Editing Living polymerization (section)

==Techniques==

===Living anionic polymerization===
{{Main|Living anionic polymerization}}
As early as 1936, [[Karl Ziegler]] proposed that anionic polymerization of styrene and butadiene by consecutive addition of monomer to an alkyl lithium initiator occurred without chain transfer or termination. Twenty years later, living polymerization was demonstrated by Szwarc through the [[anionic addition polymerization|anionic polymerization]] of [[styrene]] in [[THF]] using [[sodium naphthalene]] as an initiator.<ref>{{Cite journal | last1 = Szwarc | first1 = M. | title = 'Living' Polymers | doi = 10.1038/1781168a0 | journal = Nature | volume = 178 | issue = 4543 | pages = 1168–1169 | year = 1956 | bibcode = 1956Natur.178.1168S }}</ref><ref name="ReferenceA"/><ref>Tatemoto, Masayoshi and Nakagawa, Tsuneo "Segmented polymers containing fluorine and iodine and their production" {{US Patent|4158678}}. Priority date 30 June 1976.</ref>

The naphthalene anion initiates polymerization by reducing styrene to its radical anion, which dimerizes to the dilithiodiphenylbutane, which then initiates the polymerization.  These experiments relied on Szwarc's ability to control the levels of impurities which would destroy the highly reactive organometallic intermediates.

====Living α-olefin polymerization====

[[Alpha-olefins|α-olefins]] can be polymerized through an anionic coordination polymerization in which the metal center of the catalyst is considered the counter cation for the [[anionic]] end of the alkyl chain (through a M-R coordination). Ziegler-Natta initiators were developed in the mid-1950s and are heterogeneous initiators used in the polymerization of alpha-olefins. Not only were these initiators the first to achieve relatively high molecular weight poly(1-alkenes) (currently the most widely produced thermoplastic in the world PE([[Polyethylene]]) and PP ([[Polypropylene]])<ref name=b1>{{cite book|last=Craver|first=C.|author2=Carraher, C.|title=Applied Polymer Science: 21st Century|year=2000|publisher=Elsevier|pages=1022–1023}}</ref> but the initiators were also capable of stereoselective polymerizations which is attributed to the [[Chirality (chemistry)|chiral]] [[Crystal structure]] of the heterogeneous initiator.<ref name=Principles /> Due to the importance of this discovery Ziegler and Natta were presented with the [https://www.nobelprize.org/nobel_prizes/chemistry/laureates/1963/ 1963 Nobel Prize in chemistry]. Although the active species formed from the Ziegler-Natta initiator generally have long lifetimes (on the scale of hours or longer) the lifetimes of the propagating chains are shortened due to several chain transfer pathways ([[Beta-Hydride elimination]] and transfer to the co-initiator) and as a result are not considered living.<ref name=Principles />

Metallocene initiators are considered as a type of Ziegler-Natta initiators due to the use of the two-component system consisting of a [[transition metal]] and a group I-III metal co-initiator (for example [[methylalumoxane]] (MAO) or other alkyl aluminum compounds). The [[metallocene]] initiators form homogeneous single site [[catalyst]]s that were initially developed to study the impact that the catalyst structure had on the resulting polymers structure/properties; which was difficult for multi-site heterogeneous Ziegler-Natta initiators.<ref name=b1/> Owing to the discrete single site on the metallocene catalyst researchers were able to tune and relate how the ancillary ligand (those not directly involved in the chemical transformations) structure and the symmetry about the chiral metal center affect the microstructure of the polymer.<ref>{{cite journal|last=Coates|first=Geoffrey W.|title=Precise Control of Polyolefin Stereochemistry Using Single-Site Metal Catalysts|journal=Chemical Reviews|date=April 2000|volume=100|issue=4|pages=1223–1252|doi=10.1021/cr990286u|pmid=11749265}}</ref> However, due to chain breaking reactions (mainly Beta-Hydride elimination) very few metallocene based polymerizations are known.<ref name=Principles />

