Synthesis of Functionalized Poly(lactic acid) Using 2-Bromo-3

Mar 7, 2016 - Christopher M. Plummer , Houbo Zhou , Wen Zhu , Huahua Huang , Lixin ... Houbo Zhou , Yi Chen , Christopher M. Plummer , Huahua Huang ...
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Synthesis of Functionalized Poly(lactic acid) Using 2‑Bromo-3hydroxypropionic Acid Colin Wright, Abhishek Banerjee, Xiang Yan, William K. Storms-Miller, and Coleen Pugh* Department of Polymer Science, The University of Akron, Akron, Ohio 44325-3909, United States S Supporting Information *

ABSTRACT: Brominated poly(lactic acid) (PLB) and brominated poly(lactic acid-co-glycolic acid) (PLGB) were synthesized by acid-catalyzed melt copolyesterifications of glycolic acid and/or lactic acid with 2-bromo-3-hydroxypropionic acid, which is a brominated constitutional isomer of lactic acid. Molecular weights up to Mn = 104 Da were achieved by adding diphenyl ether to the copolymerizations (100% w/v comonomer concentration) performed at 90−130 °C in vacuo. GPCPSt underestimates the molecular weight of PLB when it contains at least approximately 6 mol % brominated units. The bromine atoms of PLB5050 (MnPSt = 5.32 × 103 Da; absolute Mn ∼ 1.70 × 104 Da) were replaced quantitatively with iodide using sodium iodide in acetone at room temperature, with a modest decrease in the molecular weight (MnPSt = 4.62 × 103 Da). PLB was also reacted with 0.25 equiv of sodium azide in DMF at 0 °C to replace the corresponding amount of bromine atoms in 88% efficiency, again with a slight decrease in the apparent molecular weight (MnPSt = 4.15 × 103 Da). The resulting azido-functionalized PLA was reacted with dimethyl acetylenedicarboxylate, an activated alkyne, via a 1,3-dipolar azide−alkyne, microwave-assisted “click” cycloaddition at 56 °C to generate poly(lactic acid) with 1,2,3-triazole rings and a corresponding increase in molecular weight (MnPSt = 5.05 × 103 Da), whereas copper-catalyzed “click” reactions involving basic amine ligands, such as a CuBr/PMDETA-catalyzed “click” reaction with propargyl alcohol, degraded the copolymer.

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petroleum-based polymers,13 it also has several disadvantages. Unmodified PLA is brittle, with less than 10% elongation at break.14 Since PLA is a hydrophobic,15 bulk-eroding material,6,13 the rate of degradation correlates with the rate of water diffusion and is relatively slow.16 Its hydrophobicity also results in low cell affinity,17,18 and upon degradation, the resulting lactic acid and carboxylic acid end groups can cause inflammation in vivo.18 Many of these negative properties of PLA could be improved, and additional properties gained, by chemically modifying the polymer, such as by incorporating biologically active molecules or other functional molecules pendant to the polymer backbone as with other, more easily functionalized, aliphatic polyesters.19−21 The two main routes used to prepare PLAs are polycondensation of lactic acid and ring-opening polymerization of lactide; each route has advantages and disadvantages.22 High molecular weight is easier to achieve, and the molecular weight distributions are typically narrower using ringopening polymerizations. However, the preparation of cyclic monomer requires an additional synthetic step, which may also limit the functional groups that can be attached to the cycle to

INTRODUCTION Bio-based polyesters are useful for a wide range of applications, including drug delivery1 and tissue engineering.2 When combined with impact modifiers and/or engineering thermoplastic resins, bio-based polyesters are also supplementing commodity plastics in durable applications.3 In the past few decades, research on the synthesis, properties, and use of biobased polyesters has increased significantly, not only because of their origin from renewable resources such as corn and other plants 4 but also because of their ability to undergo biodegradation, especially in comparison to traditional petroleum-based polymers.5 The three most widely used aliphatic polyesters that are considered “green” are poly(εcaprolactone) (PCL; although it is currently produced industrially from petrochemicals), poly(glycolic acid) (PGA), and poly(lactic acid) (PLA) as well as their corresponding copolymers.6 Because of its degradability, PLA has a wide variety of uses, ranging from biomedical applications such as scaffolds,7 sutures,8 bone screws,9 and wound dressings10 to more environmentally friendly consumer products11 such as cups, apparel, containers, durable goods, and even coffins.12 While PLA has many advantages, including its environmental friendliness, biocompatibility, ease of processing compared to other biopolymers, and low processing cost compared to © XXXX American Chemical Society

Received: February 15, 2016 Revised: February 22, 2016

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DOI: 10.1021/acs.macromol.6b00331 Macromolecules XXXX, XXX, XXX−XXX

