Borane reacts with TDM by a sequence of insertion and disproportionation reactions to yield tris-(trimethylsilyl methyl)borane. No further addition of TDM occurs. Triallylborane and tris-(4-methoxyphenylethyl)borane initiate the copolymerization of TDM and dimethylsulfoxonium methylide. The reactions afford TMS-substituted polymethylene oligomers. The resultant poly(methylidene-co-TMSmethylidene) random copolymers arise from incorporation of TMSmethylidene (CHSiMe3) and methylidene (CH2) groups into the growing polymer chain one carbon at a time.