. . . . . " with the provisos that(a) at least one of X, Y and Z represents a group wherein the atom in the main polymer chain is not Si, (b) when n=0, then X and Y are not both O or N, (c) when X and Y is GeR5 R6 or Z is Ge, then at least one of X, Y or Z is a group wherein the atom in the main polymer chain is not Ge and is not Si, (d) when one of R1-9 is alkyl-containing, then the total number of C-atoms" .