Kinetics of the polymerization of permethylcyclosiloxanes initiated by tetrakis(pentafluorophenyl)borate protic complex

The kinetics of ring opening polymerization of hexamethylcyclotrisiloxane, D-3, and octamethylcyclotetrasiloxane, D-4, in toluene initiated by protic borate complex H+(H2O)(3)B(C6F5)(4) have been studied. The reaction is first order in monomer and first order in initiator. Rate constants and activation parameters were determined. The formation of cyclic oligomers was followed. The kinetics of the formation of decamethylcyclopentasiloxane, D-5, during the polymerization of D-4 was in agreement with a back-biting mechanism. D-3n cyclics (n=2, 3,...) are exclusively formed during D-3 polymerization by intramolecular reactions between the chain ends. The amount of generated D-6 increases linearly with the monomer conversion in the range of the D-3 conversion between 0 and 60% and is almost independent of temperature. Kinetic results are interpreted in terms of a mechanism involving cyclic tertiary trisilyloxonium ion transitory intermediates.