerization was then quenched by immersing the Schlenk tube into liquid N2. The contents of the polymerization tube were thawed, opened to the atmosphere and diluted with THF (5

mL), and then precipitated into methanol (150 mL). The precipitate was collected in a fritted glass funnel reprecipitated from THF (5 mL) into cold methanol (150 mL), and recollected in a fritted glass funnel. The dry polymer was then dissolved in THF (20 mL) and passed through basic alumina to remove any remaining copper. The solvent was then removed by rotary evaporation and reprecipitated from THF (5mL) into cold methanol

(150 mL) to yield 0.60 g (30%) of PLA-g-PMMA as a white solid; GPCPSt Mn = 5.00 x

4 4 10 , Đ = 1.57; GPCLS Mn = 5.38 x 10 , Đ = 1.15.

3.3.26 Determination of Dn/Dc v Percent Conversion Graph

As an example: Graft copolymer was synthesized in 14% yield as in the following example. A solution of MMA (2.0 g, 20 mmol) and 4,4'-diheptyl-2,2'-bipryidne (140 mg,

0.40 mmol) in toluene (1.5 mL) in a Schlenk tube sealed with a rubber septum was degassed by three freeze-pump/10 min-thaw cycles. The solution was frozen in liquid

4 hydroxypropionic acid)] (77:23 LA/Br; GPCPSt Mn = 1.34 x 10 , Đ = 2.78; GPCLS Mn =

3.58 x 104, Đ = 1.64; 0.39 mg, 0.10 mmol Br) in toluene (0.5 mL) was added into the

Schlenk tube under a positive flow of N2. After degassing the polymerization mixture by two additional freeze-pump/10 min-thaw cycle and then backfilling the Schlenk tube with

N2, the polymerization mixture was stirred at 90 °C for 24 and 36 h. An aliquot was removed to determine percent conversion. The contents of the polymerization tube were 63

thawed, opened to the atmosphere and diluted with THF (5 mL), and then precipitated into methanol (150 mL). The precipitate was collected in a fritted glass funnel and reprecipitated from THF (5 mL) into cold methanol (150 mL) and recollected in a fritted glass funnel. The dry polymer was then dissolved in THF (20 mL) and passed through basic alumina to remove any remaining copper. The solvent was then removed by rotary evaporation and reprecipitated from THF (5mL) into cold methanol (150 mL) to yield

0.29 g (14%) of PLA-g-PMMA as a white solid; Dn/Dc: 0.0850 mL/g, GPCLS Mn = 5.14 x 104, Đ = 1.24.

CHAPTER IV

SYNTHESIS OF FUNCTIONALIZED POLY(LACTIC ACID) USING 2-

BROMO-3-HYDROXYPROPIONIC ACID

4.1 Introduction

Although pendant reactive groups would be useful for introducing a variety of functional molecules to PLA, the pendant functional group must not participate in side reactions with the ester, carboxylic acid or alcohol groups of the monomer and/or resulting copolymer. This requirement limits the synthesis and/or shelf life of monomers and copolyesters with pendant amine, hydroxyl, thiol, carboxylic acid derivatives and isocyanate groups. In contrast, halides are generally inert to the conditions used for both polyesterifications and ring-opening polymerizations, yet provide a site for further functionalization of the resulting copolymers. For example, pendant chlorides, bromides, and iodides have been displaced by nucleophilic substitution reactions using sodium azide, and the resulting azides subsequently reacted with alkynes by an azide-alkyne

Huisgen cycloaddition126 to introduce amines,127 ammonium salts,127 and polymeric side chains127-130 to aliphatic polyesters. Polymeric side chains have also been introduced by azide-alkyne cycloadditions to polymers containing pendant alkyne groups.69,131

Although sodium azide may also cause an E2 elimination of aliphatic polyesters based on longer hydroxyalkanoic acids, this side reaction resulted in unsaturation along the polymer backbone, which was then used as a crosslinking site under UV irradiation.132

We believe that BrH it is an ideal monomer to copolymerize with lactic acid to produce

PLA with pendant bromine atoms along the polymer backbone for further functionalization (

Scheme 4.1), while maintaining nearly the same polymer backbone as un-functionalized

PLA.

Scheme 4.1. Synthesis and functionalization of the statistical copolymer poly[(lactic acid)-co-(2-bromo-3-hydroxypropionic acid)].

4.2 Synthesis of Brominated Polyesters

We were able to synthesize a brominated poly(lactic acid) (PLB) with relatively

4 high PSt-equivalent number-average molecular weight (MnPSt = 2.03 x 10 Da, dispersity

Ð = Mw/Mn = 1.93) by an acid-catalyzed copolyesterification of equimolar amounts of lactic acid and 2-bromo-3-hydroxypropionic acid at 95 °C in vacuo in the presence of a catalytic amount of p-toluenesulfonic acid (pTSA), essentially in bulk (Scheme 4.1), if we added a small amount of high-boiling diphenyl ether. Diphenyl ether lowers the viscosity of the copolymerization system, thereby facilitating the removal of water byproduct, similar to Ajioka’s synthesis of high molecular weight PLA.133 We also

3 synthesized a larger amount (10 g) of lower molecular weight PLB (MnPSt = 5.32 x 10

Da; Ð = 2.64) in order to establish preliminary functionalization conditions. According to 1H NMR analysis, the composition of both copolymers closely matched their 1:1 comonomer feed compositions.

4.3 Comparison of GPCLS to GPCPSt

Figure 4.1a presents the refractive index increments of PLB in THF, and Figure

4.1b presents the percent difference in the absolute molecular weight, determined by GPC with a light scattering detector (GPCLS) using the refractive index increments from Figure

4.1a, and the polystyrene (PSt) equivalent molecular weights, both as a function of the amount of brominated repeat units. The refractive index increments of PLB in THF increase linearly as the concentration of brominated monomer increases. In contrast to

PLA, whose PSt-equivalent molecular weight is higher than its absolute molecular weight,134 the PSt-equivalent molecular weight of PLB is only higher than its absolute molecular weight at very low contents of the brominated monomer. The absolute number-average molecular weight (MnLS) of PLB increases relative to its MnPSt molecular weight with increasing content of the brominated monomer, such that the PSt-equivalent molecular weight of PLB underestimates its absolute molecular weight at all compositions containing greater than ~6 mol% brominated repeat units; i.e., while the

135 MnLS of PLA is only 68% of its MnPSt value, which is consistent with literature values, the MnLS and MnPSt values of PLB are approximately equal when PLB contains approximately 6 mol% brominated repeat units, and MnLS is greater than MnPSt when PLB contains greater than 6 mol% brominated repeat units. Therefore, the molecular weights

of PLB achieved by this acid-catalyzed copolyesterification are significantly higher than those indicated by GPC relative to linear polystyrene, with the 1:1 copolymer having an absolute molecular weight approximately 3.2 times greater than the PSt-equivalent molecular weight.

