Postpolymerization Functionalization of Copolymers Produced from

Article pubs.acs.org/Macromolecules

Postpolymerization Functionalization of Copolymers Produced from Carbon Dioxide and 2‑Vinyloxirane: Amphiphilic/Water-Soluble CO2‑Based Polycarbonates Donald J. Darensbourg* and Fu-Te Tsai Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States S Supporting Information *

ABSTRACT: Common CO2-based polycarbonates are known to be highly hydrophobic, and this “inert” property makes them difficult for the covalent immobilization of bioactive molecules. A practical method for modifying polymers is to introduce various functional groups that permit decoration of polymer chains with bioactive substances. In this report, CO2-based poly(2vinyloxirane carbonate) (PVIC) with more than 99% carbonate linkages is isolated from the CO2/2-vinyloxirane alternating copolymerization catalyzed by the bifunctional catalyst [(1R,2R)-SalenCo(III)(DNP)2] (1) (DNP = 2,4-dinitrophenolate) bearing a quaternary ammonium salt on the ligand framework. It was also observed that the presence of propylene oxide significantly activates 2-vinyloxirane for incorporation into the polymer chain as well as inhibits the formation of cyclic carbonate in the terpolymerization process. DSC studies demonstrate that the glass transition temperature (Tg) decreases with the increase in the content of vinyl groups in the polycarbonate. By way of thiol−ene coupling, showing mainly “click” characteristics and nearly quantitative yields, amphiphilic polycarbonates (PVIC-OH and PVIC-COOH) with multiple hydroxy or carboxy functionalities have been prepared, providing suitable reactivities for further modifications (ring-opening of L-aspartic acid anhydride hydrochloride salt and deprotonation by aqueous ammonium hydroxide (NH4OH(aq))) to successfully isolate the water-soluble CO2-based polycarbonate PVIC-COONH4, and the PVIC-OH-Asp polymer which shows particles dispersed in water with an average hydrodynamic diameter Dn = 32.2 ± 8.8 nm. It is presumed that this emerging class of amphiphilic/watersoluble polycarbonates could embody a powerful platform for bioconjugation and drug conjugation. In contrast to lower Tgs of PVIC, (PVIC-co-PC), PVIC-OH, and PVIC-COOH, the polycarbonates PVIC-OH-Asp and PVIC-COONH4 show higher Tgs as a consequence of their intrinsic ionic property (ammonium salts).

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INTRODUCTION As global energy markets become more carbon-constrained, carbon-recycling technologies will become increasingly more important elements of the overall energy portfolio. In part, this effort hinges on changing the current carbon feedstock to a new chemical building block by utilizing carbon dioxide.1 Selective transformation of carbon dioxide into biodegradable polycarbonates by the alternating copolymerization with epoxides presents a green, promising polymerization process for potential large-scale utilization of CO2 in chemical synthesis.2 Numerous homogeneous metal derivatives which could suppress ether linkage, increase polymer selectivity, and control molecular weight distribution have been developed and employed as effective catalysts for this coupling process. These single-site © 2014 American Chemical Society

metal catalysts with well-defined structures are active under mild reaction condition and in some instances provide regio- or/and stereoselective copolymer synthesis.2 However, the “inert” nature of the generated polycarbonates has largely hampered their design for biologically active materials. In order to improve material performances as well as categories of these greener polymers, attempts at synthesizing more diverse CO2-based copolymers represent important and challenging topics for research and development. Received: April 21, 2014 Revised: May 29, 2014 Published: June 6, 2014 3806

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of azobis(isobutyronitrile) (AIBN) was delineated, and the isolation of important water-soluble CO2-based polycarbonates was demonstrated.

