Anionic addition polymerization: Difference between revisions

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{{Short description|Form of addition polymerization initiated with anions}}

{{Quote box|width = 35%

|title = [[International Union of Pure and Applied Chemistry| IUPAC]] definition for '''anionic polymerization'''

|quote = [[Ionic polymerization]] in which the [[Active center (polymer science)|active center]]s are anions.

|quote = '''anionic polymerization''': An ionic polymerization in which the kinetic-chain carriers are anions.

<ref name='Gold Book "anionic polymerization"'>{{cite web |title=anionic polymerization |url=https://goldbook.iupac.org/terms/view/A00361 |website=Gold Book |publisher=IUPAC |access-date=1 April 2024 |ref=Gold Book A00361 |doi=10.1351/goldbook.A00361}}</ref>

Note 1: The anions may be free, paired, or aggregated.

Modified from the earlier definition.<ref name="Goldbook">{{GoldBookRef|title=Anionic polymerization|url=http://goldbook.iupac.org/A00361.html|accessdate=May 27, 2013}}</ref><ref name=PAC1996>{{cite journal

|url= http://iupac.org/publications/pac/68/12/2287/

|doi = 10.1351/pac199668122287

|title= Glossary of basic terms in polymer science (IUPAC Recommendations 1996)

|last1= Jenkins |first1= A. D. |last2= Kratochvíl |first2= P. |last3= Stepto |first3= R. F. T. |last4= Suter |first4= U. W.

|journal= [[Pure and Applied Chemistry]] |volume=68 |year=1996 |pages=2287–2311

|issue= 12}}</ref>

|source = [http://www.iupac.org/publications/pac/80/10/2163/ Penczek S.; Moad, G. ''Pure Appl. Chem.'', '''2008''', 80(10), 2163-2193]

|align = right

}}

'''Anionic addition polymerization''' is a form of [[chain-growth polymerization]] or [[addition polymerization]] that involves the [[polymerization]] of vinyl monomers with strong electronegative groups.<ref name=Hsieh>Hsieh, H.;Quirk, R. ''Anionic Polymerization: Principles and practical applications''; Marcel Dekker, Inc: New York, 1996.</ref><ref name=Quirk>Quirk, R. Anionic Polymerization. In ''Encyclopedia of Polymer Science and Technology''; John Wiley and Sons: New York, 2003.</ref> This polymerization is carried out through a [[carbanion]] active species.<ref>Blackeley, D.; Twaits, R. Ionic Polymerization. In ''Addition Polymers: Formation and Characterization''; Plenum Press: New York, 1968; pp. 51-110.</ref> Like all chain-growth polymerizations, it takes place in three steps: [[chain initiation]], [[chain propagation]], and [[chain termination]]. [[Living polymerization]]s, which lack a formal termination pathway, occur in many anionic addition polymerizations. The advantage of living anionic addition polymerizations is that they allow for the control of structure and composition.<ref name="Hsieh"/><ref name="Quirk"/>

In [[polymer chemistry]], '''Anionicanionic addition polymerization''' is a form of [[chain-growth polymerization]] or [[addition polymerization]] that involves the [[polymerization]] of vinyl[[monomer]]s monomersinitiated with strong[[anion]]s. The type of reaction has many manifestations, but traditionally [[Vinyl group|vinyl]] monomers electronegativeare groupsused.<ref name=Hsieh>Hsieh, H.;Quirk, R. ''Anionic Polymerization: Principles and practical applications''; Marcel Dekker, Inc.: New York, 1996.</ref><ref name=Quirk>Quirk, R. Anionic Polymerization. In ''Encyclopedia of Polymer Science and Technology''; John Wiley and Sons: New York, 2003.</ref> ThisOften anionic polymerization is carried out through ainvolves [[carbanion]] active species.<ref>Blackeley, D.; Twaits, R. Ionic Polymerization. In ''Addition Polymers: Formation and Characterization''; Plenum Press: New York, 1968; pp. 51-110.</ref> Like all chain-growth polymerizations, it takes place in three steps: [[chain initiation]], [[chain propagation]], and [[chain termination]]. [[Livingliving polymerization]]s, which lack a formal termination pathway, occur in many anionic addition polymerizations. The advantage of living anionic addition polymerizations is that they allow for theallows control of structure and composition.<ref name="Hsieh"/><ref name="Quirk"/>

Anionic polymerizations are used in the production of polydiene [[synthetic rubber]]s, solution styrene/butadiene rubbers (SBR), and styrenic [[thermoplastic elastomers]].<ref name="Hsieh"/>

In anionic polymerizations, the end group of growing macro molecule possess high activity and good stability. Polymerization process continues till the available monomers are consumed.

