In an embodiment, macromer c comprises at least one segment of the formula (I): in which (a) is a polysiloxane segment, (b) is a polyol segment which contains at least 4 C atoms, Z is a segment (c) or a group X1, (c) is defined as X2???R???X2, wherein R is a bivalent radical of an organic compound having up to 20 C atoms and each X2 independently of the other is a bivalent radical which contains a