SOLVENT EFFECTS IN THE ANIONIC POLYMERIZATION OF ALPHA, ALPHA-DISUBSTITUTED -BETA- PROPIOLACTONES

The effect of solvent polarity on the anionic polymerization rates of (alpha), (alpha)-disubstituted-(beta)-propiolactones was studied. Polymerization of (alpha)-methyl-(alpha)-propyl-(beta)-propiolactone in tetrahydrofuran was a living polymerization. In dimethyl sulfoxide, however, nucleophilic attack on the monomer by the sulfoxide oxygen initiated polymerization, obscuring the study of solvation effects. Similar behavior was observed in N, N-dimethylformamide. When other dipolar aprotic solvents were used, living polymerization systems were obtained. Polymerization rates were greatest in solvents of high dielectric constant, where cation solvation is greatest. Unlike (alpha)-methyl-(alpha)-propyl-(beta)-propiolactone, (alpha)-ethyl-(alpha)-n-butyl-(beta)-propiolactone is unreactive towards dimethyl sulfoxide. When (alpha)-ethyl-(alpha)-n-butyl-(beta)-propiolactone was polymerized in dimethyl sulfoxide, the polymerization rate decreased abruptly at low conversions because of gelation of the growing polymer. The decrease in overall polymerization rate upon gelation was caused by a sudden decrease in the number of polymer chains capable of growth. The polymerization rate of the active chains ends was unaffected by gelation. Bimodal molecular weight distributions arose because two types of chain ends were present after gelation. Anionic polymerization of (alpha)-ethyl-(alpha)-n-butyl-(beta)-propiolactone in N-methylpyrrolidone continued despite precipitation of the growing polymer at low conversions. N-Methylpyrrolidone is a better solvent than dimethyl sulfoxide despite its lower dielectric constant. Solvent-solvent interactions in dimethyl sulfoxide compete with cation-solvent interactions, resulting in decreased cation solvation. The polymerization rate of (alpha)-ethyl-(alpha)-n-butyl-(beta)-propiolactone in N-methylpyrrolidone was substantially less than that of (alpha)-methyl-(alpha)-propyl-(beta)-propiolactone. Either the increase in substituent size or the decrase in polymer solubility may have been responsible