Preparation of impact-resistant poly(alkyl)styrene

Impact-resistant translucent rubber-modified polystyrene as obtained by a process comprising:

• continuously polymerizing a reaction batch containing 70 to 97% by weight of styrene and 3 to 30% by weight of a rubbery polymer, said rubbery polymer being based on more than 80% conjugated diene, having an average molecular weight of from 20,000 to 200,000, as measured by gel permeation chromatography, and conforming to the formula P-(SH).sub.x, where x denotes the number of thiol groups per rubber molecule and is an integer from 1 to 5 and P is a polymer of more than 80% conjugated dienes of 4 or 5 carbon atoms, in the presence or absence of a solvent, with the application of heat or in the presence of free radical initiators and while stirring for from 5 to 20 hours until an end conversion of from 70 to 95%, based on the monomer, is achieved, stirring being carried out in such a way that, on completion of the polymerization, the grafted rubber particles which represent the flexible phase of the product are dispersed in the resulting polystyrene and have an average particle diameter of from 0.1 to 0.8 .mu.m.