Compounds of the formulae (I), (II), (III) and (IV), wherein Ch.sub.1 is e.g. the formula (V) or (VI); Ch.sub.2 is the formula (VII) or (VIII); Het.sub.1 is for example furyl, thienyl, pyrrolyl, pyridyl, benzothienyl, quinolyl or bithienyl; each of which is optionally substituted; Het.sub.2 and Het.sub.2' e.g. are furylene, thienylene, pyrrolylene, benzothienylene, quinolylene, furylenecarbonyl, thienylenecarbonyl, benzothienylenecarbonyl or bithienylenecarbonyl; each of which is optionally substituted; A.sub.1, and Ar.sub.1' i.a. are phenyl, naphthyl, benzoyl or naphthoyl, each of which is optionally substituted; Ar.sub.2 is for example phenylene, optionally substituted; M i.a. is C.sub.1-C.sub.20alkylene; R.sub.1 is for example C.sub.1-C.sub.12alkyl or phenyl; R.sub.2 and R.sub.2' for example are hydrogen or C.sub.1-C.sub.20 alkyl; exhibit an unexpectedly good performance in photopolymerization reactions. ##STR00001##