itiator is of formula R1-C(=O)-O-O-R2 (I) wherein R1 is an alkyl group and R2 is an alkyl group of formula -C(R3)(R4)-C(R5)(R6)(R7) (II) wherein R3 and R4 are the same or different and are an alkyl group, R5, R6 and R7 are the same or different and are a hydrogen atom or an alkyl group.