Figure 4.1. Plots of the refractive index increments (dn/dc) of poly[(lactic acid)-co-(2- bromo-3-hydroxypropionic acid)] (PLB) in THF (A), and the percent difference in the absolute molecular weights (measured by GPC using a light scattering detector in THF) 68

vs. polystyrene-equivalent molecular weights of PLB (B), as a function of the molar composition of brominated repeat units. 4.4 Iodinated Polyester

Figure 4.2. 1H NMR spectra of poly[(lactic acid)-co-(2-bromo-3-hydroxypropionic acid)] 3 (PLB; MnPSt = 5.32 x 10 Da; Ð = 2.64) and functionalized derivatives: iodinated PLA 3 (PLI; MnPSt = 4.62 x 10 Da; Ð = 1.96), azide-functionalized PLA (PLBN3; MnPSt = 4.15 3 x 10 Da; Ð = 2.33), and the “clicked” product of PLBN3 with dimethyl 3 13 acetylenedicarboxylate (MnPSt = 5.05 x 10 Da; Ð = 2.51). Inset: C NMR spectra of PLI (red) and PLB (blue). All of the bromine atoms of PLB are alpha to an electron-withdrawing carbonyl group and are therefore activated toward nucleophlic substitution.136 As outlined in

Scheme 4.1, the bromine atoms can be substituted with soft bases. For example, a

Finkelstein reaction of PLB with a slight excess of sodium iodide in acetone at room temperature for 23 h resulted in essentially quantitative substitution of bromide with iodide; such materials may be useful for medical imaging and radiation therapy applications if the appropriate isotope of iodine is used.137-139 Although the 1H NMR spectra of the brominated and iodinated copolymers are nearly identical (Figure 4.2), the 69

13C NMR spectra in the inset of Figure 4.2 confirm that the CBr resonance of PLB at 39 ppm is replaced by a new CI resonance at 13 ppm for the iodinated PLA (PLI).

A significant challenge for functionalizing PLB by nucleophilic substitution is to identify conditions that minimize degradation of the molecular weight, both by standard cleavage of the high concentration of ester groups in PLA and by competing elimination and cleavage of the 2-bromo-3-hydroxypropionate repeat units. Figure 4.3 demonstrates

3 that the nucleophilic substitution of bromine in PLB (MnPSt = 5.32 x 10 Da, Ð = 2.64)

3 with iodide (PLI MnPSt = 4.62 x 10 Da, Ð = 1.96) results in a 13% decrease in the apparent molecular weight.

Figure 4.3. Comparison of the gel permeation chromatograms of poly[(lactic acid)-co-(2- 3 bromo-3-hydroxypropionic acid)] (PLB; MnPSt = 5.32 x 10 Da; Ð = 2.64) before (blue) and after (red) reaction with sodium iodide to produce iodinated PLA (PLI; MnPSt = 4.62 x 103 Da; Ð = 1.96).

4.5 Substitution with Sodium Azide

While the reaction with sodium iodide works well, displacement of bromine with the less-soft azide is more difficult. In addition, care must be taken when introducing a

high concentration of azide groups because of the potential for explosion.140,141 In this case, the molecular weight can be maintained by using a less-than-equimolar amount of

3 azide. For example, reaction of PLB (MnPSt = 5.32 x 10 Da, Ð = 2.64) with 0.25 equivalents of sodium azide per brominated repeat unit in DMF at 0 °C for 6 h resulted in

3 only a 22% decrease in the molecular weight (MnPSt = 4.15 x 10 Da, Ð = 1.96) (Figure

3.10). The 1H NMR spectra in Figure 4.2 demonstrates that the substituted methine resonance shifts upfield from 4.6 ppm in PLB to 4.3 ppm in the azide repeat units; in the

13C NMR spectra the substituted carbon shifts downfield from 39 ppm in PLB to 60 ppm in the azide repeat units. The composition (47:42:11 LA/Br/N3 units) of the isolated product and integration of the CHBr and CHN3 methine resonances demonstrate that the azide substitution was 88% efficient, with the remaining azide presumably causing elimination and the slight decrease in molecular weight.

4.6 Attempted Azide-Alkyne Cycloaddition using Copper

Azide-functionalized polymers are very useful for introducing additional functionality under mild conditions by Huisgen azide-alkyne 1,3-dipolar cycloadditions,142 which is the most popular “click” reaction. As an example of an azide- alkyne 1,3-dipolar cycloaddition performed under standard126-128,131 copper-catalyzed conditions of a basic ligand with a Cu(I) catalyst, which can also be generated in situ,69,143 we reacted the azide-functionalized PLB with propargyl alcohol using cuprous bromide as the catalyst and N,N,N',N'-pentamethyldiethylenetriamine (PMDETA) as the copper ligand. Most of the copolymer degraded to soluble molecules that did not precipitate in methanol, with recovery of only 20 wt% of a precipitated polymeric

3 product that had a molecular weight (MnPSt = 4.45 x 10 Da, Ð = 2.58), similar to that of

3 the starting azide-functionalized PLA (PLBN3, MnPSt = 4.15 x 10 Da, Ð = 2.33) (Figure

4.5).

1 Figure 4.4. Comparison of the H NMR spectra of azide-functionalized PLA (PLBN3; 3 MnPSt = 4.15 x 10 Da; Ð = 2.33) (green), the microwave-assisted “clicked” product of 3 PLBN3 with dimethylacetylenedicarboxylate (MnPSt = 5.05 x 10 Da; Ð = 2.51) (black), and the attemped CuBr/PMDETA-catalyzed “clicked” product of PLBN3 with propargyl 3 alcohol (MnPSt = 4.45 x 10 Da, Ð = 2.58) (orange). However, as demonstrated by the 1H NMR spectrum in Figure 3.5 this polymer lacks a resonance at ~8 ppm for the [1,2,3]-triazole ring, and the CHN3 resonance at 4.3 ppm has almost disappeared, evidently because of elimination and degradation of the 2- azido-3-hydroxpropionate linkages of the polymer backbone caused by the free

PMDETA ligand.

Figure 4.5. Comparison of the gel permeation chromatograms of azide-functionalized 3 PLA (PLBN3; MnPSt = 4.15 x 10 Da; Ð = 2.33) (green), the microwave-assisted 3 “clicked” product of PLBN3 with dimethylacetylenedicarboxylate (MnPSt = 5.05 x 10 Da; Ð = 2.51) (black), and the attemped CuBr/PMDETA-catalyzed “clicked” product of 3 PLBN3 with propargyl alcohol (MnPSt = 4.45 x 10 Da, Ð = 2.58) (orange), as well as the 3 original PLB (MnPSt = 5.32 x 10 Da; Ð = 2.64) (blue).

