loalkylene groups which are connected to one another by a direct carbon-carbon bond or by a bivalent group chosen from oxygen, sulphur, alkylene with one to three carbon atoms, and a group of formulae IIIa to IIIj ##STR9## the radicals R4, R5, R6, R7, being alkyl groups with one to five carbon atoms, R8 being an alkylene group or an arylene group and B is a group which is capable of addition polym