Editing Living polymerization (section)

====Catalyst-transfer polycondensation====
{{Main|Living chain-growth polycondensation}}
[[Catalyst transfer polycondensation]] (CTP) is a chain-growth polycondensation mechanism in which the monomers do not directly react with one another and instead the monomer will only react with the polymer end group through a catalyst-mediated mechanism.<ref name="Yokozawa 1" /> The general process consists of the catalyst activating the polymer end group followed by a reaction of the end group with a 2nd incoming monomer. The catalyst is then transferred to the elongated chain while activating the end group (as shown below).<ref name="Yokozawa 2">{{cite journal|last=Miyakoshi|first=Ryo|author2=Yokoyama, Akihiro |author3=Yokozawa, Tsutomu |title=Catalyst-Transfer Polycondensation. Mechanism of Ni-Catalyzed Chain-Growth Polymerization Leading to Well-Defined Poly(3-hexylthiophene)|journal=Journal of the American Chemical Society|date=2005|volume=127|issue=49|pages=17542–17547|doi=10.1021/ja0556880|pmid=16332106|url=https://figshare.com/articles/Catalyst_Transfer_Polycondensation_Mechanism_of_Ni_Catalyzed_Chain_Growth_Polymerization_Leading_to_Well_Defined_Poly_3_hexylthiophene_/3251725|url-access=subscription}}</ref>
[[File:CTP scheme.png|center|600px]]

Catalyst transfer polycondensation allows for the living polymerization of π-conjugated polymers and was discovered by Tsutomu Yokozawa in 2004<ref name="Yokozawa 2"/> and Richard McCullough.<ref>{{cite journal|last=Iovu|first=Mihaela Corina|author2=Sheina, Elena E. |author3=Gil, Roberto R. |author4= McCullough, Richard D. |title=Experimental Evidence for the Quasi-"Living" Nature of the Grignard Metathesis Method for the Synthesis of Regioregular Poly(3-alkylthiophenes)|journal=Macromolecules|date=October 2005|volume=38|issue=21|pages=8649–8656|doi=10.1021/ma051122k|bibcode=2005MaMol..38.8649I|citeseerx=10.1.1.206.3875}}</ref> In CTP the propagation step is based on organic cross coupling reactions (i.e. [[Kumada coupling]], [[Sonogashira coupling]], [https://www.organic-chemistry.org/namedreactions/negishi-coupling.shtm Negishi coupling]) top form carbon carbon bonds between difunctional monomers. When Yokozawa and McCullough independently discovered the polymerization using a metal catalyst to couple a [[Grignard Reaction|Grignard reagent]] with an organohalide making a new carbon-carbon bond. The mechanism below shows the formation of poly(3-alkylthiophene) using a Ni initiator (L<sub>n</sub> can be [[1,3-Bis(diphenylphosphino)propane| 1,3-Bis(diphenylphosphino)propane (dppp)]]) and is similar to the conventional mechanism for [[Kumada coupling]] involving an [[oxidative addition]], a [[transmetalation]] and a [[reductive elimination]] step. However, there is a key difference, following reductive elimination in CTP, an associative complex is formed (which has been supported by intra-/intermolecular oxidative addition competition experiments<ref name="McNeil 1">{{cite journal|last=McNeil|first=Anne; Bryan, Zachary|title=Evidence for a preferential intramolecular oxidative addition in Ni-catalyzed cross-coupling reactions and their impact on chain-growth polymerizations|journal=Chem. Sci.|year=2013|volume=4|issue=4|pages=1620–1624|doi=10.1039/C3SC00090G}}</ref>) and the subsequent oxidative addition occurs between the metal center and the associated chain (an intramolecular pathway). Whereas in a coupling reaction the newly formed alkyl/aryl compound diffuses away and the subsequent oxidative addition occurs between an incoming Ar–Br bond and the metal center. 
The associative complex is essential to for polymerization to occur in a living fashion since it allows the metal to undergo a preferred intramolecular oxidative addition and remain with a single propagating chain (consistent with chain-growth mechanism), as opposed to an intermolecular oxidative addition with other monomers present in the solution (consistent with a step-growth, non-living, mechanism).<ref name=Kiriy>{{cite journal|last=Kiriy|first=Anton|author2=Senkovskyy, Volodymyr |author3=Sommer, Michael |title=Kumada Catalyst-Transfer Polycondensation: Mechanism, Opportunities, and Challenges|journal=Macromolecular Rapid Communications|date=4 October 2011|volume=32|issue=19|pages=1503–1517|doi=10.1002/marc.201100316|pmid=21800394}}</ref><ref name="Bryan 8395–8405">{{cite journal|last=Bryan|first=Zachary J.|author2=McNeil, Anne J. |title=Conjugated Polymer Synthesis via Catalyst-Transfer Polycondensation (CTP): Mechanism, Scope, and Applications|journal=Macromolecules|date=12 November 2013|volume=46|issue=21|pages=8395–8405|doi=10.1021/ma401314x|bibcode=2013MaMol..46.8395B|s2cid=101567648}}</ref> The monomer scope of CTP has been increasing since its discovery and has included poly(phenylene)s, poly(fluorine)s, poly(selenophene)s and poly(pyrrole)s.<ref name=Kiriy /><ref name="Bryan 8395–8405"/>     
[[File:CTP general scheme.png|center|400px]]