Facile Synthesis of Fluorine-Substituted Polylactides and Their

Feb 9, 2018 - We report the facile synthesis of 3-trifluoromethyl-6-methyl-1,4-dioxane-2,5-dione and ring-opening polymerization of the fluoro-lactide...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Facile Synthesis of Fluorine-Substituted Polylactides and Their Amphiphilic Block Copolymers Chang-Uk Lee,† Razieh Khalifehzadeh,‡ Buddy Ratner,*,‡,§ and Andrew J. Boydston*,† †

Department of Chemistry, ‡Department of Chemical Engineering, and §Department of Bioengineering, University of Washington, Seattle, Washington 98195, United States S Supporting Information *

ABSTRACT: We report the facile synthesis of 3-trifluoromethyl-6-methyl-1,4-dioxane-2,5-dione and ring-opening polymerization of the fluoro-lactide monomer to prepare polylactides composed of trifluoromethyl and methyl pendent groups on each repeat unit (FPLA). Molecular weights of the prepared polymers correlated well with the initial molar ratio of monomer to initiator and were found to range from 6.6 to 22.5 kDa as determined by 1H NMR spectroscopy. GPC analysis revealed an Mn of up to 16.5 kDa. 1H, 13C, and 19F NMR spectroscopy were consistent with the structures of the lactide monomer isomers, and 1H NMR analysis was consistent with polymer backbones of alternating trifluoromethyl- and methyl-substituted lactate constituents. Glass transition temperature (Tg) and decomposition temperature (Td) of the new FPLA were found to be 39 and 225 °C by DSC and TGA, respectively. Additionally, we prepared amphiphilic block copolymers of FPLA and poly(ethylene glycol) (PEG). Specifically, FPLA-b-PEG diblocks and FPLA−PEG−FPLA triblocks were synthesized by using PEG monomethyl ether (mPEG) or PEG as alcohol initiators, respectively. We observed the formation of vesicles or wormlike micelles from the particles of FPLA−PEG−FPLA in dilute aqueous solution by transmission electron microscopy (TEM), suggesting potential applications for drug delivery.

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INTRODUCTION Polylactide and polyglycolide belong to an important class of polyesters that have numerous biomedical applications such as sutures, implants, wound healing, and scaffolds for drug delivery and tissue engineering. The widespread interest in these materials stems largely from their biocompatibility, biodegradability, and biorenewable feedstock.1−4 Despite their potential, polylactides have certain less than ideal properties including relatively low Tg (35−60 °C),5 inherent brittleness,6 and poor thermal resistance2 and are generally difficult to functionalize. To extend the limited range of physicochemical properties and broaden the potential functions of polylactides, various methods including blending or copolymerization have been utilized.7,8 Additionally, substitution of one or two methyl groups of polylactides with alkyl, aryl, or other functional groups has been reported as one of the useful methods to extend the range of properties of polylactides.5,8−17 Some possible advantages of this approach include improved physical properties, such as Tg, mechanical properties, or rate of hydrolytic degradation, while maintaining degradable polyester backbones. In most cases, the preparation of these substituted polylactides was enabled by synthesis of substituted lactide monomers and their ring-opening polymerizations. For example, Baker and co-workers prepared alkyl (ethyl, hexyl, and isobutyl,9 isopropyl, cyclohexyl10) substituted or aryl-substituted polylactides (polymandelide11 or poly(phenyl lactides))12 via ring-opening polymerization of corresponding substituted lactide or glycolide monomers. They © XXXX American Chemical Society

reported the synthesis of poly(rac-dicyclohexylglycolide)s from ring-opening polymerization of the dicyclohexyl-substituted glycolides and a Tg of 98 °C from the prepared polymers, which is 43 °C higher than poly(rac-lactide)s.10 The Baker group further reported a lower rate hydrolytic degradation of poly(phenyl lactide) at one-fifth that of poly(rac-lactide)s.12 Furthermore, substitution of the methyl unit(s) of lactides by various functional groups has been reported for extended applications of polylactides. The Baker group reported preparation of oligo(ethylene oxide)-substituted glycolides and thermoresponsive oligo-(ethylene oxide)-grafted polylactides showing LCST at 19 or 37 °C for 3 or 4 ethylene oxide repeating units, respectively.3 Baker and co-workers further reported the synthesis of an acetylene-functionalized (“clickable”) glycolide monomer and decyl/PEG-grafted polypropargyl glycolide copolymers showing LCST values from room temperature to more than 60 °C.15 Additionally, Jing and Hillmyer16 reported the synthesis of norbornene-functionalized lactide monomers, spiro[6-methyl-1,4-dioxane-2,5-dione-3,2′-bicyclo[2.2.1]hept[5]ene]. They prepared norbornene-substituted poly(lactic acid) (PLA) by ROP and reported a high Tg of 113 °C, the highest (to our knowledge) among polylactide derivatives. They also prepared lactide-functionalized polynorbonenes by ROMP Received: December 1, 2017 Revised: January 25, 2018

