Sulphonium salt initiators

Compounds of the formula I, II, III and IV, wherein, R is hydrogen, C1-C20alkyl; C2-C20alkyl interrupted by one or more O; is -L-X-R2 or -L-R2; R1 has ofr example one of the meanings as given for R; R2 is a monovalent sensitizer or photoinitiator moiety; Ar1 and Ar2 for example independently of one another are phenyl substituted by C1-C20alkyl, halogen or OR3; or are unsubstituted naphthyl, anthryl, phenanthryl or biphenylyl; or are naphthyl, anthryl, phenanthryl or biphenylyl substituted by C1-C20alkyl, OH or OR3; or are -Ar4-A-Ar3 ; Ar3 is unsubstituted phenyl naphthyl, anthryl, phenanthryl or biphenylyl; or is phenyl, naphthyl, anthryl, phenanthryl or biphenylyl substituted by C1-C20alkyl, OR3 or benzoyl; Ar4 is phenylene, naphthylene, anthrylene or phenanthrylene; A is a direct bond, S, O or C1-C20alkylene; X is CO, C(O)O, OC(O), O, S or NR3 ; L is C1-C20alkylene or C2-C20alkylene interrupted by one or more O; R3 is C1-C20alkyl or C1-C20hydroxyalkyl; and Y is an anion, are suitable as photolatent acid gen