4.7 Small Molecule “Click” Reaction We also synthesized methyl 3-acetoxy-2-bromopropionate as a model of the 2- bromo-3-hydroxypropionate repeat units and then tested its stability in the presence of

PMDETA. 1H NMR spectroscopy (Figure 3.6) demonstrated that methyl 3-acetoxy-2- bromopropionate degrades to methyl α-bromoacrylate, evidently because of abstraction of the proton alpha to the carbonyl group by PMDETA, with overall elimination of acetic acid. Although methyl α-bromoacrylate is volatile, its presence was detected in an aliquot of the reaction mixture by a resonance at 3.76 ppm corresponding to the methyl ester, and the vinyl resonances at 6.19 and 6.88 ppm, in agreement with literature spectral data;144 the acetate proton corresponding to acetic acid was detected at 1.97 ppm.

Therefore, neither the 2-bromo- nor the 2-azido-3-hydroxypropionate repeat units of PLB

and/or PLBN3 tolerate basic ligands/reactants, including those used previously for the milder azide-alkyne 1,3-dipolar cycloadditions of azide-functionalized PCL-co-PLA127 and alkyne-functionalized PLA69 that proceeded without polymer degradation. The reaction of the brominated polyesters with bases and nucleophiles is discussed in more detail in Chapter 4.

4.8 Microwave-assisted Azide-Alkyne Cycloaddition

Although the azide groups of the azide-functionalized PLB can not be reacted with alkynes under copper-catalyzed conditions if basic ligands are present, they react readily with activated alkynes, such as dimethyl acetylenedicarboxylate, which do not require a catalyst. Dimethyl acetylenedicarboxylate is activated toward 1,3-dipolar cycloaddition with azides by two electron-withdrawing carboxylate groups.145,146 The reaction was performed under microwave-assisted conditions similar to those used previously with non-activated alkynes that required basic catalysts.127,147 When the azide-

3 functionalized PLB (MnPSt = 4.15 x 10 Da, Ð = 2.33; 47:42:11 LA/Br/N3 units) was reacted with dimethyl acetylenedicarboxylate in acetone in a microwave (power 65 W) at

56 °C for 4 min, including the temperature ramp, 50% of the azide groups were converted to [1,2,3]-triazole rings. The 1H NMR spectrum of the product of this reaction (Figure

4.2) confirms that the CHN3 resonance at 4.3 ppm has decreased in intensity, and the new methoxy groups resonate at 3.9 ppm. The formation of the [1,2,3]-triazole ring is confirmed by 13C NMR spectroscopy with a resonance at 127.9 ppm; the methyl ester carbons resonate at 53 ppm. In addition, the molecular weight of the product increased

3 22% to MnPSt = 5.05 x 10 Da (Ð = 2.51) (Figure 4.5), as expected for conversion of the azide groups to 4,5-dimethyl-substituted [1,2,3]-triazole rings.

4.9 Conclusion

In summary, brominated poly(lactic acid) (PLB) can be synthesized in relatively

4 high molecular weight (MnPSt = 10 Da; absolute Mn ~ 3.2 times higher for 50 mol% bromination) by an acid-catalyzed, melt copolyesterification of lactic acid and its brominated constitutional isomer, 2-bromo-3-hydroxypropionic acid, if a minimal amount of diphenyl ether is added to reduce the viscosity of the polymerization system, which also aids removal of the water byproduct. In contrast to PLA, the absolute molecular weight of PLB is higher than the polystyrene-equivalent molecular weight when the copolymer contains >6 mol% brominated repeat units. Since the bromine atoms are alpha to the carbonyl groups along the polymer backbone, these PLBs can be further functionalized by nucleophilic substitution with nucleophiles that are soft bases, such as iodide to produce iodine-labeled PLA (eg. for use in medical imaging and radiation therapy using specific isotopes of iodine), and by azide if it is not used in excess. The azide-functionalized PLAs can be reacted with alkynes in a 1,3-dipolar azide-alkyne cycloaddition, which can provide additional functionality. For example, the azide-functionalized PLA undergoes a microwave-assisted “click” reaction with dimethyl acetylenedicarboxylate, which is an activated alkyne. However, it is degraded under copper-catalyzed conditions with less active alkynes because of competing elimination caused by the amine-based copper ligands.

CHAPTER V

AZIDE EXPLORATORY FUNCTIONALIZATION OF PLA BASED POLYESTERS

5.1 Introduction

Our group has previously reported co-polymerizing lactic acid with 2-bromo-3- hydroxy propionic acid to make PLA-based polyesters with a site for post- polymerization functionalization from the backbone. One drawback to our system is the propensity for elimination to occur when attempting to functionalize the polymer with various nucleophiles. Previously we limited this problem by limiting the extent of reaction to just 25% of the bromine atoms in the backbone. In this work we will present a mechanistic study of what occurs during the nucleophilic substitution using a model compound, highlight the mechanisms at work, and present a route for functionalization that prevents elimination while allowing for compete substitution.

Figure 5.1. General scheme of how a CSR works, with the dashed portions of the arch representing the different electronic environments of the CSR, showing the chiral proton (blue) in the two different electronic environments.

Since our halide is a secondary halide it can undergo both SN1 and SN2 substitution. One-way to determine the mechanism is to start with a stereopure compound and determine the stereochemistry of the resulting product. Since isomers can have similar chemical shifts a chiral shift reagents (CSR) can be added to help resolve the two isomers. Briefly these CSR work by forming a complex with the chiral molecules that put the chiral proton into two different magnetic environments (see Figure 5.1), thus affecting their chemical shift.147

5.2 Results and Discussion

As a model for the substitution reaction on the brominated polymer, methyl-2- bromo-3-acetoxypropionic acid was synthesized as reported previously in Chapter 3. This compound was reacted with one equivalent of sodium azide to yield methyl-2-azido-3-

acetoxypropionic acid in 80% percent yield with 100% substitution. However, when the reaction was carried out on the brominated co-polymer under identical conditions, there was a large reduction in Mn from 9,700 DA to 800 DA (see Figure 5.2). The mechanism for nucleophilic substitution was investigated using sodium azide as a representative nucleophile on both the model compound and the brominated polyester.

0.8"

0.6"

0.4" Normalized+Intensity+(RI)+

0.2"

0" 0" 10" 20" 30" 40" 50" 60" Time+

Figure 5.2. Comparison of the gel permeation chromatograms of poly[(lactic acid)-co-(2- 3 bromo-3-hydroxypropionic acid)] (PLB; Mn, PSt = 6.00 x 10 DA) before (blue) and after (light blue) reaction with sodium azide to produce azide functionalized PLA (PLN3; Mn, 3 PSt = 1.20 x 10 DA).

