Equilibria and kinetics of the cationic ring‐opening polymerization of permethylated 1,4‐dioxa‐2,3,5,6‐tetrasilacyclohexane. Comparison with cyclosiloxanes

Abstract

AbstractEquilibrium constants were determined for the formation of cyclic oligomers of the general formula \documentclass{article}\pagestyle{empty}\begin{document}$ {\hbox{-\hskip-5pt-\hskip-5pt-}[({\rm CH}_3})_2 {\rm Si \hbox{---} Si(CH_3})_2\hbox{---} {\rm O\hbox{-\hskip-6.5pt-}]}_n , n = 2 - 6$\end{document}, in the polymerization of the n = 2 homologue, octamethyl‐1,4‐dioxa‐2,3,5,6‐tetrasilacyclohexane (1). The formation of a linear polymer is thermodynamically favoured in bulk and in concentrated solutions. The equilibrium concentration of cyclics is little dependent on temperature. The six‐membered ring, n = 2, used here as a monomer, is the most abundant cyclic in the equilibrium mixture. Kinetics of the conversion of 1 during the polymerization initiated with CF3SO3H, CH3SO3H and CF3COOH was studied in methylene dichloride solution. In its first stage, the reaction follows first order, but it slows down at higher monomer conversion. The order of the reaction with respect to the acid is two. Some kinetic features of the polymerization initated with CF3SO3H were similar to those exhibited by the polymerization of octamethylcyclotetrasiloxane (D4). In particular small amounts of additional water have rather small effect on the rate, but change to a considerable extent the entropy and enthalpy of activation. The concentration of cyclic oligohomologues passes through a maximum during monomer conversion, which indicates the kinetic enhancement of the cyclisation process. This observation is somewhat similar to those in the polymerization of hexamethylcyclotrisiloxane (D3). However, in contrast to the polymerization of D3 and similarly to the polymerization of D4, the excessive formation of oligomers being multiples of the monomer is not observed.