Had my chem teacher in highschool grow the PU foam in front of the class, how did he do it?
You need four chemicals:
A polyol. For foam, glycerin will work.
A diisocyanate.
A catalyst called DABCO, which has a REALLY long name.
And a blowing agent; for this we'll use water.
There are two reactions going on here. First is the urethane reaction--DABCO bonds the polyol to the diisocyanate to form the urethane molecule. Then comes the water...when diisocyanate and water are mixed, carbon dioxide is liberated. The little CO2 bubbles are trapped in the polyurethane and make it foamy.