By tuning the steric bulk and electronic properties of the ancillary ligands and their [[substituent]]s a class of initiators known as [[Chelation|chelate]] initiators (or post-metallocene initiators) have been successfully used for [[Stereospecificity|stereospecific]] living polymerizations of alpha-olefins. The chelate initiators have a high potential for living polymerizations because the ancillary ligands can be designed to discourage or inhibit chain termination pathways. Chelate initiators can be further broken down based on the ancillary ligands; ansa-cyclopentyadienyl-amido initiators, alpha-diimine chelates and phenoxy-imine chelates.<ref name=Principles />  
*Ansa-cyclopentadienyl-amido (CpA) initiators 
[[File:ACp initiators.png|thumb|300px|right|a.) Shows the general form of CpA initiators with one Cp ring and a coordinated Nitrogen b.) Shows the CpA initiator used in the living polymerization of 1-hexene (5)]]
CpA initiators have one [[cyclopentadienyl]] substituent and one or more nitrogen substituents coordinated to the metal center (generally a Zr or Ti) (Odian).  The dimethyl(pentamethylcyclopentyl)zirconium acetamidinate in figure___ has been used for a stereospecific living polymerization of 1-hexene at −10&nbsp;°C. The resulting poly(1-hexene) was [[Tacticity#Isotactic polymers|isotactic]] (stereochemistry is the same between adjacent repeat units) confirmed by <sup>13</sup>C-NMR. The multiple trials demonstrated a controllable and predictable (from catalyst to [[monomer]] ratio) M<sub>n</sub> with low Đ. The polymerization was further confirmed to be living by sequentially adding 2 portions of the monomer, the second portion was added after the first portion was already polymerized, and monitoring the Đ and M<sub>n</sub> of the chain. The resulting polymer chains complied with the predicted M<sub>n</sub> (with the total monomer concentration = portion 1 +2) and showed low Đ<ref name="Ansa chelates">{{cite journal|last1=Jayaratne|first1=K.|last2=Sita |first2=L|title= Stereospecific Living Ziegler−Natta Polymerization of 1-Hexene |journal=J. Am. Chem. Soc.|issue=5|pages=958–959|volume=122|doi=10.1021/ja993808w |year=2000}}</ref> suggesting the chains were still active, or living, as the second portion of monomer was added (5).
*α-diimine chelate initiators
α-diimine chelate initiators are characterized by having a [[diimine]] chelating ancillary ligand structure and which is generally coordinated to a late transition (i.e. Ni and Pd) metal center. 
[[File:Alphadiiminechelateinitiator.png|200px|center]]
Brookhart et al. did extensive work with this class of catalysts and reported living polymerization for α-olefins<ref name="Maurice Brookhart">{{Cite journal | doi = 10.1021/ja962516h| title = Living Polymerization of α-Olefins Using NiII−α-Diimine Catalysts. Synthesis of New Block Polymers Based on α-Olefins| journal = Journal of the American Chemical Society| volume = 118| issue = 46| pages = 11664–11665| year = 1996| last1 = Killian | first1 = C. M. | last2 = Tempel | first2 = D. J. | last3 = Johnson | first3 = L. K. | last4 = Brookhart | first4 = M. }}</ref> and demonstrated living α-olefin carbon monoxide alternating copolymers.<ref name="MAurice B 2">{{Cite journal | last1 = Brookhart | first1 = M. | last2 = Rix | first2 = F. C. | last3 = Desimone | first3 = J. M. | last4 = Barborak | first4 = J. C. | title = Palladium(II) catalysts for living alternating copolymerization of olefins and carbon monoxide | doi = 10.1021/ja00040a082 | journal = Journal of the American Chemical Society | volume = 114 | issue = 14 | pages = 5894–5895 | year = 1992 }}</ref>

