rmula CH2 ???CR--OR' in which R is hydrogen or a methyl group and R' is an alkyl group with from 1 to 18 carbon atoms, a cycloalkyl group or a polyether, acrylonitrile, methacrylonitrile, vinyl ketones of the formula CH2 ???CR--CO--CH2 --R' in which R is hydrogen or a methyl group and R' is hydrogen or a C1 -C6 alkyl group, acrylic or methacrylic acid or their esters of the formula CH2 ???CR--COOR