Kinetics of the anionic ring opening polymerization of octamethylcyclotetra- siloxane, D4, and hexamethylcyclotrisiloxane, D3, in toluene solution initiated by hexapyrrolidinediphosphazenium hydroxide, P2Pyr+ Reactions are first order both in monomer and in initiator. The specific rate of the D3 polymerization is higher than that of D4 by about 2–3 orders of magnitudes. Activation energies are 18.1 kcal mol−1 for D4 and 11 kcal mol−1 for D3. The back-biting reaction leading to decamethylcyclo- pentasiloxane, D5, was followed in the polymerization of D4. 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 propaga- tion step. 6 OH− were studied.
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