The emerging importance of amphiphilic polymers, referred to as hybrid polymers, is due to their fundamental importance and potential applicability in material science and biomedicine. Owing to their unique biocompatibility, biodegradability, and approved use in biomedical devices by the US food and drug administration (FDA), aliphatic polycarbonates are the prime synthetic biomaterials.3 Nevertheless, common polycarbonates are often challenged by their high hydrophobicity, improper degradation profile, and in particular absence of reactive centers for the covalent immobilization of bioactive molecules. A practical method for modifying the “inert” property of the generated polycarbonates to meet specific needs is to introduce various functional groups that permit incorporation of polymer chains with bioactive substances. In the past decade, significant efforts have been directed toward the development of functional aliphatic polycarbonates containing hydroxyl, carboxyl, amine, allyl, alkyne/azide, acryloyl, and oligoethylene glycol (OEG) pendant groups.4 In contrast to tremendous reports on ringopening polymerization (ROP) of functionalized six-membered cyclic carbonate monomers, the synthesis of water-soluble polycarbonates5a and the study of postpolymerization functionalization of CO2-based functional polymers are limited.5b,c Recently, the synthesis and characterization of an unsymmetrical salen ligand containing one cyclohexyl and two methyl groups and bifunctional chromium catalyst have been reported.6a,b Of importance, these bifunctional catalysts displayed negligible propensity for affording cyclic carbonates during the alternating copolymerization of CO2 and epoxides (styrene oxide and epichlorohydrin).6c,d In this report, a modified synthetic procedure for an unsymmetrical Schiff-base ligand preparation containing one methyl and two cyclohexyl groups is presented. Since it is difficult to avoid significant formation of thermally stable cyclic carbonates by intramolecular cyclic elimination via two concurrent backbiting mechanisms for the copolymerization of CO2 and terminal epoxides with electron-withdrawing groups, bifunctional electrophile−nucleophile catalysts [(1R,2R)-salenM(III)(X)2] (catalyst 1: M = Co, X = DNP = 2,4-dinitrophenolate; catalyst 2: M = Cr, X = azide) were synthesized for the asymmetric coupling of CO2 with racemic propylene oxide and/or 2-vinyloxirane (Scheme 1). In addition, synthesis of amphiphilic polymers derived from postpolymerization modification of the produced polycarbonates with 2-mercaptoethanol and thioglycolic acid in the presence

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RESULTS AND DISCUSSION Synthesis of Bifunctional Catalysts. The unsymmetrical Schiff-base ligand (1R,2R)-L of the bifunctional catalysts [(1R,2R)-LM(III)(X)2] (1, M = Co and X = DNP; 2, M = Cr, X = N3) was synthesized by the reaction of the corresponding functionalized salicyaldehyde O with the condensation product of 1,2-diaminocyclohexane mono(hydrogen chloride) and 3,5di-tert-butyl-2-hydroxybenzaldehyde.6a,b See Supporting Information for the detailed synthetic procedure as well as 1H NMR spectra of all intermediates. However, ESI-MS study indicates that compound I containing one cyclohexyl group was isolated from the reaction of compound H and boron tribromide (BBr3) (Scheme 2a). This presumably originates from the interaction between the appended amine group and BBr3 followed by the release of one bromocyclohexane. In order to prepare compound O containing two cyclohexyl groups, compound J was isolated from the reaction of compound F and BBr3. The subsequent reaction of 2 equiv of dicyclohexylamine and compound K containing one tosyl protection group affords compound L. In contrast to the previously employed chloromethyl methyl ether (MOMCl) reagent which is a potent carcinogen, the tosyl group is human and environmentally benign.6b The deprotection of compound L using 10 equiv of aqueous NaOH generates compound M containing two cyclohexyl groups quantitatively. The formylation of compound M and methylation of the appended amine group yield the corresponding functionalized salicyaldehyde O (Scheme 2b). The reaction of the unsymmetrical Schiff-base ligand (1R,2R)L with Co(OAc)2·4H2O in a 1:1 molar ratio in anhydrous MeOH at room temperature yielded the thermal stable orangered complex [(1R,2R)-LCo(II)]. Addition of 5 equiv of LiCl into the above reaction mixture followed by bubbling pure oxygen through the solution overnight generated the yellow-green complex, [(1R,2R)-LCo(III)Cl2]. Coffee-red catalyst [(1R,2R)LCo(III)(DNP)2] (1) was isolated from the reaction of [(1R,2R)-LCo(III)Cl2], AgBF4, and sodium 2,4-dinitrophenolate in CH2Cl2 at ambient temperature. In a similar manner, brown-green catalyst [(1R,2R)-LCr(III)(N3)2] (2) was synthesized from the reaction of (1R,2R)-L, CrCl2, AgBF4, and NaN3. The IR azide stretching frequencies 2044 cm−1 (s) and 2056 cm−1 (sh) in CH2Cl2 indicate that the Cr(III) center is sixcoordinated with two azide ligands on the apical positions.6a Copolymerization of CO2 and 2-Vinyloxirane. Inspired by the success with these binary (3)/bifunctional (1 and 2) catalyst systems for the alternating copolymerization of CO2 and epichlorohydrin/styrene oxide,6c,d the copolymerization of CO2 and racemic 2-vinyloxirane was conducted. As shown in Table 1, in contrast to lower TOF for catalysts 2 and 3 and poor polymer selectivity for catalyst 3 (20% at 40 °C), catalyst 1 catalyzes CO2/ 2-vinyloxirane copolymerization to selectively give the corresponding polycarbonates with more than 99% carbonate linkage. The isolated copolymers displayed monomodal molecular weight distributions with PDIs less than 1.10. However, the activity for CO2/2-vinyloxirane coupling is less than that for CO2/propylene oxide copolymerization under the same reaction condition. In efforts to enhance the coupling reactivity of 2vinyloxirane with CO2, terpolymerization of CO2, 2-vinyloxirane (VIO), and propylene oxide (PO) was investigated (Table 2). In comparison with larger PDI values for CO2/VIO/PO coupling

