Kinetic investigation and scale-up of bulk ring-opening copolymerization of L-lactide and ɛ- caprolactone in the presence of liquid tin(II) n-butoxide

This work aims to study the effectiveness of using liquid tin(II) n-butoxide (Sn(OnBu)2) as an initiator in the bulk ring-opening copolymerization (ROCOP) of L-lactide (LL) and ɛ-caprolactone (ε-CL). The effects of initiator concentration and polymerization temperature on the kinetics are investigated using proton-nuclear magnetic resonance spectroscopy (1H-NMR). The results indicate that increasing the initiator concentration enhances the final % conversion and the apparent rate constant (kapp). The findings also show that kapp increases with increasing temperature, as predicted. A comparative study between liquid Sn(OnBu)2 and the conventional initiating system of tin(II) 2-ethylhexanoate (stannous octoate, SnOct2) and n-butyl alcohol (Sn(Oct)2/n-BuOH) reveals that liquid Sn(OnBu)2 provides a higher kapp under the same polymerization conditions. Additionally, the copolymerization of LL and ɛ-CL using liquid Sn(OnBu)2 results in lower activation energy (Ea = 25.7 kJ mol−1 K−1) compared to the Sn(Oct)2/n-BuOH system (Ea = 34.6 kJ mol−1 K−1). In terms of molecular weight, using liquid Sn(OnBu)2 tends to yield poly(L-lactide-co-ɛ-caprolactone) (PLCL) copolymer with a higher molecular weight than the Sn(Oct)2/n-BuOH system under the same conditions. These combined results show that liquid Sn(OnBu)2 is a highly effective initiator in the ROCOP of LL and ɛ-CL cyclic ester monomers and can be used as a promising candidate for scaling up production.