Microstructure of the copolymer chain generated by anionic ring-opening polymerization of a model cyclotrisiloxane with mixed siloxane units

Sequencing of siloxane units was studied in the copolymer chain formed by anionic polymerization of 2,2,4,4-tetramethyl-6,6-diphenylcyclotrisiloxane. The study involved three polymerization systems: lithium silanolate in THF, potassium silanolate complexed with 18-crown-6 in toluene, and tetramethylammonium silanolate in toluene. The sequencing proved to be a very good diagnostic tool for the studies of polymerization chemoselectivity. In all the three mentioned systems the chemoselectivity was high. In particular, processes leading to chain cleavage, such as backbiting and chain fragment interchange, occurred at a much lower rate than propagation, which allowed for quantitative studies of regioselectivity in polymerization. The pentad analysis, performed using the first-order Markov statistics, completed by the determination of the unit sequence at chain extremities, permitted the determination of the contributions from the monomer ring openings at three nonequivalent sites. The regioselectivity is poor as the ring is opened in each of these three places with a significant probability. The results provide important information on the mechanism of the chain propagation. They point to the role of the counterion assistance to the monomer ring opening. The generation of a more stable silanolate propagation center may also be important for choosing the site of the ring-opening. Ab initio calculations suggest the direct interaction of the phenyl group with the metal cation.