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DOI: 10.1021/acs.macromol.7b02531 Macromolecules XXXX, XXX, XXX−XXX

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fluorinated lactide monomers also enabled us to prepare amphiphilic block copolymers of FPLA and PEG by using PEG or mPEG as an initiator. We report synthesis of FPLA− PEG diblock and FPLA−PEG−FPLA triblock copolymers and studies on the dispersion of FPLA−PEG−FPLA in aqueous solution by DLS and TEM.

and subsequently PLA composites containing polybutadiene− PLA graft copolymers for toughening PLA. More recently, Fuoco et al.17 reported preparation of lactide monomers with pendent thiol-protecting groups. They accessed a variety of copolymers of lactides and caprolactones and prepared pyridine disulfide functionalized copolymers via postpolymerization modification. This latter copolymer was utilized to bind a cysteine-containing peptide and to enable peptide-functionalized porous scaffolds. Of particular note among functionalized polymers, fluoropolymers have been of great interest due to their generally high chemical and thermal resistance, low surface energy, and low coefficient of friction.18 For biomedical applications, fluorine may provide enhanced stability in biological environments and greater blood biocompatibility than non-fluorinated analogues.19 One-fifth of all pharmaceuticals on the market today contain fluorine to increase efficacy or circulation lifetime. It is perhaps unsurprising that researchers have taken aim at combining the properties afforded by fluorination with the general biodegradability of polyesters to achieve fluorinated polylactides. In most cases thus far, fluorine has been incorporated at the end groups of polylactides by utilizing alcohols containing fluorine as an initiator in ring-opening polymerization of lactides.4,20−22 For example, Lee et al.4 reported the preparation of perfluoroalkylterminated polylactides (F-polyesters) and poly(lactide-coglycolide) copolymers. These polymers showed a retarded initial degradation rate at the surface of F-polyester films or blend films of F-polyester and PLA where fluorine end groups were surfacesegregated, and thus water repellency was enhanced. Frediani and co-workers20 also reported that perfluoroalkyl-terminated polylactides showed more enhanced, water-repellent and photostable properties than polylactides. The Frediani group21 further reported the preparation of perfluoroalkyl-terminated copolymers of lactic acid and mandelic or salicyclic acid with increased hydrophobicity and stability compared to polylactides. In addition to incorporation of fluorine at the end groups, Borkar and co-workers23 targeted incorporation of fluorocarbons as pendent groups in the main polyester chains. They reported preparation of poly(caprolactone-alt-fluororohexene) alternating copolymers by free radical ring-opening polymerization of 2methylene-1,3-dioxepane and fluoroalkenes. They observed fluorine-enriched surfaces from the copolymer films ascribed to segregation of the perfluoroalkyl pendent tails toward the airexposed side, while maintaining hydrolytic degradability from the ester groups of carprolactones. Interestingly, investigation on polylactides with fluorine substitution in the main chain has rarely been reported. McKie et al.24,25 reported the fully fluorinated polylactides, poly((R,S)3,3,3-trifluorolactic acid), synthesized by self-esterification of (R,S)-3,3,3-trifluorolactic acid using 1,3-diisopropylcarbodiimide as a stoichiometric coupling reagent. Their work inspired us to address potential anticipated challenges with regard to controlling polymer molecular weight and preparation of block copolymers containing the fully fluorinated polylactides. Herein, we report controlled synthesis of alternating polylactides composed of trifluoromethyl and methyl pendent groups in the polyester backbones with controlled molecular weights. This work was enabled by successful synthesis of lactide monomers with one of the methyl groups substituted for a trifluoromethyl group and controlled ring-opening polymerization of the monomers using benzyl alcohol as an initiator and tin(II) 2ethylhexanoate as a catalyst. We report spectroscopic and thermal characterizations of the prepared fluorine-substituted lactide monomers and polymers. The successful synthesis of

