The present invention relates to a polymerisable plasticiser formed by a hydrocarbon chain, one end of which bears more than one isocyanate function, said hydrocarbon chain comprising and/or being substituted by an aromatic ring and/or an aliphatic ring and/or said hydrocarbon chain is substituted by at least two hydrocarbon chains that can comprise an unsaturation, and the number of isocyanate functions being strictly greater than 1, preferably greater than 1.2, and, more preferably, grater than 1.5 but less than or equal to 2.2. The invention also relates to liquid polyurethane resin compositions and, in particular, stable, ready-to-use liquid polyurethane resin compositions comprising said plasticiser. The invention further relates to methods for replacing all or part of the exogenous plasticiser and/or solvent in a polyurethane resin by adding said polymerisable plasticiser.