Regio- and Stereospecific Cyclopolymerization of Bis(2-propenyl)diorganosilanes and the Two-State Stereoengineering of 3,5-cis,isotactic Poly(3,5-methylene-1-silacyclohexane)s.

Abstract

Transition-metal-mediated coordination cyclopolymerization of bis(2-propenyl)dimethylsilane (1a) using the C1-symmetric, group 4 metal preinitiator, (η5-C5Me5)Zr(Me)2[N(Et)C(Me)N(tBu)] (I), in combination with 1 equiv of the borate coinitiator, [PhNHMe2][B(C6F5)4] (II), proceeds in a regio- and stereospecific manner to provide highly stereoregular 3,5-cis,isotactic poly(3,5-methylene-1,1-dimethyl-1-silacyclohexane) (2a). Successful stereoengineering of 2a to eliminate undesirable crystallinity while preserving a high Tg value of >120 °C was subsequently accomplished by employing a "two-state" propagation system that uniquely produces an isotactic stereoblock microstructure of decreasing stereoblock length with decreasing percent level of "activation" of I with II. The controlled character of cyclopolymerization of 1a using the less sterically encumbered preinitiator, (η5-C5Me5)Hf(Me)2[N(Et)C(Me)N(Et)] (III), and 1 equiv of II was used to prepare well-defined poly(1-hexene)-b-poly(3,5-methylene-1-silacyclohexane) block copolymers through sequential monomer additions.