(Co)polymerisation of styrene or mixtures thereof with 50% or less of comonomer(s) is carried out in suspension, in presence of a foaming agent and a suspension agent and 0.01 to 0.3%, with respect to weight of monomer(s) of a polysiloxane. The (co)polymer obtained may be coated with an emulsifier. Addition of the polysiloxane results in production of a finely divided polymer having excellent flow characteristics without tendency to agglomeration of the particles. Esp. suitable for prodn. of thin-walled foamed mouldings having homogeneous structure and good mechanical strength.
