Compositions of expandable vinylaromatic polymers which contain a polymer, of at least one vinylaromatic monomer, having an average molecular weight Mw ranging from 50,000 to 170,000, which comprise: a) 0-50% by weight of a copolymer obtained by polymerizing one or more vinylaromatic monomers and 0.1-15% by weight of an α-alkylstyrene; b) 0-10% by weight of a compatible polymer having a crystallinity lower than 10% and a glass transition temperature (Tg) higher than 100° C., the remaining percentage consisting of the vinylaromatic polymer; c) 2-10% by weight, calculated with respect to the total, of an expanding agent; with the proviso that at least one of (a) or (b) is present in the polymeric mixture.