Mechanism of the cationic ring opening polymerization of cyclosiloxanes - Interpretation of new results

Mechanism of the cationic ring opening polymerization of cyclosiloxanes is still not fully understood. For example, the role of tertiary silyloxonium ions as the intermediates in this process is still controversial. Formation of trisilyloxonium ions generated from hexamethylcyclotrisiloxane, D-3, and octamethylcyclotetrasiloxane, D-4, was observed by Si-29 NMR spectroscopy at low temperatures and the ring opening polymerization (ROP) of these monomers in the presence of such ions was demonstrated. Kinetics of the polymerization of cyclic oxymultisilylene, [(Me2Si)(2)O](2), D-2(2), and the direct observation of trisilyloxonium ions in the polymerization system under low nucleophilic conditions is in favour of this mechanism [equations (3), (4), (8)]. The results of sequence analysis in the polymer formed by cationic ROP of cyclotrisiloxanes, containing different siloxane units in the molecule (Markov statistics - Table 1, Schemes B and C), were interpreted in terms of the chain extension mechanism involving reversible deactivation, in which the cyclic trisilyloxonium ion is not a persistent active propagation center but only a transient intermediate in each act of monomer addition. Polymerizations of strained cyclotrisiloxanes (such as D3), unstrained cyclotetrasiloxanes (such as D-4) and cyclic oxymultisilylenes (such as D-2(2)) seem to proceed according to the common mechanism. The differences in kinetics and thermodynamics of the polymerization of these monomers can be explained by the differences in the ring strain and in the nucleophilicity of these monomers relative to that of the oxygen atoms in a polymer.