New precursors for prodn. of compsns. for protecting materials (e.g. ships) against the growth of undesired organisms are cpds. of formula (I). In this formula, m is 1-10; each X (same or different) is =5C alkyl, =5C alkoxyalkyl or a gp. Y; Y is a residue -SnR1R2R3 in which R1, R2 and R3 are alyl, cycloalkyl or aryl, the total carbon content of R1, R2 and R3 being =18C-atoms. The residues X are so chosen so that the Sn:Si ratio of (I) is =1:50. New antifouling polysiloxanes consist of randomly crosslinked groups of formula (II), (III) and (IV) in which the the ratio of Sn to Si is ca 1:50 to 2:5, and the branchings of the polysiloxanes are terminated by H, 6C alkyl, 6C alkoxyalkyl or Y.
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