===Living cationic polymerization===
{{Main|Living cationic polymerization}}
Monomers for living cationic polymerization are electron-rich alkenes such as vinyl ethers, [[isobutylene]], [[styrene]], and N-vinylcarbazole. The initiators are binary systems consisting of an electrophile and a Lewis acid. The method was developed around 1980 with contributions from Higashimura, Sawamoto and Kennedy.  Typically, generating a stable carbocation for a prolonged period of time is difficult, due to the possibility for the cation to be quenched by a β-protons attached to another monomer in the backbone, or in a free monomer.  Therefore, a different approach is taken<ref name=Cowie>{{cite book|last=Cowie|first=J.M.G.|title=Polymers chemistry and physics of modern materials|year=2007|publisher=Taylor & Francis|location=Boca Raton|isbn=9780849398131|edition=3rd ed / J.M.G. Cowie and Valeria Arrighi}}</ref><ref name=Principles /><ref name=Goethals />

[[File:Wiki Cation.png|right|400px|This is an example of a controlled/living cationic polymerization.  Note that the "termination" step has been placed in equilibrium with an "initiation" step in either direction.  Nu: is a weak nucleophile that can reversibly leave, while the MXn is a weak Lewis acid M bound to a halogen X to generate the carbocation.]]

In this example, the carbocation is generated by the addition of a Lewis acid (co-initiator, along with the halogen "X" already on the polymer – see figure), which ultimately generates the carbocation in a weak equilibrium.  This equilibrium heavily favors the dormant state, thus leaving little time for permanent quenching or termination by other pathways.  In addition, a weak nucleophile (Nu:) can also be added to reduce the concentration of active species even further, thus keeping the polymer "living".<ref name=Cowie /><ref name=Principles /><ref name=Goethals>{{cite journal|last=Goethals|first=E|author2=Duprez, F |title=Carbocationic polymerizations|journal=Progress in Polymer Science|date=2007|volume=32|issue=2|pages=220–246|doi=10.1016/j.progpolymsci.2007.01.001}}</ref> However, it is important to note that {{em|by definition, the polymers described in this example are not technically living}} due to the introduction of a dormant state, as termination has only been decreased, not eliminated (though this topic is still up for debate).  But, they do operate similarly, and are used in similar applications to those of true living polymerizations.

===Living ring-opening metathesis polymerization===
Given the right reaction conditions [[ring-opening metathesis polymerization]] (ROMP) can be rendered living. The first such systems were described by [[Robert H. Grubbs]] in 1986 based on [[norbornene]] and [[Tebbe's reagent]] and in 1978 Grubbs together with [[Richard R. Schrock]] describing living polymerization with a [[tungsten]] carbene complex.<ref>{{cite journal | last1 = Schrock | first1 = R. R. | last2 = Feldman | first2 = J. | last3 = Cannizzo | first3 = L. F. | last4 = Grubbs | first4 = R. H. | year = 1987 | title = Ring-opening polymerization of norbornene by a living tungsten alkylidene complex | journal = [[Macromolecules (journal)|Macromolecules]] | volume = 20 | issue = 5| pages = 1169–1172 | doi = 10.1021/ma00171a053 | bibcode = 1987MaMol..20.1169S }}</ref>

Generally, ROMP reactions involve the conversion of a cyclic olefin with significant ring-strain (>5 kcal/mol), such as cyclobutene, norbornene, cyclopentene, etc., to a polymer that also contains double bonds.  The important thing to note about ring-opening metathesis polymerizations is that the double bond is usually maintained in the backbone, which can allow it to be considered "living" under the right conditions.<ref name=Grubbs>{{cite journal|last=Bielawski|first=Christopher W.|author-link1= Christopher Bielawski |author2=Grubbs, Robert H. |title=Living ring-opening metathesis polymerization|journal=Progress in Polymer Science|date=2007|volume=32|issue=1|pages=1–29|doi=10.1016/j.progpolymsci.2006.08.006}}</ref>

For a ROMP reaction to be considered "living", several guidelines must be met:<ref name=Grubbs />
# Fast and complete initiation of the monomer. This means that the rate at which an initiating agent activates the monomer for polymerization, must happen very quickly.
# How many monomers make up each polymer (the degree of polymerization) must be related linearly to the amount of monomer you started with.
# The [[dispersity]] of the polymer must be < 1.5.  In other words, the distribution of how long your polymer chains are in your reaction must be very low.
With these guidelines in mind, it allows you to create a polymer that is well controlled both in content (what monomer you use) and properties of the polymer (which can be largely attributed to polymer chain length).  It is important to note that living ring-opening polymerizations can be anionic ''or'' cationic.

