The present invention relates to a copolymer, which is the reaction product of (a) a first prepolymer, comprising a bioactive moiety and at least one radically polymerizable group, and (b) a second prepolymer which is a polyvinyl alcohol having a weight average molecular weight of at least about 2000 that, based on the number of hydroxy groups of the polyvinyl alcohol, comprises from 0.5 to 80% structural units of formula (I) wherein: R is alkylene having up to 8 carbon atoms, R1 is hydrogen or alkyl having up to seven carbon atoms, and R2 is an olefinically unsaturated, electron-attracting copolymerizable radical.