, equal to or different from each other, are linear or branched, saturated or unsaturated C1-C20-alkyl radicals, optionally containing heteroatoms belonging to groups 13-17 of the Periodic Table of the Elements; preferably R3 and R4 equal to or different from each other are C1-C10-alkyl radicals; more preferably R3 is a methyl, or ethyl radical; and R4 is a methyl, ethyl or isopropyl radical; (A)