[[File:Wiki LivingROMP2.png|center|500px|The catalytic cycle of a living ring-opening metathesis polymerization with a metal catalyst.  Note that the ring can be any size, but should contain some significant ring strain on the alkene.]]

Because living polymers have had their termination ability removed, this means that once your monomer has been consumed, the addition of more monomer will result in the polymer chains continuing to grow until all of the additional monomer is consumed.  This will continue until the metal catalyst at the end of the chain is intentionally removed by the addition of a quenching agent.  As a result, it may potentially allow one to create a [[block copolymer|block]] or [[gradient copolymers|gradient copolymer]] fairly easily and accurately.  This can lead to a high ability to tune the properties of the polymer to a desired application (electrical/ionic conduction, etc.)<ref name=Principles>{{cite book|last=Odian|first=George|title=Principles of polymerization|year=2004|publisher=Wiley-Interscience|location=Hoboken, NJ|isbn=978-0471274001|edition=4.}}</ref><ref name=Grubbs />

==="Living" free radical polymerization===
{{Main|Reversible-deactivation radical polymerization}}  
Starting in the 1970s several new methods were discovered which allowed the development of living polymerization using [[radical (chemistry)|free radical]] chemistry. These techniques involved [[catalytic chain transfer]] polymerization, iniferter mediated polymerization, stable free radical mediated polymerization (SFRP), [[atom transfer radical polymerization]] (ATRP), [[reversible addition-fragmentation chain transfer]] ([[RAFT (chemistry)|RAFT]]) polymerization, and iodine-transfer polymerization.

In "living" radical polymerization (or controlled radical polymerization (CRP)) the chain breaking pathways are severely depressed when compared to conventional radical polymerization (RP) and CRP can display characteristics of a living polymerization. However, since chain termination is not absent, but only minimized, CRP technically does not meet the requirements imposed by IUPAC for a living polymerization (see introduction for IUPAC definition). This issue has been up for debate the view points of different researchers can be found in a special issue of the Journal of Polymer Science titled   [http://onlinelibrary.wiley.com/doi/10.1002/(SICI)1099-0518(20000515)38:10%3C%3E1.0.CO;2-C/issuetoc Living or Controlled ?]. The issue has not yet been resolved in the literature so it is often denoted as a "living" polymerization, quasi-living polymerization, pseudo-living and other terms to denote this issue.

There are two general strategies employed in CRP to suppress chain breaking reactions and promote fast initiation relative to propagation. Both strategies are based on developing a dynamic equilibrium amongst an active propagating radical and a dormant species.<ref name="Braunecker CRP" />

The first strategy involves a reversible trapping mechanism in which the propagating radical undergoes an activation/deactivation (i.e. [[Atom-transfer radical-polymerization]]) process with a species X. The species X is a persistent radical, or a species that can generate a stable radical, that cannot terminate with itself or propagate but can only reversibly "terminate" with the propagating radical (from the propagating polymer chain) P*. P* is a radical species that can propagate (k<sub>p</sub>) and irreversibly terminate (k<sub>t</sub>) with another P*.  X is normally a nitroxide (i.e. [[TEMPO]] used in [[Nitroxide Mediated Radical Polymerization]]) or an organometallic species. The dormant species (P<sub>n</sub>-X) can be activated to regenerate the active propagating species (P*) spontaneously, thermally, using a catalyst and optically.<ref name="Braunecker CRP">{{cite journal|last=Braunecker|first=Wade A.|author2=Matyjaszewski, Krzysztof |title=Controlled/living radical polymerization: Features, developments, and perspectives|journal=Progress in Polymer Science|date=2007|volume=32|issue=1|pages=93–146|doi=10.1016/j.progpolymsci.2006.11.002}}</ref><ref name=Matyjaszewski>{{cite web|last=Matyjaszewski|title=Features of Controlled "Living" Polymerization|url=http://www.cmu.edu/maty/crp/feature-development-crp/features.html#feature%202|url-status=dead|archiveurl=https://web.archive.org/web/20140314175012/http://www.cmu.edu/maty/crp/feature-development-crp/features.html#feature%202|archivedate=14 March 2014}}</ref>  
[[File:reversible trapping.png|center|400px]]

