Preparation of High-Molecular-Weight Aliphatic Polycarbonates by

Apr 16, 2013 - A synthetic strategy was developed for the condensation polymerization of aliphatic diols with dimethyl carbonate to produce ...
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Preparation of High-Molecular-Weight Aliphatic Polycarbonates by Condensation Polymerization of Diols and Dimethyl Carbonate Ji Hae Park, Jong Yeob Jeon, Jung Jae Lee, Youngeun Jang, Jobi Kodiyan Varghese, and Bun Yeoul Lee* Department of Molecular Science and Technology, Ajou University, Suwon 443-749, Korea S Supporting Information *

ABSTRACT: A synthetic strategy was developed for the condensation polymerization of aliphatic diols with dimethyl carbonate to produce high-molecular-weight aliphatic polycarbonates. In the first step, oligomers were formed bearing almost equal numbers of hydroxyl and methyl carbonate end-groups. In the second step, the condensation reaction was conducted at a high temperature (>180 °C) to connect the −OH and −OC(O)OCH3 chain-ends while removing the generated methanol under reduced pressure. Small amounts of sodium alkoxide (0.02−0.5 mol %) were used as a catalyst. Using an anhydrous diol was crucial for increasing the reaction rate and also for obtaining reproducible results. In the second step, the pressure was gradually reduced and the temperature was optimized, in order to minimize side products. Using this strategy, high-molecular-weight poly(1,4-butylene carbonate) (PBC) and its copolymers incorporating various other diols (2−10 mol %) were prepared, with weight-average molecular weight (Mw) of 100 000−200 000, in a short reaction time, totaling 6.5 h. This strategy was also effective for producing other high-molecular-weight aliphatic polycarbonates (Mw ∼ 200 000) using 1,6-hexanediol and cyclohexane-1,4-dimethanol. When the [−OH]/[−OCH3] ratio of the oligomers generated in the first step deviated from ∼1, it was hard to attain such a high molecular weight.

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derivatives (Scheme 1b).10−12 This ring-opening polymerization can also provide a high-molecular-weight poly(trimethylene carbonate) (Mw of up to 618 000). This polymer is also amorphous (Tg = −19 °C) but shows strain-induced crystallization (Tm = 36 °C).13 However, cyclic monomers are expensive, and the resulting ring-opened polymers have been studied mainly to find biomedical applications. Aliphatic polycarbonates in which carbonate linkages are connected by more than three carbon atoms, such as poly(1,4butylene carbonate) (PBC) and poly(hexamethylene carbonate) (PHC), can also be prepared through ring-opening polymerization of the corresponding cyclic carbonate dimers (Scheme 1b). This method also provides high-molecular-weight polymers (Mw 119 000 and 399 000 for PBC and PHC, respectively).14 However, access to the cyclic monomers is restricted because of their very low synthetic yields, which precludes large-scale preparation of PBC and PHC by this method.15,16 The best strategy for preparing aliphatic polycarbonates in which the carbonate linkages are connected by more than three carbon atoms is the condensation polymerization of dimethyl carbonate (DMC) and aliphatic diols (Scheme 1c). DMC was previously prepared industrially using phosgene, which is hazardous, but it is now produced on a large scale using carbon monoxide or, more attractively, using carbon dioxide.17,18 The aliphatic diol 1,4-butanediol (BD) is an inexpensive chemical produced industrially on a large scale.

INTRODUCTION Aliphatic polycarbonates are attractive as biodegradable materials. Aliphatic polycarbonates in which the carbonate linkages are connected by two carbon atoms can be prepared by copolymerization of CO2 and an epoxide (Scheme 1a).1,2 Scheme 1. Preparation of Aliphatic Polycarbonates

Catalyst development has been an issue in such copolymerizations,3−7 and a highly efficient catalyst, which can produce highmolecular-weight polymers (Mn up to 300 000), has recently been developed. These polymers are currently in the early stage of commercialization.8,9 The CO2/propylene oxide and CO2/ ethylene oxide copolymers are amorphous, with glass-transition temperatures (Tg) of 40 and 20 °C, respectively. Aliphatic polycarbonates in which the carbonate linkages are connected by three carbon atoms can be prepared through ring-opening polymerization of cyclic trimethylene carbonate and its © 2013 American Chemical Society

Received: February 21, 2013 Revised: April 1, 2013 Published: April 16, 2013 3301