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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 ringopening 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 reacted with alkynes by an azide−alkyne Huisgen cycloaddition32 to subsequently introduce amines,19 ammonium salts,19 and polymeric side chains19,21,33 to aliphatic polyesters. Polymeric side chains have also been introduced by azide−alkyne cycloadditions to polymers containing pendant alkyne groups.20,34 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 cross-linking site under UV irradiation.35 We recently designed and synthesized hyperbranched polyacrylates by homopolymerization of acrylate inimers in which the key intermediate is a 2-halo-3-hydroxypropionic acid (halo = Cl or Br).36−38 Since 2-bromo-3-hydroxypropionic acid (C3H5BrO3) is a brominated constitutional isomer of lactic acid (C3H6O3), yet has a primary alcohol group like glycolic acid, we believe that it is an ideal monomer to copolymerize with glycolic acid and/or lactic acid to produce PLA and PLGA with pendant bromine atoms along the polymer backbone (PLB and PLGB, respectively) for further functionalization (Scheme 1),

produce functional polymers. Ring-opening polymerizations are also more sensitive to impurities, and the amount of moisture must be controlled or eliminated in order to control the amount and/or type of initiating species and therefore the molecular weight and chain ends. In addition to condensation polymers having broader molecular weight distributions, the greatest challenge of the polycondensation route is achieving high molecular weight polymer, which requires efficient removal of the condensation byproduct, such as water. Nevertheless, polycondensations do not require stringently pure and dry starting conditions and are therefore simple to perform. Condensation polymerizations are also useful for screening and developing new materials, such as functionalized PLAs. PLA is difficult to functionalize both because it lacks specific reactive sites along the polymer backbone and because the polymer backbone is extremely susceptible to cleavage due to the very high concentration of ester groups. Nevertheless, several routes have been investigated to impart functionality, including nonspecific surface modification, chemical modification of bulk material, and copolymerization of lactic acid or lactide with functional comonomers. For example, the surface of PLA has been coated with an apatite/collagen composite to increase its cell affinity.23 Its cell affinity has similarly been increased by treating its surface with a dilute alkaline solution of NaOH (0.25 M),24 which must be monitored to minimize competing bulk degradation of the polymer. The surface of PLA can also be modified by grafting it photochemically under mild reaction conditions.13 Grafting of PLA with poly(Nvinylpyrrolidone) (in almost 80% efficiency) decreases its water contact angle from 74° to 38°.25 Photochemical cross-linking of PLLA with triallyl isocyanurate results in a bioinert material with increased tensile mechanical properties.26 In addition to being photochemically labile, the protons alpha to the carbonyl group of PLA are acidic and can be removed with a strong base such as lithium N,N-diisopropylamide to form an enolate anion, which can then react with an electrophile to introduce functionality along the polymer backbone.27 However, the concentration of base must be low (99%) was distilled from KOH under N2 and stored over KOH. All other reagents and solvents were commercially available and were used as received. 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. All spectra were recorded in CDCl3, and the resonances were measured relative to residual solvent resonances and referenced to tetramethylsilane (0.00 ppm). A PerkinElmer Pyris 1 differential scanning calorimeter was used to determine the glass transition temperatures (Tgs), which were read as the middle of the change in heat capacity. All heating and cooling rates were 10 °C/min. Transition temperatures were calibrated using indium and benzophenone standards. Number- (Mn) and weight-average (Mw) molecular weights and dispersities (Đ = Mw/Mn) were determined by gel permeation chromatography (GPC) relative to linear polystyrene (GPCPSt) from calibration curves of log Mn vs elution volume at 35 °C using tetrahydrofuran (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 μm 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 GPC with a light scattering detector (GPCLS) 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 aqueous 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. Synthesis of Poly[(lactic acid)-co-(2-bromo-3-hydroxypropionic acid)] PLB9010 in Bulk Followed by Chain Extension. A melted solution of 85% D,L-lactic acid (1.2 g, 11 mmol), BrA (0.21 g, 1.2 mmol), diphenyl ether (1 mL), and pTSA· H2O (0.05 g, 0.3 mmol) in a sealed Schlenk flask was stirred at 130 °C, while gradually decreasing the pressure over 10 min, and then stirred at 130 °C under a dynamic vacuum (5 mmHg) for 48 h to produce a prepolymer with MnPSt = 5.67 × 103 Da and Đ = 2.40. CH2Cl2 (40 mL) was added, and the solution was heated to reflux for 18 h at atmospheric pressure to dehydrate the polymerization mixture, with the solvent refluxing through oven-dried 4 Å molecular sieves for 18 h before reentering the Schlenk flask. DiPC (0.20 g, 1.6 mmol) was added to the polymerization solution at room temperature, and preformed DMAP·pTSA (0.10 g, 0.34 mmol) was added while cooling in an ice bath. After stirring the polymerization mixture at room temperature for 7 h, an aliquot was withdrawn, concentrated by rotary evaporation, precipitated in 1:1 hexanes/isopropanol, and analyzed by GPCPSt: MnPSt = 2.17 × 104 Da and Đ = 2.29.

