Polymer Journal (2011) 43, 1003–1007 & The Society of Polymer Science, Japan (SPSJ) All rights reserved 0032-3896/11 $32.00 www.nature.com/pj 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