Controlled polymerization reagents are chemicals used to regulate polymer chain growth during the polymerization process. They are essential to achieve a specific molecular weight and polydispersity index for polymers. These reagents typically work by controlling the rate of monomer addition or terminating polymer chain growth at a specific point. They are widely used in the synthesis of advanced materials such as block copolymers and hydrogels. Examples of controlled polymerization reagents include activated catalysts, chain transfer agents, and free radical scavengers.