== History ==

[[File:ET-coupledStyrene.png|thumb|Product of the reductive coupling of styrene with lithium, 1,4-dilithio-1,4-diphenylbutane. In the original work, Szwarc studied the analogous disodium compound.<ref>{{cite book|chapter=Ionic Polymerization|author=Sebastian Koltzenburg|author2=Michael Maskos|author3=Oskar Nuyken|title=Polymer Chemistry|isbn=978-3-662-49279-6|publisher=Springer|date=2017-12-11}}</ref>]]

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. The early work of [[Michael Szwarc]] and co – workers in 1956 was one of the breakthrough events in the field of [[polymer science]]. When Szwarc learned that the [[electron transfer]] between [[radical anion]] of [[naphthalene]] and [[styrene]] in an [[aprotic solvent]] such as [[tetrahydrofuran]] gave a messy product, he started investigating the reaction in more detail. He proved that the electron transfer results in the formation of a [[dianion]] which rapidly added styrene to form a "two – ended living polymer." Being a [[physical chemist]], Szwarc set forth in understanding the mechanism of such living polymerization in greater detail. His work elucidated the [[chemical kinetics|kinetics]] and the [[thermodynamics]] of the process in considerable detail. At the same time, he explored the structure property relationship of the various [[ion pair]]s and radical ions involved. This had great ramifications in future research in polymer synthesis, because Szwarc had found a way to make polymers with greater control over [[molecular weight]], molecular weight distribution and the architecture of the polymer.<ref>Smid, J. Historical Perspectives on Living Anionic Polymerization. ''J. Polym. Sci. Part A.''; '''2002''', ''40'',pp. 2101-2107. [https://archive.today/20121012113202/http://www3.interscience.wiley.com/journal/94515609/abstract DOI=10.1002/pola.10286]</ref>

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 [[Michael Szwarc]] and coworkers.<ref>{{cite journal|title=Polymerization Initiated by Electron Transfer to Monomer. A New Method of Formation of Block Polymers|first1=M.|last1=Szwarc|first2=M.|last2= Levy|first3=R.|last3=Milkovich|journal=J. Am. Chem. Soc.|year=1956|volume=78|issue=11|pages=2656–2657

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|doi=10.1021/ja01592a101}}</ref><ref>{{cite Twenty years later, living polymerization was demonstrated by Szwarcjournal|author=M. The early work of [[Michael Szwarc]] and|year=1956|title="Living" co – workers in 1956polymers|journal=Nature|volume=178|issue=4543|page=1168|doi=10.1038/1781168a0|bibcode=1956Natur.178.1168S}}</ref> wasIn one of the breakthrough events in the field of [[polymer science]]. When, Szwarc learnedelucidated that the [[electron transfer]] betweenoccurred from [[radical anion]] of [[sodium naphthalene]] andto [[styrene]] in an [[aprotic solvent]] such as [[tetrahydrofuran]] gave a messy product, he started investigating the reaction in more detail. He proved that the electron transferThe results in the formation of aan [[dianion]]organosodium species, which rapidly added styrene to form a "two – ended living polymer." Being aAn [[physicalimportant chemist]],aspect Szwarcof sethis forthwork, inSzwarc understandingemployed the mechanism[[aprotic ofsolvent]] such[[tetrahydrofuran]]. livingBeing polymerizationa in[[physical greaterchemist]], detail. His workSzwarc elucidated the [[chemical kinetics|kinetics]] and the [[thermodynamics]] of the process in considerable detail. At the same time, he explored the structure property relationship of the various [[ion pair]]s and radical ions involved. This hadwork greatprovided ramificationsthe infoundations futurefor research in polymerthe synthesis, because Szwarc had found a way to makeof polymers with greaterimproved control over [[molecular weight]], molecular weight distribution, and the architecture of the polymer.<ref>Smid, J. Historical Perspectives on Living Anionic Polymerization. ''J. Polym. Sci. Part A.''; '''2002''', ''40'', pp. 2101-2107. [https://archive.today/20121012113202/http://www3.interscience.wiley.com/journal/94515609/abstract DOI=10.1002/pola.10286]</ref>

