Anionic addition polymerization: Difference between revisions

Revision as of 05:19, 16 October 2021

Revision as of 04:56, 28 November 2021

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{{Quote box

|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.

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'''Anionic addition polymerization''' is a form of [[chain-growth polymerization]] or addition polymerization that involves the [[polymerization]] of monomers initiated with anions. The type of reaction has many manifestations, but traditionally 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 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]] [[sodium naphthalene]] to [[styrene]]. The results in the formation of an 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 synthesis of polymers with improved control over [[molecular weight]], molecular weight distribution, and the architecture.<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>

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[[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 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^-−) [[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"/>

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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==

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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 remains 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/>

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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|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}}