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Scheme 2. Strategies for Preparing a High-Molecular-Weight Poly(1,4-butylene carbonate)

oligomers bearing −OH end-groups and unreacted diol after the first step to isolate the oligomers mostly end-capped with methyl carbonate, with which the second step was conducted.21 In a patent, a mixture of mono- and bis(methyl carbonate) esters of BD was formed in the first step, and these were reacted at an elevated temperature under evacuation to perform the second step.34 Li reported the adoption of this strategy.29,30 In some reports, the bis(methyl carbonate) of an aliphatic diol was isolated for the polycondensation.21,35 Most trials using this strategy resulted in the formation of low-molecular-weight polymers with Mn less than 10 000. We designed a slightly different strategy for preparing highmolecular-weight aliphatic polycarbonates (Scheme 2b). The difference is the formation of oligomers in the first step bearing almost equal numbers of methyl carbonate and hydroxyl endgroups, instead of oligomers end-capped mostly with methyl carbonate. In the first step, DMC and BD are reacted at the boiling point of DMC (90 °C) with atmospheric pressure distillation of the generated methanol along with some DMC, which forms an azeotrope with methanol. The desired [−OCH3]/[−OH] ratio of ∼1.0 in the oligomers formed in the first step may be achieved by adjusting either the feed [DMC]/[diol] ratio or the reaction time. In the second step, the chains grow mostly by reaction between the methyl carbonate and hydroxyl end-groups, with formation of methanol as a byproduct. Catalyst Screening. Various catalysts such as Ti(OR)4,27,28,36 calcined MgAl hydrotalcites,26 Sn compounds,21 Ca,25 Li,35 Na,37 K2CO3,38 tertiary amine or ammonium salts,34 a complex system composed of TiO 2 /SiO 2 /poly(vinylpyrrolidone) (TSP-44),29 and even enzymes23 have been used in the condensation polymerization of dialkyl carbonates and diols. Most of them gave rather slow reaction rates, and long reaction times of more than 10 h were required, even in the preparation of low-molecular-weight macrodiols. We screened the catalysts previously used in the condensation polymerizations. The conversions of −CH2OH to −CH2OC(O)O− groups were determined from 1H NMR spectra after reacting BD and DMC (2.5 equiv) at the boiling point of DMC (90 °C) for 1 h in the presence of 0.01 mol % catalyst. Ti(OEt)4 and Sb2O3, which are active catalysts in PET synthesis, were inactive under the aforementioned conditions. A Sn compound, Sn(Oct)2, which is typically used in the ringopening polymerization of lactones, was also inactive. A complex system composed of TiO2/SiO2/poly-

Recently, its production capacity has been increased because it is an indispensable ingredient in biodegradable aliphatic polyesters.19,20 PBC is an attractive semicrystalline polymer (Tm = 60 °C) and is also biodegradable. However, there have been few reports describing the successful preparation of highmolecular-weight aliphatic polycarbonates through condensation polymerization of DMC and an aliphatic diol (Scheme 1c). Sivaram reported the preparation of PBC using 1,3diphenoxytetra-n-butyldistannoxane as a catalyst, but the Mn was only 8000.21 Picquet and Plasseraud recently reported the preparation of PBC with a molecular weight of similar magnitude (Mn 6600) using 1-n-butyl-3-methylimidazolium-2carboxylate as a catalyst.22 Gross reported the preparation of PHC in diphenyl ether solvent using lipase as a catalyst; the Mw was 27 000.23,24 Because of the difficulty of preparing highmolecular-weight aliphatic polycarbonates, the emphasis in industry has been on the preparation of low-molecular-weight macrodiols for use as intersegments in polyurethanes.25−28 Recently, Li et al. reported the preparation of a high-molecularweight PBC and a PHC (Mn up to 94 000) using complex catalytic system composed of TiO 2 , SiO 2 , and poly(vinylpyrrolidone).29 In other publications, they reported the preparation of PBCs of Mn 51 000 or 38 000, using the same catalyst and under almost identical conditions.30,31 In this work, we report a new strategy for the preparation of high-molecularweight aliphatic polycarbonates with Mw values in the range 100 000−250 000.

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RESULTS AND DISCUSSION

Strategy for Generating High-Molecular-Weight Aliphatic Polycarbonates. In the commercial production of conventional aromatic polycarbonates, equimolar amounts of bisphenol A and diphenyl carbonate are condensed while removing the byproduct phenol by evacuation,32,33 but a twostep strategy has been adopted in the condensation polymerization of BD and DMC (Scheme 2a), mimicking the strategy used in a commercial synthesis of poly(ethylene terephthalate) (PET). The first step is transesterification, and the second step is polycondensation under evacuation. In the first step, the oligomers, mostly capped with methyl carbonate (−OC(O)OCH3), were generated using excess DMC, while distilling off the generated methanol at an atmospheric pressure. In the second step, the chain length was increased through elimination of DMC by the reaction between the two methyl carbonate chain-ends (Scheme 2a). Sivaram washed out the water-soluble 3302

