A compound of formula (I) in which formula Q is a C1-C6 hydrocarbylene diradical; Y is either a single bond, a carbonyl group or a methylene, ethylene, -CH(OH)-, -O-(C 6 H 4)- (ortho, meta, para) or -S-(C 6 H 4)-(ortho, meta, para) diradical; R 1 and R 2, ...