An electropolymerization of haloalkylhalosilanes (Cl-R-SiCH2Cl2) that possess two types of electroactive sites, that is, the C-Cl and Si-Cl bond is described. The one-pot synthesis method is shown to yield branched polycarbosilanes having a regular carbon block-spaced silicon backbone structure. A series of branched polycarbosilanes, [-R-SiCH3-](n) with R being -CH2-, -C2H4-, -C3H6-, and -CH2-C6H4-C2H4-, have been successfully electropolymerized with Mu up to 42,600 Dalton. Experimental and simulation cyclic voltammetry of these monomers and the computational examination of their LUMOs are applied to study the electropolymerization mechanism. The results suggest that polymerization proceeds by iterating steps involving (1) electroreduction of a C-Cl bond to a carbanion, which is catalyzed by silylanion radical [Cl (center dot center dot center dot) under bar SiCl(CH3)RCl](degrees) under bar and/or Ni(O)/TDA-1; and (2) nucleophilic attack of carbanions to Si-Cl bonds of a second monomer or oligomer to extend the polymer chain. The investigation reveals that the R spacer has a considerable impact on the polymerizability of the corresponding monomer. Such interfacial polymerization resembles a template polymerization, leading to unique microstructures that were preserved even after converted to silicon carbide ceramics at high temperatures. (C) 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7677-7689, 2008