Expandable polyacrylates and their foams

The present invention provides expandable plastics having a weight-average molecular weight of 50,000 to 500,000 which comprises the following structural units (I), (II) and (III) giving a foam having heat resistance and fine structure by heating, the molar ratio of (I+II) to (III) being 1 to 0.6-1.5 and the proportion of (I) being 5 to 50 wt.% of the total components: wherein R represents a hydrogen atom or a methyl group. When the copolymer, the expandable plastics, of the present invention is used, white foams having a very fine texture are obtained without using special foaming agents, and besides the heat resistance of the foams is high and the amount of water absorbed thereby is little. The foams of the present invention, therefore, can be used in a wide range as soundproofing materials and heat insulators for which the conventional foams cannot be used because of their low heat resistance, and substitutes for honeycomb structures used in airplanes.