Conventional free radical polymerization (FRP) has been a widely used technique for manufacturing of polymeric materials due to widely available vinyl monomers, tolerance to functional groups, and mild reaction conditions. However, highly reactive radical species cause rapid bimolecular termination reactions that lead to poor control over molecular weight and polydispersity. Since polymer properties and applications depend upon molecular weight distribution, composition, and topology, a polymerization method that affords control over these molecular features was desired. Controlled radical polymerization (CRP) is such a method that is also industrially applicable. IUPAC recommends the term reversible-deactivation radical polymerization (RDRP) to describe this technique and hence it will be used in this chapter. Compared to FRP, RDRP minimizes the side reactions and allows synthesis of unprecedented functional polymers with narrow molecular weight distribution while retaining the tolerance of FRP to various reaction conditions. 1 The control over molecular weight is achieved by a dynamic equilibrium between active (growing radical) and dormant (deactivated) species so that active species is present in low concentrations at any moment. The process is characterized by almost quantitative initiation so that rate of propagation is much lower than rate of initiation. Based on the mechanism involved in the deactivation process to maintain the equilibrium, there are three types of RDRP. The first approach involves reversible deactivation 162of propagating radicals to form the dormant species by the coupling between propagating (transient) radical and a stable or persistent free radical. Due to persistent radical effect 2 the coupling of propagating and stable radical is favored over the self-coupling of propagating radicals, thereby giving controlled polymerization. Examples of this approach include nitroxide-mediated polymerization (NMP) 3 and organometallic-mediated radical polymerization (OMRP) 4 that involves reversible homolytic cleavage of the weak bond between an alkyl group and a transition metal catalyst. In the second type, degenerate transfer (thermodynamically neutral bimolecular exchange) takes place between propagating radicals and dormant species. The degenerative radical transfer polymerization does not involve a stable radical and the kinetics of polymerization is similar to FRP. Examples include reversible addition-fragmentation chain-transfer (RAFT) polymerization, 5 macromolecular architecture design by interchange of xanthates (MADIX), 6 and iodine transfer polymerization. 7 In the third type of CRP, called as atom transfer radical polymerization (ATRP), 8,9 the reversible deactivation is accomplished by atom transfer catalyzed by a transition metal catalyst. This chapter will focus on ATRP since it is a metal-catalyzed CRP. General mechanism of ATRP is presented in Figure 5.1.