Kinetics of the anionic ring opening polymerization of cyclosiloxanes initiated with a superbase

Kinetics of the anionic ring opening polymerization of octamethylcyclotetrasiloxane, D-4, and hexamethylcyclotrisiloxane, D-3, in toluene solution initiated by hexapyrrolidinediphosphazenium hydroxide, P(2)Pyr(6)(+)OH(-) were studied. Reactions are first order both in monomer and in initiator. The specific rate of the D-3 polymerization is higher than that of D-4 by about 2-3 orders of magnitudes. Activation energies are 18.1 kcal mol(-1) for D-4 and 11 kcal mol(-1) for D-3. The back-biting reaction leading to decamethylcyclopentasiloxane, D-5, was followed in the polymerization of D-4. This reaction is retarded by the presence of monomer. The kinetics is interpreted in terms of a mechanism in which the active propagation center appears mostly as a monomer separated ion pair, which is also the intermediate in the propagation step.