5.3 Determination of Mechanism

The bromine in our polyester is a secondary halide that can undergo nucleophilic substitution by either the Sn1 or Sn2 mechanism, or a combination of both. We can synthesize the stereopure (S) isomer of our model compound. Starting with (L) serine, the diazatation and subsequent displacement with bromine are both stereopure reactions with the amine undergoing a cyclic intermediate resulting in a double inversion (net retention)

at the stereocenter. The esterification and aceylation both take place away from the chiral center and should have no effect on the chiral center.

Figure 5.3. Chiral shift reagent Europium tris[3-(heptafluoropropylhydroxymethylene)-(- )-camphorate In order to determine the ratio of the two isomers, Europium tris[3-

(heptafluoropropylhydroxymethylene)-(-)-camphorate(Figure 5.3) which is a CSR, was added to both the model compound, and the azide derivative. The ratio of S to R isomers in the starting model compound was found to be 80:20 when it should be 100% S based on the chemistry chosen.

Figure 5.4. Stacked proton NMR spectra of model compound with CSR added. Top (black): (R) model compound with 1.5 eq CSR. Bottom (blue): Azide substituted model compound with 1.5 eq CSR. Insert: Methyl ester resonance showing the splitting from different isomers. The appearance of two stereoisomers indicates that there is some Sn1 character of our secondary halide, in which the bromine leaves generating a carbocation, that the bromine can either attack from the Ri or the Si face, to give the R and S enantiomer respectively. Reacting the (R) model compound with sodium azide and adding a chiral shift agent (CSR), we see the splitting of the acetoxy resonance and the methylester resonance (see insert Figure 5.4). The splitting of the methoxy and the acetate protons are roughly 2:1, although the resonances are not resolved to baseline. There are three possible products: the (R) isomer from the Sn2 mechanism, the (R) product from the Sn1 mechanism and the (S) isomer from the Sn1 mechanism. Since the methyl ester and

acetate resonances are both split in the presence of the CSR we can conclude that both the

Sn1 and Sn2 mechanisms are occurring simultaneously. From the relative ratio of the two isomers 2:1 we can conclude that both mechanisms are occurring in equal parts, though we cannot assign which isomer is which.

5.3.1 E2 Elimination

Figure 5.5. Crude aliquots of the model compound after reacting with various eq NaN3 0.5 (blue), 0.88 (red), 1.0 (green), and 1.6 (black). Since the model compound undergoes both Sn1 and Sn2 substitution mechanisms, that means that it can also undergo the corresponding E1 or E2 elimination reactions, with the E2 pathway resulting in chain cleavage in the polymer system. As reported in our previous work, (Chapter 3) if the reaction were to undergo E2, the leaving group would be an acetate, which is a good leaving group since it is resonance-stabilized with

the negative charge distributed over two electronegative oxygen atoms. In order to determine if sodium azide acts as a nucleophile (Sn1 or 2) or as a base (E1 or E2), the model compound was reacted with various equivalents of sodium azide (Figure 5.5).

Crude aliquots of the reaction were taken because of the volatility of the resulting methyl

2-azidoacrylate that is formed.148 As shown in Figure 5.5, when 0. 5 equivalents of azide are used relative to the bromide, there is 100% substitution with no detectable elimination. However, above 0.9 equivalents of sodium azide, two doublets appear at 5.4 and 5.8 ppm corresponding to the vinyl protons of the methyl 2-azidoacrylate.148 Above 1 equivalence of sodium azide the amount of elimination remains 3%.

Table 5.1. Various halogenated polyesters reacting with sodium azide. - means that the integration was not resolved enough to determine the degree of substitution.

63 Halide T((˚C) Eq(NaN3 Mn(*10 (Da Đ Mn(decrease((%) Subsitution((%) Yield((%) 0.25 4.5 2.64 25 36 89 0.50 2.7 1.89 55 60 78 23 0.75 1.6 1.37 74 6 57 1.00 1.2 1.24 80 76 57 Br 0.25 6.0 2.96 0 32 24 0.50 3.5 2.24 52 54 36 0 0.75 2.1 1.75 75 60 21 1.00 1.2 1.25 80 12 16 0.25 3.1 2.01 28 26 47 0.50 2.7 1.83 37 40 69 23 0.75 2.0 1.65 53 6 74 1.00 2.5 2.12 42 6 76 I 0.25 3.2 2.87 26 10 67 0.50 2.3 1.94 47 14 71 0 0.75 2.2 1.71 49 6 64 1.00 1.3 1.26 70 6 48

5.3.2 Brominated Polyester at Room Temperature

The brominated polyester was reacted with various equivalents of sodium azide at room temperature (see Table 5.1). Even at 0.25 equivalents of sodium azide, only 36% of the azide displaces the bromine with a 25% decrease in Mn. As the amount of sodium 82

azide is increased, however, there is a corresponding increase in the loss of Mn from 25% to 80%. Similarly the yield of the precipitated product also decreases with an increase in the amount of azide present, as more of the polymer is cleaved to soluble oligomers.

5.3.3 Brominated Polyester at 0 ˚C

The reaction was carried out at 0 ˚C (see Table 5.1) to limit the amount of sodium azide in solution. Aliquot 336 is added as a phase transfer catalyst to help with the solubility of sodium azide, as well as a way in control the amount of sodium azide in solution. When 0.25 equivalents of azide are added, the degree of substitution is 32%, a

4% decrease from the RT reaction. There is, however, no change in Mn but the peak molecular weight shifts from 15,800 Da to 13,700 Da. When the amount of sodium azide is increased to 0.5 equivalents per bromine, there is 54% substitution with a 52% decrease in Mn, similar to the reaction being carried out at RT. Reactions with 0.75 and

1.0 equivalents show no difference between RT and 0 ˚C.

5.3.4 Iodated Polyester at Room Temperature

The larger size of the iodide atom as compared to the bromine atom, makes iodide a better leaving group. The reaction with sodium azide was carried out using the iodated polyester. When 0.25 equivalents of sodium azide are added (see Table 5.1), the substitution is only 26% efficient with a 28% decrease in Mn. When 1.0 equivalents are added, the resonances in the 1H spectrum merge and make it difficult to determine the degree of substitution, even though there is a 70% decrease in the Mn. On comparing the brominated to the iodated polyester at room temperature, the degree of substitution slightly favors the bromide as opposed to the iodide. However, when comparing the 83

decrease in Mn, the iodated polyester undergoes less elimination than the brominated polyeseter. One possible explanation for why the degree of substitution is lower for the iodated polyester but shows less elimination is the E1 elimination. Unlike E2 elimination which results in chain cleavage, E1 elimination would result in unsaturation along the polymer backbone, preserving the molecular weight.

