Cyclotetrasilanes with methyl and phenyl substituents Si4MenPh8-n(n=3,4,5,6) have been prepared by dearylation of octaphenylcyclotetrasilane and subsequent nucleophilic displacement with methylmagnesium bromide. All monomers are sufficiently strained to be polymerized to high molecular weight linear polysilanes. Ring Opening Polymerization, ROP, of cyclotetrasilanes proceeds with considerable stereoselectivity and regioselectivity. Silylcuprates lead to two inversions of configuration at both the attacked Si atom and at the newly developed active center. The assignment of29Si NMR signals for poly(methylphenylsilylene) to heterotactic (−38.5 ppm), syndiotactic (−39.0 ppm) and isotactic (−41.0 ppm) triads has been determined based on the polymerization of various mixtures of stereoisomers