Compounds of the formulae I, II and III ##STR00001## wherein R.sub.1 is for example hydrogen, C.sub.3 C.sub.8cycloalkyl, C.sub.1 C.sub.12alkyl, phenyl unsubstituted or substituted; R.sub.2 and R.sub.2' for example are hydrogen, C.sub.1 C.sub.20alkyl, C.sub.3 C.sub.8cycloalkyl or phenyl, unsubstituted or substituted, or are ##STR00002## Ar.sub.1 is for example phenyl, optionally substituted by e.g. ##STR00003## Ar.sub.2 is for example phenylene, optionally substituted e.g. by --(CO)R.sub.7, (D), (E) or (F); Ar.sub.3 is for example phenyl; M.sub.1, M.sub.2 and M.sub.3 are, for example, C.sub.1 C.sub.20alkylene; M.sub.4 is for example direct bond, --O--, --S--, --Y--(C.sub.1 C.sub.10alkylene)-Y'--, optionally substituted; Y and Y' are for example a direct bond or --O--; R.sub.7 is for example hydrogen, C.sub.1 C.sub.20alkyl or phenyl, optionally substituted; R.sub.8, R.sub.9, R.sub.8' and R.sub.9' are for example hydrogen or C.sub.1 C.sub.12alkyl, exhibit an unexpectedly good performance in photopolymerization reactions.