Synthesis of polylactides in the presence of co-initiators with different numbers of hydroxyl groups

Linear and star-shaped polylactides were prepared by using alcohols with different numbers of hydroxyl groups as co-initiators. 1H-NMR analysis of low molecular weight polymers showed that the number of hydroxyl groups initiating polymerisation was near theoretical for 1,4-butanediol (2 OH groups) and pentaerythritol (4 OH groups). The numbers for polyglycerines were somewhat lower than theoretical (8 or 12 OH groups), but they clearly suggested the star-shaped structure, with more arms than in polymers initiated with pentaerythritol. The preparation of high molecular weight polymers showed that the polymerisation rate increases with the number of hydroxyl groups in the co-initiator. Along with the faster polymerisation, higher molecular weight polymers were obtained. High hydroxyl group content in the polymer did not cause a drop in the conversion level or enhanced backbiting during extended polymerisation. Furthermore, the co-initiator did not affect the thermal properties of the polymers except that slightly lower melting temperatures were measured for star-shaped than linear polylactides.