, or a mixture of said monomers, wherein R3 is hydrogen or methyl, and R12 is a linear or branched aliphatic, cycloaliphatic or aromatic alkyl group with from 1 to 21 carbon atoms and which may contain ether or thioether linkages or a --CO-- group; or is a heterocyclic alkyl group containing oxygen, sulfur or nitrogen atoms, or a polypropylene oxide or poly-n-butylene oxide group with from 2 to 50