A method of preparing cellular pellets from a prepolymer comprising a blowing agent, the method comprising the steps of: a) extruding the prepolymer through a die, the die maintained at conditions such that the blowing agent remains in the condensed phase in the prepolymer prior to emerging from the die, and b) upon emergence of the prepolymer through the die, substantially simultaneously cooling the prepolymer by contacting the prepolymer with a cooling agent and cutting the prepolymer; the conditions outside the die being maintained such that the blowing agent vaporizes in the prepolymer to form pores.