This results in a product with a wide range of molecular weights. The average molecular weight builds up slowly through the entire polymerization and a high-molecular-weight product is only formed after a sufficiently long reaction time when the conversion is more than 98% (Figure 2).
There are four different ways to initiate chain-growth polymerization: free radical, cationic, anionic, and coordination. This synthesis used benzoyl peroxide as a free radical initiator to polymerize styrene monomer. First, the benzoyl peroxide must be split to become a peroxy radical. The chain is then initiated when a peroxy radical combines with a styrene monomer, forming an active center on the end opposite the initiator. Until termination, the chain continues to grow one styrene monomer at a time. The reaction used to form polystyrene in this lab is shown in Figure 3.