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(vinylpyrrolidone) (TSP-44), reported to be able to produce high-molecular-weight PBC, was inactive. The effective catalysts were simple bases of alkali metals. The countercation of the base influenced the rate, presumably as a result of coordination to the carbonyl facilitating nucleophilic attack of the alkoxide anion. tBuOLi gave the highest conversion (39%). tBuONa gave a slightly lower conversion (30%). tBuOK and [Bu4N]+[OH]− were inferior (17% and 12% conversions, respectively) to Li- and Na-based bases. DMC was not miscible with BD at the initial stage, but they formed a homogeneous phase as a result of the transesterification reaction. Bases of divalent metals, such as Ca(OCH3)2 and Zn(OPh)2, were inactive. Preparation of High-Molecular-Weight PBC. Although lithium alkoxide showed slightly higher activity than sodium alkoxide, we chose sodium alkoxide as the catalyst because it is cheaper. Sodium alkoxide was generated in situ by addition of NaH to BD. It was reported in a patent that a high amount (1.0 mol %) of sodium alkoxide was used as the catalyst in the preparation of low-molecular-weight macrodiols.37 After polymerization, the basic catalyst was neutralized by washing with diluted aqueous acid after dissolving the products in CH2Cl2. We reduced the catalyst amount used in order to prepare a high-molecular-weight polymer because the attainable DP is theoretically limited by the amount of basic catalyst by the equation DPmax = 2 × [BD]/[base]. At a feed level of 0.20 mol % NaH per BD, the theoretical limiting DP was sufficiently high up to 1000, and the sodium content in the resulting polymer would be as low as 400 ppm, presumably not affecting the polymer properties, even without removal of catalyst residue. When a two-phase mixture of BD (10.0 g, 110 mmol) and DMC (15.0 g, 167 mmol) was stirred, after addition of 0.20 mol % of NaH, in a bath of 120 °C for 1.0 h, while distilling off the generated methanol at atmospheric pressure using a simple distillation apparatus, the extent of the transesterification reaction was unsatisfactorily low (entry 1 in Table 1). The 1 H NMR spectrum indicated that the [−OCH3]/[−OH] ratio was 0.48, i.e., lower than the desired value of ∼1, and that only 54% of the −CH2OH groups were converted to −CH2OC(O)O− groups. Furthermore, the [−OCH3]/[−OH] ratio and the conversion were not reproducible when the experiments were repeated, even using the same bottle of BD. We thought that water contamination of the BD might be a factor in the poor results. When the BD was dried and handled under an inert atmosphere, the transesterification reaction rate in the first step was significantly improved, and the desired [−OCH3]/ [−OH] ratio of 1.01 was achieved with 79% conversion of −CH2OH to −CH2OC(O)− groups (entry 2 in Table 1; Figure 1a). This result was consistently reproducible once dried DB was used. In the first step, some excess DMC (1.57 equiv) was fed because it was distilled off together with the generated methanol. The feed amount of DMC was smaller than the amounts (2−3 equiv per BD) fed by other researchers, whose strategy was preparation of oligomers mostly end-capped with methyl carbonate (Scheme 2a).21,22,29 The DMC feed amount may be reduced further using a well-designed distillation apparatus, with which methanol can be efficiently fractionated from DMC. In this study, we used a simple and short-path distillation setup. The reaction rate was fairly fast using anhydrous BD, and most of the methanol was generated and distilled off at an early stage, within 20 min. Lengthening the

Table 1. 1,4-Butanediol/Dimethyl Carbonate Condensation Polymerization Resultsa entry

base/mol %

[−OCH3]/ [−OH]b

yield (%)

Mwc × 10−3

Mw/Mn

1d 2e 3 4f 5e 6e 7 8 9 10 11

NaH/0.20 NaH/0.20 NaH/0.20 NaH/0.20 NaH/0.20 NaH/0.20 NaH/0.50 NaH/1.0 NaH/0.10 NaH/0.050 NaH/0.020

0.48 1.01 1.10 1.10 4.51 0.48 1.10 1.08 1.09 1.09 1.15

82 90 86 89 37 86 87 93 90 85

77 146 248 38 32 145 87 135 124 107

1.50 1.53 1.65 1.48 1.61 1.60 1.63 1.66 1.65 1.57

a

Polymerization conditions: dried BD (10.0 g, 110 mmol) and DMC (15.7 g, 174 mmol) in a bath of 120 °C for 1.0 h, while removing methanol at atmospheric pressure, in the first step, then at 190 °C successively under 570 mmHg for 0.50 h, 380 mmHg for 1.0 h, 190 mmHg for 2.0 h, and finally 0.3 mmHg for 2 h, in the second step. b The integration ratios of −OC(O)OCH3 to −CH2OH signals observed in the 1H NMR spectra of the oligomers generated in the first step. cMeasured on GPC at 40 °C using THF solvent and polystyrene standards. dBD was used as received without drying. e DMC amounts were 15.0, 25.0, and 10.0 g for entries 2, 5, and 6, respectively. fFull evacuation time of 8 h. gLow yield as a result of formation of significant amount of THF.

Figure 1. 1H NMR spectra of oligomers formed in the first step (a) and of high-molecular-weight poly(1,4-butylene carbonate) (b) (entry 3 in Table 1).

reaction time to over 1.0 h only marginally influenced the [−OCH3]/[−OH] ratio and the conversion. Undesirable side reactions might occur in the second step at a high temperature (Scheme 3). In the reaction between the −OH and −OC(O)OCH3 end-groups, the two chains could be connected by generation of methanol and also by generation of the mono(methyl carbonate) of BD, which was volatile under 3303

