If X is an alkyl group substituted by groups containing O, N or S, X can be, for example, the following groups: hydroxyalkyl such as --CH2 CH2 OH or --CH2 CHOH--CH3 ; carboxyalkyl such as --CH2 COOH; alkyl substituted by ester groups, for example alkoxycarbonylalkyl such as --CH2 COOCH3, --CH2 COOC2 H5, --CH2 COOC8 H17, --CH2 COOC12 H25 or --CH2 COOC18 H37, or alkylcarbonyloxyalkyl such as --CH2 C