5.3.5 Iodated Polyester at 0 ˚C

When the substitution reaction was carried out at 0 ˚C (see Table 5.1) for 0.25 equivalents, there was only 10% substitution with a 26% decrease in Mn. At 1.0 equivalents, the resonances again merge to make it difficult to estimate the degree of substitution but there is a large drop (70%) in Mn. At lower temperatures the iodated polyester shows a decrease in substitution to only 10% versus the 26% obtained at room temperature, with a larger decrease in Mn. The low temperatures may suppress the nucleophilic substitution reaction while the chain ends can continue to act as a base causing elimination.

5.3.6 Reason for Elimination

Figure 5.6. Acid-base equilibrium.

While sodium azide itself can act as a base and cause elimination, in the model compound it only caused ~3% elimination, a result which does not explain the large 84

decrease in molecular weight under identical conditions for the polyester. Our model compound does not account for the chain ends present in our polyester, which are an alcohol and a carboxcylic acid. The carboxcylic acid chain end is in equilibrium with sodium azide to produce the conjugate base (the carboxylate) and the conjugate acid

(hydroazidic acid) (Figure 5.6). The resulting carboxylate chain end can act as a base, abstracting the alpha proton via the E2 mechanism to cause chain scission. The amount of carboxylate present in the reaction would depend on the pKa of the chain ends. There are two possible acidic chain ends, either the lactic acid chain end or the 2-bromo-3-hydroxy propionic chain ends.

5.3.7 Determination of pKa

Figure 5.7. Titration of lactic acid with KOH as a control experiment

In order to determine the pKa of our chain ends, the corresponding acids (BrH and

LA) were titrated with NaOH. A third order polynomial equation was fit to the buffer region (from start to just before the equivalence point). The first derivative of this equation was taken and the minimum value was found which corresponds to the inflection point or the ½ equivalence point in which the pH is equal to the pKa. This value is then substituted into the third order polynomial to determine the pKa of the acid. As a control and to verify our procedure, we determined the pKa of lactic acid (Figure 5.7) to be 3.68, a value which is in good agreement with literature (3.85).149

O O O K OH HO OH HO O 10.000# K OH HO O K H2O K KBr Br Br OH

8.000#

6.000# pH#

4.000#

2.000#

0.000# 0# 2# 4# 6# 8# 10# 12# NaOH#(mL)#

Figure 5.8. Titration of 2-bromo-3-hydroxypropionic acid with NaOH done in triplicate. The black box highlights the buffer region and the red box is from the presumed reaction shown in red in which the second equivalence of hydroxide acts as a nucleophile to displace the bromine.

When 2-bromo-3-hydroxypropionic acid was titrated with NaOH the pKa (Figure

5.8) was determined to be 2.12, a value indicating it is a stronger acid than lactic acid. It should be noted that above the equivalence point (pH = 7) the pH value did not increase to 12 as expected, but only went to 9. This result is presumably caused by the second equivalence of NaOH acting as a nucleophile and displacing the bromide to give 2,3- dihydroxy propionate (shown in red in Figure 5.8).

5.3.8 Model Compound with Various Acids

Elimina9on$ 3%$ H2C=CN3$

3%$

>0.5%$ No$Acid$

0%$ 10%$GA$

10%$BrH$

10%$TFA$

Figure 5.9. Crude aliquots of the reaction of model compound with one equivalence of sodium azide. in the presence of various acids. Insert: Enhancement of the vinylic region showing the presence of elimination. In order to mimic our polyester system more effectively the model compound was stirred with one equivalent of sodium azide but with the addition of 10 mol% of various acids (see Figure 5.9). Glycolic acid (pKa: 4.78) was chosen since its pKa is above that hydroazidic acid, and triflic acid (pKa: 0.23) was chosen as an example of a strong acid.

When no external acid is present, there is 3% elimination that occurs. When 10% glycolic acid is added, the elimination remains unchanged at 3%. When 10% of 2-bromo-3- hydroxypropionic acid was added to the reaction, the degree of elimination was reduced to less than 0.5% (via 1H NMR). When 10% trifilic acid was stirred with the model compound, the elimination reaction was completely suppressed.

The equilibrium in the presence of glycolic acid favors sodium azide remaining deprotonated and thus without an affect on the sodium azide the elimination that is seen is most likely entirely caused by sodium azide. When 2-bromo-3-hydroxypropionic acid is present, the equilibrium shifts to favor the azide being protonated. The resulting carboxylate is less active as a base since it is a stronger acid (lower pKa), so it does not cause elimination as readily since the negative charge is better stabilized. When trifilic acid is present, the resulting triflate is unreactive as a base since the negative charge is stabilized. In our polymer, triflic acid should be strong enough to shift the equilibrium to favor protonating both the lactic and bromo chain ends, thereby limiting the degree of elimination that occurs by preventing the formation of basic carboxylates at the chain ends. From the small molecule work, we can conclude that the lactic acid chain end is primarily responsible for the elimination in our polymer.

5.3.9 Chain End Suppression

0.8"

0.6"

RI#Response#(AU)# 0.4"

0.2"

0" 0" 10" 20" 30" 40" 50" 60" Time#

Figure 5.10. Stacked GPC traces of the starting brominated polyester (blue) (Mn, PSt = 9.7 3 2 x 10 DA) with no TFA (red) (Mn, PSt = 8.0 x 10 DA), 10% TFA (green) (Mn, PSt = 1.0 x 3 3 10 DA), 1% TFA (purple) (Mn, PSt = 1.8 x 10 DA), 0.1% TFA (light blue) (Mn, PSt = 1.8 x 103 DA). We attempted to replicate the small molecule work with the brominated polyester

(Figure 5.10). By adding TFA, we are attempting to suppress the lactic acid chain end in the polyester. Based on the work with the model compound, we thought that we could prevent elimination by adding TFA when attempting nucleophilic substitution. When

10% TFA was added to the polyester, there was still a large decrease in molecular weight, but a slight improvement from Mn: 800 Da to Mn: 1,000 DA. The concentration of acid was reduced to 0.1% with further improvement in the molecular weight to Mn:

1,800 DA. When the amount of TFA was reduced to 0.001%, there was no further improvement in Mn. While the addition of TFA shifts the equilibrium to favor the

protination of the chain ends, there presumably are some carboxylates that are formed that still cause elimination.

5.3.10 Eliminating the Chain Ends

O - HCl H O O O HO R H O O O O S O S N O Cl S N H HCl Cl N O O R R Nuc N H S O Nuc

Scheme 5.1. Formation of the thionyl chloride/DMF adduct and reaction with a carboxcylic acid.

