1 to 4 carbon atoms, such as a methyl, ethyl, propyl or isobutyl group, said alkyl group being substituted by one or more substituents chosen from hydroxyl groups (such as 2-hydroxypropyl methacrylate or 2-hydroxyethyl methacrylate) and halogen atoms (Cl, Br, I or F), such as trifluoroethyl methacrylate, methacrylates of formula CH2???C(CH3)???COOR9, R9 representing a linear or branched C6 to C12