Compounds of the formula (I), wherein A1 is formula (II); A2 is formula (III); A3 is formula (IV); A4 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; M1, M2, M3 and M4 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; R1 is for example hydrogen, C1-C20alkyl or phenyl or naphthyl; R2 and R″2 for example are is hydrogen or C1-C20alkyl; R3, R4, R′3, R′4, R″3 and R″4 for example are hydrogen, halogen, phenyl, or C1-C20alkyl; and R24 is for example a direct bond; exhibit an unexpectedly good performance in photopolymerization reactions.