As reported by Palomo's group DMF will react with thionyl chloride to form a very electrophilic molecule that a neutral carboxcylic acid will attack (See Scheme 5.1).150 A nucleophile will then attack the carbonyl of the acid to form a new bond with the formation of SO2 and regeneration of DMF. In addition, adding the DMF/SOCl2 adduct to the polymer system will increase the polarity of the system to favor the Sn1 mechanism.

Figure 5.11. gHSQC of poly[(lactic acid)-co-(3-hydroxy-2-azidopropionic acid)] (MnLS: 3.39 x 104, Đ =1.73) showing the overlap of the methyene and methylene of the azide repeat unit from 4.4-4.7 ppm.

After the DMF/thionyl chloride adduct formed, it was added to a cooled solution of brominated polyester in DMF and allowed to react for five minutes sodium azide was then added all at once. Care should be taken since the reformation of DMF and the

1 release of SO2 are both exothermic reactions. After addition of sodium azide, the H spectrum remained unchanged, while the 13C showed the disappearance of the CHBr

(39.9 ppm) and the appearance of the CHN3 (53.0 ppm). There was a slight upfield shift

in the 13C spectra from 60.0 ppm in the model compound to 53.0 ppm in the fully substituted polymer. gHSQC (Figure 5.11) showed that there are resonances from 4.4-4.7 ppm. They do not correspond to CHBr (39.0 ppm) but rather to the new CHN3 at 52.8 ppm. It is important to note there is no signal at 39.0 ppm, a result indicating full substitution.

Figure 5.12. Stacked GPCLS traces of the starting brominated polyester (blue) (Mn,LS = 2.34 x 104 DA, Đ =1.97) (blue) and poly[(lactic acid)-co-(3-hydroxy-2-azidopropionic 4 acid)] (MnLS: 3.39 x 10 , Đ =1.73) (red).

GPCLS of the azide-containing polyester shows a 10,200 Da increase in Mn from

23,400 Da to 33,900 DA and a narrowing of the polydispersity from 1.95 to 1.73, both results indicating that there is chain coupling. The coupling can come from the alcohol chain end first reacting with the thionyl chloride/DMF adduct and then a carboxcylic acid chain end can attack the methylene adjacent to the chain end to regenerate DMF and SO2.

Since the ratio of LA to BrH is 50:50 (or 1:1), we can combine the weight of one unit of LA (72 g/mol) and the weight of the BrH (150.97 g/mol) into one repeat unit with

a weight of 222.97 g/mol. Dividing the Mn of the brominated polyester 23,400 Da by

222.97 gives an Xn of 105. Thus our starting polymer has 105 combined monomer units.

After substitution, the new combined repeat unit weighs 185.07 g/mol, which gives

Mntheo of 19,400 Da (185*105). The actual Mn is 33,900 Da which gives an Xn of 183, almost double starting material. The idea of chain coupling is further supported by the decrease in the Đ.

5.4 Conclusions

Our brominated polyesters are susceptible to elimination in the presence of basic compounds. In order to determine the cause of the elimination, reactions were carried out on a model compound that showed that nucleophilic substitution occurs by both the Sn1 and Sn2 mechanisms. Furthermore while sodium azide will cause a trace amount of elimination on its own, it is the acid chain ends (specifically the free carboxylic acid of lactic acid) that causes elimination. Our initial attempt to suppress the chain ends by the addition of a strong acid resulted in minor improvements in the Mn. By eliminating the chain ends, we were able to achieve full substitution with a doubling of the molecular weight via a simultaneous chain-coupling reaction.

CHAPTER VI

RADICAL GRAFTING OF PLA BASED POLYESTERS

6.1 Introduction

The overall goal of this project is to synthesize silicon-containing graft copolymers from our brominated polyester. Studies have shown that silicon is important for bone formation, and the goal of this project is to study how having silicon present will affect bone growth. A former student in our group, Xiang Yan, originally started this project in collaboration with Dr. William Landis and another graduate student in his group, Qing Yu.

6.2 Grafting using Cu(I)Cl and PMDETA

O O Br O 1 O O 1 Cu(I)Cl / 1 PMDETA O HO H 200 m n 90˚C N2 Toluene 3h O O O O O HO H m n O

Scheme 6.1. Attempted grafting using the brominated copolymer with PMDETA as the ligand.

Figure 6.1. Crude aliquot of the PLB-g-PMMA using PMDETA showing 20% conversion. Previous work in our group151 attempted to make PLB-g-PMMA using PMDETA as a ligand (Scheme 6.1). As mentioned in Chapter 3, free PMDETA will act as a base and cause elimination. Despite the complexation of PMDETA to the cuprous halides, the ligand can still act as a base and cause elimination along the polyester backbone. At high conversions of PMMA it is very difficult to see the polyester backbone via 1H NMR spectroscopy. To determine if the ligand was acting as a base, the grafting reaction (using

PMDETA) using the macroiniator was stopped after 3 h (20% conversion of MMA via

1H NMR)(Figure 6.1). 96

Figure 6.2. 1H spectrum of the precipitated graft copolymer showing the methyene of lactic acid at 5.16 ppm.

0.8

0.6 RI Response (A.U) 0.4

0.2

0 -5 5 15 25 35 45 55 Time

Figure 6.3. Staked GPCPSt traces of the macroinitiator (blue) (Mn: 13,400 Da), and the attempted grafting using PMDETA (red) stopped at 20% conversion (Mn: 19,200 Da).

Figure 6.4. Staked GPCLS traces of the macroinitiator (red) (Mn: 35,800 Da, Mp: 47,000 DA), and the attempted grafting using PMDETA (blue) stopped at 20% conversion (Mn: 36,000 Da, Mp: 40,900 DA). 1H spectroscopy showed a small amount of the methyne of lactic acid (Figure

6.2). While GPCPSt showed a 6K DA increase in the Mn (Figure 6.3) the trace was narrower and fell underneath the starting polyester. As reported in our previous work

(See Chapter 3), GPCPSt underestimates the actual molecular weight of our polyesters, and so light scattering is needed to determine the actual molecular weight. GPCLS (Figure

6.4) indicated that at 20% conversion the Mn of the macroinitiator (35.8 KDa) was the same as the graft co-polymer (36.0 KDa). However the Mp of the macroinitiator was 47.0

KDa while the graft co-polymer showed a lower Mp of 40.9 kDa. This result indicates that PMDETA is still acting as a base and causing elimination. In order to prevent the

elimination from occurring, the ligand was initially changed to 2,2'-bypyridine (Bipy), a much less basic ligand than PMDETA. Bipy when complexed to cuprous halides is not completely soluble in organic solvents, and to avoid the inhomogeneity 4,4'-diheptyl-2,2'- bypyridine (HB) was synthesized, HB remains soluble even when complexed to cuprous halides. Kinetic studies were carried out on both the model compound as well as the macroinitiator.

