Microwave-Assisted Low-Temperature Dehydration Polycondensation of Dicarboxylic Acids and Diols

RAPID COMMUNICATION

Microwave-assisted low-temperature dehydration polycondensation of dicarboxylic acids and diols

Polymer Journal (2011) 43, 1003–1007; doi:10.1038/pj.2011.107; published online 26 October 2011

INTRODUCTION time (4100 h). Therefore, we next focused on has been no report concerning a Currently, because of increasing concerns identifying more active catalysts and found that non-thermal effect in microwave-assisted about damage to the environment, the devel- scandium and thulium bis(nonafluorobutane- polycondensation reactions,33,34 although opment of new, eco-friendly (industrially sulfonyl)imide ((Sc(NNf2)3) and (Tm(NNf2)3)) there has been a report that non-thermal relevant) chemical reactions and materials is were more efficient catalysts and allowed us microwaves have a role in the chain polymer- crucial. Aliphatic polyesters have attracted to obtain high-molecular-weight polyesters ization of a lactone.32 Therefore, we studied 4 much interest as environmentally benign, (Mn42.0Â10 ) from adipic acid (AdA) and microwave-assisted syntheses of polyesters at biodegradable polymers.1,2 In general, alipha- 3-methyl-1,5-pentanediol (MPD) at 60 1Cina a relatively low temperature (80 1C) using a tic polyesters are commercially produced by shortperiodoftime(24h)andwithasmaller microwave chamber equipped with a tem- polycondensation of a dicarboxylic acid and a amount of catalyst (0.1 mol%) than had pre- perature control, and the results are reported 1.1–1.5 mol excess of a diol at a temperature viously been possible.26 herein. We compared the rates of the micro- 4250 1C and under an extremely reduced Rapid syntheses that depend on microwave wave-assisted polymerizations with those pressure.3–6 These severe reaction conditions irradiation have attracted interest because obtained using conventional heating, which preclude the syntheses of aliphatic polyesters they are environmentally benign. Since first allowed us to characterize the contribution of with low thermostabilities and the use of reported in 1986,27,28 microwave irradiation the non-thermal effects of the microwaves. thermally unstable monomeric reagents. has been used to shorten the reaction time of This is the first report of a catalyzed step Polycondensation reactions are catalyzed organic syntheses, to decrease the levels of polymerization for which the second-order by Lewis acids,7–13 but only a few of these side products and to improve the yields rate constant was increased by a non-thermal acids are suitable catalysts because most are and/or chemoselectivities of the products.29 microwave-induced effect. labile in the presence of protic substances, for The fact that there is an exponentially example, carboxylic acids, alcohols and water, increasing number of publications dealing EXPERIMENTAL PROCEDURE making these reagents unsuitable for dehyd- with microwave-assisted chain polymeriza- Materials ration polycondensation reactions. Recently, tions, for example, anionic polymerization AdA, glutaric acid, sebacic acid and succinic we reported that at or near room temperature of acrylamides,30 ring-opening cationic acid were purchased from Nacalai Tesque, direct polycondensation of diols and dicar- polymerization of 2-oxazolines31 and ring- Inc., (Kyoto, Japan). Methyl succinic acid boxylic acids, catalyzed by scandium trifluor- opening polymerization of e-caprolactones,32 was purchased from Aldrich Co., Ltd. (Mil- 14 omethanesulfonate (triflate) (Sc(OTf)3) or indicates the remarkable interest in this techni- waukee, WI, USA). Sc(OTf)3 was purchased scandium trifluoromethanesulfonimide (tri- que.33,34 In addition to chain polymerizations, from Tokyo Chemical Industry Co., Ltd. 15,16 flylimide) (Sc(NTf2)3), affords aliphatic microwave-assisted step-growth polymeriza- (Tokyo, Japan). MPD and decahydro- polyesters with number-average molecular tions have been successfully attempted in a naphthalene (mixture of cis and trans) were 4 33,34 weights (Mn) 410 (room-temperature poly- domestic microwave oven. For example, purchased from Wako Pure Chemical Indus- condensation17–21). We also demonstrated Scherf et al.35 synthesized donor-acceptor tries, Ltd. (Osaka, Japan). Scandium bis(no- that these polycondensation systems can pi-conjugated polymers using microwave nafluorobutanesulfonyl)imide (Sc(NNf2)3) incorporate thermally unstable monomers that irradiation at 150 1C for 15 min. Nagahata was prepared in our laboratory according to contain a carbon–carbon double bond,16 a and colleagues36 obtained poly(butylene the procedure described in the literature.26 bromo group,16 hydroxyl groups,22 mercapto succinate) that had a weight-average molecu- 23 24,25 4 group and/or a disulfide linkage and lar weight (Mw)of2.90Â10 in the incredibly Measurements that these reactions are under kinetic control short time of 10 min using microwave 1H-NMR spectra were recorded at 27 1C (chemoselective dehydration polycondensation). irradiation. However, because of the high using a Bruker Analytik DPX200 spectro- Although the polycondensation reactions were temperatures that they used (200–260 1C), meter (Bruker BioSpin, Kanagawa, Japan, run as one-step reactions under mild condi- it is difficult to assess the importance of a 200 MHz). The number-average molecular tions (35 1C), they required large amounts of non-thermal effect on their polycondensation weight (Mn) and the polydispersity index the catalyst (ca. 1 mol%) and long reaction reactions. To the best of our knowledge, there (Mw/Mn) of each polyester were estimated Rapid Communication 1004

