Polymerization of α, α-disubstituted-β-propiolactones and lactams. 9. ABA block copolymers of α-methyl-α-butyl-β-propiolactone and pivalolactone

ABA block copolymers were prepared by the anionic polymerization of α-methyl-α-butyl-β-propiolactone, MBPL (B block), and pivalolactone, PL (A blocks). The MBPL block had a very low decree of crystallinity and a glass temperature of ? 13°C, so phase separation with extensive crystallization of the PL blocks gave thermoplastic elastomers when the MBPL block constituted the principal and continuous phase. The observed crystallinity and melting point of 40–45°C in the MBPL homopolymer have not been previously reported. Measurements were obtained by electron microscopy of the initial size distribution of the PL domains as a function of copolymer composition and degree of polymerization, and on the effect of annealing on this parameter. Tensile strengths and elongations at break were both less than those previously observed for equivalent ABA block copolymers of PL and α-methyl-α-propyl-β-propiolactone.