A polysiloxane copolymer that is useful for forming biomedical devices
comprises blocks (I) and (II) and is terminated at each end with an
ethylenically unsaturated radical: (*Dii*Diol*Dii*PS).sub.x (I)
(*Dii*PS).sub.y (II) wherein: each Dii is independently a diradical
residue of a diisocyanate; each Diol is independently a diradical residue
of a diol having 1 to 10 carbon atoms; each PS is independently a
diradical residue of a polysiloxane-containing diol or diamine; each * is
independently --NH--CO--NH--, --NH--COO-- or --OCO--NH--: x represents
the number of blocks (I) and is at least 2, and y represents the number
of blocks (II) and is at least 1.