ANIONIC-POLYMERIZATION OF BETA-LACTONES INITIATED WITH ALKALI-METAL ALKOXIDES - REINVESTIGATION OF THE POLYMERIZATION MECHANISM

The mechanism of anionic polymerization of beta-propiolactone (PL) and beta-butyrolactone (BL) initiated by potassium alkoxides (methoxide and tert-butoxide) with 18-crown-6 was reinvestigated on the basis of polymer end-group analysis and model reactions. It has been found that corresponding unsaturated esters (unreactive in further polymerization) and potassium hydroxide are the initial products of the initiation step. The latter acts as the real initiator in this system. Polyesters with hydroxy or unsaturated dead end groups are produced, with carboxylate anions formed due to the alkyl-oxygen bond cleavage of the monomer molecule being the exclusive propagation species. The proposed novel mechanism seems to be a general one as far as the polymerization of unsubstituted and beta-substituted beta-lactones by alkali-metal alkoxides is concerned.