Synthesis of Poly(lactic acid) in Bulk Followed by Chain Extension. A melted solution of 85% D,L-lactic acid (1.2 g, 11 mmol) was polymerized exactly as described above for the 90:10 mixture of LA and BrA to produce a prepolymer with MnPSt = 1.49 × 104 Da, Đ = 1.67 and a chain extended polymer with MnPSt = 3.12 × 104 Da, Đ = 1.72. Syntheses of Poly[(lactic acid)-co-(2-bromo-3-hydroxypropionic acid)] (PLB) and Poly[(lactic acid)-co-(glycolic acid)co-(2-bromo-3-hydroxypropionic acid)] (PLGB) in the Presence of Diphenyl Ether. The PLB and PLGB copolymers were synthesized in 44−70% yield as in the following example for PLGB801010. A Schlenk tube containing a mixture of D,L-lactic acid (0.93 g, 9.0 mmol), GA (85 mg, 1.1 mmol), BrA (0.19 g, 1.1 mmol), and pTSA (51 mg, 0.30 mmol) in diphenyl ether (1.0 mL) was sealed with a glass stopper and placed in an oil bath at 95 °C. After 1 h, the pressure was reduced to 5 mmHg, and the solution was stirred at 95 °C in vacuo for 48 h. After releasing the vacuum, CH2Cl2 (10 mL) was added to dissolve the copolymer. Diphenyl ether crystallized out of solution and was collected in a fritted glass funnel. The filtrate was precipitated into rapidly stirring methanol (50 mL), and the resulting copolymer was collected in a fritted glass funnel and reprecipitated four times from CH2Cl2 (10 mL) into methanol (50 mL) to yield 0.53 g (59%) of PLGB801010 as a white solid; MnPSt = 2.99 × 104, Đ = 2.51; composition 79 mol % lactate units, 7 mol % glycolate units, 14 mol % 2-bromo-3-hydroxypropionate units. 1H NMR: 1.4−1.8 (m, CH3), 4.4−4.7 (m, CHBrCH2), 4.7−4.9 (m, CH2CO2), 5.10−5.30 (m, CH(CH 3 )CO 2 ). 13 C NMR: 16.6 (CH 3 ), 39.4 (CBr), 60.8 (OCH2CO2), 64.49 (OCH2CHBr), 69.0 (CH(CH3)CO2), 166.5 (CH2CO2), 166.5 (CHBrCOO), 169.3 (C(CH3)CO2). Large Scale (10 g) Synthesis of Lower Molecular Weight Poly[(lactic acid)-co-(2-bromo-3-hydroxypropionic acid)] (PLB5050) for Further Functionalization. A melted solution of D,L-lactic acid (4.1 g, 38 mmol), BrA (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 mmHg) 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 PLB5050 as a white solid; MnPSt = 5.32 × 103 Da, Đ = 2.64; composition 49 mol % lactate units, 51 mol % 2-bromo-3-hydroxypropionate units. 1H NMR: 1.4−1.7 (bm, CH3), 4.4−4.7 (m, CH2CHBr), 5.1−5.3 (m, CH(CH3)CO2). 13C NMR (Figure S1): 16.8 (CH3), 39.5 (CBr), 64.8 (CH2CHBr), 65.5 (CH2CHBr), 69.1 (CHCH3), 166.6 (CHBrCO2), 169.3 (C(CH3)CO2). Synthesis of Poly[(lactic acid)-co-(2-iodo-3-hydroxypropionic acid)] (PLI5050). A solution of PLB5050 (MnPSt = 5.32 × 103 Da, Đ = 2.64; 0.40 g, 1.8 mmol Br) and sodium iodide (0.30 g, 2.0 mmol) in 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 in vacuo on a Schlenk line to yield 0.49 g (92%) of copolymer as a yellow solid; MnPSt = 4.62 × 103 Da, Đ = 1.96; composition 49 mol % lactate units, 51 mol % 2-iodo-3-hydroxypropionate units. 1H NMR: 1.5−1.7 (b, CH3), 4.4−4.7 (CH2CHI), 5.1−5.3 (b, CHCH3). 13C NMR (Figure S5): 12.6 (CHI), 16.7 (CH3), 66.0 (CH2), 69.0 (CHCH3), 169.0 (O2CCHI), 169.4 (O2CCHCH3). 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 PLB5050 (MnPSt = 5.32 × 103 Da, Đ = 2.64; 49:51 LA/Br units) (1.6 g, 7.2 mmol Br) in a 14 mM solution of Aliquat 336 (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 the solvent was removed by rotary evaporation. The residue was precipitated twice from CH2Cl2 (5 mL) into 1:1 (v:v) hexanes/Et2O (200 mL) to yield 0.77 g (50%) of azideC

