. . "al from group IVb, Vb or VIb of the Periodic Table, R1 and R2 are identical or different and are a hydrogen atom, a C1 -C10 -alkyl group, a C1 -C10 -alkoxygroup, a C6 -C10 -aryl group, a C6 -C10 -aryloxy group, a C2 -C10 -alkenyl group, a C7 -C40 -arylalkyl group, a C7 -C40 -alkylaryl group, a arylalkenyl group or a halogen atom, R3 and R4 are identical or different and are a mono- or polynuclear" . . . . . .