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) (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. 2.