DOI: 10.1021/acs.macromol.6b00331 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 2. Comparison of the Polystyrene-Equivalent Molecular Weights of Poly(lactic acid) and Poly[(lactic acid)-co-(2bromo-3-hydroxypropionic acid)] Achieved by Acid-Catalyzed Melt (Co)polycondensation and Then by Further Dehydration by Refluxing an Added Solvent (CH2Cl2) through 4 Å Molecular Sieves and Chain Extension Using Diisopropyl Carbodiimide

Table 1. Synthesis of Poly(lactic acid) (PLA) and Its PLB and PLGA Copolymers with 2-Bromo-3-hydroxypropionic Acid (BrA) and/or Glycolic Acid (GA), Respectively, by Acid-Catalyzed Melt Copolyesterification in the Presence of Diphenyl Ethera feed composition (mol %)

copolymer composition (mol %)

GPCPStc

sampleb

LA

GA

BrA

LA

GA

BrA

yield (%)

Mn × 10−4

DPn

Đ

Tgd (°C)

PLA PLGA9010 PLGA8020 PLB9010 PLB8020 PLB7030 PLB6040 PLB5050 PLGB801010 PLGB701020 PLGB601030 PLGB702010 PLGB602020 PLGB502030

100 90 80 90 80 70 60 50 80 70 60 70 60 50

0 10 20 0 0 0 0 0 10 10 10 20 20 20

0 0 0 10 20 30 40 50 10 20 30 10 20 30

100 89 78 89 78 67 56 55 79 68 61 70 60 50

0 11 22 0 0 0 0 0 7 7 8 16 17 16

0 0 0 11 22 33 44 45 14 25 31 14 23 34

54 50 64 60 48 53 56 44 59 61 63 63 58 70

3.16 2.29 2.01 2.08 1.80 1.68 1.71 2.03 2.99 2.37 2.30 2.32 2.20 1.89

439 324 291 260 205 176 165 182 381 274 244 301 259 203

1.62 1.53 1.51 1.99 2.20 2.18 2.01 1.93 2.51 3.99 4.35 2.52 4.13 3.86

51 50 47 43 41 41 37 35 48 45 43 47 41 39

Copolymerizations on a 1 g scale were performed at 95 °C for 48 h under a dynamic vacuum (5 mmHg) in the presence of 3 mol % ptoluenesulfonic acid and 1 mL of diphenyl ether. bThe numbers in the sample name correspond to the composition of the copolymers according to the comonomer feed. cNumber- (Mn) and weight-average (Mw) molecular weights and dispersities (Đ = Mw/Mn) were measured by gel permeation chromatography relative to linear polystyrene (GPCPSt) in tetrahydrofuran at 35 °C. dThe glass transition temperatures (Tg) were measured by differential scanning calorimetry on heating at 10 °C/min. a

containing copolymer as a white solid; MnPSt = 4.15 × 103 Da, Đ = 2.33; composition 47 mol % lactate units, 42 mol % 2-bromo-3hydroxypropionate units, 11 mol % 2-azido-3-hydroxypropionate units. 1H NMR: 1.4−1.7 (CH3), 4.2−4.4 (m, CH2CHN3), 4.4−4.8 (m, CHN3, CH2CHBr), 5.1−5.3 (CHCH3). 13C NMR (Figure S6): 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). 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-azido-3-hydroxypropionic acid)] (MnPSt = 4.15 × 103 Da, Đ = 2.33; 47:42:12 LA/Br/N3 units) (0.31 g, 1.5 mmol N3) and dimethyl acetylenedicarboxylate (0.22 g, 1.5 mmol) in acetone (10 mL) was heated in a microwave (power 65 W) in air (protected by a drying tube) for 4 min (3 min ramp to 56 °C, 1 min hold at 56 °C). The solution was concentrated by rotary evaporation, CH2Cl2 (2 mL) was added, and the resulting solution was precipitated in 1:1 (v:v) hexanes/Et2O (80 mL) to yield 0.16 g (72%) of copolymer as a white

solid; Mn = 5.05 × 103 Da, Đ = 2.51; composition 56 mol % lactate units, 32 mol % 2-bromo-3-hydroxypropionate units, 6 mol % 2-azido3-hydroxypropionate units, 6 mol % “clicked” [1,2,3]-triazole units. 1H NMR (Figure S8): 1.5−1.7 (CH3), 3.9−4.0 (CH3O2C), 4.2−4.4 (CH2CHN3), 4.4−4.8 (CHN3, CH2CHBr), 5.0−5.3 (CHCH3). 13C 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)).