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EXPERIMENTAL SECTION

Materials and General Considerations. Trifluorolactic acid was purchased from Matrix Scientific, and all other chemicals were purchased from Aldrich. Methoxy-PEG (5000) or PEG (6000) were dried by azeotropic distillation in toluene. Lactide was purified by recrystallization in toluene. Benzyl alcohol was dried over CaH2 and followed by distillation. Tin(II) 2-ethylhexanoate was purified by distillation. Acetonitrile was dried by passing through neutral alumina and storage over 3 Å molecular sieves. Dry toluene and dichloromethane were obtained from a Glass Contour solvent purification system. Ethyl acetate and hexanes were dried over molecular sieves. Premium grade silica gel (porosity: 60 Å; particle size: 40−75 μm) was purchased from Sorbent Technologies and dried at 150 °C under high vacuum overnight prior to using it for column chromatography. Characterization. 1H, 13C NMR, and 19F NMR spectra were recorded on a Bruker AV 300, 500, and DRX 499 spectrometer, respectively. Chemical shifts are reported in delta (δ) units, expressed in parts per million (ppm) downfield from tetramethylsilane using the residual protio-solvent as an internal standard (CDCl3, 1H: 7.26 ppm and 13C: 77.16 ppm, CD3CN, 1H: 1.94 ppm and 13C: 118.36 ppm). For 19 F NMR spectra, hexafluorobenzene (−164.9 ppm) was used as an internal standard in deuterated acetonitrile for monomer and deuterated chloroform for the polymer. Molecular weights of the synthesized fluorine-substituted polylactides or PEG were determined by gel permeation chromatography (GPC) equipped with three MZ gel 10 μm columns of pore sizes of 103, 103, and 105 Å, a DAWN-HELEOS II 18-angle multiangle laser light scattering detector, and an OptiLab T-rEx refractive index (RI) detector from Wyatt Technologies Corporation. Chloroform was used as the mobile phase for FPLA homopolymers and THF for PEG or block copolymers composed of PEG and FPLA. Each eluent was used at a flow rate of 1 mL/min. The refractive index increment (dn/dc) values of the prepared FPLA polymers were measured using the OptiLab T-rEx RI detector and Astra software dn/dc template in a batch mode. Five solutions of the polymer in chloroform with precisely known concentrations (0.46−2 mg/mL) were prepared by diluting a stock solution using volumetric flasks and injected to the RI detector using a syringe pump. The dn/dc value was determined as −0.0335 ± 0.0018 mL/g from the linear fit to a plot of differential refractive index versus concentration (Figures S1 and S2). The absolute weight-average molecular weights were determined by using the predetermined dn/dc value. Number-average molecular weights and molecular weight dispersities were then calculated using the onboard Astra software. Differential scanning calorimetry (DSC) studies of the polymer were conducted a TA DSC Q200 calorimeter under nitrogen. A powder sample sealed in aluminum pans was first heated from room temperature to 180 °C at 10 °C/min to remove any thermal history in the sample and cooled to 0 °C at 5 °C/min. Then the sample was second heated to 180 °C at 10 °C/min, and heat flow as watts from cooling and the second heating was recorded and reported after normalizing by mass of the sample (W/g). Thermogravimetric analysis (TGA) of the prepared monomer and polymer was conducted by a TA TGA Q50 under nitrogen from room temperature to 500 or 600 °C at a rate of 10 °C/min. The decomposition temperature of the polymer was determined from the onsets of weight loss. Dynamic light scattering (DLS) measurements of aqueous solutions of di- and triblock copolymers (0.1 wt %) were performed using Malvern Zetasizer Nano-ZS (Malvern Instrument; Westborough, MA). The He−Ne laser operating at 633 nm was utilized, and light scattered by the sample was detected at an external angle of 173° for a backscatter detector and at 12.8° for forward-scatter detector. Data from three B

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Macromolecules Scheme 1. Synthesis of 3-Trifluoromethyl-6-methyl-1,4-dioxane-2,5-dione (Fluorine-Substituted Lactides, 2)

