Catalyst transfer polymerization (CTP), or Catalyst Transfer Polycondensation, is a type of living chain-growth polymerization that is used for synthesizing conjugated polymers. Benefits to using CTP over other methods are low polydispersity and control over number average molecular weight in the resulting polymers. Very few monomers have been demonstrated to undergo CTP. History The first reports of CTP came simultaneously from the labs of Yokozawa and McCullough in 2004, with the recognition that polythiophene can be synthesized with low dispersity and with control over molecular weight. This recognition sparked interest in polymerization mechanism so that it could be expanded to other monomers. Few polymers can be synthesized via CTP, so most conjugated polymers are synthesized via step-growth using palladium catalyzed cross-coupling reactions. Characteristics CTP is exclusively performed on arene monomers to give conjugated polymers. The polymers obtained from CTP are often low dispersity due to its living, chain growth nature. Mass spectrometry can be used to identify end-groups on the polymer to determine if the polymer was synthesized via chain growth. Types CTP utilizes cross coupling reactions (see Mechanism below) with monomers containing magnesium-, zinc-, boron-, and tin-based transmetallating groups, giving rise to Kumada CTP, Negishi CTP, Suzuki CTP, and Stille CTP reactions. Mechanism The mechanism of CTP has been debated. The living chain-growth nature of CTP can be explained by the existence of a π-complex (as described in this section) but can also be explained via polymer reactivity. Initiation Initiation from a metal(II) species (either Ni or Pd) involves two monomers transmetalating onto the metal center to form a complex that can undergo reductive elimination. The complex formed after reductive elimination is referred to as a π-complex because the catalyst bound to the π system of the monomer. The catalyst can isomerize to other π-complexes via a process known as "ring-walking" to the π-bond adjacent to a C-X bond at the end of the chain allowing oxidative addition to occur. The product of oxidative addition is an active polymer-metal(II)-halide, and it can react with monomers in the propagation reaction. Propagation The propagation steps of CTP occurs through a cycle of transmetalation, reductive elimination, ring walking, and oxidative addition. The existence of a π-complex allows for the polymerization to be controlled as it ensures that the catalyst cannot dissociate from the polymer chain (and start new chains). This means that the number of polymer chains at the end of the