Thioxanthone derivatives, and their use as cationic photoinitiators

76)ay2by2b(y-1)76a6 7 123 4 1 3 5 Photoinitiator compounds of formula (I): {where: A represents a direct bond or a group of formula -[O(CHRCHR]-, -[O(CH)CO]-, or -[O(CH)CO]-[O(CHRCHR)]-, where one of Rand Ris hydrogen and the other is hydrogen or methyl; a is a number from 1 to 2; b is a number from 4 to 5; Q is a residue of a polyhydroxy compound having 2 to 6 hydroxy groups; x is a number greater than 1 but no greater than the number of available hydroxyl groups in Q; y is a number from 1 to 10; R, R, Rand Rare individually the same or different and each is hydrogen, hydroxy, or alkyl; or Rand Rare joined to form a fused ring system with the benzene rings to which they are attached; and Rrepresents a direct bond, an oxygen atom or a methylene group} are useful as cationic photoinitiators, especially for use in surface coating applications, such as printing inks and varnishes, and which are intended to be cured by polymerisation initiated by radiation.