using a size exclusion chromatography system placed into a multimode microwave reactor ferent conditions to compare the effects of that included a Tosoh DP8020 pump system, (MWO-1000S, EYELA, 2.45 GHz, maximum microwave and oil bath heating. The product, an RI (Tosoh, Tokyo, Japan, Tosoh RI-8020) power 500 W). MPD, AdA and a catalyst poly(3-methylpentamethylene adipate), has a detector and a Tosoh TSKgel SuperMultipor- were mixed at 80 1C until a homogeneous low glass transition point (Tg¼À63 1C). eHZ-M column calibrated with polystyrene state was observed. The pressure was then Because this product was an amorphous standards. The eluent was CHCl3,theflow gradually decreased to 3.0 kPa, and we solid at the temperature of our experiments, rate was 0.35 ml minÀ1 and the temperature defined this point as t¼0, at which we the molecular rotations of the chain should was 40 1C. Differential scanning calorimetry obtained oligoesters with an Xn of ca 10 have allowed the polycondensation reactions using a DSC6220S calorimeter (Seiko Instru- without reducing the pressure. A 200 W to proceed as bulk reactions. A microwave- ments Inc., Chiba, Japan) was performed microwave was used to irradiate the reaction assisted polycondensation of AdA with MPD from À130 to 80 1C, with the temperature mixture at 80 1C in the temperature control was performed at 80 1C (temperature-con- increased or decreased at a rate of 10 1Cper mode using decahydronaphthalene as the trolled microwave irradiation for 6 h at a min. The instrument was calibrated using cooling medium until a homogeneous state maximum power of 200 W) with Sc(OTf)3 indium and tin samples. For all experimental was observed. The internal temperatures of as the catalyst (0.5 mol% relative to the total samples, the heating cycle from À130 to 80 1C polycondensation reaction mixtures exposed number of moles of reactants), which yielded 3 and back to À130 1C was reproducible. Each to microwave irradiation were measured a polyester with a Mn of 9.8Â10 (Table 1, sample weighed between 4 and 6 mg and was using a thermocouple equipped with a run 3). The Mn of that polyester was larger by placed into an aluminum pan that was cov- microwave reactor (MWO-1000S) and were B53% than the one obtained by conven- 3 ered with a lid within the calorimeter. The controlled using a proportional–integral– tional heating (Mn¼6.4Â10 ,run1).Addi- glass transition temperature (Tg)wastakenas derivative control and PC software (EYELA, tionally, polyesters with similar Mn values the inflection point of the corresponding heat Tokyo, Japan). The temperature and irradia- were obtained by microwave heating and by capacity jump of the differential scanning tion power profile (Supplementary Figure conventional heating when using a reaction calorimetry trace. The melting temperature S1) of the representative example showed time for the former was half that of the (Tm) was defined as the minimum point of that the temperature was maintained at latter (3 h, run 2 vs 6 h, run 1). We also the endothermic trough. Matrix-assisted laser 80 1C during the polycondensation. The observed substantial increases in Mn when desorption/ionization time-of-flight mass pressure was gradually decreased to 3.0 kPa, the Sc(OTf)3 concentration was reduced spectra were recorded using a Kratos PCAx- at which point the polycondensation com- (0.1 mol%, Table 1, runs 4–6) for the micro- ima CFRplus V2.4.0 mass spectrometer menced. When the reaction was complete, wave runs in comparison with the conven- (Kratos, Manchester, UK) using 1,8,9-anthra- the yield of the polyester was calculated by tional heating runs and when the more cenetriol as the matrix reagent. NaI was subtracting the known weight of the catalyst effective catalyst, scandium bis(nonafluoro- included to generate sodium cations of the from the total weight of the solid present. butanesulfonyl)imide (Sc(NNf2)3), was used polycondensation products ([M+Na]+). (Table 1, runs 7–12). When the polymeriza- RESULTS AND DISCUSSION tions using Sc(OTf)3 as the catalyst were Polycondensation of dicarboxylic acids and Microwave-assisted polycondensation carried out for 24 h, we obtained polyesters 4 diols catalyzed by scandium catalysts in an reactions of AdA and MPD catalyzed by with Mn values of 1.5Â10 (Table 1, runs 13). oil bath Sc compounds To examine the effects of microwave For poly(3-methylpentamethylene adipate), a Dehydration polycondensation reactions of heating on the polycondensation reaction, typical polycondensation was carried out as AdA with MPD were performed under dif- 1H-NMR spectra of poly(3-methylpenta- follows. In a 30 ml three-necked test tube, MPD (1.65 g, 14.0 mmol), AdA (2.05 g, 14.0 mmol) and catalysts were stirred at Table 1 Direct polycondensations of AdA and MPD under reduced pressure at 80 1Ca 80 1C (760 mm Hg) until a homogeneous b c 3 c state was observed. We confirmed that both Run Catalyst Mol% Heating method Time (h) Yield (%) Mn (crude) Â10 Mw/Mn the temperatures of oil bath and reaction 1Sc(OTf)3 0.5 OB 6 99 6.4 1.87 mixture in the flask were 80 1C using thermo- d 2Sc(OTf)3 0.5 MW 3— 6.71.83 couples. The pressure was gradually decreased d 3Sc(OTf)3 0.5 MW 6 95 9.8 1.85 to 3.0 kPa, at which point polycondensation 4Sc(OTf)3 0.1 OB 6 99 3.8 1.91 commenced (t 0), and we confirmed that d ¼ 5Sc(OTf)3 0.1 MW 3— 4.41.89 d oligoesters with an Xn of ca 10 were formed. 6Sc(OTf)3 0.1 MW 6 96 5.6 2.01 When the reaction was finished, the yield of 7Sc(NNf2)3 0.1 OB 6 99 7.9 1.84 d thepolyesterwascalculatedbysubtracting 8Sc(NNf2)3 0.1 MW 4— 8.31.82 d the known weight of the catalyst from the 9Sc(NNf2)3 0.1 MW 6 97 9.6 1.80