Scheme 1

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Scheme 2

electron-withdrawing effect of the vinyl group (Figure 2). The C NMR suggests that the resulting copolymer has a head-to-tail content of 52%, implicating that ring-opening of 2-vinyloxirane during the copolymerization with CO2 occurs almost equally at both Cα−O and Cβ−O bonds (Figure 3). As shown in Figure 4, MALDI-TOF studies reveal one series of signals with a regular interval of 114.1 m/z (CO2-alt-VIO) being observed, and each signal matches [183.3 (2,4-dinitrophenolate) + (114.1 × n) (CO2-alt-VIO) + 70.1 (VIO) + 1.0 (H) + 39.0 (K+)], confirming the highly alternating nature of the resulting copolymer. In the 1 H NMR of the resulting CO2/VIO/PO terpolymers (P(VIC-coPC)), one (δ = 5.81 ppm) of the two peaks was assigned to methine CH in 2-vinyloxirane carbonate unit and the other (δ = 4.99 ppm) was attributed to methine CH in propylene carbonate unit (Supporting Information, Figure S19a,b). No signal at δ =

catalyzed by catalyst 3 and poor catalytic activity for catalyst 2, the selectivity for polymer formation in the VIO/PO/CO2 terpolymerization catalyzed by catalyst 1 is 100% without any formation of the corresponding cyclic carbonates. The isolated polymers have narrow molecular weight distributions with PDIs less than 1.10 and exhibit monomodal distributions. This result indicates that the presence of propylene oxide significantly activates 2-vinyloxirane and inhibits cyclic carbonate formation and presumably arises from preferentially ring-opening of 2vinyloxirane by a propylene carbonate polymer chain end. The main-chain sequence of the resulting poly(2-vinyloxirane carbonate) (PVIC) was characterized by 1H and 13C NMR spectroscopy (Figures 2 and 3) and MALDI-TOF mass spectroscopy (Figure 4). In the 1H NMR spectrum, the resonance of methine CH is found at 5.81 ppm owing to the

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Table 1. Coupling Reaction of CO2 and 2-Vinyloxirane (VIO)a

cat.

time (h)

temp. (oC)

convd (%)

TOF (h−1)

21 21 21 21 72 21 21 21

25 25 40 25 25 40 25 40

0 16.0 25.1 57.5 89.3 85.2 0 18.7

0 3.8 5.9 27.4 12.4 40.6 0 4.4

b

3 3c 3c 1b 1b 1b 2c 2c

selectivityd (polymer %)

carbonated linkages (%)

Mn(GPC)e (g/mol)

PDI

79 20 100 93 92

100 100 100 100 100

13 800 27 700 26 600

1.048 1.055 1.053

80

100

a c

The coupling reaction was conducted in neat 2-vinyloxirane in a 25 mL autoclave at 33 bar of CO2 pressure. bCatalyst:2-vinyloxirane = 1:1000. Catalyst:2-vinyloxirane = 1:500. dBased on 1H NMR spectroscopy. eTHF was used as eluent for GPC measurements.

