Simplify the following text for a Grade 13 native English-speaking person.
Original: 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 .
Simplified: 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 .