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|file=A00361|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

}}

In [[polymer chemistry]], '''Anionicanionic addition polymerization''' is a form of [[chain-growth polymerization]] or addition polymerization that involves the [[polymerization]] of monomers[[monomer]]s initiated with anions[[anion]]s. The type of reaction has many manifestations, but traditionally [[Vinyl group|vinyl]] monomers are used.<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> Often anionic polymerization involves [[living polymerization]]s, which allows control of structure and composition.<ref name="Hsieh"/><ref name="Quirk"/>

== 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 [[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|pagepages=2656–2657

|doi=10.1021/ja01592a101}}</ref><ref>{{cite journal|author=M. Szwarc |year=1956|title="Living" polymers|journal=Nature|volume=178|issue=4543|page=1168|doi=10.1038/1781168a0|bibcode=1956Natur.178.1168S}}</ref> In one of the breakthrough events in the field of [[polymer science]], Szwarc elucidated that [[electron transfer]] occurred from [[radical anion]] of [[sodium naphthalene]] to [[styrene]]. The results in the formation of aan [[dianion]] (or equivalently disodio-)organosodium species, which rapidly added styrene to form a "two – ended living polymer." An important aspect of his work, Szwarc employed the [[aprotic solvent]] [[tetrahydrofuran]]. Being a [[physical chemist]], Szwarc 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 work provided the foundations for the rational synthesis of polymers with improved 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]].

The use of [[alkali metals]] to initiate polymerization of 1,3-[[diene]]s led to the discovery by 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 initiatiors.<ref name="Quirk"/>

== 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"/>

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

[[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]]

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== 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 naphthalene]] proceeds by [[electron transfer]] from the [[naphthalene]] [[radical anion]] to the monomer. The resulting radical dimerizes to give a dilithiodisodium 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 ==

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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>{{cite journal|last1=Hong|first1=K.|last2=Uhrig|first2=D.|last3=Mays|first3=J.|title=Living Anionic Polymerization|journal= Current Opinion in Solid State and Materials Science|year=1999|volume=4|issue=6|pagepages=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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Chain transfer can occur when an agent can act as a [[Acid#Brønsted-Lowry acids|Brønsted acid]]. In this case, the [[pKa]] value of the agent is similar to the conjugate acid of the propagating carbanionic chain end. Spontaneous termination occurs because the concentration of carbanion centers decay over time and eventually results in hydride elimination.<ref name=Odian/>

-->

==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|issue=12|pagepages=1068–1132|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|issue=2|pagepages=171–179|doi=10.1002/pi.1994.210330208}}

*{{cite book|author=Rempp, P.; |author2=Franta, E.; |author3=Herz, J.|s2cid=92176703|title=Polysiloxane Copolymers/Anionic Polymerization|chapter=Macromolecular Engineering by Anionic Methods|year=1998|volume=4|pagepages= 145–173|doi=10.1007/BFb0025276|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|issue=13|pagepages=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}}