The Polymerization of Free Enols with Electron-Deficient Comonomers

Abstract

The balance between kinetics and thermodynamics is illustrated herein by the first direct polymerization of vinyl alcohol, the thermodynamically unstable tautomer of acetaldehyde, at a rate faster than it can tautomerize. Vinyl alcohol was formed through the acid catalyzed hydrolysis of ketene methyl vinyl acetal. With excess water present, the kinetics of tautomerization first order dependence upon vinyl alcohol (k_obs = 2.73 × 10⁻⁴ s⁻¹). Under water starved conditions, however, the kinetics now show a zero order dependence upon the concentration of vinyl alcohol (k_obs = 3.5 × 10⁻⁶ M/s). Under these latter conditions, the half life of vinyl alcohol is nearly 24 hours at room temperature. Although cationic and homo free radical polymerization of vinyl alcohol failed, we found that this meta-stable species could be quantitatively polymerized in a copolymerization (AIBN, hυ, -10 to 25°C) with maleic anhydride. The k_obs for copolymerization was found to be 4.41 × 10⁻⁴ sec⁻¹ at −10°C. Since the rate of polymerization is far greater than that of tautomerization under these conditions (ca. 30 times faster at −10°C), there is no significant increase in acetaldehyde concentration during polymerization.