6.3 Kinetic Study using HB and the Model Compound O O O O 2 Cu(I)Cl / 2 HB O 200 1 O O O O 90˚C N2 Toluene 24h HO H O O m n Br O

Scheme 6.2. Establishing grafting kinetics using the model compound as an initiator with MMA.

100# 4.0# 3.5# 80# 3.0#

60# 2.5# /[M])* 2.0# 0 40# 1.5# Ln([M] %*Conversion* 1.0# 20# 0.5# 0# 0.0# 0# 5# 10# 15# 20# 25# Time*(h)*

Figure 6.5 Conversion and first order monomer conversion in the atom transfer radical polymerizations of methyl methacrylate in toluene at 90 °C using CuCl, as the catalyst and 2,2'-diheptyl-4,4'-bypyridine as the ligand; [MMA]:[I]:[CuCl]:[Heptyl Bipy] = 200:1:1:1. 100

Figure 6.5 plots the conversion and first-order monomer conversions as a function of time for the ATRP polymerization of 200 equivalents of MMA initiated by methyl 3- acetoxy-2-bromopropionate in toluene at 90 ˚C using CuCl and HB (Scheme 6.2). Each monomer conversion was determined by 1H NMR spectroscopy by comparing the integrals of the vinyl resonances of the monomer (5.62 and 6.22 ppm) and the methyl resonances of the polymer (1.7 to 1.0 ppm). There is an induction period (~2h) when HB is used. After the induction period the first-order monomer conversion is linear with time, a result indicating that there is no termination occurring.

97%#Mn=17.9#k,#Đ=1.35#

91%#Mn=16.5#k,#Đ=1.36#

70%#Mn=14.2#k,#Đ=1.34#

28%#Mn=8.0#k,#Đ=1.32#

7%#Mn=2.3#k,#Đ=1.25#

20# 22# 24# 26# 28# 30# 32# 34# 36# Elu$on'Volume'(mL)'

Figure 6.6. Gel permeation chromatograms and the corresponding monomer conversions of aliquots taken from the atom transfer radical polymerizations of methyl methacrylate presented in Figure 6.5 using CuCl, as the catalyst and 2,2'-diheptyl-4,4'-bypyridine as the ligand in toluene at 90 °C; [MMA]:[I]:[CuX]:[Heptyl bipy] = 200:1:1:1.

101

Figure 6.6 presents the corresponding GPCPSt chromatograms of five representative aliquots using HB as the catalyst. Figure 6.7 plots the number-average molecular weights and polydispersity of PMMA as a function of time for the systems presented in Figure 6.5. The upward curve of the Mn vs percent conversion graph shown in Figure 6.7 indicates that there is some chain transfer occurring.

18.0 2.00 16.0 1.90 14.0 1.80 1.70 12.0

-4 1.60 10.0

1.50 Đ X 10 n 8.0

M 1.40 6.0 1.30 4.0 1.20 2.0 1.10 0.0 1.00 0 20 40 60 80 100 Conversion (%)

Figure 6.7. Number-average molecular weights (Mn) and polydispersity (Đ) corresponding to the atom transfer radical polymerizations of methyl methacrylate presented in Figure B using CuCl, as the catalyst and 2,2'-diheptyl-4,4'-bypyridine as the ligand in toluene at 90; [MMA]:[I]:[CuX]:[PMDETA] = 200:1:1:1.

6.4 Graft Copolymers using Heptyl Bipyridine and Cu(I)Cl

O O O Br 2 Cu(I)Cl / 2 HB O 1 O O 200 HO H O O m n 90˚C N2 Toluene 66h HO H O O O m n O

Scheme 6.3. Establishing grafting kinetics using the model compound as an initiator with MMA.

102

100%# 3.0# 90%# 2.5# 80%# 70%# 2.0# 60%# /[M])* 50%# 1.5# 0 40%# Conversion* 1.0# Ln([M] 30%# 20%# 0.5# 10%# 0%# 0.0# 0# 10# 20# 30# 40# 50# 60# 70# Time*(h)*

Figure 6.8. Conversion and first order monomer conversion in the atom transfer radical grafting of methyl methacrylate in toluene at 90 °C using CuCl, as the catalyst and 2,2'- diheptyl-4,4'-bypyridine as the ligand; from the brominated copolymer [MMA]:[I]:[CuCl]:[Heptyl bipy] = 200:1:2:2.

Figure 6.8 plots the conversion and first-order monomer conversions as a function of time for the ATRP polymerization of 200 equivalents of MMA initiated by the brominated copolymer in toluene at 90 ˚C using CuCl and HB (Scheme 6.3). Each monomer conversion was determined by 1H NMR spectroscopy by comparing the integrals of the vinyl resonances of the monomer (5.62 and 6.22 ppm) and the methylester resonances of the polymer (3.43 to 3.72 ppm). There is an induction period

(~10 h) when HB is used, which is longer than with the model compound but in agreement with what was observed when PMDETA was used as the ligand.151 As with the model compound, after the induction period the first-order monomer conversion is linear with time, a result indicating that there is no termination occurring (Figure 6.8).

103

6.4.1 Chain Transfer using GPCPSt

2.50 300

250

2.00 200

PSt nPSt Đ X 150

100 1.50

0 1.00 0% 20% 40% 60% 80% 100% Conversion

Figure 6.9. Number-average molecular weights (Mn) and polydispersity (Đ) determined by GPCPSt corresponding to the atom transfer radical polymerizations of methyl methacrylate presented in Figure F using CuCl, as the catalyst and 2,2'-diheptyl-4,4'- bypyridine as the ligand in toluene at 90; [MMA]:[I]:[CuX]:[PMDETA] = 200:1:1:1. To determine chain transfer of the graft copolymers, the number of PMMA units

(Xn) added was plotted versus percent conversion. To determine Xn of the graft, the Mn of the starting polyester was subtracted from the graft copolymer and then divided by the weight of the MMA repeat unit (100). Similar to the model compound there is a slight upward curve of the XnPMMA vs conversion graph (Figure 6.9) a result indicating that there is chain transfer occurring. The XnPMMA at 12 h is -8 a value indicating that even with HB there may still be some elimination that is occurring. Again GPCPSt does give us the absolute molecular weight so GPCLS is needed.