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Scheme 3. Side Reactions in the 1,4-Butanediol/Dimethyl Carbonate Condensation Polymerization

ratio fell below 1.0 as the second step proceeded; the [−OCH3]/[−OH] ratio was 0.70 at the starting point of the final full evacuation. In this situation, the final chain growth was achieved by formation of either methanol (Scheme 2b) or BD (Scheme 3c). The formation of BD resulted in a low yield (82%). Furthermore, chain growth by formation of BD was not effective limiting the increase in the molecular weight (see below). To compensate for the reduction in the [−OCH3]/ [−OH] ratio by formation of some DMC in the second stage, we prepared oligomers in which the [−OCH3]/[−OH] ratio was slightly higher than 1.0 (i.e., 1.10) by slightly increasing the [DMC]/[BD] feed ratio. When the second step was conducted according to the same procedure with the oligomers at a ratio [−OCH3]/[−OH] = 1.10, the [−OCH3]/[−OH] ratio became 0.96 at the starting point of the final full evacuation, and eventually a higher-molecular-weight PBC was generated (Mw, 146 000; Mw/Mn = 1.53) with a higher yield, i.e., 90% (entry 3). The polydispersity index (Mw/Mn) was 1.53, still far below Flory’s theoretical asymptotic value 2.0, which might be attributed to loss of some oligomeric portions during the sample preparation for the GPC studies. For the GPC study, polymer sample was thoroughly washed with water to remove the catalyst-related salts. In the 1H NMR spectrum of the generated PBC, only two signals were observed, at 4.15 and 1.77 ppm, with equal integration values, and end-group signals were not detected (Scheme 2b). By lengthening the reaction time of the full evacuation stage from 2 h to 4, 6, and 8 h, the molecular weight (Mw) was gradually increased from 120 000 to 155 000, 218 000, and 248 000, respectively (entry 4). As the molecular weight of PBC increased, the enthalpy of melting (ΔH) decreased, but the melting temperature was constant at 68 °C (see Supporting Information Table S1 and Figure S2). The high-molecular-weight PBCs showed a typical thermoplastic stress−strain curve with a high tensile strength and a high elongation at break, while these values were lower for a low-molecular-weight PBC (see Supporting Information Table S1 and Figure S3). Our strategy was valid for the preparation of high-molecularweight PBC. For comparison, we prepared oligomers mostly end-capped with methyl carbonate, i.e., oligomers in which the [−OCH3]/[−OH] ratio was fairly high, namely 4.51, and the second step of the polycondensation was conducted using identical conditions and procedures (entry 5). The molecular weight attained was low, Mw = 38 000, under identical reaction conditions and a total reaction time of 6.5 h; in the 1H NMR

evacuation (Scheme 3a). The reaction shown in Scheme 3b was the main chain-growth reaction in the strategy used by others, in which the volatile byproducts evacuated were DMC and the bis(methyl carbonate) of BD. In the reaction shown in Scheme 3c, the byproduct was BD, which was also volatile. The formation of cyclo(tetramethylene carbonate) was thermodynamically unfavorable, but its dimeric form, cyclobis(tetramethylene carbonate), was generated to some extent and sublimed above 180 °C under full evacuation (Scheme 3d). All these reactions (Scheme 3a−d) were reversible, and there was no effect if the byproducts remained in the reaction medium. In order to make the byproducts remain in the reaction medium as much as possible, the evacuation level was increased stepwise: 570 mmHg for 0.50 h, 380 mmHg for 1.0 h, 190 mmHg for 2.0 h, and finally full evacuation at 0.3 mmHg for 2 h. Even for this stepwise action, loss of DMC generated by the side reaction in Scheme 3b was inevitable because of its low boiling point (90 °C), but the other compounds, cyclobis(tetramethylene carbonate)s, and BD and its mono- and bis(methyl carbonate)s remained in the reaction medium at the evacuation stages of 570, 380, and 190 mmHg. At the final stage of full evacuation (0.3 mmHg), cyclobis(tetramethylene carbonate), BD, and its mono- and bis(methyl carbonate)s were volatile to be condensed in the cooling zone of the distillation apparatus or at the upper part of the reactor wall. However, the evaporated amounts of these byproducts at the final full evacuation stage were not significant, and satisfactorily high yields were achieved (85−90% based on added BD). The detrimental side reaction was formation of THF by the alkylation of alkoxide anion, which was irreversible (Scheme 3e). However, THF was generated only above 190 °C under our base-catalyzed conditions. At 180 °C, THF was not detectable at all in analysis of the liquid collected in the cold trap. At 190 °C, a negligible amount of THF was generated, but a substantial amount of THF was detected at 200 °C. Therefore, we set the second-step reaction temperature at 180−190 °C. When the condensation reaction was conducted with the free-flowing oily oligomers at a ratio of [−OCH3]/[−OH] = 1.0, prepared in the first step according to the procedure described above, a high-molecular-weight PBC was obtained, with Mw 77 000 and a polydispersity index (Mw/Mn) of 1.50 (entry 2 in Table 1). In the second step, the undesirable formation of DMC (Scheme 3b) was inevitable because of its low boiling point (90 °C). Therefore, the [−OCH3]/[−OH] 3304

