Anionic polymerization of 4-methyl-2-oxetanone (β-butyrolactone)

Polymerization of 4-methyl-2-oxetanone ( 1 ) initiated with potassium acetate-dibenzo-18-crown-6 complex ( 2 ) in THF as solvent, was studied. Transfer reactions, leading to both crotonate anions and carboxylic acid formation, have been observed. Two kinetic effects of these reactions, hampering the living polymerization, have been established. The first results from reinitiation with the crotonate anions and thereby lowers the polymer molecular weight. The second is the decrease in the overall polymerization rate due to complexation of the growing carboxylate anions with carboxylic acid moieties. Kinetic scheme of polymerization involves propagation accompanied by transfer followed by slow reinitiation. This scheme, including complexation of the active species has been solved numerically. The apparent rate and equilibrium constants (k_p, k_tr, k_ri, and K_ass and respectively) have been determined. Although these kinetic parameters depend strongly on the polymerization conditions, but the ratio of the rate constants k_p : k_t : k_ri is fairly constant and equal to 10⁻⁴ : 10⁻⁶ : 10⁻⁶, respectively (at 20°C). Conditions of the controlled anionic synthesis of the amorphous poly(4-methyl-2-oxetanone) with $bar M_n$ as high as 1.7 × 10⁴ and ${{ le bar M_n } mathord{left/ {vphantom {{ le bar M_n } {bar M_n }}} right. kern-nulldelimiterspace} {bar M_n }} le 1.20$ have also been elaborated.