One of the most common and useful reaction for making polymers is free radical polymerization. It is used to make polymers from vinyl monomers, that is, from small molecules containing carbon-carbon double bonds. Polymers made by free radical polymerization include polystyrene, poly(methyl methacrylate), poly(vinyl acetate) and branched polyethylene. But enough introduction. What is this reaction, and how does it work?