1,239,495. Solid state polycondensation. MOBIL OIL CORP. 12 May, 1969 [10 July, 1968], No. 23995/69. Heading C3R. In a process for polycondensing a polyester prepolymer in the solid state, an inert gas or foaming agent is dispersed in the molten saturated polyester prepolymer having an intrinsic viscosity of from 0.3 to 1À2, and this is followed by extrusion of the prepolymer to form a solid foam which is formed into particles having a maximum dimension of <SP>1</SP>/ 16 to ¢ inches and polycondensation of the foamed particles in the solid state without reduction in particle size. The inert gas is suitably nitrogen. Specified polyesters are polyethylene terephthalate, poly - (ethylene 2,6 - naphthalate), poly-(1,4 cyclohexanedimethylene terephthalate) and copoly-(90-98 mole per cent ethylene terephthalate and 10-2 mole per cent ethylene succinate).