Preferred units containing a crosslinkable group conform to the formula III ##STR3## in which R is lower alkylene, R1 is hydrogen or lower alkyl, p has the value zero or one, q has the value zero or one, R4 is an olefinically unsaturated, crosslinkable radical having 2 to 8 carbon atoms, and R5 and R6, independently of one another, are lower alkylene having 2 to 8 carbon atoms, arylene having 6 to