Monitoring of the Course of the Silanolate-Initiated Polymerization of Cyclic Siloxanes. A Mechanism for the Copolymerization of Dimethyl and Diphenyl Monomers

Homopolymerization of octamethylcyclotetrasiloxane, D 4, and two copolymerizations of this monomer with octaphenylcyclotetrasiloxane, D 4 Ph2, and hexaethylcyclotrisiloxane, D 3 Et2, were investigated by monitoring the courses of their occurrence using three different methods: size exclusion chromatography (SEC), thermal gravimetric analysis (TGA), and 29 Si NMR spectroscopy. The results obtained showed that while homopolymerization of D 4 and its copolymerization with D 3 Et2 occurred in a nearly identical manner, suggesting that reactivities of the two monomers toward the silanolate active species were practically identical, the copolymerization of D 4 with D 4 Ph2 exhibited a distinct initial induction period resulting from two factors: (a) very low solubility of solid D 4 Ph2 in D 4 and (b) significantly higher reactivity of D 4 Ph2 than D 4 toward the dimethylsilanolate. On the basis of these results, we propose a new three-stage mechanism for the copolymerization of D 4 with D 4 Ph2, which includes (a) initial formation of B–A–B triblocks of dimethylsiloxane (A) and diphenylsiloxane (B) segments, followed by (b) subsequent formation of A–B–A–B–A pentablock species, and (c) redistribution of the former multiblocks through a siloxane equilibration reaction into copolymers containing single diphenylsiloxy units separated by extended polydimethylsiloxane segments.