The electroreductive polymerization of dichloromethylphenylsilane in the presence of triphenylsilyl group-containing disilanes such as hexaphenyldisilane followed by the electroreductive termination with chlorotriphenylsilane afforded triphenylsilyl group-terminated polymethylphenylsilane in 15-32% yield. The isolated polymethylphenylsilane (M n = 3350 g mol −1 , M w /M n = 1.4) was found to react as a macroinitiator to copolymerize with dibutyldichlorosilane under electroreductive conditions producing the corresponding block copolymer (M n = 4730 g mol −1 , M w /M n = 1.2) in 38% yield. The ratio of monomer units (-MeSiPh-to-BuSiBu-) of the copolymer was determined to be 75 : 25 using 1 H NMR analysis, which was in good agreement with the calculated ratio (74 : 26) on the assumption that molecular weight of the macroinitiator was not changed. The block structure of the resulting copolymer, poly(methylphenylsilane)-block-poly(dibutylsilane), was also confirmed by comparing its 1 H NMR and UV absorption spectra with those of polymethylphenylsilane, polydibutylsilane and a statistical copolymer prepared by electroreductive polymerization of dichloromethylphenylsilane with dibutyldichlorosilane. This method is applicable to the preparation of other types of macroinitiator such as triphenylsilyl group-terminated polydibutylsilane, and polydibutylsilane-block-polymethylphenylsilane was also obtained using this macroinitiator.