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Scheme 4. Chain-Growth Mechanisms in the Base-Catalyzed Aliphatic Diol/Dimethyl Carbonate Condensation Polymerization

integration value for the −CH2OH signal decreased, but its rate of decrease rate slowed down by time, with integration values of 1.46, 1.21, and 1.11 at additional reaction times of 4, 6, and 8 h, respectively; the increase of molecular weight was not high, with Mw = 32 000, 42 000, and 53 000 for each additional reaction time, respectively. Variations in the catalyst amount in the range 0.02−1.0 mol % did not influence the [−OCH3]/[−OH] ratio in the first step. At a high amount of catalyst (1.0 mol %), the generated polymer melt became turbid in the second stage as a result of low solubility of alkoxide anions in the polymer melt medium, and the molecular weight of the generated polymer was relatively low (Mw 87 000, entry 8). By gradually lowering the catalyst amount from 0.20 mol % to 0.10, 0.05, and 0.02 mol %, the molecular weight was gradually, but not significantly, decreased from Mw 145 000 to 135 000, 124 000, and 107 000, respectively, probably because of lower condensation rate in the second step as a result of using a lower amount of catalyst (entries 2 and 9−11). Condensation Polymerization of Other Aliphatic Diols with DMC. Inexpensive, readily available aliphatic diols (1−5 in Chart 1) were screened for condensation polymerization

spectrum, the −OCH3 end group signal was detected with an integration value of 1.13 when the integration value for the −CH2O(CO)− signal was given as 100. The −CH2OH signal was absent. At the early stage of the second step, the depletion rate of the −OCH3 signal was fairly fast in the 1H NMR studies, indicating that chain growth occurred well through the generation of methanol and/or DMC (Scheme 2b and/or Scheme 3b). However, the rate slowed down after complete disappearance of the −CH2OH signal. When the condensation reaction was conducted further under conditions of full evacuation and 190 °C, the integration value of the −OCH3 signal decreased slowly from 1.13 to 0.89, 0.66, 0.51, and 0.49 for additional reaction times of 2, 4, 6, and 8 h, respectively. The molecular weight also increased slowly from an Mw of 38 000 to 67 000, 87 000, 110 000, and 126 000 for the additional reaction times of 2, 4, and 6, and 8 h, respectively. In our strategy of base-catalyzed condensation polymerization, chain growth occurs mostly by the transesterification reaction between −OH and −OC(O)OCH3 end-groups, with formation of methanol. The added basic catalyst becomes a chain-end alkoxide anion, which eventually attacks the carbonyl of the −OC(O)OCH3 chain-end, forming a tetrahedral intermediate. Liberation of methoxide anions from the tetrahedral intermediate results in connection of the two chain-ends (Scheme 4). The liberated methoxide anions are protonated by −OH end-groups to produce volatile methanol, with regeneration of chain-end alkoxide anions (Scheme 4a). In the strategy of other researchers, chain growth occurs mostly by connecting the two methyl carbonate chain-ends, with formation of DMC. For the formation of DMC, the liberated methoxide anions should subsequently attack another methyl carbonate chain-end (Scheme 4b). The probability of these successive attacks on the methyl carbonate chain-ends decreased dramatically with increasing chain-length, resulting in slowing down of the condensation rate as the chain-length increased. In the presence of both methyl carbonate and hydroxyl end-groups, the liberated methoxide anions were protonated to become volatile methanol by a rapid protonexchange reaction with the −OH end-group, allowing a faster condensation rate even at a high conversion (Scheme 4a). With the oligomers enriched with hydroxyl end-groups, it was harder to prepare a high-molecular-weight polymer (entry 6). When the second step was conducted under identical reaction conditions and total reaction time of 6.5 h, with an oligomer of [−OCH3]/[−OH] = 0.48, a low-molecular-weight polymer of low viscosity was generated. In the 1H NMR spectrum, the −CH2OH end-group was observed with an integration value of 7.32 relative to an integration value of 100 for the −CH2O(CO)O− signal. The −OCH3 end-group signal was absent. When the condensation reaction was further conducted under conditions of full evacuation at 190 °C, the

Chart 1. Aliphatic Diols Used for Condensation Polymerization with Dimethyl Carbonate

with DMC, adopting the same strategy. Because there was little chance of losing reactant diol in the second step of the evacuation, because of the high boiling points of 1−5, the [−OCH3]/[−OH] ratio was set at a slightly higher value of ∼1.3 in the first step. The [DMC]/[diol] ratio in the feed was adjusted case by case to obtain [−OCH3]/[−OH] = ∼1.3, and the reaction time and temperature were fixed at 1 h and 120 °C, respectively. There were no side reactions such as formation of THF as was the case with BD (Scheme 3e), and we could increase the reaction temperature above 190 °C. Using this strategy, a high-molecular-weight PHC (Mw 201 000) was prepared in a short reaction time totaling 6 h (entry 1 in Table 3305

