Compounds of the formulae (I), (II), (III) and (IV), wherein Ch1 is e.g. the formula (V) or (VI); Ch2 is the formula (VII) or (VIII); Het1 is for example furyl, thienyl, pyrrolyl, pyridyl, benzothienyl, quinolyl or bithienyl; each of which is optionally substituted; Het2 and Het2′ e.g. are furylene, thienylene, pyrrolylene, benzothienylene, quinolylene, furylenecarbonyl, thienylenecarbonyl, benzothienylenecarbonyl or bithienylenecarbonyl; each of which is optionally substituted; A1, and Ar1′ i.a. are phenyl, naphthyl, benzoyl or naphthoyl, each of which is optionally substituted; Ar2 is for example phenylene, optionally substituted; M i.a. is C1-C20alkylene; R1 is for example C1-C12alkyl or phenyl; R2 and R2′ for example are hydrogen or C1-C20 alkyl; exhibit an unexpectedly good performance in photopolymerization reactions.