Abstract: The invention relates to a cross-linking monomer with at least one sulfur atom, representable by a structure of formula (I) wherein the symbols have the following meaning: L1 is a linear, branched or cyclic, saturated or unsaturated, aliphatic or heteroaliphatic group with 2 to 10 carbon atoms; an aromatic or heteroaromatic group with 4 to 12 carbon atoms or a group with 6 to 14 carbon atoms, which comprises at least one saturated or unsaturated, aliphatic and/or heteroaliphatic residue and at least one aromatic and/or heteroaromatic residue, wherein the phosphorus atom is directly bonded with a carbon atom; V1 is a saturated or unsaturated, aliphatic or heteroaliphatic group with 2 to 50 carbon atoms; an aromatic or heteroaromatic group with 5 to 50 carbon atoms or a group with 6 to 50 carbon atoms, which comprises at least one saturated or unsaturated, aliphatic and/or heteroaliphatic residue and at least one aromatic and/or heteroaromatic residue; X1 is the same or different, O or NR3, each time it o