total weight of the solid present. 10 Sc(NNf2)3 0.05 OB 6 99 5.8 2.06 d 11 Sc(NNf2)3 0.05 MW 3— 6.52.08 d Polycondensation of dicarboxylic acids and 12 Sc(NNf2)3 0.05 MW 6 94 9.3 1.85 d diols catalyzed by scandium catalysts under 13 Sc(OTf)3 0.5 MW 24 — 15.1 1.96

microwave irradiation Abbreviations: AdA, adipic acid; Mn, number-average molecular weights; MPD, 3-methyl-1,5-pentanediol; Mw, weight-average For poly(3-methylpentamethylene adipate), molecular weight; MW, microwave; OB, oil bath. aAll runs are performed by bulk condensation at 3.0 kPa and 80 1C. MPD (1.65 g, 14.0 mmol), AdA (2.05 g, bWithout reprecipitation. c Determined by size exclusion chromatography with CHCl3 as the eluent. Values are reported relative to those of poly(styrene) 14.0 mmol) and catalysts were mixed in a standards. 30 ml three-necked test tube that was then dTemperature-controlled irradiation at a maximum power of 200 W.

Polymer Journal Rapid Communication 1005

50 70 60 40 Sc(OTf)3 Sc(NNf2)3 50 30 40 n n X X 20 30 20 Microwave 10 Microwave 10 Oil bath Oil bath 0 0 0 10 20 30 40 50 60 0 10 20 30 40 50 60 Time (min) Time (min)

Figure 1 Plots of number-average molecular weights (Mn) vs time for the polycondensation of adipic acid and 3-methyl-1,5-pentanediol. Left panel, 3 kPa, 80 1C, 0.5 mol% Sc(OTf)3. Samples were removed every 10 min outside of the temperature-controlled environment (solid lines; microwave heating, diamonds; oil-bath heating, squares), or a sample was removed after 60 min (circle, dashed line). The lines are linear interpolations of the data points. Right panel: conditions were as for the experiments shown in the left panel except that 0.5 mol% Sc(NNf2)3 was the catalyst. A full color version of this ﬁgure is available at Polymer Journal online.

ab60 50

50 40 40 30 n n

30 X X 20 20 Microwave Microwave 10 10 Oil bath Oil bath 0 0 0 102030405060 0 102030405060 Time (min) Time (min)

cd50 50

40 40

30 30 n n X X 20 20 Microwave 10 Microwave 10 Oil bath Oil bath 0 0 0 102030405060 0 102030405060 Time (min) Time (min)

Figure 2 Plots of number-average molecular weights (Mn) vs time for direct polycondensations of 3-methyl-1,5-pentanediol and various dicarboxylic acids ((a) succinic acid, (b) glutaric acid, (c) methyl succinic acid, (d) sebacic acid) under reduced pressure at 80 1C catalyzed by Sc(OTf)3 ((catalyst)¼0.5 mol%). A full color version of this ﬁgure is available at Polymer Journal online.