Table 2. Terpolymerization of CO2, 2-Vinyloxirane (VIO) and Propylene Oxide (PO)a

cat.

time (h)

temp (oC)

3b

21

25

3b

21

40

1c

21

25

1c

21

40

2b 2b

21 21

25 40

conve (%) 80.9 (VIO: 67.6; PO: 93.2) 84.9 (VIO: 75.2; PO: 93.4) 83.0 (VIO: 69.9; PO: 96.1) 96.7 (VIO: 94.3; PO: 99.0) 0 22.1 (VIO: 16.7; PO: 28.3)

TOF (h−1)

selectivitye (polymer %)

carbonate linkagese (%)

Mn(GPC)f (g/mol)

PDI

19.1

100

100

12 500

1.095

2:3

20.1

90

100

9 100

1.155

10:17

39.5

100

100

18 000

1.061

2:3

46.0

100

100

23 000

1.054

1:1

0 5.4

100

100

n:m (isolated polymer)e

2:3d

a

The terpolymerization was conducted in neat 2-vinyloxirane/propylene oxide in a 25 mL autoclave at 33 bar of CO2 pressure. bCatalyst:2vinyloxirane:propylene oxide = 1:250:250. cCatalyst:2-vinyloxirane:propylene oxide = 1:500:500. dCharacterized by the 1H NMR spectrum of crude product. eBased on 1H NMR spectroscopy. fTHF was used as eluent for GPC measurements.

The 1H NMR spectrum of the amphiphilic polymer containing the pendant hydroxyl groups (PVIO-OH) is shown in Figure S20 of the Supporting Information, where the chemical shift at 3.50 ppm assigned to the methylene protons (−SCH2CH2OH) conjoint with the end OH group. Concomitantly, complete disappearance of the chemical shifts at 5.33 and 5.48 ppm due to the vinyl groups was noted. The methine proton of the polycarbonate backbone shifted from 5.81 to 4.99 ppm as a consequence of the loss of the electron-withdrawing vinyl substituent. A similar upfield shift was observed of the methine proton in the amphiphilic polymer containing the pendant carboxylic acid function (PVIC-COOH) from 5.81 to 4.91 ppm. This observation along with the absence of chemical shifts at 5.33 and 5.48 ppm of the vinyl groups demonstrates that thiol−ene coupling between the vinyl groups and thioglycolic acid is quantitative. The chemical shift observed at 3.24 ppm is assigned to the methylene protons (−SCH2COOH). Because of the large number of pendant hydroxyl/carboxylic acid groups in the resulting polymers, PVIC-OH and PVICCOOH are not soluble in CH2Cl2 but are completely soluble in THF, DMSO, and methanol. GPC (THF) analysis of each of these polymer shows a single peak (PVIC-OH: Mn = 64 700 g/ mol, PDI = 1.182; PVIC-COOH: Mn = 56 300 g/mol, PDI =

Figure 1. (Salen)CoX binary catalyst system (3).

3.3−3.6 ppm assignable to ether linkage units was observed, demonstrating that both copolymer and terpolymer have >99% carbonate linkage. Postpolymerization Functionalization of 2-Vinyloxirane Copolymers. The functionalization of the VIO/CO2 copolymer has been achieved via the azobis(isobutyronitrile) radical initiated additions of the thiols 2-mercaptoethanol or thioglycolic acid (Scheme 3). This thiol−ene reaction for the selective functionalization of polymer side chains occurs with anti-Markovnikov regioselectivity.7 In order to avoid possible cross-linking reactions, excess thiol (40-fold of the number of vinyl groups) was employed in the process. The resulting amphiphilic polymers were purified by repeated precipitation from THF/diethyl ether. 3809

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Figure 2. 1H NMR spectrum (CDCl3) and GPC (THF) trace of copolymer poly(2-vinyloxirane carbonate) (PVIC).

Scheme 3

Figure 3. 13C NMR spectrum (CDCl3) of carbonate region for copolymer poly(2-vinyloxirane carbonate) (PVIC).

may be ascribed to the interaction of functional groups (OH/ COOH) with the resin within the GPC column. In order to synthesize water-soluble polycarbonates, the reaction of PVIC-OH and L-aspartic acid anhydride hydrochloride salt and the deprotonation of PVIC-COOH carboxylic

1.156) without any tailing at the high molecular weight part (Supporting Information Figures S20 and S21), confirming that interchain radical coupling reactions were completely depressed. It is presumed that the tailing at lower molecular weight region

Figure 4. MALDI-TOF mass of poly(2-vinyloxirane carbonate) end-capped with OH group. 3810

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polycarbonates show higher Tgs than the others. An examination of the mechanical behavior of these newly synthesized polymers is beyond the scope of this contribution.

