Lipase‐catalyzed polymerization of L‐lactide in ionic liquids

Lipase-catalyzed polymerizations of L-lactide were carried out in four kinds of ionic liquids (ILs) in order to investigate the effect of ILs on the conversion, molecular weight, and yield of the obtained poly(lactide) (PLLA). PLLA could be synthesized in ILs of 1-butyl-3-methylimidazolium ([C4mim]) except in [C4mim][N(CN)2]. While the number-average molecular weight (Mn) of the PLLA obtained in [C4mim][PF6] was below 4000 g mol−1 and the yield was below 10%, the Mn and yield of the PLLA obtained in [C4mim][BF4] reached 55000 g mol−1 and 35%, respectively. The Mn values of PLLAs in the ILs were clearly higher than those of PLLAs obtained in bulk and in toluene (<44000 g mol−1). The polymerization in the ILs proceeds even at lower temperatures as compared with that in bulk. The effect of lipase content on the Mn of PLLA was also investigated. When [C4mim][BF4] was used as the solvent, the highest Mn was obtained at a lipase content of 10 wt%. In the case of [C4mim][NTf2], the most suitable lipase content depended on the polymerization temperature. [C4mim][BF4] was suitable to obtain higher molecular weight PLLA and higher polymer yield at lower lipase contents. Copyright © 2008 John Wiley & Sons, Ltd.