Since the majority of conventional organic/inorganic composite materials are obtained by mechanical blending of a silsesquioxane and an organic polymer or other means, it was extremely difficult to control the structure of the composite as a molecular agglomerate. In order to solve such a problem, the invention is to provide a silicon compound represented by Formula (1). This novel silicon compound has a living radical polymerization initiating ability for addition polymerizable monomers of a wide range. In Formula (1), R.sup.1 is hydrogen, an alkyl, an aryl, or an arylalkyl; R.sup.2 is an alkyl, phenyl, or cyclohexyl; and A is a group having a polymerization initiating ability for addition polymerizable monomers ##STR00001##