acid side chains were conducted in dry THF under an argon atmosphere. Since it is difficult to know the exact molecular weight of each amphiphilic polymer, the stoichiometric amount of L-aspartic acid anhydride hydrochloride salt or aqueous ammonium hydroxide (30 wt % NH4OH(aq)) is calculated from the mole of pendant vinyl groups within the former polycarbonate PVIC multiplied by 0.9. This was done to avoid the presence of unreacted reagents from contaminating the produced polymers. The ring-opening of L-aspartic acid anhydride by hydroxyl groups generates water-soluble polycarbonate PVIC-OH-Asp containing pending ammonium salts and carboxylic acid groups (Scheme 4). In the 1H NMR

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CONCLUSIONS The bifunctional (salen)Co(III) catalyst (1) bearing a quaternary ammonium salt on the ligand framework was shown to be very effective at catalyzing the copolymerization of CO2 and 2-vinyloxirane (VIO). The process was demonstrated to be highly selective for copolymer formation at ambient temperature to provide the polycarbonate with greater than 99% carbonate linkages. An increase in reaction temperature to 40 °C resulted in a slight decrease in selectivity for copolymer formation, as evident by an 8% production to cyclic carbonate. On the other hand, the binary cobalt(III) analogue (3) provided 80% selectivity for cyclic carbonate formation at 40 °C. In the presence of catalyst 1 conversion of 2-vinyloxirane to polymer was enhanced upon the addition of propylene oxide and the terpolymerization process was 100% selective for polymer formation. That is, the presence of propylene oxide significantly enhanced the rate of incorporation of 2-vinyloxirane into the growing polymer chain but also served to inhibit the backbiting process which led to cyclic carbonate production. DSC studies revealed the glass transition temperature to decrease with increasing content of 2-vinyloxirane in the polycarbonate. Postpolymerization functionalization of these well-defined aliphatic polycarbonates, PVIC, was achieved by the radical addition of thiols containing −OH and −COOH functional groups to the pendant vinyl groups. GPC (THF) studies of these amphiphilic polymers revealed a monomodal distribution of molecular weights with low polydispersity indexes. There was not tailing in the high molecular weight portion of the GPC traces, indicative of the lack of interchain radical coupling process occurring. The further modifications (ring-opening of L-aspartic acid anhydride hydrochloride salt and deprotonation by aqueous ammonium hydroxide (NH4OH(aq))) of PVIC-OH and PVICCOOH led to the successful isolation of water-soluble CO2based polycarbonates, PVIC-OH-Asp and PVIC-COONH4. In comparison with the amphiphilic polymer’s PVIC-COONH4 complete solubility in water, DLS studies demonstrate that the amphiphilic polymer PVIC-OH-Asp forms dispersed particles in water with an average hydrodynamic diameter Dh (DLS, number) = 32.2 ± 8.8 nm. Potentially, this emerging class of amphiphilic/water-soluble polycarbonates could represent a powerful platform for polyvalent conjugation in general and are ideal candidates for polymer therapeutics in particular. Moreover, in contrast to lower Tgs of PVIC, P(VIC-co-PC), PVIC-OH, and PVIC-COOH, polycarbonates PVIC-OH-Asp and PVIC-COONH4 show higher Tgs as a consequence of their intrinsic ionic property (ammonium salts).

