Cationic polymerization of a model cyclotrisiloxane with mixed siloxane units initiated by a protic acid. Mechanism of polymer chain formation

The structure of polysiloxane chain generated by the quenched cationic ring-opening polymerization (CROP) of 2,2-diphenyl-4,4,6,6-tetramethylsiloxane (1) initiated by trifluoromethanesulfonic acid was studied(1) by Si-29 NMR and by statistical methods. The sequential analysis was performed at the pentad level using the first-order Markov chain statistics. Sequences at chain extremities were studied in the polymer obtained by quenched polymerization of I and in polymers obtained by polymerization of this monomer with chain transfer to hexamethyldisiloxane. The results indicate that the randomization processes, such as intermolecular exchange of chain fragments and backbiting, do not occur during the polymerization of monomer 1. Dominating is the monomer addition giving the symmetrical arrangement of siloxane units -->OSiMe2.OSiPh2.OSiMe2-->(ca. 68%) by the ring opening at oxygen bridging methyl-substituted silicons. Only one of the two possible unsymmetrical additions occurs, i.e., -->OSiMe2.OSiMe2.OSiPh2- (ca. 32%). The results are interpreted in terms of propagation accompanied by the fast deactivation mechanism. This pathway involves a formation of the terminal cyclic trisilyloxonium ion intermediate which is transformed to terminal silyl ester with incorporation of its cyclic fragment to the polymer chain. Chain growth proceeds stepwise with competitive cyclization by the intermolecular reaction of the silyloxonium intermediate with the silanol end group. Permanent initiation of new chains by the acid dominates the reaction pattern.