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RESULTS AND DISCUSSION Optimization of the Copolymerization of 2-Bromo-3hydroxypropionic Acid with Lactic Acid. 2-Bromo-3hydroxypropionic acid was prepared by a deaminohalogenation of D,L-serine via a diazonium intermediate as described previously.38 As a brominated constitutional isomer of lactic acid, it is an ideal monomer to copolymerize with LA to produce PLA with pendant bromine atoms along the polymer D

DOI: 10.1021/acs.macromol.6b00331 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules backbone (PLB). It also has a primary alcohol group like glycolic acid and can therefore be copolymerized with a mixture of LA and GA to produce brominated PLGA (PLGB). The pendant bromine atoms offer sites for further functionalization of both PLB and PLGB. Although a living ring-opening copolymerization of lactide and a cyclic monomer based on BrA would presumably produce high molecular weight PLB more readily and with narrow molecular weight distribution, direct polycondensation of BrA with LA is a simple method for determining the properties of PLB and screening its functionalization reactions. Scheme 2 compares the acid-catalyzed (3 mol % pTSA) homopolymerization of LA in bulk at 130 °C under dynamic vacuum (5 mmHg) to that of a 9:1 mixture of LA and BrA. The presence of BrA decreased the molecular weight of the resulting copolymer under identical conditions. While PLA reached a molecular weight of MnPSt = 1.49 × 104 Da, the copolymerization only reached MnPSt = 5.67 × 103 Da. Since the synthesis of high molecular weight polymer from reversible polycondensations requires the rigorous removal of the condensation byproduct, we investigated additional methods for removing water. As also outlined in Scheme 2, after producing both PLA and PLB9010 by direct melt polycondensation in vacuo, we added CH2Cl2 to the two (co)polymerizations and refluxed the solvent through 4 Å molecular sieves to completely dehydrate the polymerization systems, similar to Ajioka’s synthesis of high molecular weight PLA using high boiling point solvents,40 and then chain extended the resulting (co)polymers using diisopropylcarbodiimide at room temperature under conditions developed by Moore and Stupp for room temperature polyesterifications.41 This dehydration and chain extension procedure doubled the molecular weight of PLA to MnPSt = 3.12 × 104 Da and quadrupled the molecular weight of PLB to MnPSt = 2.17 × 104 Da, such that the two (co)polymers had similarly high molecular weights in the Mn = 104 range. Next, we examined simply adding a minimal amount of a high boiling solvent to the direct melt (co)polycondensations in vacuo. Table 1 summarizes the copolymerizations of 2-bromo3-hydroxypropionic acid with lactic acid in comparison to the homopolymerization of LA; copolymerizations of BrA with both GA and LA to produce PLGB are also included. All (co)polymerizations were performed on a 1 g scale at 95 °C for 48 h using a dynamic vacuum (5 mmHg) in the presence of 3 mol % pTSA and 1 mL of diphenyl ether. Molecular weights in the Mn = 104 Da range were achieved in a single step in the presence of diphenyl ether. Diphenyl ether lowers the viscosity of the copolymerization system, thereby facilitating the removal of water. As expected for a step polymerization, the dispersities (Đ) are close to 2 but tend to be broader for the brominated copolymers. Figure 1 presents the 1H NMR spectra of PLB, and Figure S1 in the Supporting Information presents the 1H NMR spectra of PLGB. The compositions of the copolymers were calculated by comparing the methine resonance of the lactate units at 5.2 ppm, the methylene resonance of the glycolate units at 4.8 ppm, and the overlapping resonances of the methylene and methine (CH2CHBr) protons of the 2-bromo-3-hydroxypropionate units at 4.5 ppm. According to 1H NMR analyses, the compositions of the copolymers closely matched their comonomer feed compositions (Table 1), and copolymers containing 10−45 mol % brominated repeat units were readily prepared.

Figure 1. 1H NMR (300 MHz) spectra of poly[(lactic acid)-co-(2bromo-3-hydroxypropionic acid)] (PLB) copolymers synthesized by acid-catalyzed melt polycondensation in the presence of diphenyl ether (Table 1); ∗ = CHCl3..

