Disclosed is a compound of the formula ##STR00001## wherein R.sub.1 is an alkylene, arylene, arylalkylene, or alkylarylene group, R.sub.2 and R.sub.2' each independently of the other, are alkylene, arylene, arylalkylene, or alklarylene groups, R.sub.3 and R.sub.3' each, independently of the other, are either (a) photoinitiating groups, or (b) groups which are alkyl, aryl, arylalkyl, or alkylaryl groups, provide that at least one of R.sub.3 and R.sub.3' is a photoinitiating group, and X and X' each, independently of the other, is an oxygen atom or a group of the formula --NR.sub.4--, wherein R.sub.4 is a hydrogen atom, an alkyl group, an aryl group, or an alkylaryl group.