The use of [[alkali metals]] to initiate polymerization of 1,3-[[diene]]s led to the discovery by [[Frederick W. Stavely|Stavely]] and co-workers at Firestone Tire and Rubber company of cis-1,4-[[polyisoprene]].<ref name=Odian>Odian, G. Ionic Chain Polymerization; In '' Principles of Polymerization''; Wiley-Interscience: Staten Island, New York, 2004, pp. 372-463.</ref> This sparked the development of commercial anionic polymerization processes that utilize alkyllithium initiatiorsinitiators.<ref name="Quirk"/>

[[Roderic Quirk]] won the 2019 [[Charles Goodyear Medal]] in recognition of his contributions to anionic polymerization technology. He was introduced to the subject while working in a [[Phillips Petroleum]] lab with [[Henry Hsieh]].

Living polymerization was demonstrated by Szwarc and co workers in 1956. Their initial work was based on the polymerization of styrene and dienes.

== Monomer characteristics ==

Two broad classes of monomers are susceptible to anionic polymerization.<ref name="Quirk"/>

Vinyl monomers have the formula CH₂=CHR, the most important are styrene (R = C₆H₅), butadiene (R = CH=CH₂), and isoprene (R = C(Me)=CH₂). A second major class of monomers are acrylate esters, such as [[acrylonitrile]], [[methacrylate]], [[cyanoacrylate]], and [[acrolein]]. Other vinyl monomers include [[vinylpyridine]], vinyl [[sulfone]], vinyl [[sulfoxide]], [[vinyl silane]]s.<ref name="Quirk"/>

===Vinyl monomers===

[[File:Ex polar monomers.png|thumb|300px|right|Examples of polar monomers]]

Vinyl monomers have the formula CH₂=CHR and CH₂=C(R)Z. Some of the most important are styrene, butadiene, and isoprene. Many acrylate esters are ammenable.

[[File:Example Vinyl monomer.png|thumb|200px|right|Examples of vinyl monomers]]

===Cyclic monomers===

[[File:Wiki65656.tif|thumb|600px|center|The anionic ring-opening polymerization of ε-caprolactone, initiated by alkoxide]]

Many cyclic compounds are susceptible to [[ring-opening polymerization]]. [[Epoxide]]s, cyclic [[trisiloxane]]s, some lactones, [[lactide]]s, [[cyclic carbonate]]s, and [[amino acid N-carboxyanhydride]]s.

[[file:Hexamethylcyclotrisiloxan.svg|thumb|140px|right|Hexamethylcyclotrisiloxane is a cyclic monomer that is susceptible to anionic polymerization to [[siloxane]] polymers.]]

Many cyclic compounds are susceptible to [[ring-opening polymerization]]. [[Epoxide]]s, cyclic tri[[trisiloxanesiloxane]]s, some lactones, [[lactide]]s, [[cyclic carbonate]]s, and [[amino acid N-carboxyanhydride]]s.

In order for polymerization to occur with [[vinyl group|vinyl]] [[monomer]]s, the [[substituent]]s on the [[double bond]] must be able to stabilize a [[negative charge]]. Stabilization occurs through [[delocalization]] of the negative charge. Because of the nature of the [[carbanion]] propagating center, substituents that react with bases or nucleophiles either must not be present or be protected.<ref name="Quirk"/>

[[Image:Example Vinyl monomer.png|thumb|200px|left|Examples of vinyl monomers.]]Vinyl monomers with substituents that stabilize the negative charge through charge delocalization, undergo polymerization without termination or chain transfer.<ref name="Quirk"/> These monomers include [[styrene]], [[diene]]s, [[methacrylate]], vinyl [[pyridine]], [[aldehyde]]s, [[epoxide]], [[episulfide]], cyclic [[siloxane]], and [[lactone]]s.

Polar monomers, using controlled conditions and low temperatures, can undergo anionic polymerization. However, at higher temperatures they do not produce living stable, carbanionic chain ends because their polar substituents can undergo side reactions with both initiators and propagating chain centers. The effects of counterion, solvent, temperature, Lewis base additives, and inorganic solvents have been investigated to increase the potential of anionic polymerizations of polar monomers.<ref name="Quirk"/> Polar monomers include [[acrylonitrile]], [[cyanoacrylate]], [[propylene oxide]], vinyl [[ketone]], [[acrolein]], vinyl [[sulfone]], vinyl [[sulfoxide]], [[vinyl silane]] and [[isocyanate]].[[Image:Ex polar monomers.png|thumb|300px|center|Examples of polar monomers.]]

