Suppression of Crystallization in Polydimethylsiloxanes and Chain Branching in Their Phenyl-Containing Copolymers

While incorporation of only a few mol % of diphenylsiloxy-, DiPhS, repeat units into polydimethylsiloxane, PDMS, chains is enough to completely suppress their crystallization, it also leads, in polymers obtained by silanolate-initiated ring-opening polymerization, to a puzzling and yet unexplained chain branching which significantly distorts polymer molecular weight distributions and affects their chain conformation. In contrast to this, we found that introduction of comparable amounts (ca. 5 mol %) of diethylsiloxy-, DiEtS, units into the same type of polymers also suppresses polymer crystallization but does not lead to any detectable branching, yielding polymers with the most probable molecular weight distribution and typical random coil conformation in a thermodynamically good solvent, such as toluene. On the basis of the results of a Si-29 NMR and SEC-MALS study, we propose that branching in DiPhS-containing polymers is caused by a nucleophilic attack of initiating silanolate anions on their Si-C-Ar, side bonds and a resulting formation of phenyltrisiloxysilane T-branches. We also propose that because branching may seriously disrupt mechanical properties of elastomers if such DiPhS-containing PDMSs are used for cross-linking, the DiEtS-modified derivatives represent polymers of choice if applications of such elastomers at extremely low temperatures are desired.