Kinetic and mechanistic studies of bulk copolymerization of l-lactide and glycolide initiated by liquid tin(Ii) n-butoxide

Abstract

The main objective of this study was to investigate the effectiveness of liquid tin(II) n-butoxide (Sn(On-Bu)2) as an initiator in the bulk ring-opening copolymerization (ROCP) of L-lactide (LL) and glycolide (GL) in mol% ratio 80:20. The effects of initiator concentration and copolymerization temperature on the kinetics of the reaction were studied by gravimetric analysis. The initiator concentrations used were 0.010, 0.025, 0.050, 0.075 and 0.100 mol% while the copolymerizations were carried out for 96 h at temperatures ranging from 120-150°C. Increasing the liquid Sn(On-Bu)2 initiator concentration increased both the apparent first-order copolymerization rate constant (kapp) and the final %conversion. The 0.075 mol% liquid Sn(On-Bu)2 concentration was chosen to study the effect of copolymerization temperature. The results showed that kapp increased with increasing temperature as expected. A comparative study between the liquid Sn(On-Bu)2 and the conventional initiating system of tin(II) 2-ethylhexanoate (Sn(Oct)2) and n-butyl alcohol (n-BuOH) showed that the liquid Sn(On-Bu)2 gave a higher kapp under the same conditions of copolymerization. Furthermore, copolymerization of LL and GL using the liquid Sn(On-Bu)2 initiator resulted in a lower activation energy (Ea = 9.57 kJ mol-1) than that for the Sn(Oct)2/n-BuOH system (Ea = 15.14 kJ mol-1). In terms of molecular weight, the liquid Sn(On-Bu)2 tended to yield poly(L-lactide-co-glycolide) (PLG) copolymers with higher molecular weights than the Sn(Oct)2/n-BuOH system under the same conditions. These combined results showed that the liquid Sn(On-Bu)2 was a highly effective initiator in the ROCP of LL and GL cyclic ester monomers.