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Living cationic polymerization

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<div id="mw-content-text" class="mw-body-content mw-content-ltr" lang="en" dir="ltr"><div class="mw-content-ltr mw-parser-output" lang="en" dir="ltr">Living cationic polymerization is a <a href="Living_polymerization" title="Living polymerization">living polymerization</a> technique involving <a href="Cationic" class="mw-redirect" title="Cationic">cationic</a> propagating species.<a href="#cite_note-1">[1]</a><a href="#cite_note-2">[2]</a> It enables the synthesis of very well defined polymers (low <a href="Molar_mass_distribution" title="Molar mass distribution">molar mass distribution</a>) and of polymers with unusual architecture such as star polymers and <a href="Block_copolymer" class="mw-redirect" title="Block copolymer">block copolymers</a> and living cationic polymerization is therefore as such of commercial and academic interest.

<div class="mw-heading mw-heading2"><h2 id="Basics">Basics</h2></div>

In carbocationic polymerization the active site is a carbocation with a counterion in close proximity. The basic reaction steps are:

<ul><li><a href="Chain_propagation" title="Chain propagation">Chain propagation</a>:</li></ul>

<ul><li><a href="Chain_termination" title="Chain termination">Chain termination</a>:</li></ul>

<ul><li><a href="Chain_transfer" title="Chain transfer">chain transfer</a>:</li></ul>

Living cationic polymerization is characterised by defined and controlled initiation and propagation while minimizing side-reactions termination and chain transfer. Transfer and termination do occur but in ideal living systems the active ionic propagating species are in <a href="Chemical_equilibrium" title="Chemical equilibrium">chemical equilibrium</a> with the dormant covalent species with an exchange rate much faster than the propagation rate. Solution methods require rigorous purification of monomer and solvent although conditions are not as strict as in anionic polymerization.

Common <a href="Monomer" title="Monomer">monomers</a> are <a href="Enol_ether" title="Enol ether">vinyl ethers</a>, alpha-methyl vinyl ethers, <a href="Isobutene" class="mw-redirect" title="Isobutene">isobutene</a>, <a href="Styrene" title="Styrene">styrene</a>, <a href="Methylstyrene" title="Methylstyrene">methylstyrene</a> and <a href="N-vinylcarbazole" class="mw-redirect" title="N-vinylcarbazole">N-vinylcarbazole</a>. The monomer is nucleophilic and substituents should be able to stabilize a positive <a href="Carbocation" title="Carbocation">carbocationic</a> charge. For example, para-methoxystyrene is more reactive than styrene itself.

Initiation takes place by an initiation/coinitiation binary system, for example an alcohol and a <a href="Lewis_acid" class="mw-redirect" title="Lewis acid">Lewis acid</a>. The active electrophile is then a proton and the counter ion the remaining <a href="Alkoxide" title="Alkoxide">alkoxide</a> which is stabilized by the Lewis acid. With organic acetates such as cumyl acetate the initiating species is the carbocation R+ and the counterion is the acetate anion. In the <a href="Iodine" title="Iodine">iodine</a>/<a href="Hydrogen_iodide" title="Hydrogen iodide">hydrogen iodide</a> system the electrophile is again a proton and the carbocation is stabilized by the <a href="Triiodide" title="Triiodide">triiodide</a> ion. Polymerizations with <a href="Diethylaluminium_chloride" title="Diethylaluminium chloride">diethylaluminium chloride</a> rely on trace amounts of water. A proton is then accompanied by the counterion Et2AlClOH−. With <a href="Tert-butyl_chloride" class="mw-redirect" title="Tert-butyl chloride">tert-butyl chloride</a> Et2AlCl abstracts a chlorine atom to form the tert-butyl carbocation as the electrophile. Efficient initiators that resemble the monomer are called cationogens. Termination and chain transfer are minimized when the initiator counterion is both non-nucleophilic and non-basic. More <a href="Solvent_polarity" class="mw-redirect" title="Solvent polarity">polar solvents</a> promote ion dissociation and hence increase molar mass.

Common additives are electron donors, salts and proton traps . Electron donors (e.g. nucleophiles, Lewis bases) for example <a href="Dimethylsulfide" class="mw-redirect" title="Dimethylsulfide">dimethylsulfide</a> and <a href="Dimethylsulfoxide" class="mw-redirect" title="Dimethylsulfoxide">dimethylsulfoxide</a> are believed to stabilize the carbocation. The addition of salt for example a <a href="Ammonium_salt" class="mw-redirect" title="Ammonium salt">tetraalkylammonium salt</a>, prevents dissociation of the ion pair that is the propagating reactive site. Ion dissociation into free ions lead to non-living polymerization. Proton traps scavenge protons originating from protic impurities.

