Making polyurethane foam. Image 1 in a series of 5. In this demonstration, two viscous liquid chemicals are mixed together. One contains a polyether polyol, a silicone surfactant, and the catalyst tertiary amine. The other contains diphenylmethane diisocyanate and oligomers of diisocyanate. The polymerization reaction occurs when the two substances are mixed. In addition, water reacts with the diisocyanate in a decomposition reaction to produce CO2 gas. This gas causes the foaming and expansion. After the increase in volume the polymer hardens and forms pourous and rigid polyurethane foam.