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Communications Lipase-Catalyzed Degradation of Polyesters in Organic Solvents. A New Methodology of Polymer Recycling Using Enzyme as Catalyst Shiro Kobayashi,* Hiroshi Uyama, and Tetsufumi Takamoto Department of Materials Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan Received December 6, 1999
Enzymatic hydrolytic degradation of aliphatic polyesters in organic solvents has been examined. The degradation of poly(-caprolactone) took place using Candida antarctica lipase as catalyst in toluene at 60 °C to give oligomers with molecular weight of less than 500. The degradation behavior catalyzed by lipase was quite different than an acid-catalyzed degradation (random bond cleavage of polymer). After the removal of the solvent from the reaction mixture, the residual oligomer was polymerized in the presence of the same catalyst of lipase. These data provide a basic concept that the degradation-polymerization could be controlled by presence or absence of the solvent, providing a new methodology of plastics recycling. Worldwide potential demands for recycling of polymeric materials are quite significant from environmental viewpoints.1 Among recycling methods, chemical recycling is the most important because starting materials can be reproduced only by a chemical process.2 However, industrial examples of chemical recycling are limited and their processes need much energy (high temperature and/or pressure). Therefore, development of environmentally friendly processes of chemical recycling is strongly desired. Biodegradable polymers are expected as an alternative to traditional non- or low-biodegradable plastics such as polyethylene, polypropylene, and polystyrene. Biodegradable polymers are subjected to degradation (hydrolysis) by living organisms, whereas the degradation products are not directly converted to the original polymers in nature. Aliphatic polyesters, one of the most promising biodegradable plastics, are synthesized by fermentation processes3 or chemical polymerizations.4 The former produces poly(hydroxyalkanoate)s showing high biodegradability, and the latter provides a variety of polyesters. Studies on enzymatic degradation of these polymers in aqueous media and degradation in soils have been extensively carried out.5 These studies show environmental degradability of polyesters. Polymer syntheses using isolated enzymes as catalyst have received much attention as environmentally benign processes of polymer production under mild reaction conditions.6 So far, biodegradable aliphatic polyesters have been synthesized by lipase catalyst from various monomer combinations.6 We * To whom correspondence may be addressed. E-mail: kobayasi@mat. polym.kyoto-u.ac.jp.
Chart 1
have found that random copolyesters were formed by lipasecatalyzed ring-opening copolymerization of lactones,7 despite difference of the enzymatic polymerizability of the monomers. This suggests the frequent occurrence of the transesterification, i.e., the bond cleavage at the ester group of the polymer chain. These results strongly encouraged us to investigate the enzymatic degradation (hydrolysis) of polyesters in nonaqueous medium. This study deals with lipasecatalyzed degradation of aliphatic polyesters in organic solvents and proposes a new concept of one-pot chemical recycling of polymers using an enzyme as the catalyst.8 In this study, lipase derived from Candida antarctica (lipase CA) was used as catalyst, which was reported to afford a variety of aliphatic and aromatic polyesters.9 As to -caprolactone, in particular, a smaller amount of lipase CA catalyzed the polymerization much faster than other lipases showing high catalytic activity for ring-opening polymerization of lactones.9a At first, degradation of poly(-caprolactone) (PCL) with molecular weight of 6.0 × 104 (Chart 1) was carried out in dry toluene at 60 °C.10 Degradation was monitored by size exclusion chromatography (SEC). The water content of lipase CA was approximately 1 wt %.11 During the degradation, PCL and the resulting products were soluble in toluene. SEC traces of the reaction mixture
10.1021/bm990007c CCC: $19.00 © 2000 American Chemical Society Published on Web 02/18/2000
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Figure 2. MALDI-TOF MS spectrum of the degradation product obtained by using lipase CA catalyst in toluene at 60 °C for 120 h: C, cyclic product; L, linear product.
Figure 1. SEC traces of degradation products of (A) PCL using lipase CA catalyst, (B) PCL using p-toluenesulfonic acid (5 mol % for monomer unit of PCL) as catalyst, (C) PDDL using lipase CA catalyst, and (D) PBA using lipase CA catalyst. The degradation was performed in toluene at 60 °C.