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0.94−1.37; the data were not optimized values but obtained in a trial using a feed with appropriate [DMC]/[diol] molar ratio (1.50 or 1.60). When the [DMC]/[diol] feed ratio was 1.50 (entries 1−3 and 10−12), the [−OCH3]/[−OH] ratios after the first step were slightly lower (0.94−1.14). The molecular weights were fairly high in the range Mw = 100 000−170 000 but, in these cases, the high molecular weights were attained at the expense of the yield. The yields were relatively low, 71− 79%, as a result of the reaction generating BD (Scheme 3c) at the latter stage of the second step. When the [DMC]/[diol] feed ratio was slightly higher at a 1.60 (entries 4−9), the [−OCH3]/[−OH] ratios in the first step were 1.13−1.37, and the molecular weights were satisfactorily high, mainly in the range Mw = 150 000−200 000 (entries 4−5 and 8−9), and with better yields (80−84%). In the case of a 10 mol % feed of 1,4benzenedimethanol (3), the molecular weight was relatively low (Mw 93 000), probably because of the high viscosity of the resulting polymer (entry 6). By incorporating a small amount (2 mol %) of a diol (2, 3, 4, or 5), the melting temperature of the resulting polymer was lowered from that of PBC (68 °C) to 54, 57, 51, or 52 °C, respectively, and the enthalpy of melting (ΔH) was also lowered. The effects of lowering the melting temperature and the ΔH value were minimal for 1,4-benzenedimethanol (3), and the melting endotherm was observed even at 5 mol % incorporation (entry 5). For 5 mol % incorporation of the other diols (2, 4, and 5), the polymers were amorphous. When the incorporated amount was 10 mol %, all the copolymers were amorphous. The Tg increased not only with increasing incorporated amount but also with increasing bulkiness of the comonomer diol.

2). When cyclohexane-1,4-dimethanol (2) was used, a highmolecular-weight aliphatic polycarbonate (Mw 174 000) was Table 2. Results for Condensation Polymerizations of Dimethyl Carbonate and Various Diolsa entry diol 1 2 3

[DMC]/ [diol]

[−OCH3]/ [−OH]b

yield (%)

Mwc × 10−3

Mw/Mn

Tgd (°C)

1.56 1.65 1.80

1.28 1.31 1.29

95 96 98

201 174 45

1.87 1.90 1.81

−51 45 73

1 2 4

a

Polymerization conditions: diol (10.0 g) and NaH (0.20 mol % per diol) in bath of 120 °C for 1.0 h, while distilling off methanol at atmospheric pressure, in the first step; 380 mmHg at 180 °C for 1 h and then full evacuation at 180 °C (entry 1) or 210 °C (entries 2 and 3) for 2 h, and at 200 °C (entry 1) or 240 °C (entries 2 and 3) for 2 h, in the second step. bThe integration ratio of −OC(O)OCH3 to −CH2OH signals observed in the 1H NMR spectra of the oligomers generated in the first step. cMeasured on GPC at 40 °C using THF solvent and polystyrene standards. dMeasured on DSC in the second heating cycle.

also generated in a total reaction time of 6 h (entry 2). In this case, the final reaction temperature in the second step was 240 °C to overcome the stirring problems caused by the high viscosity at the latter stage. For 1,4-benzenedimethanol (3), the oligomers formed in the first step were solids, which did not melt even at 240 °C, hampering further condensation. When 4,8-bis(hydroxymethyl)tricyclodecane (4) was used, a relatively a low-molecular-weight polymer was obtained. At the latter stage of the second step, the polymer melt became turbid because of the low solubility of the alkoxide anions in the resulting hydrophobic polymer melt; this limited further chain growth. The hydrogenated compound obtained from bisphenol A, 4,4′-isopropylidenedicyclohexanol (5), remained as a solid in DMC even at high temperature, and it was impossible even to conduct the first step. Copolymerization Using a Mixture of BD and Various Diols. Adopting the strategy described above, high-molecularweight aliphatic copolymers (Mw 90 000−210 000) were also prepared by feeding BD mixed with a minor amount of diol, selected from 2−5 (2, 5, and 10 mol %; Table 3). The [−OCH3]/[−OH] ratios after the first step were in the range

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EXPERIMENTAL SECTION

General Remarks. BD was dried by stirring overnight at 80 °C after addition of Na and phthaloyl dichloride (0.25 equiv per Na) and was then vacuum-distilled to a reservoir. Other high-boiling-point diols were dried by evacuation overnight. The NMR spectra were recorded on a Varian Mercury Plus 400 instrument. The gel permeation chromatograms (GPCs) were obtained in THF at 40 °C using a Waters Millennium apparatus with polystyrene standards. The Tm data were determined at the offset point of the endotherm signal from the first heating, at rate of 10 °C/min, in differential scanning calorimetry

Table 3. Results for Condensation Copolymerization Using a Mixture of 1,4-Butanediol and Various Diolsa entry

diol

[diol]/[BD]

[−OCH3]/[−OH]b

yield (%)

Mwc × 10−3

Mw/Mn

Tmd (°C)

Tgd (°C)

ΔHd (J/g)

1 2 3 4 5 6 7 8 9 10 11 12

2 2 2 3 3 3 4 4 4 5 5 5

0.02 0.05 0.10 0.02 0.05 0.10 0.02 0.05 0.10 0.02 0.05 0.10

0.94 1.09 1.08 1.27 1.15 1.13 1.20 1.32 1.37 1.03 1.08 1.14

71 73 76 80 82 84 80 84 83 77 76 79

170 133 115 156 158 93 113 210 163 125 120 104

1.58 1.66 1.59 1.61 1.94 1.96 1.60 1.63 1.68 1.67 1.72 1.67

54 n.d.e n.d. 57 54 n.d. 51 n.d. n.d. 52 n.d. n.d.