Figure 2 compares the 13C NMR spectra of the PLB5050 copolymer and PLA. The methyl carbon of the lactate unit resonates at 16.6 ppm, and the methine (CHBr) carbon of the 2-bromo-3-hydroxypropionionate unit resonates at 39.3 ppm. As shown in the expanded region of the spectrum, there are two types of lactate methine carbons in the PLB5050 copolymer. We have assigned the resonances at 69.1 ppm to the methine (CHCH3) carbon of a lactate homodyad (LL) and its stereoisomers because the PLA methine carbon also resonates at 69 ppm. The resonance at 70.0 ppm therefore corresponds to the methine carbon of a BL heterodyad. Similarly, there are two types of 2-bromo-3-hydroxypropionate methylene (CH2CHBr) carbons that resonate at 64.5 and 65.3 ppm, presumably due to the BL heterodyad and BB homodyad, respectively. The corresponding 13C NMR spectra of the PLGB copolymers are presented in Figures S2 and S3 of the Supporting Information. The glass transition temperatures of the PLB and PLGB copolymers are listed in Table 1. The glass transition temperatures of both types of copolymers decrease with increasing concentrations of BrA. Introducing 10 mol % 2bromo-3-hydroxypropionate units decreased the glass transition temperature from 51 °C for PLA to 43 °C for PLB9010, which is a greater decrease than that caused by 10 mol % glycolate units (Tg = 50 °C for PLGA9010). Similarly, introducing 20 mol % 2-bromo-3-hydroxypropionate units decreased the Tg of PLB8020 to 41 °C, which is a greater decrease than that caused by 20 mol % glycolates units (Tg = 47 °C for PLGA8020). At constant 20 mol % GA content, the Tg of the PLGB copolymers decreased from 47 °C for PLGB702010 to 41 °C for PLGB602020 and 39 °C for PLGB502030. Similarly, at constant 10 mol % GA content, the Tg of the PLGB copolymers decreased from Tg = 48 °C for PLGB801010 to E

DOI: 10.1021/acs.macromol.6b00331 Macromolecules XXXX, XXX, XXX−XXX

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equivalent degrees of polymerization and molecular weights of the two PLGA9010 (DPn = 324; MnPSt = 2.29 × 104 Da) and PLGA8020 (DPn = 291; MnPSt = 2.01 × 104 Da) copolymers of LA with GA are also lower than those of PLA. Since both GA and BrA have primary alcohols and LA has a secondary alcohol, it would be surprising if the reactivity of either monomer is less than that of LA. Instead, the decreasing molecular weight and degree of polymerization of LA copolymers with increasing concentrations of BrA may be due to changes in the hydrodynamic volumes of the copolymers and may therefore be due to GPC effects. We therefore determined the effect of the brominated monomer on the absolute molecular weights of brominated PLA using GPC with a light scattering detector (GPCLS).42 Figure 3A and Table

Figure 2. 13C NMR (75 MHz) spectrum and an expanded region (showing the methine (CH) and methylene (CH2) carbons) of the spectrum for poly[(lactic acid)-co-(2-bromo-3-hydroxypropionic acid)] PLB5050 copolymer synthesized by acid-catalyzed melt polycondensation in the presence of diphenyl ether (Table 1); ∗ = CHCl3. The corresponding expanded region of the 13C NMR spectrum of poly(lactic acid) is also shown for comparison.

45 °C for PLGB701020 and 43 °C for PLGB601030. Nevertheless, the glass transition temperatures of PLB and PLGB are in a similar temperature region as those of PLA and PLGA. PLB5050 has the lowest Tg at 35 °C, which is only 16 °C lower than that of PLA prepared under identical conditions. Absolute Molecular Weight of PLB. Compared to PLA (DPn = 439; MnPSt = 3.16 × 104 Da), the PSt-equivalent degrees of polymerization and number-average molecular weights of the copolymers of LA and BrA generally decreased with increasing concentrations of BrA in the comonomer feed (Table 1); for example, PLB9010 reached DPn = 260 (MnPSt = 2.08 × 104 Da), and PLB6040 reached DPn = 165 (MnPSt = 1.71 × 104 Da). Compared to PLGA801010 (DPn = 381; MnPSt = 2.99 × 104 Da), replacing LA with increasing amounts of BrA decreased the PSt-equivalent degree of polymerization to DPn = 274 (Mn = 2.37 × 104 Da) for PLGB701020 and DPn = 244 (MnPSt = 2.30 × 104 Da) for PLGB601030. Similarly, at a constant 20 mol % GA in the comonomer feed, the DPn of PLGA702010 decreased from 301 (MnPSt = 2.32 × 104 Da) to DPn = 259 (MnPSt = 2.20 × 104 Da) for PLGB602020 and DPn = 203 (MnPSt = 1.89 × 104 Da) for PLGB502030. This indicates that BrA has a lower reactivity than LA. However, the PSt-

Figure 3. 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. The curve in (B) is meant to guide the eye.

