Synthesis of star-shaped poly(d,l-lactic acid-alt-glycolic acid) with multifunctional initiator and SnOct2 catalyst

d,l-3-methylglycolide (MG) was successfully polymerized with multifunctional initiator (trimethylolpropane (TMP) or pentaerythritol (PTOL)) and stannous octoate (SnOct2) catalyst in bulk at 110°C. The effects of molar ratios of monomer to initiator, monomer to catalyst and monomer conversion on the molecular weight of polymer were studied. For the homopolymerization of MG with TMP initiator and SnOct2 catalyst, the molecular weight of polymer increases from 6840 to 35 010 with the molar ratio of monomer to initiator (45–450), and the molecular weight distribution is from 1.15 to 1.35. The results indicate that in the homopolymerization of MG, the molecular weight of polymer is proportional to the molar ratio of monomer to initiator and the monomer conversion. The molar ratio of monomer to catalyst has no influence on the molecular weight of polymer at least within the range of 500–4000. 1H NMR spectra of the resulting polymers obtained from the homopolymerization of MG show that the homopolymerization of MG with TMP or PTOL initiator and SnOct2 catalyst produced two types of three-arm or four-arm star-shaped polymers. The bulk ring-opening homopolymerization of MG proceeds through a “coordination–insertion” mechanism and follows the selective acyl–oxygen bond cleavage reaction. 13C NMR spectroscopy indicates that the obtained poly(d,l-lactic acid-co-glycolic acid) (50:50, in molar ratio; d,l-PLGA50) has an alternating structures of lactyl and glycolyl units.