Polymer blends composed of two different polymers P1 and P2, which form compatible polymer mixtures where polymer P1 is composed of at least 20 and up to 100% by weight of styrene and polymer P2 is composed of at least 20 and up to 100% by weight of monomers with Formula I ##STR1## where R.sub.1 is hydrogen, methyl or a group ##STR2## and where R.sub.2 or R.sub.2 ', are a group ##STR3## where n is 0-3, R.sub.3 is a 5-8 membered heterocyclic ring, optionally substituted, containing at least 2 hetero atoms in the ring, one heteroatom being oxygen, and R.sub.4 is hydrogen or an alkyl group with 1-6 carbon atoms.