"lkyl group containing an alkyl group with 1 to 2 carbon atoms, the above alkyl groups being of linear or branched chain; R2 is a hydrogen atom, a linear or branched chain alkyl group containing 1 to 20 carbon atoms, a cyclohexyl group or a phenyl group; and R4 and R5, independently from each other, are a hydrogen atom or a linear or branched chain alkyl group containing 1 to 5 carbon atoms, and im" . . . .