Figure 1. 1H NMR spectrum of fluorine-substituted lactides in acetonitrile-d3. measurements were recorded with at least 10 runs at 25 or 37 °C. The autocorrelation function from the scattered intensities was obtained for each sample, and the average hydrodynamic diameter was determined using the Stokes−Einstein equation. Transmission electron microscopy (TEM) of particles from FPLA− PEG−FPLA or PLA−PEG−PLA triblock terpolymers in water was conducted on a FEI Tecnai (model: G2 F20) with an acceleration voltage of 200 kV. Synthesis of Fluorine-Substituted Lactide Monomers. To a dried round-bottom flask was added a solution of trifluorolactic acid (6.2 g, 43 mmol) in dry acetonitrile (70 mL) and trimethylamine (5.6 g, 7.8 mL, 56 mmol) under nitrogen gas flow. This solution was then cooled in an ice bath, and then a solution of 2-bromopropionyl bromide (11.6 g, 5.7 mL, 54 mmol) in 10 mL of dry acetonitrile was added dropwise into the reaction flask. Then, the ice bath was removed, and the mixture was stirred at room temperature for 3 h. The reaction mixture was then filtered through a pad of Celite to remove salts that had formed during the reaction, and then the filtrate was concentrated under reduced pressure. The resulting product was dissolved in ethyl acetate, and any remaining salts were removed by filtration. The filtrate was concentrated under reduced pressure to obtain 1 as a brown oil, which was used without further purification for the next step. For the synthesis of 2, sodium hydride (60%, 2.6 g, 64.5 mmol) was stirred using a magnetic stirbar in dry acetonitrile (800 mL) in an ice bath under nitrogen gas flow. Next, a solution of 1 in dry acetonitrile (100 mL) was added dropwise to the sodium hydride mixture over a period of 30 min. The ice bath was then removed, and the reaction mixture was stirred at room temperature for 1 h under nitrogen. The mixture was then heated in an oil bath preset to 70 °C and stirred for 17 h. After confirming complete consumption of 1 by TLC analysis, the mixture was removed from the oil bath and stirred until it reached room temperature. White solids were then removed by filtration through a pad of Celite, and then the filtrate was concentrated under reduced pressure. The resulting product was then dissolved in dry ethyl acetate and filtered through a pad of Celite to remove any residual salts. The filtrate was then concentrated under reduced pressure and purified by silica gel column chromatography using dry premium grade silica gel and a mixture of 30% dry ethyl acetate/hexanes as an eluent under nitrogen gas flow. The resulting product was further purified by recrystallization in dry CH2Cl2/hexanes to give a white solid as a. mixture of R,S (88%, 7/8) and R,R/S,S (12%, 1/8) stereoisomers (2.27 g, 27% yield). 1H NMR (500 MHz, CD3CN): δ 5.71 (q, J = 7.5 Hz, 1H × 1/8), 5.69 (q, J = 6.3

Hz, 1H × 7/8), 5.31 (q, J = 7.0 Hz, 1H × 1/8), 5.26 (q, J = 6.7 Hz, 1H × 7/8), 1.65 (d, J = 7.0 Hz, 3H × 1/8), 1.58 (d, J = 6.7 Hz, 3H × 7/8). 13C NMR (126 MHz, CD3CN): δ 165.8, 161.0, 121.4 (q, 1JCF = 278 Hz, CF3), 73.7, 73.0 (q, 2JCF = 33 Hz, CF3C−), 15.6. 19F NMR (471 MHz, CD3CN, C6F6): δ −75.3. Solution Polymerization of Fluorine-Substituted Lactide Monomers. The solution polymerization was conducted in toluene at 110 °C using benzyl alcohol as an initiator and tin(II) 2ethylhexanoate (Sn(Oct)2) as a catalyst. The initiator and catalyst were purified by distillation prior to polymerization, and their stock solutions in dry benzene (0.1 M) were prepared. A representative procedure to prepare fluorine-substituted polylactides is described as follows: In a drybox, monomer (2, 198 mg, 1 mmol) was placed into a vial with dry toluene (4 mL, 0.25 M). Then stock solutions of Sn(Oct)2 (4, 100 μL, 0.01 mmol) and benzyl alcohol (3, 100 μL, 0.01 mmol) were added to the monomer solution. The vial was sealed with a Teflon cap and heated at 110 °C for 24 h. The mixture was then removed from heating. Once at room temperature, the solution was concentrated under vacuum to remove solvent, catalyst, and unreacted initiators. The crude product was analyzed by 1H NMR spectroscopy to determine polymerization conversion. The crude polymer was then dissolved in chloroform, causing precipitation of unreacted monomer. The solids were then removed by filtration, and then the filtrate was concentrated under reduced pressure. Then, the solid product was stirred in methanol, which presumably reacted with any residual monomer to produce a volatile ester product. The resulting mixture was then dried under high vacuum. The resulting polymer was then dissolved in chloroform and precipitated into an excess of hexanes. The final product was obtained after removing hexanes by decantation and drying under vacuum to yield a white powder (135 mg, 68% yield). Bulk Polymerization of Fluorine-Substituted Lactide Monomers. Solvent-free polymerization was carried out in a sealed vial in a drybox. The vial was charged with monomer (2, 50 mg, 0.25 mmol), a stir bar, and stock solutions of Sn(Oct)2 (25 μL, 2.5 μmol) and benzyl alcohol (25 μL, 2.5 μmol) and then sealed with a PTFE-lined screw cap. The reaction mixture was then heated at 140 °C for 12 h. At the end of polymerization time, unreacted monomer, catalyst, and initiator were removed by applying the same purification process as described above for solution polymerization. Synthesis of Amphiphilic Block Copolymers of FPLA and PEG. Block copolymerizations were carried out in dry toluene at 110 °C using Sn(Oct)2 as a catalyst and either methoxy PEG (mPEG) macroinitiator C

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Figure 2. 13C NMR spectrum of fluorine-substituted lactides in acetonitrile-d3.