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6.4.2 Chain Transfer using GPCLS

300

250 1.75

200

150 1.25

PMMA 100 Đ Xn 50 0.75 0 0 10 20 30 40 50 60 70 -50

-100 0.25 Time (h)

Figure 6.10. Number-average molecular weights (Mn) and polydispersity (Đ) determined by GPCLS corresponding to the atom transfer radical polymerizations of methyl methacrylate presented in Figure E using CuCl, as the catalyst and 2,2'-diheptyl-4,4'- bypyridine as the ligand in toluene at 90; [MMA]:[I]:[CuX]:[PMDETA] = 200:1:1:1.

Because of the sensitivity of the GPCLS to sample concentration, the aliquots that were taken from each reaction were not concentrated enough to give a reliable signal.

Instead the reaction was run 4 times and quenched at the appropriate time interval, worked up and then run on the GPCLS. To determine the number of MMA units that are being added, the molecular weight the starting brominated polyester is subtracted from each sample and then divided by 100 (weight of the MMA repeat unit). When plotted against time (Figure 6.10),the first data point is -81 units of MMA. This value indicates that there is still elimination occurring even with using the less basic ligand. However, the downward slope of the last three data points in the graph indicate that even though there is elimination that occurs throughout the grafting process, there is still graft

105

copolymer present after 66 h. While this result is an improvement over the PMDETA system, there are still ways to attempt to improve the conditions.

6.5 Graft Copolymers using Heptyl Bipyridine and Cu(I)Br

O O O Br 2 Cu(I)Br / 2 HB O 1 O O 200 HO H O O m n 90˚C N2 Toluene 48h HO H O O O m n O

Scheme 6.4. Grafting of MMA using the brominated copolymer with two equivalents of

Cu(I)Br and HB.

4 Figure 6.11. GPCLS of the graft copolymer (Mn 4.30 x 10 ) stopped at 48 h (61% conversion) and precipitated 3x [MMA]:[I]:[CuBr]:[HB] = 200:1:2:2. Since the ligand is still causing elimination, we then switched the metal from

Cu(I)Cl to Cu(I)Br (Scheme 6.4). While presumably this change will give us less control over the graft polydispersity, it will increase the rate of polymerization (Rp) and hopefully increase the rate to make it faster than the rate of elmination (Re). The reaction was

106

stopped after 48 h (61% conversion). GPCLS (see Figure 6.11) gave an Mn,LS (see was

4.30 x 104 with 198 units of PMMA added.

6.6 Graft Copolymers using Bipyridine and Cu(I)Br

O O O Br 1 Cu(I)Br / 1 Bipy O 1 O O 200 HO H O O m n 90˚C N2 Toluene 48h HO H O O O m n O

Scheme 6.5. Grafting of MMA using the brominated copolymer with one equivalent of

Cu(I)Br and Bipy.

While initially not wanting to use Bipy because it is inhomogeneous and noting that HB was causing elimination, we decided to try Bipy, with the idea that having less ligand in solution would also help to prevent elimination. This route also has the trade off of possibly sacrificing control of the grafting (Scheme 6.5). In attempting to keep the reaction conditions the same as with HB, it was discovered that with two equivalent of

Cu(I)Br and Bipy, the reaction became a gel after only 3h. The amount of copper was reduced to one equivalent and after 48h the conversion was 41%. As expected, this conversion is lower than when HB is used since bipy is less active as a ligand compared to HB.

107

Figure 6.12. 1H NMR spectrum of MMA grafted from the brominated polyester stopped after 48 h, and precipitated 3x [MMA]:[I]:[CuBr]:[Bipy] = 200:1:1:1.

The proton spectra (Figure 6.12) showed the methine of the lacitc acid in the backbone at

4 5.15 ppm. GPCLS (Figure 6.13) showed an increase in the Mn from 2.32 x10 to 8.46 x104. This corresponds to 614 units of PMMA added which it about 53% of the total

11,440 unit of PMMA that should be added at 100% conversion.

108

4 Figure 6.13. Stacked GPCLS of the starting brominated repeat unit (red) (Mn 2.34 x 10 ), 4 and the graft-copolymer (blue) (Mn 8.46 x 10 ) stopped at 40% conversion and precipitated 3x [MMA]:[I]:[CuBr]:[Bipy] = 200:1:1:1.

6.7 Graft Copolymers using Bipyridine and Cu(I)Cl

As a final trial the metal was switched to Cu(I)Cl here, the idea being tested is that, while slower than the Cu(I)Br Bipy system the use of Cu(I)Cl may give more control over the graft. However, after stopping the reaction at 48 h the conversion was only 10% (see Figure 6.14). Presumably this result is due to Cu(I)Cl being less active than Cu(I)Br. Because of the long reaction times this route will not be further investigated, although the next step would be to try with two equivalents of Cu(I)Cl per bromine.

109

Figure 6.14. Crude 1H NMR spectrum of 1H NMR spectrum of MMA grafted from the brominated polyester stopped after 48 hours [MMA]:[I]:[CuCl]:[Bipy] = 200:1:1:1.

110

CHAPTER VII

CONCLUDING THOUGHTS

In this thesis we have established that the mechanism of nucleophilic substitution occurs by both the SN1 and SN2 mechanisms simultaneously. While attempting nucleophilic substitution with sodium azide there was a large decrease in the molecular weight, an unexpected result based on the small molecule work. It was determined that the chain ends, specifically lactic acid, were causing elimination. Conditions were then established that allowed substitution without elimination.

In addition to the work done on nucleophilic substitution radical grafting was investigated. Issues with elimination were discovered with our previous conditions for grafting. After changing ligands to HB we were able to make graft-copolymers but still saw elimination. The grafting conditions were then optimized by using Bipy and Cu(I)Br to give graft-copolymers with no apparent elimination.

7.1 Future Directions

With conditions established that will allow for complete substitution of the bromine atom with sodium azide, the "click" reaction can now be investigated. Routes for further improvement can involve optimization of microwave parameters such as power and time of reaction. Based on Chapter 6, the copper-catalyzed azide-alkyne click reaction can be tried using Cu/Bipy or non-amine-based ligands can be

111

investigated. Various alkynes can be used and analyzed to determine affects on the properties of this PLB system.

With conditions established for radical grafting from the brominated polyester without backbone degradation, silicon-containing monomers can be incorporated along with MMA to study the affects of silicon on bone growth; a current project in collaboration with Dr. Landis' group. In addition to radical grafting of monomers, radical traps such as TEMPO can be used to quench the radical. By functionalizing a radical trap with PEG, this polymer can be used for drug delivery by encapsulating the drug in a polymer micelle. Being able to make graft copolymers offers a way to increase the amount of PEG added that traditional block copolymers cannot. In addition, the radical trap can be functionalized with the drug directly allowing for covalent attachment of the drug to the backbone of the polymer.

112

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