Abstract: A graft polymer obtainable by grafting a polymer substrate with a compound of formula (I), in which the groups R are the same or different and each is a straight or branched C.sub.1 -C.sub.4 alkyl group; X is an aryl group or a straight or branched C.sub.1 -C.sub.20 alkylene group, optionally containing one or more carbon-carbon double or triple bonds, ether linkages or aryl groups; the aryl groups being unsubstituted or substituted by one or more C.sub.1-4 alkyl groups; n is from 2 to 4; and A is a reactive group, is useful as a biocompatible material in contact lenses, biomedical devices and blood-containing devices. In a preferred embodiment A is a polymerizable group and the graft polymer is produced by radical initiated polymerization of the compound of formula (I) on the substrate which is initiated by a radical-forming group on the substrate.