Figure 3. 19F NMR spectrum of fluorine-substituted lactides in acetonitrile-d3. for diblock copolymer 7 or PEG macroinitiator for triblock terpolymer 9. A glass vial was charged with monomer (2, 50 mg, 250 μmol) and dry toluene (1.5 mL, 0.17 M) in a drybox. Then, stock solutions of Sn(Oct)2 (4, 83 μL, 8.3 μmol) and PEG6000 (50 mg, 8.3 μmol) were added to this monomer solution. The vial was sealed with a Teflon cap and heated at 110 °C for 24 h. The reaction mixture was then removed from heating and then concentrated under vacuum. The crude product was analyzed by 1H NMR spectroscopy to determine polymerization conversion. The resulting polymer was then dissolved in chloroform and precipitated into an excess of ether. The final product was obtained after filtering and drying under vacuum as a light yellow powder (65 mg, 65% yield). Sample Preparation for DLS. Aqueous solutions of amphiphilic block copolymers were prepared by the thin-film hydration technique as previously described.26 An appropriate amount of polymer powder was dissolved in chloroform in a vial. Chloroform was then removed under vacuum to render a thin film on the walls of the vial. Afterward, the thin film was hydrated by adding water at a concentration of 0.1 wt %, and the solution was heated at 80 °C for 20 min during which time the solution became clear. The polymer solution was then removed from heat until it reached room temperature and was then filtered through a 0.2 μm filter. Hydrodynamic diameters were measured at designated time points of 0, 1, 5, and 14 days after filtration. Sample Preparation for TEM. Carbon films for TEM were plasma cleaned with the Gatan Solarus 950 plasma cleaner to increase hydrophilicity. A drop of aqueous polymer solution (0.1 wt %) was placed on a carbon film supported by copper grids for 5 or 15 min, and excess solution was removed by a piece of filter paper. Then the film was negatively stained by phosphotungstic acid aqueous solution (1 wt %) by placing it on the carbon film treated with the polymer solution for 2 min. After removing excess of the acid solution by filter paper, the carbon film was washed three times with deionized water and dried in

the air. Particle size was determined by analyzing TEM images using ImageJ software.

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RESULTS AND DISCUSSION Monomer Synthesis. To the best of our knowledge, the synthesis of discrete fluorine-substituted lactide monomers has

Scheme 2. Polymerization of Fluorine-Substituted Lactide Monomers in Dry Toluene

not been previously reported. The synthesis procedure is shown in Scheme 1. Trifluorolactic acid was reacted with 2bromopropionyl bromide in the presence of trimethylamine to produce linear bromo esters (1). The crude linear ester was then cyclized under dilute conditions (47.8 mM) using sodium hydride as base, which provided a higher overall yield than another inorganic bases such as sodium bicarbonate21 or organic bases such as diisopropylamine. For purification, flash column chromatography on silica gel gave higher isolated yields than sublimation. Dried, premium grade silica gel was found to be advantageous over standard silica gel, as did use of nitrogen gas versus house air.8 Following column chromatography, fractions containing the desired product were collected and further D

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Figure 4. 1H NMR spectrum of the purified FPLA 5 in CDCl3.

Figure 5. 19F NMR spectrum of the purified FPLA 5 in CDCl3.

Scheme 3. Synthesis of FPLA-b-PEG (7) and FPLA−PEG−FPLA (9) Block Copolymers

Table 1. Representative Results from Solution Polymerizationa entry 1 2 3

[2]:[3]:[4] 25:1:1 50:1:1 100:1:1

conva (%) 96 93 80

DPn,NMRb

Mn,theoc (kDa)

33 62 113

4.9 9.3 16.0

Mn,NMRb (kDa)

Mn,GPCd (kDa)

ĐGPCd

6.6 12.4 22.5

− 12.0 16.1

−e 1.06 1.18

e

a Polymerization was conducted in toluene at 110 °C ([2] = 0.25 M). bDPn,NMR and Mn,NMR were calculated using end-group analysis by 1H NMR spectroscopy. cTheoretical molecular weights were calculated by initial ratio of monomer to initiator and conversion of monomer. dMolecular weights were determined by GPC using the predetermined dn/dc value. eThe molecular weights by GPC were not determined due to weak signal intensities.

