Novel monomers P1.2.1 are vinylaromatic monomers of, preferably, not more than 20 carbon atoms, for example styrene, ??-methylstyrene or styrenes alkylated on the nucleus, such as p-methylstyrene or p-tert-butylstyrene, styrene, ??-methylstyrene or p-methylstyrene or mixtures thereof being preferably used and styrene being particularly preferably used.