more heteroatoms from groups 13-17 of the Periodic Table of the Elements or one or more unsaturated bonds, with the two radicals X also being able to be joined to one another, L is a divalent bridging group selected from the group consisting of C1-C20-alkylidene, C3-C20-cycloalkylidene, C6-C20-arylidene, C7-C20-alkylarylidene and C7-C20-arylalkylidene radicals which may contain heteroatoms from g