Polylactones, 15. Reactions of δ-valerolactone and ϵ-caprolactone with acidic metal bromides†

δ Valerolactone and ϵ-caprolactone were reacted with BBr3, AIBr3, TiBr4, SnBr4 or Bu2SnBr2 in dichloromethane or chloroform in a 1 : 1 mole ratio. When these exothermic reactions were conducted with cooling, complexation of the lactones at the exocyclic oxygen was detectable by means of IR and 1H NMR spectroscopy as the first reaction step. However, heating to 60°C causes in all cases ring cleavage, and after hydrolytic removal of the metal bromides ω-bromocarboxylic acids or oligomers with ω-bromoalkanoyl end groups were obtained. SnBr4, Bu2SnBr2 and ZnBr2 proved to be good initiators of the polymerization of both lactones. At low monomer/initiator (M/I)-ratios (high initiator concentrations) most of the initiator remained unchanged, and the average degrees of polymerization of the resulting polylactones largely exceeded the value expected for the M/I-ratio. By means of viscosity and GPC measurements weight-average molecular weights up to 70 000 were found. All polylactones obtained by initiation with these three metal bromides contain ϵ-bromoalkanoyl end groups. Furthermore, high-molecular-weight poly(ϵ-caprolactone) could be prepared at polymerization temperatures up to 150°C. The results do not indicate a cationic polymerization mechanism, but an insertion mechanism involving the metal-O bond formed after the first cleavage of the lactone rings.