<div class="mw-heading mw-heading2"><h2 id="History">History</h2></div>

The method was developed starting in the 1970s and 1980s with contributions from Higashimura on the polymerization of p-methoxystyrene using iodine or acetyl perchlorate,<a href="#cite_note-3">[3]</a> on the polymerization of isobutyl vinyl ether by iodine <a href="#cite_note-4">[4]</a> and with Mitsuo Sawamoto by iodine/<a href="Hydrogen_iodide" title="Hydrogen iodide">HI</a><a href="#cite_note-5">[5]</a> and on the formation of p-methoxystyrene - isobutyl vinyl ether <a href="Block_copolymer" class="mw-redirect" title="Block copolymer">block copolymers</a>.<a href="#cite_note-6">[6]</a>

Kennedy and Faust studied <a href="Methylstyrene" title="Methylstyrene">methylstyrene</a> / <a href="Boron_trichloride" title="Boron trichloride">boron trichloride</a> polymerization (then called quasi-living) in 1982 <a href="#cite_note-7">[7]</a> and that of <a href="Isobutylene" title="Isobutylene">isobutylene</a> (system with cumyl acetate, 2,4,4-trimethylpentane-2-acetate and BCl3) in 1984 <a href="#cite_note-8">[8]</a><a href="#cite_note-9">[9]</a>

Around same time Kennedy and Mishra discovered very efficient living polymerization of isobutylene (system with Tertiary Alkyl (or Aryl) Methyl Ether and BCl3)[<a href="#cite_note-10">[10]</a> that paved the way for rapid development of macromolecularly engineered polymers.

<div class="mw-heading mw-heading2"><h2 id="Isobutylene_polymerization">Isobutylene polymerization</h2></div>

Living <a href="Isobutylene" title="Isobutylene">isobutylene</a> polymerization typically takes place in a mixed solvent system comprising a non-polar <a href="Solvent" title="Solvent">solvent</a>, such as <a href="Hexane" title="Hexane">hexane</a>, and a polar solvent, such as <a href="Chloroform" title="Chloroform">chloroform</a> or <a href="Dichloromethane" title="Dichloromethane">dichloromethane</a>, at temperatures below 0 °C. With more polar solvents <a href="Polyisobutylene" class="mw-redirect" title="Polyisobutylene">polyisobutylene</a> solubility becomes a problem. Initiators can be <a href="Alcohols" class="mw-redirect" title="Alcohols">alcohols</a>, <a href="Halides" class="mw-redirect" title="Halides">halides</a> and <a href="Ethers" class="mw-redirect" title="Ethers">ethers</a>. Coinitiators are <a href="Boron_trichloride" title="Boron trichloride">boron trichloride</a>, <a href="Tin_tetrachloride" class="mw-redirect" title="Tin tetrachloride">tin tetrachloride</a> and organoaluminum halides. With ethers and alcohols the true initiator is the chlorinated product. Polymer with <a href="Molar_mass" title="Molar mass">molar mass</a> of 160,000 g/mole and <a href="Polydispersity_index" class="mw-redirect" title="Polydispersity index">polydispersity index</a> 1.02 can be obtained.

<div class="mw-heading mw-heading2"><h2 id="Vinyl_ether_polymerization">Vinyl ether polymerization</h2></div>

Vinyl ethers (CH2=CHOR, R = <a href="Methyl" class="mw-redirect" title="Methyl">methyl</a>, <a href="Ethyl_group" title="Ethyl group">ethyl</a>, <a href="Isobutyl" class="mw-redirect" title="Isobutyl">isobutyl</a>, <a href="Benzyl" class="mw-redirect" title="Benzyl">benzyl</a>) are very reactive vinyl monomers. Studied systems are based on I2/HI and on zinc halides <a href="Zinc_chloride" title="Zinc chloride">zinc chloride</a>, <a href="Zinc_bromide" title="Zinc bromide">zinc bromide</a> and <a href="Zinc_iodide" title="Zinc iodide">zinc iodide</a>.

