Stereoregular Polysiloxanes via Ring-Opening Polymerization, a Review

Although polysiloxanes are not ordinarily considered stereoregular, a few references describe ring-opening polymerizations of cyciosiloxanes that can produce polymers enriched in stereoregularity. Most are made from the cis-isomers of unsymmetrically substituted strained cyclosiloxanes by methods where ring–chain and chain–chain equilibria are suppressed. The monomers insert randomly at the reactive chain ends with equal probabilities of forming new meso (m) or raceme (r) siloxane links while preserving the stereoconfiguration of the original monomer. The resulting polysiloxanes are enriched in isotactic configurations but are limited in their run lengths by the new randomly configured siloxane links formed between the original monomer units. One citation, however, describes a ring–chain equilibrium where the isotactic conformation was the preferred conformation. Another illustrates the profound influence of stereoregularity on the mechanical properties of a silicone elastomer.