Communication pubs.acs.org/Biomac
Dehydration Polycondensation of Dicarboxylic Acids and Diols Using Sublimating Strong Brønsted Acids Takaya Moyori, Tang Tang, and Akinori Takasu* Department of Frontier Materials, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan S Supporting Information *
ABSTRACT: We investigated catalytic activities of strong brønsted acids for dehydration polycondensations of dicarboxylic acids and diols, which were carried out at low temperature ( 19000) but also aromatic polyester (Mn > 7000). The used Nf2NH was sublimated from the reaction flask during polycondensation, and the sublimate, Nf2NH, was extra pure so that we can reuse the catalyst without loss of the activity in the dehydration polycondensations.
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monomers that contain a carbon−carbon double bond,9a a bromo group,9a hydroxyl groups,10 a mercapto group,9b and/or a disulfide linkage9c,d and are under kinetic control (chemoselective dehydration polycondensation). Although the polycondensations were run as one-step reactions and under mild conditions (35 °C), they required large amounts of catalyst (ca. 1 mol %) and long reaction times (>100 h). Therefore, we next focused on identifying more active catalysts and found that scandium and thulium bis(nonafluorobutanesulfonyl)imide ([Sc(NNf2)3] and [Tm(NNf2)3]) were more efficient catalysts that allowed us to obtain high-molecular-weight polyesters (Mn > 2.0 × 104) from adipic acid (AdA) and 3-methyl-1,5pentanediol (MPD) at 60 °C in a shorter period of time (24 h) and with a smaller amount of catalyst (0.1 mol %) than had previously been possible.11 Considering recent high-fidelity supplies for electric devices containing polyesters and the biomedical applications of such devices, the presence of residual metals in the polyesters is a more serious problem. Therefore, a survey of organic catalysts is still desirable from both environmental and industrial viewpoints. We recently found that bis(1,1,2,2,3,3,4,4,4-nonafluoro-1-butanesulfonyl)imide (Nf2NH) has a high activity in the ring-opening polymerization of ε-caprolactone,12 and Nf2NH can be purified by washing12 or sublimation. This background prompted us to assess the Nf2NH-catalyzed direct polycondensation of diols and dicarboxylic acids and the
INTRODUCTION Aliphatic polyesters are an attractive class of polymer that can be used in biomedical and pharmaceutical applications.1 One reason for the growing interest in this type of degradable polymer is that the physical and chemical properties can be varied over a wide range, for example, through copolymerization and advanced macromolecular architecture.1 The synthesis of novel polymer structures through ring-opening polymerization has been studied for a number of years because it allows better control over molecular weight and molecular weight distribution and provides access to more complex topologies such as block copolymers, star polymers, and so on, and synthesis via the polycondensation of dicarboxylic acids and diols requires excess amounts of diols (1.1 to 1.5 equiv), temperatures above 250 °C, and extremely reduced pressures.2,3 These severe reaction conditions preclude the synthesis of aliphatic polyesters with low thermostabilities and the use of thermally unstable monomeric reagents. Although polycondensations are catalyzed by some Lewis acids,4 only a few rare-earth Lewis acids are not deactivated by alcohols, water,5 and carboxylic acid.6 Therefore, it had been difficult to survey the catalysts suitable for dehydration polycondensations. We already reported that at around room temperature, direct polycondensations of diols and dicarboxylic acids catalyzed by scandium trifluoromethanesulfonate (triflate) [Sc(OTf)3]7a or scandium trifluoromethanesulfonimide (triflylimide) [Sc(NTf2)3]7b afford aliphatic polyesters with number-average molecular weights (Mn) > 104 (room-temperature polycondensation).8 We also demonstrated that these polycondensation systems can incorporate thermally unstable © 2012 American Chemical Society
