Flexible high density polyurethane foam is produced by a quasiprepolymerisation technique in which a ''''prepolymer composition'''' which is obtained from a diisocyanate and a copolymer of an Epsilon -caprolactone, an epoxide and a monomeric diol chain initiator; is reacted with a ''''reactant mixture'''' which is a mixture of the copolymer and monomeric diol. The prepolymer composition and reactant mixture are stable, easily handled liquids which intermix easily and react quickly with each other, the presence of the diol in the reactant mixture facilitating mixing with the prepolymer composition to ensure good foam formation.