NEW MATERIAL:A compound of the formula (Z is halogen; Ma and Mb are alkoxycarbonyl group; m is a positive number).

EXAMPLE: Diethyl(γ-trichlorosilyl)propylmalonate.

USE: A raw material compound for polysilsesquioxane which is a carcinostatic agent.

PREPARATION: A trihalogenosilane, e.g. trichlorosilane, is added to an alkylmalonic acid derivative having a terminal double bond e.g. diethyl allylmalonate, in the presence of a catalyst, e.g. a platinum compound, e.g. chloroplatinic acid hexahydrate, by the so-called hydrosilylation reaction.