S1 in the Supporting Information present the refractive index increments of PLB in THF, and Figure 3B presents the percent difference in the absolute molecular weights using the refractive index increments from Figure 3A and the 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,43 the PSt-equivalent molecular weight of PLB is only higher than its absolute molecular weight at very low contents of the brominated monomer (Table S1). The absolute number-average molecular weight (MnLS) of PLB increases relative to its MnPSt molecular weight with increasing F

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Scheme 3. Synthesis and Functionalization of the Statistical Copolymer Poly[(lactic acid)-co-(2-bromo-3-hydroxypropionic acid)]

Figure 4. 1H NMR spectra of poly[(lactic acid)-co-(2-bromo-3-hydroxypropionic acid)] (PLB5050; MnPSt = 5.32 × 103 Da) and functionalized derivatives: iodinated PLA (PLI; MnPSt = 4.62 × 103 Da), azide-functionalized PLA (PLBN3; MnPSt = 4.15 × 103 Da), and the “clicked” product of PLBN3 with dimethyl acetylenedicarboxylate (MnPSt = 5.05 × 103 Da). Inset: 13C NMR spectra of PLI (red) and PLB (blue).

5.32 × 103 Da, Đ = 2.64; absolute Mn ∼ 1.70 × 104 Da) for use in establishing preliminary functionalization conditions. All of the bromine atoms of PLB are alpha to an electron-withdrawing carbonyl group and are therefore activated toward nucleophlic substitution.44 As outlined in Scheme 3, 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; such materials may be useful for medical imaging and radiation therapy applications if the appropriate isotope of iodine is used.45 Although the 1H NMR spectra of the brominated and iodinated copolymers are nearly identical (Figure 4), the 13C NMR spectra in Figures S4 and S5 of the Supporting Information and the inset of Figure 4 confirm that the CBr resonance of PLB at 39.3 ppm is replaced by a new CI resonance at 12.9 ppm for the iodinated PLA (PLI).

content of the brominated monomer, such that the PStequivalent molecular weight of PLB underestimates its absolute molecular weight at all compositions containing greater than ∼6 mol % brominated repeat units; i.e., while the MnLS of PLA is only 68% of its MnPSt value, which is consistent with literature values,43 the MnLS and MnPSt values of PLB are equal when PLB contains approximately 6 mol % brominated repeat units, and MnLS is greater than MnPSt when PLB contains greater than approximately 6 mol % brominated repeat units. Therefore, the actual molecular weights of PLB achieved by this acid-catalyzed copolyesterification are significantly higher than those indicated by GPCPSt at most bromine contents, with the 1:1 PLB5050 copolymer having an absolute molecular weight approximately 3.2 times greater than its PSt-equivalent molecular weight. Functionalization of PLB by Nucleophilic Substitution and Subsequent Click Reactions. We synthesized a larger amount (5 g) of lower molecular weight PLB5050 (MnPSt = G

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Figure 5. Comparison of the gel permeation chromatograms of poly[(lactic acid)-co-(2-bromo-3-hydroxypropionic acid)] (PLB5050; MnPSt = 5.32 × 103 Da; Đ = 2.64) before (blue) and after (red) reaction with sodium iodide to produce iodinated PLA (PLI; MnPSt = 4.62 × 103 Da; Đ = 1.96).

Figure 6. Comparison of the gel permeation chromatograms of azide-functionalized PLA (PLBN3; MnPSt = 4.15 × 103 Da; Đ = 2.33) (green), the microwave-assisted “clicked” product of PLBN3 with dimethylacetylene dicarboxylate (MnPSt = 5.05 × 103 Da; Đ = 2.51) (black), and the attemped CuBr/PMDETA-catalyzed “clicked” product of PLBN3 with propargyl alcohol (MnPSt = 4.45 × 103 Da) (orange) as well as the original PLB5050 (MnPSt = 5.32 × 103 Da; Đ = 2.64) (blue).