The second strategy is based on a degenerative transfer (DT) of the propagating radical between transfer agent that acts as a dormant species (i.e. [[Reversible addition−fragmentation chain-transfer polymerization]]). The DT based CRP's follow the conventional kinetics of radical polymerization, that is slow initiation and fast termination, but the transfer agent (Pm-X or Pn-X) is present in a much higher concentration compared to the radical initiator. The propagating radical species undergoes a thermally neutral exchange with the dormant transfer agent through atom transfer, group transfer or addition fragment chemistry.<ref name="Braunecker CRP" />             
[[File:radical degenerative transfer.png|center|500px]]

===Living chain-growth polycondensations===

Chain growth polycondensation polymerizations were initially developed under the premise that a change in substituent effects of the polymer, relative to the monomer, causes the polymers end group to be more reactive this has been referred to as "reactive intermediate polycondensation". The essential result is monomers preferentially react with the activated polymer end groups over reactions with other monomers.  This preferred reactivity is the fundamental difference when categorizing a polymerization mechanism as chain-growth as opposed to [[step-growth polymerization|step-growth]] in which the monomer and polymer chain end group have equal reactivity (the reactivity is uncontrolled). Several strategies were employed to minimize monomer-monomer reactions (or self-condensation) and polymerizations with low D and controllable Mn have been attained by this mechanism for small molecular weight polymers.<ref name="Yokozawa 1">{{Cite journal | doi = 10.1016/j.progpolymsci.2006.08.001| title = Chain-growth polycondensation: The living polymerization process in polycondensation| journal = Progress in Polymer Science| volume = 32| pages = 147–172| year = 2007| last1 = Yokozawa | first1 = T. | last2 = Yokoyama | first2 = A. }}</ref> However, for high molecular weight polymer chains (i.e. small initiator to monomer ratio) the Mn is not easily to controlled, for some monomers, since self-condensation between monomers occurred more frequently due to the low propagating species concentration.<ref name="Yokozawa 1"/>

====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]]

===Living group-transfer polymerization===
'''Group-transfer polymerization''' also has characteristics of living polymerization.<ref>Davis, Fred J. (2004) ''Polymer chemistry: a practical approach''. Oxford University Press. {{ISBN|978-0-19-850309-5}} .</ref> It is applied to alkylated [[methacrylate]] monomers and the initiator is a [[silyl ketene acetal]]. New monomer adds to the initiator and to the active growing chain in a [[Michael reaction]]. With each addition of a monomer group the trimethylsilyl group is transferred to the end of the chain. The active [[chain-end]] is not ionic as in anionic or cationic polymerization but is covalent. The reaction can be catalysed by bifluorides and bioxyanions such as ''tris(dialkylamino)sulfonium bifluoride'' or ''tetrabutyl ammonium bibenzoate''. The method was discovered in 1983 by [[Owen Webster]]<ref>{{cite journal | last1 = Webster | first1 = O. W. | last2 = Hertler | first2 = W. R. | last3 = Sogah | first3 = D. Y. | last4 = Farnham | first4 = W. B. | last5 = RajanBabu | first5 = T. V. | year = 1983 | title = Group-transfer polymerization. 1. A new concept for addition polymerization with organosilicon initiators | journal = [[J. Am. Chem. Soc.]] | volume = 105 | issue = 17| pages = 5706–5708 | doi = 10.1021/ja00355a039}}</ref> and the name first suggested by [[Barry Trost]].