Polymer blends having a high tenacity and impact strength comprising(a) from 10 to 98% by weight of a vinyl-aromatic copolymer (A) containing at least 50% by weight of one or more vinyl-aromatic compounds of the formula:FORMULAin which X is hydrogen or an alkyl radical containing 1 to 4 carbon atoms; n is zero or an integer ranging from 1 to 5 and Y is a halogen or an alkyl radical having 1 to 4 c