Compounds of the formula (I), wherein A.sub.1 is formula (II); A.sub.2 is formula (III); A.sub.3 is formula (IV); A.sub.4 is formula (V); w, x, y and z independently of each other are an integer from 0-4, provided that the sum of x+y+z is an integer from 2-4, corresponding to the valency of Q; M.sub.1, M.sub.2, M.sub.3 and M.sub.4 for example are a direct bond, CO or O; Y for example is a direct bond or S; Q is a (x+y)-valent linking group; R.sub.1 is for example hydrogen, C.sub.1-C.sub.20alkyl or phenyl or naphthyl; R.sub.2 and R''.sub.2 for example are is hydrogen or C.sub.1-C.sub.20alkyl; R.sub.3, R.sub.4, R'.sub.3, R'.sub.4, R''.sub.3 and R''.sub.4 for example are hydrogen, halogen, phenyl, or C.sub.1-C.sub.20alkyl; and R.sub.24 is for example a direct bond; exhibit an unexpectedly good performance in photopolymerization reactions. ##STR00001##