Kinetics and Mechanism of the Stannous Octoate-Catalyzed Bulk Polymerization of ε-Caprolactone

Bulk polymerizations of ε-caprolactone (CL) were conducted at 130 °C, in which the structure and amount of initiator were varied. Polymerization induction periods were observed and attributed to slow heat transfer and to the type of alcohol used as initiator. Induction periods persisted until virtually complete consumption of both ethylene glycol (EG) and 1,3-propanediol (PD). 1-Butanol displayed no detectable induction. 1H NMR suggested that induction was a result of strong and unique interactions between the diols and stannous octoate, which lead to the formation of more stable, less reactive stannous alkoxides compared to stannous alkoxides derived from the polymer chain end. Only after virtually all free diol was consumed did chain propagation commence at the normal rate. The rate of polymerization was insensitive to the [CL]/[EG] ratios chosen for this study. The results were consistent with a mechanism in which stannous alkoxide initiator is formed in situ via reaction between stannous octoate and alcohol, and stannous alkoxide chain ends are the actively propagating species.