wherein R1, R2 and R4, each independently of one another, stand for a hydrogen atom or a C1-C4 alkyl group, R3 stands for a branched or unbranched, saturated or unsaturated C1-C12 alkyl group, R5 stands for a branched or unbranched, saturated or unsaturated C6-C14 alkyl group, A stands for oxygen, sulfur or an NH group, and c) a foaming agent.