Compounds of formula (I): wherein Ar represents an, optionally substituted, aryl or heteroaryl group, R represents an, optionally substituted, aryl or heteroaryl group, an optionally substituted straight or branched chain C1.10-alkyl, R1 is H or methyl, X is an extender group, n is 0 or an integer between 1 and 12 inclusive, A is a polyol residue wherein the unsubstituted polyol from which the residue is derived has at least y OH groups, and y is an integer>1, can easily be prepared, and are useful as synergists in radiation curing.