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38,39 methylene adipate) synthesized by Sc(OTf)3 Data for the polymerization of AdA with tion. We established that the microwave- catalysis under conditions of microwave and MPD catalyzed by Sc(OTf)3 or Sc(NNf2)3 are assisted polycondensation reactions were at conventional heating were recorded. These shown in Figure 1. The data follow equation 5 least 1.4-fold more rapid than were the spectra showed that all polyesters had the with values for Xn that increase linearly as a corresponding conventional polycondensa- expected structures and that peaks that function of reaction time. The Mw/Mn values tion reactions: poly(3-methylpentamethylene could be ascribed to side-reaction products that were obtained from the plots are between succinate), 1.6-fold; poly(3-methylpentamethy- were absent (Supplementary Figure S2). 1.8 and 2.1 and are approximately those lene glutarate), 1.4-fold; poly(3-methylpenta- Matrix-assisted laser desorption/ionization predicted by Flory’s theory of polycondensa- methylene methylsuccinate), 2.5-fold; and poly 38,39 time-of-flight spectra were recorded to char- tion, where Mw/Mn¼1+p and p is the (3-methylpentamethylene sebacate), 1.8-fold. acterize the absolute molecular weights and extent of reaction. In addition, we found that Interestingly, when MSA was used, the Mn for À1 repeat units of the polyesters. In the spectra of the proportionality constant (c0k¼0.633 s ) the polyester produced by microwave-assisted the polyesters obtained by each heating for the microwave-assisted polycondensation polycondensation with more than two times method, two peak patterns separated by an was 1.6 times greater than that of the con- greater than that for the polyester produced À1 m/z of ±229 were observed. The value of 229 ventional-heating experiment (c0k¼0.397 s ; by the conventional heating was attained. is the expected m/z for the repeating unit Figure 1, left panel). Additionally, with In summary, for the work reported in this (Supplementary Figure S3). The differences Sc(NNf2)3 as the catalyst, the polycon- communication, we demonstrated that among the m/z values of the three peaks densation proportionality constant obtai- microwave heating accelerates the rate of within a group suggest that the polyesters ned when microwave irradiation was used AdA/MPD polycondensation catalyzed by À1 terminated with an a-hydroxyl and an (c0k¼0.876 s ) was 1.5 times greater than Sc(OTf)3 at a moderate temperature and o-carboxyl, an a-ando-hydroxyl or an when conventional heating was used that a smaller amount of catalyst can be À1 a-ando-carboxyl. For example, the peaks (c0k¼0.580 s ) (Figure 1, right panel). used than in a conventional polycondensa- at 2546, 2647 and 2674 would be derived To conclusively demonstrate that a non- tion. We also investigated how microwave from polyesters having an a-hydroxyl and an thermal effect was the cause of the increased heating affects the kinetics of polycondensa- o-carboxyl, an a-ando-hydroxyl, and an rate, we had to eliminate the possibility that tion in detail by determining the second- a-ando-carboxyl at the termini, respectively. the rate difference was the result of removing order proportionality constants from plots Peaks at 2582, which correspond to cyclic the samples outside of the oil bath and the of Xn as a function of time. Finally, we also polyesters, were also observed. microwave