ula Rl-C (=0) -0-0-R2 (I) wherein Rl is an alkyl group and R2 is an alkyl group of formula -C(R3) (R4)-C(R5') (R6')-C(R7') (R8') ( R9 ) (III) wherein R3 and R4 are the same or different and are an alkyl group, R5 ' and R6 ' are the same or different and are a hydrogen atom or an alkyl group, R7 ' , R8 ' and R9 ' are the same or different and are a hydrogen atom or an alkyl group, with the proviso