purified by recrystallization. The final product was obtained as a mixture of R,S (88%) and R,R/S,S (12%) diastereomers (Figure 1). Analysis of the mixture by 1H NMR spectroscopy revealed a doublet from the methyl protons of the R,S isomer with the coupling constant of JHH = 6.7 Hz and that from R,R or S,S

isomers of JHH = 7.0 Hz (Figure 1). The greater coupling constant from protons of R,R or S,S isomers than that from R,S ones was validated by other reports on substituted lactide monomers.14,27 Additionally, pure R,S diastereomers were obtained in small fractions (Figure S3). A molecular structure E

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spectroscopy (1H decoupled) in Figure 3 shows a singlet for the trifluoromethyl groups of the monomers. Solution Polymerization of FPLA. Ring-opening polymerization of the fluorolactide monomers was conducted in toluene using tin(II) catalyst. Bulk polymerization without solvent above the melting temperature of monomers is a common method to polymerize lactides or substituted lactides since it is relatively fast and efficient. In this study, bulk polymerization was first attempted, but about half of the monomer mass (assessed visually) was sublimed to the top of the reaction vessels at the beginning of the polymerizations. Those monomers remained unreacted during polymerization. For this reason, we selected solution polymerization using toluene as a solvent, benzyl alcohol as an initiator, and tin(II) 2-ethylhexanoate as a catalyst (Scheme 2). The initial molar ratio of the initiator and catalyst was fixed at 1:1 for all reactions in this study. The monomer was found to have limited solubility in toluene at room temperature but became fully soluble at 110 °C. The reaction solutions remained homogeneous at 110 °C during polymerization. Figure 4 shows a representative 1H NMR spectrum of one of the isolated high molecular weight FPLA polymers. The methine protons next to the trifluoromethyl groups are clearly shown from δ = 5.47 to 5.7 ppm, and methine protons next to the methyl groups are found at δ = 5.2 to 5.45 ppm. The ratio of integration of the peaks from protons next to the trifluoromethyl groups to those next to the methyl groups is 0.9, indicating essentially equal portions of trifluoromethyl and methyl groups in the backbones of polymer. This NMR analysis suggested to us that the polymer microstructures contained alternating units of trifluoromethyllactate and lactate units. Additionally, Figure 5 shows an 19F NMR spectrum of the polymer. Compared to that of the monomer (Figure 3), the peaks became broad, indicating distribution of polymer chains with different molecular weights. Table 1 summarizes results from solution polymerization of fluorine-substituted lactides. At initial molar ratios of monomer to initiator ([monomer]0/[initiator]0) of 25 and 50, high conversions (>90%) were reproducibly observed (entries 1 and 2). At [monomer]0/[initiator]0 = 100, a range of conversions from 68 to 80% were observed (entry 3) under identical conditions. End-group analysis by 1H NMR spectroscopy was used to calculate number-average molecular weights (Mn,NMR) by comparing the integrations of peaks from the methine groups (δ = 5.47−5.7 ppm) and methyl groups (δ = 1.47−1.71 ppm) against those from peaks assigned to the benzyl end groups (δ = 7.29−7.3 ppm) (Figure 4). Molecular weights by end-group analysis were found to reproducibly close to or higher than theoretical values calculated based on conversions. Molecular weights of the polymers could be controlled by altering [monomer]0/[initiator]0 from 25 to 100 and reached from 6.6 to 22.5 kDa as determined by 1H NMR analysis. GPC analysis using multiangle light scattering (MALS) for direct determination of Mw gave calculated Mn values that were generally consistent with those obtained from 1H NMR analysis. The deviations that were observed appeared to be sporadic, and

Figure 6. Representative DSC thermograms of FPLA 5 (Mn,NMR = 18.1 kDa).

Figure 7. Representative TGA thermogram of FPLA 5 (Mn,NMR = 21.1 kDa).

of the prepared lactide monomers was confirmed by 1H, 13C, and 19 F NMR spectroscopy (Figures 1−3). In 1H NMR spectroscopy, the methine protons next to the trifluoromethyl groups of the R,S isomers are deshielded due to the electron-withdrawing trifluoromethyl groups (δ = 5.7 ppm) compared to the methine protons next to the methyl groups at δ = 5.25 ppm (Figure 1). Also, the coupling constant (JHF = 6.3 Hz) of the quartet from the methine protons next to the trifluoromethyl groups is distinct from that (JHH = 6.7 Hz) next to the methyl groups. The equal integration from those methine groups is also consistent with the structure of the fluorine-substituted lactide monomers. 13C NMR spectroscopy further supported the existence of trifluoromethyl groups in the lactide monomers (Figure 2). A quartet at δ = 121.4 ppm (2JCF = 278 Hz) from carbons of the trifluoromethyl groups was ascribed to carbon−fluorine couplings. Additionally, another quartet at δ = 73 ppm with 1 JCF = 33 Hz was assigned to carbons adjacent to the trifluoromethyl groups, which is distinct from the singlet δ = at 73.7 ppm from carbons next to the methyl groups. 19F NMR Table 2. Results of Block Copolymerization block copolymer

