Ring-opening Polymerization of Lactide by Bifunctional Organocatalyst at Ambient Conditions

Ring-opening polymerization of lactide (LA) is one of the most important techniques to synthesize poly(lactic acid) (PLA). In this work, a series of organocatalysts have been prepared for both solution polymerization and bulk polymerization of LA at ambient conditions. Derived from the facile reactions between phthalimide and quaternalyammonium salt, these catalysts are inexpensive and stable in air. Tetraethylammonium 2-aminobenzoate (TEACB) (catalyst a) was first applied to polymerize LLA in toluene solvent, and a conversion of 42.7% was achieved after the reaction proceeded at 25 degrees C for 1 h. Orthogonal experiments suggested that the optimum condition was reaction temperature of 75 degrees C, reaction time of 4 h, and the molar ratio of lactide:catalyst a:alkoxide equal to 200:10:1, which afforded PLA product with molecular weight of 8.03 kg.mol(-1), polydispersity index (PDI) of 1.53, and a high conversion of 88.5%. Next, bulk polymerization of LLA was carried out at different temperatures to explore the effects of initiator, alcohol salt, reaction time, and the molar ratio of lactide:catalyst:alkoxide. The catalytic activity of the catalysts depended largely on their chemical structures. Under the same reaction temperature, catalyst b with larger cation part led to a higher conversion; meanwhile, catalysts a and b with phenyl group in the anionic part were more active than catalyst c bearing an aliphatic group although the latter produced PLA with higher molecular weight and narrower molecular weight distribution. The catalysts developed in this study worked well in the absence of alkoxide, whilst alkoxide and alcohol could improve the performance of the catalyst system. LLA polymerizations could be conveniently performed under atmospheric conditions and increasing temperature resulted in PLA products with higher molecular weight and narrower molecular weight distribution. The conversion reached up to 95.7% after polymerization at 150 degrees C, in which the M-n and PDI of the PLA product equaled 2.57 kg.mol(-1) and 1.24, respectively. DSC measurements indicated that PLAs obtained via varied methods displayed similar melting temperatures in the range of 130 -134 degrees C. Further, cooperative dual activation of both the monomer and the initiator/chain-end could be confirmed based on MALDI-TOF-MS analyses. This novel catalyst system possesses specific monocomponent hetero-bifunction with H-bonding capability.