Propagation Mechanisms in Ring-Opening Polymerization of Small Phosphorus Heterocycles: Toward Free-Radical Polymerization of Phosphines?

Abstract

The mechanism and kinetics of the propagation step in the radical ring-opening polymerization of methylphosphirane, -phosphetane, and -phospholane, and also phenylphosphetane, were studied via high-level ab initio calculations. It was found that radical ring-opening polymerization should occur via attack of the carbon-centered propagating radical at the phosphorus center of the ring. This is a facile process (with reaction rates of the order of 104−106 L mol-1 s-1 at 298.15 K), driven by the creation of a transition structure that resembles a (relatively stable) stretched phosphoranyl radical. The reaction is fastest for the four-membered phosphetanes, reflecting the best compromise between the strain in the transition structure and the strain released by the partially broken ring bond. Though slightly slower, radical ring opening of the three-membered phosphirane should also occur, but the reaction of the five-membered phospholane is not thermodynamically favorable. On the basis of the calculations for t...