4.3 ppm in the azide repeat unit; in the 13C NMR spectra, the substituted carbon shifts downfield from 39.3 ppm in PLB5050 (Figure 4 inset and Figure S4) to 60.0 ppm in the azide repeat units (Figure S6). The composition (47:42:11 LA/BrA/N3 units) of the isolated product and integration of the CHBr and CHN3 methine resonances demonstrate that the azide substitution was only 88% efficient, with the remaining azide presumably causing elimination and the 22% decrease in molecular weight. Azide-functionalized polymers are very useful for introducing additional functionality by Huisgen azide−alkyne 1,3-dipolar cycloadditions under mild conditions.47 As an example of a standard copper-catalyzed “click” reaction, we reacted the azidefunctionalized PLB with propargyl alcohol using cuprous bromide as the catalyst and 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 product with MnPSt = 4.45 × 103 Da,

A significant challenge for functionalizing PLB by nucleophilic substitution is to identify conditions that minimize degradation of the molecular weight; i.e., competing elimination results in cleavage of the polymer backbone. Figure 5 demonstrates that the nucleophilic substitution of bromine in PLB5050 (MnPSt = 5.32 × 103 Da) with iodine (PLI MnPSt = 4.62 × 103 Da) results in minimal change in the apparent molecular weight. However, 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 due to the potential for explosion.46) In this case, the molecular weight can be maintained by using a less-than-equimolar amount of azide. For example, reaction of PLB5050 (MnPSt = 5.32 × 103 Da) with 0.25 equiv of sodium azide per brominated repeat unit in DMF at 0 °C for 6 h resulted in only a slight decrease in the molecular weight (MnPSt = 4.15 × 103 Da) (Figure 6). The 1H NMR spectra in Figure 4 demonstrate that the substituted methine resonance shifts upfield from 4.6 ppm in PLB5050 to H

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functionalities. For example, the azide-functionalized PLA undergoes a microwave-assisted “click” reaction with dimethyl acetylenedicarboxylate, which is an activated alkyne. However, the azide-functionalized PLA is degraded under coppercatalyzed conditions with less active alkynes due to competing elimination caused by the amine-based copper ligands to form acrylate.

similar to that of the starting azide-functionalized PLA (PLBN3 MnPSt = 4.15 × 103 Da) (Figure 6). As demonstrated by the 1H NMR spectrum in Figure S7, this polymer lacks a resonance at ∼8 ppm for the [1,2,3]-triazole ring, although the CHN3 resonance at 4.3 ppm has almost disappeared, presumably due to elimination and degradation of the polymer backbone caused by the free PMDETA ligand. We therefore 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 S8) demonstrated that methyl 3-acetoxy-2-bromopropionoate degrades to methyl α-bromoacrylate, evidently due to 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;48 the acetate proton corresponding to acetic acid was detected at 1.97 ppm. Although the azide groups of the azide-functionalized PLB cannot be reacted with alkynes under copper-catalyzed conditions if basic ligands are present, they react readily with activated alkynes, such as dimethyl acetylenedicarboxylate, that do not require a catalyst. Dimethyl acetylenedicarboxylate is activated toward 1,3-dipolar cycloaddition with azides by two electron-withdrawing carbonyl groups.49 The reaction was performed under microwave-assisted conditions similar to those used previously with nonactivated alkynes that required basic catalysts.19,30 When the azide-functionalized PLB (MnPSt = 4.15 × 103 Da; 47:42:11 LA/Br/N3 units) was reacted with dimethyl acetylenedicarboxylate in acetone in a microwave50 (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 in Figure 4 of the product of this reaction 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 (Figure S9), with a resonance at 127.9 ppm; the two methyl ester carbons resonate at 52.9 and 53.6 ppm. In addition, the molecular weight of the product increased slightly to MnPSt = 5.05 × 103 Da (Figure 6), as expected for conversion of the azide groups to 4,5-dimethyl-substituted [1,2,3]-triazole rings.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00331. Synthesis of methyl 3-acetoxy-2-bromopropionate and test of its stability to PMDETA; attempted coppercatalyzed Huisgen alkyne−azide cycloaddition of poly[(lactic acid)-co-(2-azido-3-hydroxypropionic acid)-co(2-bromo-3-hydroxypropionic acid)] with propargyl alcohol; details of samples corresponding to Figure 3; 1 H and 13C NMR spectra of PLGB; 13C NMR and remaining 1H spectra of functionalized PLAs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel (330) 972-6614 (C.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support from the National Science Foundation (DMR-1006195 for partial support of A.B. and W.K.S-M.) and the National Institutes of Health (ARRA Supplement for GM86895-2). We thank Prof. George Newkome, Dr. Charles Moorefield, and Dr. Tony Schultz for their assistance with the use of their microwave reactor.

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REFERENCES

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CONCLUSIONS Brominated poly(lactic acid) (PLB) can be synthesized in relatively high molecular weight with polystyrene-equivalent molecular weights in the range of MnPSt = 104 Da by an acidcatalyzed, 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. In contrast to PLA, the absolute molecular weight of PLB is higher than the polystyrene-equivalent molecular weight when the copolymer contains greater than approximately 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 and by azide if it is not used in excess. The azidefunctionalized PLAs can be reacted with alkynes in a 1,3-dipolar azide−alkyne cycloaddition to provide a number of other I

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