Polyurethane polymers are formed through step-growth polymerization . In this process , a monomer containing at least two isocyanate functional groups reacts with another monomer containing at least two hydroxyl ( alcohol ) groups in the presence of a catalyst . Polyurethane polymers are formed through step-growth polymerization by reacting a monomer containing at least two isocyanate functional groups with another monomer containing at least two hydroxyl ( alcohol ) groups in the presence of a catalyst .