== Initiation ==

Initiators isare selected based on the reactivity of the monomers. Highly electrophilic monomers such as cyanoacrylates require only weakly nucleophilic initiators, such as amines, phosphines, or even halides. Less reactive monomers such as styrene require powerful nucleophiles such as [[butyl lithium]]. ReactionReactions of intermediate strength are used for monomers of intermediate reactivity such as [[vinylpyridine]].<ref name="Quirk"/>

The solventsolvents used in anionic addition polymerizations are determined by the reactivity of both the initiator and nature of the propagating chain end. Anionic species with low reactivity, such as [[heterocyclic]] monomers, can use a wide range of solvents.<ref name="Quirk"/>

===Initiation by electron transfer===

Initiation of styrene polymerization with [[sodium naphthenatenaphthalene]] proceeds by [[electron transfer]] from the [[naphthalene]] [[radical anion]] to the monomer. The resulting radical dimerizes to give a disodium compound, which then functions as the initiator. Polar solvents are necessary for this type of initiation both for stability of the anion-radical and to solvate the cation species formed.<ref name=Odian/> The anion-radical can then transfer an electron to the monomer.

Initiation can also involve the transfer of an electron from the alkali metal to the monomer to form an anion-radical. Initiation occurs on the surface of the metal, with the reversible transfer of an electron to the adsorbed monomer.<ref name="Quirk"/>

===Initiation by strong anions===

[[Nucleophilic]] initiators include covalent or ionic metal [[amide]]s, [[alkoxide]]s, [[hydroxide]]s, [[cyanide]]s, [[phosphine]]s, [[amine]]s and organometallic compounds ([[alkyllithium]] compounds and [[Grignard reagents]]). The initiation process involves the addition of a neutral (B:) or negative (B:-B⁻) [[nucleophile]] to the monomer.<ref name=Odian/>

The most commercially useful of these initiators has been the [[alkyllithium]] initiators. They are primarily used for the polymerization of styrenes and dienes.<ref name="Quirk"/>

Monomers activated by strong electronegative groups may be initiated even by weak anionic or neutral [[nucleophiles]] (i.e. amines, phosphines). Most prominent example is the curing of [[cyanoacrylate]], which constitutes the basis for [[superglue]]. Here, only traces of basic impurities are sufficient to induce an anionic addition polymerization or [[zwitterionic addition polymerization]], respectively.<ref>Pepper, D.C. Zwitterionic Chain Polymerizations of Cyanoacrylates. ''Macromolecular Symposia''; '''1992''',''60'', pp. 267-277. [http://onlinelibrary.wiley.com/{{doi/|10.1002/masy.19920600124/abstract]}}</ref>

== Propagation ==

[[File:RLi+Styrene.png|center|640px|thumb|Organolithium-initiated polymerization of styrene.]]

Propagation in anionic addition polymerization results in the complete consumption of monomer. ItThis stage is veryoften fast, and occurseven at low temperatures. This is due to the anion not being very stable, the speed of the reaction as well as that heat is released during the reaction. The stability can be greatly enhanced by reducing the temperatures to near 0˚C. The propagation rates are generally fairly high compared to the decay reaction, so the overall polymerization rates is generally not affected.<ref name="Hsieh"/>

==Living anionic polymerization==

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* Chain end functionalization can be carried out quantitatively.

However, in practice, even in the absence of terminating agents, the concentration of the living anions will reduce with time due to a decay mechanism termed as spontaneous termination.<ref name=Odian>Odian, G. Ionic Chain Polymerization; In '' Principles of Polymerization''; Wiley-Interscience: Staten Island, New York, 2004, pp. 372-463.</ref>

==Consequences of living polymerization==

===Block copolymers===

Synthesis of block copolymers is one of the most important applications of living polymerization as it offers the best control over structure. The [[nucleophilicity]] of the resulting carbanion will govern the order of monomer addition, as the monomer forming the less nucleophilic propagating species may inhibit the addition of the more nucleophilic monomer onto the chain. An extension of the above concept is the formation of triblock copolymers where each step of such a sequence aims to prepare a block segment with predictable, known molecular weight and narrow molecular weight distribution without chain termination or transfer.<ref>Hsieh, H.;Quirk, R. Anionic Polymerization: Principles and practical applications; Marcel Dekker, Inc.: New York, 1996.</ref>