in different reaction time are shown in Figure 1A. A very small amount of water contained in the reaction mixture is probably involved in the hydrolysis of PCL. Intensity of a peak due to PCL at elution volume of 6.5 mL gradually decreased, and peaks due to oligomers (number-average molecular weight of less than 500) newly appeared. After 24 h, the polymer peak almost disappeared. Interestingly, no formation of the polymer whose molecular weight was intermediate between those of the starting material and the oligomer was observed. For reference, the degradation of PCL by p-toluenesulfonic acid (5 mol % for monomer unit of PCL) was examined (Figure 1B). The SEC peak of the degradation mixture was nearly unimodal and the molecular weight gradually decreased, indicating that the ester-bond cleavage by acid catalyst randomly took place. These data suggest that the hydrolysis behavior of PCL by lipase CA was quite specific. The degradation of PCL also took place in dry isopropyl ether, in which PCL was scarcely soluble, and the hydrolysis behavior was very similar to that in toluene (data not shown). The terminal structure of the degradation product was analyzed by matrix-assisted laser desorption/ionization timeof-flight mass spectroscopy (MALDI-TOF MS). MALDITOF MS has been successfully utilized for the characterization of biomolecules. This method has been extensively applied to characterization of synthetic polymers, especially determination of terminal structure.12 In the spectrum of the degradation product obtained in toluene (Figure 2), there were mainly sets of two peaks with regular peak-to-peak distance (114). For example, peaks with mass of 936.5 and 954.4 are described to cyclic and linear octamers cationized
with Na+. From the intensity of these peaks, it was found that the product was a mixture of cyclic and linear oligomers and the content of the cyclic oligomer was larger than that of a linear one. In the case of the product obtained in isopropyl ether, the cyclic oligomer was mainly formed, which was also confirmed by 1H NMR spectroscopy. In the lipasecatalyzed polymerization of -caprolactone (CL) in organic solvents, formation of macrocycles by intramolecular condensation was observed.13 These data indicate that cyclic oligomers were enzymatically formed in organic solvents during the hydrolysis of high molecular weight PCL as well as the polymerization of CL. Interestingly, the degradation in water-saturated isopropyl ether (water content of ca. 0.2%) mainly produced the linear oligomer. These data qualitatively indicate that the terminal structure of the degradation product could be controlled by the water content in the solvent. In the lipase-catalyzed polymerization of lactones, the water content in the reaction system greatly affected the polymerization behaviors.14 Lipase-catalyzed degradation of polyesters from 12dodecanolide (13-membered) lactone and 1,4-butanediol/ adipic acid (PDDL and PBA, respectively) were tested in toluene (parts C and D of Figure 1). Weight-average molecular weight (Mw) and its index (Mw/Mn) of PDDL are 1.3 × 104 and 2.0, respectively. Mw and Mw/Mn of PBA are 8.3 × 103 and 1.7, respectively. Both samples were enzymatically degraded, although the degradation rate was smaller than that of PCL. The pattern of the SEC traces was quite different with that of PCL. These data indicate that the polymer structure strongly affected the degradation behaviors; the polymer peak gradually shifted to higher elution volume, suggesting the occurrence of random degradation, behavior very similar to the acid-catalyzed degradation of PCL (Figure 1B). As a possible application, one-pot degradation-polymerization of PCL using lipase CA catalyst was examined (Figure 3). First, the degradation of PCL was carried in toluene at 60 °C. After 24 h, the solvent was removed under reduced pressure to give a mixture of the waxy oligomer (Figure 3A) and lipase CA, which was subsequently kept at 60 °C for 8 h without solvent, yielding a polymer with a molecular weight of 8 × 103 (Figure 3B). The cycle of degradation-polymerization could be performed repeatedly in a similar manner (parts C and D of Figure 3). These data indicate that the reaction pathway of degradation/polymerization was controlled by the presence or absence of the
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tration, amount of lipase, and temperature affected the degradation behaviors. Further investigations on the specific degradation mechanism of PCL in an organic solvent with relation to water content in the reaction mixture and on the enzymatic hydrolytic degradation of not only polyesters but also other polymers such as polycarbonates and polyamides are under way in our laboratory. Acknowledgment. This work was supported by a Grantin-Aid for Specially Promoted Research (No. 08102002) from the Ministry of Education, Science, and Culture, Japan. We acknowledge the gift of lipase CA from Novo Nordisk Bioindustry. References and Notes
Figure 3. SEC traces of (A) first degradation of PCL, (B) first polymerization of the degradation product, (C) second degradation, and (D) second polymerization. The degradation was performed using lipase CA as catalyst in toluene at 60 °C for 24 h. The degradation product was polymerized using lipase CA as catalyst in bulk at 60 °C for 8 h. Scheme 1
solvent using the same catalyst in one pot. On the basis of these data, we propose a new concept of chemical recycling of polymers using enzyme as catalyst (Scheme 1). This process is very facile and proceeds under mild reaction conditions without use of toxic reagents. Therefore, the present method is expected to be an environmentally benign process of polymer recycling, giving an example system of green polymer chemistry.6b In conclusion, lipase-catalyzed degradation of aliphatic polyesters took place smoothly in organic solvents. A very small amount of water contained in lipase CA and the solvent is probably involved in the hydrolysis of the ester bond in polyesters. The degradation behavior of PCL was quite specific, as compared with that by an acid catalyst. The obtained data qualitatively showed the dependence of the hydrolysis behavior on the polymer structure and water content in the reaction system. The degradation/polymerization of PCL could be controlled by presence or absence of the solvent, providing a new chemical recycling system using enzyme as catalyst. We have preliminarily found that in the enzymatic hydrolysis of PCL in organic solvents, the polymer concen-
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