[M]:[I]:[C]a

conv (%)

Mn,PEGb (kDa)

Mn,FPLAc (kDa)

Mn,copolymerd (kDa)

NPEG

NFPLA

PEG121-b-FPLA10 FPLA10-PEG140-FPLA10

18:1:1 30:1:1

97 95

5.3 6.1

2 4.1

7.3 10.2

121 140

10 20

a c

[M]: molar concentration of FLA monomers; [I]: concentration of mPEG or PEG; [C]: concentration of Sn(Oct)2 (catalyst). bBy GPC analysis. By NMR spectroscopy. dSum of Mn,PEG and Mn,FPLA. F

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Figure 8. 1H NMR spectrum of FPLA10−PEG140−FPLA10 triblock terpolymer 9 in CDCl3.

Figure 11. TGA thermogram of FPLA10−PEG140−FPLA10.

Figure 9. GPC trace of PEG and FPLA10−PEG140−FPLA10 triblock terpolymer 9 using THF as an eluent.

Figure 10. DSC thermogram of FPLA10−PEG140−FPLA10.

Figure 12. Hydrodynamic sizes of particles from FPLA10−PEG140− FPLA10 in dilute aqueous solution (0.1 wt %) over time.

we note that GPC analysis was somewhat complicated by low signal intensities from the detectors. The FPLAs showed negative refractive index (RI) signals and a negative refractive index increment (dn/dc) value (−0.0335 mL/g) (Figures S1, S2, and S4). Chloroform was selected as an eluent, as it led to stronger signals from both LS and RI detectors than THF and toluene. The small absolute value of the dn/dc is likely the main cause of the weak signals from MALS detection, as the intensity of scattered light is proportional to the square of dn/dc (Iscattered ∝ Mc(dn/dc)2 where c is concentration of polymer in solution and M is molecular weight).28 Additionally, RI signals from the FPLA are weak and negative. As a result, it should be noted in this study that weak signals from both LS and RI may cause errors in

determining absolute molecular weights and molecular weight distribution by GPC, and therefore GPC analysis needs to be compared to NMR analysis (Table S1). Polymer Characterization. Figure 6 shows DSC thermograms of one of the prepared polymers 5 (Mn,NMR = 18.1 kDa) during cooling from 180 to 0 °C and second heating to 180 °C. As can be seen, the Tg of the polymer was found to be 39 °C, which is lower than polylactides (55 °C for Mn = 22.7 kDa).29 During the cooling or heating cycle, no crystallization or melting from the polymer was observed. FPLA prepared from pure R,S isomers (Figure S3) was also analyzed by DSC. No crystallization was observed from these polymers either. G

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Figure 13. TEM image of vesicles (left) or wormlike micelles (right) formed from FPLA10−PEG140−FPLA10 in dilute aqueous solution (0.1 wt %).

FPLA22, with comparable molecular weight to FPLA10− PEG140−FPLA10, in water was studied by DLS under the same conditions (see Supporting Information). Figure S10 shows that PLA−PEG−PLA did not form uniform or stable particles or micelles even after 2 weeks in water. TEM was used to investigate the shapes and sizes of the particles from FPLA10−PEG140−FPLA10 in water. Figure 13 shows TEM images of particles from FPLA10−PEG140−FPLA10 in dilute aqueous solution after 2 weeks. FPLA10−PEG140− FPLA10 formed oval-shaped vesicles with a long diameter of 380 nm, short diameter of 300 nm, and wall thickness of 2.3 nm. Also, wormlike micelles with a diameter matching the wall thickness of the vesicles were observed. We postulate that the vesicles formed by closing the wormlike micelles (Figure 13). Interestingly, these self-assembled nanostructures were obtained from racemic FPLA-based copolymers. To our knowledge, the formation of vesicles or wormlike micelles has not been reported from amphiphilic block copolymers composed of racemic PLA in aqueous solution. Petzetakis et al.30 reported the formation of wormlike micelles from diblock copolymers composed of poly(acrylic acid) and crystalline, enantiomerically PLLA or PDLA but reported that spherical micelles were obtained from racemic PLA. The formation of spherical micelles or particles was also reported from triblock terpolymers, PLA−PEG−PLA, of enantiomerically pure PLLA.31,32 Du et al.33 reported formation of vesicles or wormlike micelles from PLA−PEG in mixtures of THF/H2O or dioxane/H2O with 30 or 40 wt % of water, but not in 100% water. Our control experiment on the dispersion of PLA−PEG−PLA in water did not show any formation of vesicles or wormlike micelles when studied by TEM. We speculate that the increased hydrophobicity from the trifluoromethyl groups in FPLA−PEG−FPLA could be contributing to the formation of vesicles or wormlike micelles.

