AbstractThree styrene derivatives, para‐substituted pentamethyldisilyl (Si‐Si), heptamethyltrisilyl (Si‐Si‐Si), and nonamethyltetrasilyl (Si‐Si‐Si‐Si) groups 1 ‐ 3 were synthesized and polymerized in tetrahydrofuran (THF) at −78°C and in benzene at 40°C. The polymerizations of 1 and 2 in THF were found to proceed without transfer and termination reactions to afford stable living polymers. The Si‐Si and Si‐Si‐Si bonds are found to be completely stable under the conditions. Under the same conditions, novel block copolymers with well‐defined structures were synthesized by sequential addition of 1 or 2 and styrene. By contrast, side reactions occurred during the polymerizations of 1 and 2 in benzene at 40°C, although polymer yields were quantitative. More seriously, polymer was not obtained in the polymerization of 3 both in THF and in benzene. Thus, the effect of chain length of the oligosilyl group is found to be critical in the anionic polymerization of 1 ‐ 3.