Ring-opening polymerization of thiolactide by using thiol-amine combination

Abstract

This is the first report about the ring-opening polymerization of thiolactide, which is the thioester analogue of lactide. The cyclic condensation of thiolactic acid gave rac-thiolactide, which underwent the ring-opening polymerization by a combination of thiol as the initiator and DBU as the catalyst. The polymerization was in equilibrium, showing that the monomer conversions were as low as 20% in solvents. The bulk polymerization at r.t. led the monomer conversions around 50%, and the molecular weights of the polymer products increased with decreasing the amount of the thiol initiator. The MALDI-TOF mass spectra revealed that the hexane-insoluble polythiolactide (Mn = 3000–3500, PDI = 1.3–1.6, 20–30%yield) had not only the linear but also the macro cyclic structures. A much weaker base, pyridine, worked as the catalyst, showing the lower activity but leading the polymerization to a more controllable fashion. DFT calculation suggested that rac-thiolactide has the smaller ring-strain than rac-lactide, which agreed with the lower polymerizability of rac-thiolactide. Polythiolactide showed the higher degradability for alkaline hydrolysis and UV photolysis than polylactide. Copolymerization of thiolactide with thioglycolide was also studied.