Figure 7 shows TGA thermograms of one of the prepared polymers 5 (Mn,NMR = 21.1 kDa). The decomposition temperature (Td) was found to be 225 °C, which was determined from the onset of the weight loss. The Td of this FPLA is within close range of that of polylactides (180 to ca. 240 °C).29 Additionally, the Td of the lactide monomer (2) was also determined by TGA and found to be 95 °C (Figure S5). Notably, the Td of the monomer as determined by TGA could actually be the onset of sublimation during the analysis. Block Copolymerization. One of the advantages of the successful synthesis of fluorine-substituted lactides would be preparation of block copolymers using any macroinitiators with alcohol end groups. In this study, amphiphilic block copolymers composed of FPLA and PEG were prepared; key results are summarized in Table 2. The analogous non-fluorinated copolymers were also prepared for comparison using readily available lactides to form PLA blocks (see Supporting Information). Diblock and triblock copolymerizations were achieved with high conversion (>95%) of fluorinated monomer 2. The 1H NMR spectrum for FPLA10−PEG140−FPLA10 triblock terpolymer 9 (Figure 8) and comparison of GPC traces for PEG and 9 (Figure 9) support the successful chain extension of the PEG macroinitiator with FPLA chains. DSC data of FPLA−PEG−FPLA triblock terpolymer 9 (Figure 10) show melting (Tm) and crystallization (Tc) transitions from PEG chains at 51 and 20 °C, respectively. A Tg ascribed to the FPLA chains is also visible at 9 °C. TGA data of the polymer (Figure 11) show first a degradation of FPLA chains at 210 °C and a second weight loss from the PEG segments at 366 °C. TGA data also revealed the weight % of FPLA to be 37% and that of PEG to be 63%. These values match closely with those obtained from GPC and NMR data of the block copolymer. Dispersion of Amphiphilic FPLA−PEG−FPLA Block Copolymers in Aqueous Solution. Dispersion of amphiphilic FPLA10−PEG140−FPLA10 block copolymers in aqueous solution was studied by DLS at room temperature. Figure 12 shows DLS data from the polymer in aqueous solution after 0, 1, 5, and 14 days. We observed that small and large micelle particles were forming at the beginning and that these particles combined to form relatively uniform and stable particles over time. The average diameter of the particles after 5 days was found to be 75 nm as determined by DLS analysis. The DLS measurements were repeated at 37 °C, and no changes in hydrodynamic sizes were observed compared to those at room temperature. As a control experiment, the dispersion of copolymer PLA22−PEG140−

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CONCLUSIONS In this study, we report successful synthesis of fluorinesubstituted, cyclic diester monomers and alternating copolylactides of trifluoromethyl groups and methyl groups by ringopening polymerization of the monomers with controlled molecular weights. Additionally, we report synthesis of amphiphilic block copolymers composed of FPLA and PEG and their formation of vesicles or wormlike micelles in dilute aqueous solution. We expect this study opens a new pathway to synthesize fluorine-substituted, biodegradable polylactides and other polyesters and to study their biomedical applications. H

DOI: 10.1021/acs.macromol.7b02531 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

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Future studies will address surface analysis, degradation kinetics, protein adsorption, and biological properties.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02531. GPC and characterization data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (A.J.B.). *E-mail [email protected] (B.R.). ORCID

Andrew J. Boydston: 0000-0001-7192-4903 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by Abbott Vascular, Inc., and by the UWEB21 Engineering Research Center. Part of this work was conducted at the Molecular Analysis Facility, a National Nanotechnology Coordinated Infrastructure site at the University of Washington which is supported in part by the National Science Foundation (grant ECC-1542101), the University of Washington, the Molecular Engineering & Sciences Institute, the Clean Energy Institute, and the National Institutes of Health. Special thanks to Ellen Lavoie at the Molecular Analysis Facility for help with the TEM experiments.

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REFERENCES

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DOI: 10.1021/acs.macromol.7b02531 Macromolecules XXXX, XXX, XXX−XXX