Scheme 4

spectrum (D2O) (Figure 5a), the chemical shift at 3.58 ppm is assigned to the methylene protons (−CHCH2COOH) adjacent to the end COOH group. Moreover, the resonances of αmethine proton (α-CHNH3+Cl−) and methylene protons (−CHCH2COOH) are found at 3.57 and 2.86 ppm in the 1H NMR (DMSO:D2O = 25:1 in volume ratio) spectrum (Supporting Information Figure S22), confirming the attachment of L-aspartic acid onto the pendant hydroxyl groups. For the further modification of PVIC-COOH, the polymer was deprotonated with aqueous ammonium hydroxide. The resulting water-soluble polycarbonate PVIC-COONH4 that precipitates in THF was characterized using 1H NMR spectroscopy and elemental analysis (EA). As shown in Figure 5b, the chemical shifts at 4.98 and 4.10−4.17 ppm are attributed to methine proton and methylene protons of the polycarbonate backbone. Also, elemental analysis data show that the measured nitrogen content (5.76%) is close to the calculated value (6.27%), indicating ammonium carboxylate side chains. Dynamic light scattering (DLS) analysis shows no hydrodynamic diameter distribution observed in aqueous solution of PVIC-COONH4, demonstrating the amphiphilic polymer PVIC-COONH 4 dissolves in water completely. However, DLS studies (Supporting Information Figure S23) of amphiphilic polymer PVIC-OHAsp in aqueous solutions at 25 °C show the hydrodynamic diameter Dh (DLS, number) = 32.2 ± 8.8 nm; Dh (DLS, volume) = 44 ± 20 nm; Dh (DLS, intensity) = 88 ± 49 nm, implicating the formation of dispersed particles in water. For the understanding of the thermal properties of the isolated polycarbonates, DSC studies were conducted. As shown in Table 3, the glass transition temperatures (Tg), which are a function of changes in polymer properties such as modulus, decrease with an increase in the content of vinyl groups in the polycarbonates PVIC and P(VICco-PC). In comparison with Tgs of PVIC and P(VIC-co-PC), Tgs of polymers PVIC-OH and PVIC-COOH decrease significantly as side chains of the polycarbonate become longer leading to more flexibility in the side groups. Because of the intrinsic ionic property (ammonium salts) of PVIC-OH-Asp and PVICCOONH4 causing stronger interchain interactions, these two

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EXPERIMENTAL SECTION

General Information. All manipulations involving air- or/and moisture-sensitive compounds were carried out in a glovebox or with standard Schlenk technique under an argon atmosphere. Racemic 2vinyloxirane (97%, Alfa) and propylene oxide (98%, Alfa) were distilled over CaH2 under reduced pressure prior to use. L-Aspartic acid anhydride hydrochloride salt was synthesized according to the published procedure.8 Tetrahydrofuran (THF) was purified by an MBraun manual solvent purification system packed with Alcoa F200 activated alumina desiccant. Bone-dry carbon dioxide supplied in a high-pressure cylinder and equipped with a liquid dip tube was purchased from Scott Specialty Gases. 3811

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Figure 5. 1H NMR spectra (D2O) of water-soluble polycarbonates (a) PVIC-OH-Asp and (b) PVIC-COONH4. α-Cyano-4-hydroxycinnamic acid (J&K, 97%), was used as a matrix. CH3COOK (Aldrich, 98%) was added for ion formation. Gel Permeation Chromatography (GPC). Molecular weight determinations (Mn and Mw) were carried out with a Malvern modular GPC apparatus equipped with ViscoGEL I-series columns (H+L) and Model 270 dual detector composed of RI and light scattering detectors. The curve was calibrated using monodisperse polystyrene standards covering the molecular weight rage from 580 to 460 000 Da. In addition, THF was used as eluent for all GPC measurements. Dynamic Light Scattering (DLS). Dynamic light scattering (DLS) measurements were conducted using Delsa Nano C (Beckman Coulter, Inc., Fullerton, CA) equipped with a laser diode operating at 658 nm. All measurements were made in water (n = 1.3328, η = 0.8878 cP) at 25 ± 1 °C. Each concentration of the PVIC-COONH4 and PVIC-OH-Asp aqueous solutions was 1 mg/mL. Scattered light was detected at 15° angle and analyzed using a log correlator over 70 accumulations for a 0.5 mL of sample in a glass size cell (0.9 mL capacity). Prior to measurement, solutions were filtered through a 0.2 μm PTFE membrane filter to remove dust particles. The photomultiplier aperture and the attenuator were automatically adjusted to obtain a photon counting rate of ca. 10 kcps. The calculation of the particle size distribution and distribution averages was performed using CONTIN particle size distribution.

Table 3. Glass Transition Temperature (Tg) for Polycarbonate polycarbonates a

PVIC P(VIC-co-PC)a (1:1) P(VIC-co-PC) (2:3) PVIC-OH PVIC-COOH PVIC-OH-Asp PVIC-COONH4

Mn (g/mol) (GPC (THF))

Tg (oC) (DSC)

27 700 23 000 20 100 64 700 56 300

18 25 30 −10 8 49 61

a

PVIC: poly(2-vinyloxirane carbonate); PPC: poly(propylene carbonate).

