Compounds of formula I ##STR1## wherein n is 0 or 1, p is a number from 1 to 8, q is 2 or 3, and R.sub.1 is unsubstituted phenyl or phenyl which is substituted by one to three halogen, C.sub.1 -C.sub.8 alkyl, C.sub.1 -C.sub.8 alkoxy, C.sub.2 -C.sub.8 alkoxyalkyl or/and C.sub.1 -C.sub.4 alkylthio, are suitable for use as initiators for photopolymerising compounds containing ethylenically unsaturated double bonds.