−30 −28 −22 −31 −29 −26 −29 −25 −14 −28 −19 −4

28

31 15 27

19

a Polymerization conditions: BD + diol (110 mmol), DMC (16.0 g for entries 4−9, 15.0 g for the other entries) and NaH (5.3 mg, 2.2 mmol) in bath at 120 °C for 1.0 h, while distilling off methanol at atmospheric pressure, in the first step; 380 mmHg at 180 °C for 2 h and then full evacuation at 180 °C for 2 h and at 190 °C for 2 h, in the second step. bThe integration ratio of −OCH(O)OCH3 to −CH2OH signals observed in the 1H NMR spectra of the oligomers generated in the first step. cMeasured on GPC at 40 °C using THF solvent and polystyrene standards. dMeasured on DSC. e Not detected (amorphous).

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(DSC) using a Thermal Analysis Q10 instrument. The Tg data were determined from a second heating at a heating rate of 10 °C/min. For measurements of the enthalpy of melting (ΔH), the samples were treated identically: They were kept at 90 °C for 1 h, then cooled gradually to 30 °C over 5 h, and finally kept at 30 °C overnight. Representative BD/DMC Condensation Polymerization. A three-necked flask (∼50 mL) was equipped with a mechanical stirrer and distillation apparatus and connected to a manifold equipped with vacuum and N2 gas lines. BD (10.0 g, 111 mmol), NaH (5.3 mg, 0.20 mol %), and DMC (15.7 g, 174 mmol) were successively added to the flask under inert conditions, i.e., N2 gas. The flask was immersed in a hot-oil bath (120 °C), and the first step of the reaction was conducted for 1 h, while distilling off volatiles under atmospheric pressure. Before moving to the second step, an aliquot was removed and evacuated for 1 H NMR spectroscopy, based on which the [−OCH3]/[−OH] ratio was calculated. The pressure was reduced to 570 mmHg, and the bath temperature was increased to 190 °C. The condensation reaction was conducted for 0.5 h at that temperature and pressure while the generated volatiles were condensed using a dry ice/acetone bath. After the pressure was further reduced to 380 mmHg, the condensation reaction was conducted for 2 h at 190 °C. The pressure was further reduced to 190 mmHg, and the condensation reaction was conducted for another 1 h. Finally, the reaction was conducted for a further 2 h with full evacuation at 0.3 mmHg. At the final stage, some cyclobis(tetramethylene carbonate) (∼0.3 g) was deposited as crystals on the upper part of the reactor wall; these were distilled off to the distillation condenser, using a heat gun, under evacuation. The monoand bis(methyl carbonate) of BD were condensed in the distillation condenser as a wetted solid mixed with BD. In the 1H NMR spectrum in CDCl3, two signals were observed, at 4.17 and 1.78 ppm, with equal integration values, and the signals for −CH2OH and −C(O)OCH3 end-groups were not detected. Analytical data for the crystals deposited on the upper part of the reactor wall were in agreement with those for cyclobis(tetramethylene carbonate); mp 179 °C. 1H NMR (CDCl3): δ 4.27−4.16 (m, 8H, CH2OC(O)), 1.93−1.82 (m, 8H, CH2CH2OC(O)) ppm. 13C NMR (CDCl3): δ 154.80, 67.50, 25.17 ppm. 1H and 13C NMR spectra of the compounds condensed in the cooling zone of the distillation apparatus as a wetted solid indicated that they were a mixture of BD and its mono- and bis(methyl carbonate)s. 1H NMR (CDCl3): δ 4.20−4.12 (m, CH2OC(O)), 3.76 (s, (O)COCH3), 3.70−3.63 (m, CH2OH), 2.23 (s, OH), 1.82−1.71 (m, CH2CH2OC(O)), 1.71−1.60 (m, CH2CH2OH) ppm. 13C NMR (CDCl3): δ 155.68 (carbonyl), 68.02 (mono-), 67.51 (bis-), 62.86 (BD), 62.34 (mono-), 54.91 (bis-), 54.87 (mono-), 30.09 (BD), 29.06 (mono-), 25.40 (bis-), 25.35 (bis-) ppm. After the polymerization, the base catalyst was effectively neutralized by feeding phthaloyl dichloride or melamine phenylphosphonate. After quenching, the polymer showed the same level of thermal stability as that of the pure polymer (see Supporting Information Figure S1). Thus, the polymerization pot was cooled to 100 °C, and a THF solution (1.0 mL) of phthaloyl dichloride (22 mg, 0.11 mmol) was added using a syringe. The viscous polymer melt was stirred at 160 °C for 1 h. 1,6-Hexanediol/DMC Condensation Polymerization. The first step was conducted with 1,6-hexanediol (10.0 g, 85 mmol), NaH (4.1 mg, 0.17 mmol), and DMC (12.0 g, 133 mmol), using identical conditions and procedures to those used for DB/DMC condensation polymerization. In the second step, the condensation reaction was conducted at 180 °C under a reduced pressure of 380 mmHg for 1 h, at 190 °C under full evacuation at 0.3 mmHg for 2 h, and finally at 200 °C under full evacuation for 2 h. The basic catalyst was neutralized with phthaloyl dichloride, using the same procedures and conditions applied as those for DB/DMC condensation polymerization. The polymerization results are summarized in Table 2. 1H NMR (CDCl3): δ 4.11 (br, 4H, CH2OC(O)), 1.66 (br, 4H, CH2CH2OC(O)), 1.39 (br, 4H, CH2CH2CH2OC(O)) ppm. No end-groups were detected. Representative (BD + Diol)/DMC Condensation Copolymerization. The first step was conducted with a mixture of BD and a diol chosen among 2−5 (total, 110 mmol), NaH (5.3 mg, 0.2 mol %), and DMC (15.0 g, 167 mmol, or 16.0 g, 178 mmol), using identical conditions and procedures as those used for DB/DMC condensation