Received: February 13, 2012 Revised: April 23, 2012 Published: April 26, 2012 1240
dx.doi.org/10.1021/bm300231d | Biomacromolecules 2012, 13, 1240−1243
Biomacromolecules
Communication
Scheme 1. Low-Temperature Polycondensation of Adipic Acid and 3-Methyl-1,5-pentanediol
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RESULTS AND DISCUSSION The dehydration polycondensation of AdA with MPD served as a model for amorphous polyester production. Various strong Brønsted acids were used as catalysts, and the polymerization conditions were surveyed. The product, poly(3-methylpentamethylene adipate), has a low-temperature glass point (Tg = −63 °C). Therefore, as it is an amorphous solid at the temperature of our experiments, the increased molecular motions of the chain (in comparison with a crystalline solid) should allow the polycondensation to proceed further at lower temperatures. The polycondensation of AdA with MPD proceeded at 60 °C when catalyzed by Nf2NH to give a polyester with Mn = 10.2 × 103, even though the reaction was run at a low catalyst concentration (0.05 mol % compared with the total moles of reactants) (run 1). When the concentration was increased to 0.1 mol % (run 3), a polyester with Mn = 19.1 × 103 was obtained (Mw/Mn = 2.26). The structure was confirmed by 1H and 13C NMR spectra (Figure S1a of the Supporting Information and Figure S1b). MALDI-TOF mass spectrum of the polyester was also used to determine the structure (Figure S3a of the Supporting Information), in which there is one set of peaks with repeat of 229 m/z, which corresponds to the molecular weight of the repeating unit consisting of AdA and MPD. To check the effect of the degree of reduced pressure on the Mn, we also investigated the polycondensation under 10 mmHg of reduced pressure, but it hardly influenced the molecular weight (Mn). For comparison of the catalytic efficiencies of Nf2NH with other strong Brønsted acids, rare-earth catalysts, and PTSA (run 10) as a general-use acid catalyst, bulk polycondensations of AdA and MPD to afford poly(3-methylpentane adipate) were run at 60 °C for 18 h. For the strong Brønsted acids Tf2NH (run 7), NfOH (run 8), and TfOH (run 9) (0.1 mol % compared with the total number of moles of reactants), the Mn values estimated by SEC were (14.3, 13.9, and 12.3) × 103, respectively, which were greater than that with PTSA (Mn = 5.8 × 103) or the rare-earth catalyst Sc(NNf2)311(Mn = 10.3 × 103). This result indicated that the strong Brønsted acids were more effective catalysts than the rare-earth catalysts7,9−11 or PTSA for bulk polycondensations at moderate temperatures (Table 1). Furthermore, a remarkable increase in Mn was observed when Nf2NH was used as the catalyst. Compared with the reported Lewis acids Sc(NNf2)311 (run 11) and Sc(OTf)37a (run 12), it is clear that Nf2NH is an excellent new candidate for the dehydration polycondensation catalyst. To demonstrate the ability of this catalyst, we performed polycondensations of some monomers (Table 2) under moderate temperatures. In particular, the polycondensation of diethylene glycol (run 3, Tg = −46 °C, Tm = −2 °C) and triethylene glycol (run 4, Tg = −54 °C, Tm = −9 °C) also proceeded to afford the corresponding water-soluble polyesters, although the Sc(OTf)3-catalyzed polycondensations were very
subsequent recovery and reuse of the catalyst via sublimation. In this study, the catalytic abilities are compared with other strong Brønsted acids, including bis(trifluoromethanesulfonyl)imide (Tf2NH), 1,1,2,2,3,3,4,4,4-nonafluoro-1-butanesulfonic acid (NfOH), and 1,1,1-trifluoro-1-methanesulfonic acid (TfOH), for the synthesis of high-molecular-weight (Mn > 10.0 × 103) polyesters in short periods of time and under mild conditions.