NMR Experiments. 1H and 13C NMR spectra were recorded on Mercury 300 MHz and Inova 300 MHz spectrometers. The peak frequencies were referenced versus the internal standard (TMS) shift at 0 ppm for 1H NMR and against the solvent chloroform-d at 77.0 ppm for 13 C NMR. MALDI-TOF. MALDI-TOF mass spectrometric measurements were performed on a Waters MALDI Micro MX mass spectrometer, equipped with a nitrogen laser delivering 3 ns laser pulses at 337 nm. 3812

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Representative Procedures for the Polymerization of CO2 with 2-Vinyloxirane/Propylene Oxide. A 25 mL Parr autoclave was heated to 120 °C under vacuum for 8 h, cooled under vacuum to room temperature, and moved to a drybox. Catalyst 1 (11.5 mg, 0.01 mmol) and 2-vinyloxirane (8.0 mL, 10 mmol, 1000 equiv) (for terpolymerization, 2-vinyloxirane (4.0 mL, 5 mmol, 500 equiv) and propylene oxide (3.5 mL, 5 mmol, 500 equiv)) were placed in an autoclave equipped with a magnetic stirrer. The autoclave was placed in a bath at the desired temperature (25 or 40 °C) and pressurized to appropriate pressure with CO2. After the allotted reaction time, a small amount of the resultant polymerization mixture was removed from the autoclave for 1H NMR analysis to quantitatively give the conversion of 2-vinyloxirane and was also used for GPC analysis. The crude polymer was dissolved in a 10 mL CH2Cl2/MeOH (5/1, v/v) mixture with 0.5% HCl solution and precipitated from methanol. This process was repeated 3−5 times to completely remove the catalyst, and white polymer was obtained by vacuum-drying. Thiol−Ene Click Reaction between Copolymer and Thiol. A typical procedure for synthesis of amphiphilic polymer was started with the ratio of reagents [CC]0/[thiol]0/[AIBN]0 = 1/40/0.33. Thiol− ene click reaction between polycarbonate (0.5 g, 4.3 mmol of CC groups) and thiol (2-mercaptoethanol: 13 mL, 175 mmol; thioglycolic acid: 13 mL, 178 mmol) was conducted in a 100 mL Schlenk flask under an argon atmosphere with 25 mL of THF as solvent and AIBN (0.27 g, 1.5 mmol) as initiator. The reaction mixture was stirred for 24 h at 70 °C. After filtration, the solvent was removed by rotary evaporation. The crude product was dissolved into THF and precipitated in diethyl ether. The process was repeated for several times, and amphiphilic polymer was obtained by vacuum-drying. Reaction of Amphiphilic Polymer PVIC-OH and L-Aspartic Acid Anhydride Hydrochloride Salt. To a 50 mL round-bottom flask containing 0.9 equiv (based on mole of pendant vinyl groups of former polycarbonate PVIC) of L-aspartic acid anhydride hydrochloride salt was added THF solution of PVIC-OH under a positive Ar atmosphere. The reaction mixture was stirred for 24 h at ambient temperature. The resulting suspension was filtered, and the white solid was collected and vacuum-dried. Deprotonation of Amphiphilic Polymer PVIC-COOH Using Aqueous Ammonium Hydroxide. 0.9 equiv (based on mole of pendant vinyl groups of former polycarbonate PVIC) of aqueous ammonium hydroxide (30 wt % NH4OH(aq)) was added into the THF solution of PVIC-COOH dropwisely via syringe under a positive Ar atmosphere. The reaction mixture was stirred for 5 min at ambient temperature. The resulting suspension was filtered, and the white solid was collected and vacuum-dried. It is noticed that the resulting polymer PVIC-COONH4 is highly hydroscopic and becomes gluey in air. Anal. Calcd for (C7H13NSO5)n: C, 37.67; H, 5.83 ; N, 6.27. Found: C, 38.75; H, 6.02; N, 5.76.

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their DLS instrument and help with the light scattering measurements.

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ASSOCIATED CONTENT

S Supporting Information *

Figures, experimental details, and characterization data for CO2based polymers. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (D.J.D.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the National Science Council of Taiwan (NSC 102-2917-I-564-057), the National Science Foundation of the USA (CHE-1057743), and the Robert A. Welch Foundation (A-0923). We are also thankful to Professor Karen L. Wooley and Gyu Seong Heo for use of 3813

dx.doi.org/10.1021/ma500834r | Macromolecules 2014, 47, 3806−3813