ethylene groups, R1 and R2 can be identical or different and the free valencies of the oxygen atoms are saturated either by silicon atoms of further units of the formula (2) and/or by crosslinking bridge members: ##STR6## in which R' is a methyl or ethyl group and in which the ratio of the silicon atoms in (2) to the bridge atoms silicon, titanium and aluminum can be 1:0 to 1:10, R3 has the same g