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EXPERIMENTAL SECTION
Materials. Nf2NH was obtained from Mitsubishi Materials Electronic Chemicals (Akita, Japan). NfOH and Tf2NH were purchased from Aldrich (Milwaukee, WI). TfOH, dodecanedioic acid, and 1,12-dodecanediol were purchased from Tokyo Chemical Industry (Tokyo, Japan). AdA, p-toluenesulfonic acid (PTSA), ethylene glycol, maleic acid, and phthalic acid were purchased from Nacalai Tesque (Kyoto, Japan). MPD, diethylene glycol, triethylene glycol, succinic acid, and 4,4′-isopropylidenebis(2-phenoxyethanol) were purchased from Wako Pure Chemical Industries (Osaka, Japan). Polycondensation. For poly(3-methylpentamethylene adipate) and other polyesters, a detailed procedure is given here as an example. In a 10 mL round-bottomed flask, MPD (0.83 g, 7.0 mmol), AdA (1.02 g, 7.0 mmol), and Nf2NH (8.2 mg, 0.1 mol %) were stirred at 80 °C (760 mmHg) until a homogeneous state was observed. The pressure and temperature were gradually decreased to 0.3−3 mmHg and 60 °C, respectively, at which point polycondensation commenced. When the reaction was finished, the yield of the polyester was calculated by subtracting the known weight of the catalyst from the total weight of the solid present (>99% yield). Before DSC measurements, the produced polyesters were purified by repricipitation using chloroform and hexane as the good and poor solvents, respectively. Measurements. 1H and 13C NMR spectra were recorded at 27 °C using a Bruker DPX200 spectrometer (200 MHz for 1H and 50 MHz for 13C). Chemical shifts were referenced to tetramethylsilane (δ = 0). The number-average molecular weight (Mn) and polydispersity index (Mw/Mn) of each polyester was estimated using a size exclusion chromatography (SEC) system that included a Tosoh DP8020 pump system, an RI (Tosoh RI-8020) detector, and a Tosoh TSK-GEL SUPERMULTIPOREHZ-M column calibrated with polystyrene standards. The eluent was CHCl3, the flow rate was 0.35 mL/min, and the temperature was 40 °C. Differential scanning calorimetry (DSC), which used a DSC6220S calorimeter (Seiko Instruments, Chiba, Japan), was performed from −90 to 180 to −90 °C at 10 °C/ min. The instrument was calibrated with indium and tin. Each polyester sample weighted between 5 and 7 mg and was contained in an aluminum pan that was covered with a lid within the calorimeter. The glass-transition temperature (Tg) was taken as the inflection point of the DSC heat-capacity jump. The melting temperature (Tm) was defined as the minimum point in the endothermic trough. Matrixassisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectra were recorded by using a JMS-S3000 mass spectrometer (JEOL), with dithranol as the matrix reagent. CF3COONa was used to generate sodium-cationized ions of the copolymers ([M+Na]+). 1241
dx.doi.org/10.1021/bm300231d | Biomacromolecules 2012, 13, 1240−1243
Biomacromolecules
Communication
Table 1. Direct Polycondensations of AdAa and MPDb under Reduced Pressure at 60 °Cc time
yieldd
Mn (crude)e
run
catalyst
mol %
h
%
×103
Mw/Mne
1 2 3 4 5 6 7 8 9 10 11 12
Nf2NH Nf2NH Nf2NH Nf2NH Nf2NH Nf2NH Tf2NH NfOH TfOH PTSA Sc(NNf2)3 Sc(OTf)3
0.05 0.1 0.1 0.3 0.5 0.5 0.1 0.1 0.1 0.1 0.1 0.1
18 12 18 18 12 18 18 18 18 18 18 18
99 99 99 98 99 99 99 99 99 >99 98 >99
10.2 11.7 19.1 15.0 15.5 15.1 14.3 13.9 12.3 5.8 10.3 5.6
2.11 2.28 2.26 2.30 2.45 2.40 2.25 2.21 2.13 2.11 2.56 2.02
We also report a new approach for the recovery and purification of the catalyst by sublimation. Nf2NH could be sublimated at 60 °C under reduced pressure ( 7.0 × 103.
Figure 1. Photographs of the reaction vessel showed the sublimation of Nf2NH in the case of the bulk polycondensation.
Table 2. Direct Polycondensations of Dicarboxylic Acids and Diols under Reduced Pressures at Low Temperatures (