Sequential monomer addition is the dominant method, also this simple approach suffers some limitations.</ref>

Moreover, this strategy, enables synthesis of linear block copolymer structures that are not accessible via sequential monomer addition. For common A-b-B structures, sequential block copolymerization gives access to well defined

block copolymers only if the crossover reaction rate constant is significantly higher than the rate constant of the homopolymerization

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===End-group functionalization/termination===

One of the remarkable features of living anionic polymerization is the absence of a formal termination step. In the absence of impurities, the carbanion would remainsremain active, awaiting the addition of new monomer. Termination can occur through unintentional quenching by impurities, often present in trace amounts. Typical impurities include [[oxygen]], [[carbon dioxide]], or [[water]]. Termination intentionally allows the introduction of tailored end groups.

Living anionic polymerization allow the incorporation of functional [[end-group]]s, usually added to quench polymerization. End-groups that have been used in the functionalization of α-haloalkanes include [[hydroxide]], -NH₂, -OH, -SH, -CHO,-COCH₃, -COOH, and epoxides.

[[Image:AAP End Group Add.png|thumb|400px|center|Addition of hydroxide group through an epoxide.]]

An alternative approach for functionalizing end-groups is to begin polymerization with a functional anionic initiator.<ref name=HongK>Hong{{cite journal|last1=Hong|first1=K.; |last2=Uhrig, |first2=D.;|last3=Mays, |first3=J. |title=Living Anionic Polymerization.|journal= Current ''CurrOpinion Opinin Solid State Materand Sci.''Materials Science|year=1999,|volume=4, 531-538. |issue=6|pages=531–538|doi=10.1016/S1359-0286(00)00011-5|bibcode=1999COSSM...4..531H}}</ref> In this case, the functional groups are protected since the ends of the anionic polymer chain is a strong base. This method leads to polymers with controlled molecular weights and narrow molecular weight distributions.<ref>Quirk, R. Anionic Polymerization. In Encyclopedia of Polymer Science and Technology; John Wiley and Sons: New York, 2003.</ref>

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==Additional reading==

*Cowie, J.; Arrighi, V. ''Polymers: Chemistry and Physics of Modern Materials''; CRC Press: Boca Raton, FL, 2008.

*{{cite journal|author=Hadjichristidis, N.; |author2=Iatrou, H.; |author3=Pitsikalis, P.; |author4=Mays, J.|title=Macromolecular architectures by living and controlled/living polymerizations|journal=Prog. Polym. Sci.|year=2006|volume=31|pageissue=12|pages=1068-11321068–1132|doi=10.1016/j.progpolymsci.2006.07.002}}

*{{cite journal|author=Efstratiadis, V.; |author2=Tselikas, Y.; |author3=Hadjichristidis, N.; |author4=Li, J.; |author5=Yunan, W.; |author6=Mays, J.|title=Synthesis and characterization of poly(methyl methacrylate) star polymers|journal=Polym Int.|year=1994|volume=4|pageissue=2|pages=171-179171–179|doi=10.1002/pi.1994.210330208}}

*{{cite journalbook|author=Rempp, P.; |author2=Franta, E.; |author3=Herz, J.|s2cid=92176703|title=Polysiloxane Copolymers/Anionic Polymerization|chapter=Macromolecular Engineering by Anionic Methods|journal=Adv. Polym. Sci.|year=1998|volume=4|pagepages= 145-173145–173|doi=10.1007/BFb0025273BFb0025276|series=Advances in Polymer Science|isbn=978-3-540-18506-2}}

*{{cite journal|title=Universal Methodology for Block Copolymer Synthesis|first1=Vasilios|last1=Bellas|first2=Matthias|last2=Rehahn|s2cid=96556942|date=2 July 2007|journal=Macromolecular Rapid Communications|volume=28|pageissue=13|pages=1415–1421|doi=10.1002/marc.200700127}}

*{{cite book|title=Anionic Polymerization Principles, Practice, Strength, Consequences and Applications|editor=Nikos Hadjichristidis|editor2=Akira Hirao|year=2015|isbn=978-4-431-54186-8|publisher=Springer}}

==References==

{{reflistReflist}}

{{DEFAULTSORT:Anionic Addition Polymerization}}