chamber. Therefore, we also car- found that the non-thermal microwave effect ried out polycondensation reactions for might affect the rates of polycondensation Polycondensation kinetics 60 min before sampling (Figure 1, left panel, reactions for dicarboxylic acids other than We also investigated the effect of micro- dash lines). Given that there was no apparent AdA. These fundamental results provide wave irradiation on the second-order difference between the sampling methods for new guidelines for the microwave-assisted, kinetics of the catalyzed polycondensation both sets of experiments, we concluded that eco-friendly production of materials. reactions.37 In general, when equimolar the microwave-assisted polycondensation amounts of dicarboxylic acids and diols reactions were accelerated by a non-thermal CONFLICT OF INTEREST (c¼[COOH]¼[OH]) are mixed, polyconden- microwave-induced effect. The authors declare no conflict of interest. sation in the presence of a catalyst is a second-order reaction, and the number-aver- Microwave-assisted polycondensation ACKNOWLEDGEMENTS of MPD with dicarboxylic acids other aged degree of polymerization (Xn)is This work was funded by the Ministry of Educa- 37 than AdA expected to increase linearly with time. We tion, Science and Culture of Japan (Grant-in-Aid removed aliquots from the conventional- To determine whether the non-thermal for Development Scientific Research, no. heating and microwave-heating reactions microwave-induced effect was independent 22750104). We are grateful to Dr Sadatsugu every 10 min, which were then used to deter- of the type of monomer, dicarboxylic acids Takayama of the National Institute for Fusion (succinic acid, glutaric acid, methyl succinic Science for valuable advice and discussions. mine the Mn values (by size exclusion chro- acid and sebacic acid) were individually poly- matography) and to calculate Xn. condensed with MPD at 80 1C and 3.0 kPa Shinji Yamada and Akinori Takasu d½COOH R ¼À ¼ k½COOH½OH with 0.5% mol Sc(OTf)3 (these conditions dt were used for a polycondensation of AdA and Department of Frontier Materials, dc MPD). All polycondensation reactions pro- Graduate School of Engineering, Nagoya !À ¼ kc2 ð1Þ Institute of Technology, Nagoya, Japan dt ceeded smoothly as expected because all of the polyester products are amorphous and E-mail: [email protected] have a glass transition point lower than room 1 ¼ kt+const: ð2Þ temperature: (poly(3-methylpentamethylene c succinate), À51 1C; poly(3-methylpentamethy- c0 À c lene sebacate), À88 1C; poly(3-methylpenta- 1 Lenz, R. W. Biodegradable polymers. Adv. Polym. Sci. p ¼ ! c ¼ c0ð1 À pÞ 107, 1–40 (1993). c0 methylene methylsuccinate), À64 1C; poly 2 Koeshak, V. V. & Vinogradora, S. V. Polyester (Pergamon (3-methylpentamethylene glutarate), À64 1C). Press, New York, NY, 1995). ðp : extent of polymerizationÞð3Þ 3 Carothers, W. H. & Dorough, G. L. Studies on polymer- We always obtained linear relationships ization and ring formation. IV.. J. Am. Chem. Soc. 52, c0 1 1 between Xn and time (Figure 2), and for 711–721 (1930). Xn ¼ ¼ ! 1 À p ¼ ð4Þ 4 Takiyama, E., Niikura, I. & Hatano, Y. Japan Patent c 1 À p X Mw/Mn, we obtained values between 1.8 and n 189823 (1992). 2.1, which are approximately the values 5 Miura, M., Watanabe, H. & Fujiwara, M. Japan Patent Xn ¼ c0kt+const: ð5Þ predicted by Flory’s theory of polycondensa- 53695 (1995).

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