Terpolymerization of cyclosiloxanes with various substituents at the silicon atoms

With the use of 29 Si NMR spectroscopy, gas–liquid chromatography, viscometry, refractometry, and gel permeation chromatography, the process of copolymer formation during anionic copolymerization of octamethylcyclotetrasiloxane, 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetrasiloxane, and 1,3,5-trimethyl-1,3,5-triphenylcyclotrisiloxane in the presence of the molecular-weight regulators hexamethyldisiloxane or trimethylsilanol has been studied. It has been shown that anionic copolymerization of the above cyclosiloxanes proceeds in two stages. At the first stage, poly(methylphenylmethylvinyl)siloxane with a structure varying from block to random is formed, and, at the second stage, octamethylcyclotetrasiloxane enters into the reaction to yield a random copolymer of a desired composition via chain transfer reactions. Under conditions of anionic copolymerization, hexamethyldisiloxane is inefficient as a chain-terminating agent owing to its low reactivity, whereas trimethylsilanol participates in copolymerization and controls the molecular weight of the copolymer through the reaction of proton–cation exchange with the propagating macroanion.