polymerization. In the second step, the condensation reaction was conducted at 180 °C and 380 mmHg for 2 h, at 180 °C and 0.3 mmHg for 2 h, and finally at 190 °C and 0.3 mmHg for 2 h. The catalyst was neutralized with phthaloyl dichloride using the same procedures and conditions as those used for DB/DMC condensation polymerization. The polymerization results are summarized in Table 3. 1H NMR (CDCl3) for [BD + 2 (10 mol %, a mixture of two isomers in 7:3 ratio)]/DMC condensation copolymerization: δ 4.15 (br, 4H, CH2OC(O)), 4.04 (d, J = 7.2 Hz, 0.12H, cis-2−CH2OC(O)), 3.94 (d, J = 6.4 Hz, 0.28H, trans-2−CH2OC(O)), 1.77 (br, 4H, CH2CH2OC(O)), 1.98−0.82 (m, 1.0H, 2-CH2 and CH) ppm. 1H NMR (CDCl3) for [BD + 3 (10 mol %)]/DMC condensation copolymerization: δ 7.38 (br, 0.40H, 3-C6H4), 5.14 (br, 0.40H, 3CH2OC(O)), 4.15 (br, 4H, CH2OC(O)), 1.77 (br, 4H, CH2CH2OC(O)) ppm. 1H NMR (CDCl3) for [BD + 4 (10 mol %, a mixture of two isomers)]/DMC condensation copolymerization: δ 4.16 (br, 4H, CH2OC(O)), 4.02−3.82 (m, 0.40H, 4−CH2OC(O)), 2.60−0.90 (m, 1.4H, 4-CH2 and CH), 1.77 (br, 4H, CH2CH2OC(O)) ppm. 1H NMR (CDCl3) for [BD + 5 (10 mol %, a mixture of two isomers in 2:1 ratio)]/DMC condensation copolymerization: δ 4.87 (br, 0.07H, cis5−CHOC(O)), 4.58−4.40 (m, 0.13H, trans-5−CHOC(O)), 4.14 (br, 4H, CH2OC(O)), 2.20−0.73 (m, 2.4H, 5-CH2 and CH), 1.77 (br, 4H, CH2CH2OC(O)) ppm.

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CONCLUSIONS High-molecular-weight aliphatic polycarbonates of Mw 100 000−250 000 were successfully prepared by condensation polymerizations of DMC and various diols, in a reasonably short reaction time, i.e., a total 6.5 h. The success was the result of using a new two-step strategy. In the first step, lasting 1 h, oligomers bearing almost equal numbers of −OH and −OC(O)OCH3 end-groups were prepared, and then the polycondensation was carried out in the second step, lasting 5.5 h, at a high temperature (>180 °C) under evacuation; the polymer chains grew mainly by the reaction between −OH and −OC(O)OCH3 end-groups, generating methanol. In order to minimize side reactions such as formation of cyclic compounds, the diol, and its mono- and bis(methyl carbonate)s, the pressure was reduced stepwise. Drying the diol before use was crucial for increasing the reaction rate and also for obtaining consistent and reproducible results. Catalysts were screened, and sodium alkoxide was used in amounts of 0.5−0.02 mol % per diol. Our new strategy was valid for the preparation of highmolecular-weight polymers; it was impossible to prepare such high-molecular-weight polymers when the polycondensation was conducted using identical conditions and reaction times with the oligomers prepared in the first step enriched with −OH or −OC(O)OCH3 end-group.

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ASSOCIATED CONTENT

S Supporting Information *

Cyclohexane-1,4-dimethanol/DMC and 4,8-bis(hydroxymethyl)tricyclo[5.2.1.02,6]decane/DMC polymerization conditions, catalyst-quenching studies, and thermal and tensile properties of the polymers. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 3307

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ACKNOWLEDGMENTS

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REFERENCES

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This work was supported by the Korea CCS R&D Center (KCRC) grant (No. 2012-0008935) and Priority Research Centers Program (No. 2012-0006687) funded by the Korea Government (Ministry of Education, Science and Technology).

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