<div class="mw-heading mw-heading2"><h2 id="Living_cationic_ring-opening_polymerization">Living cationic ring-opening polymerization</h2></div>

In Living cationic ring-opening polymerization the monomer is a <a href="Heterocycle" class="mw-redirect" title="Heterocycle">heterocycle</a> such as an <a href="Epoxide" title="Epoxide">epoxide</a>, <a href="THF" class="mw-redirect" title="THF">THF</a>, an <a href="Oxazoline" title="Oxazoline">oxazoline</a> or an <a href="Aziridine" title="Aziridine">aziridine</a> such as t-butylaziridine.<a href="#cite_note-11">[11]</a> The propagating species is not a carbocation but an <a href="Oxonium_ion" title="Oxonium ion">oxonium ion</a>. Living polymerization is more difficult to achieve because of the ease of termination by nucleophilic attack of a heteroatom in the growing polymer chain. Intramolecular termination is called backbiting and results in the formation of cyclic oligomers. Initiators are strong electrophiles such as <a href="Triflic_acid" title="Triflic acid">triflic acid</a>. <a href="Triflic_anhydride" class="mw-redirect" title="Triflic anhydride">Triflic anhydride</a> is an initiator for bifunctional polymer.

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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</style><cite id="CITEREFAoshimaKanaoka2009" class="citation journal cs1">Aoshima, Sadahito; Kanaoka, Shokyoku (2009). <a rel="nofollow" class="external text" href="https://doi.org/10.1021%2Fcr900225g">"A Renaissance in Living Cationic Polymerization"</a>. Chemical Reviews. 109 (11): 5245–87. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1021%2Fcr900225g">10.1021/cr900225g</a>. <a href="PMID_(identifier)" class="mw-redirect" title="PMID (identifier)">PMID</a> <a rel="nofollow" class="external text" href="https://pubmed.ncbi.nlm.nih.gov/19803510">19803510</a>.</cite>

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<li id="cite_note-2"><a href="#cite_ref-2">^</a> Controlled and living polymerizations: methods and materials

2009 Krzysztof Matyjaszewski,Axel H. E. Muller

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<li id="cite_note-3"><a href="#cite_ref-3">^</a> Possible formation of living polymers of p-methoxystyrene by iodine Higashimura, Toshinobu; Kishiro, Osamu Polymer Journal (Tokyo, Japan) (1977), 9(1), 87-93 <a rel="nofollow" class="external text" href="http://www.jstage.jst.go.jp/article/polymj/9/1/87/_pdf">pdf</a>

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<li id="cite_note-4"><a href="#cite_ref-4">^</a> Studies on the nature of propagating species in cationic polymerization of isobutyl vinyl ether by iodine Ohtori, T.; Hirokawa, Y.; Higashimura, T. Polym. J. 1979, 11, 471. <a rel="nofollow" class="external text" href="http://www.jstage.jst.go.jp/article/polymj/11/6/471/_pdf">pdf</a>

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<li id="cite_note-5"><a href="#cite_ref-5">^</a> <cite id="CITEREFMiyamotoSawamotoHigashimura1984" class="citation journal cs1">Miyamoto, Masaaki; Sawamoto, Mitsuo; Higashimura, Toshinobu (1984). "Living polymerization of isobutyl vinyl ether with hydrogen iodide/iodine initiating system". Macromolecules. 17 (3): 265. <a href="Bibcode_(identifier)" class="mw-redirect" title="Bibcode (identifier)">Bibcode</a>:<a rel="nofollow" class="external text" href="https://ui.adsabs.harvard.edu/abs/1984MaMol..17..265M">1984MaMol..17..265M</a>. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1021%2Fma00133a001">10.1021/ma00133a001</a>.</cite>

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<li id="cite_note-6"><a href="#cite_ref-6">^</a> <cite id="CITEREFHigashimuraMitsuhashiSawamoto1979" class="citation journal cs1">Higashimura, Toshinobu; Mitsuhashi, Masakazu; Sawamoto, Mitsuo (1979). "Synthesis of p-Methoxystyrene-Isobutyl Vinyl Ether Block Copolymers by Living Cationic Polymerization with Iodine". Macromolecules. 12 (2): 178. <a href="Bibcode_(identifier)" class="mw-redirect" title="Bibcode (identifier)">Bibcode</a>:<a rel="nofollow" class="external text" href="https://ui.adsabs.harvard.edu/abs/1979MaMol..12..178H">1979MaMol..12..178H</a>. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1021%2Fma60068a003">10.1021/ma60068a003</a>.</cite>

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<li id="cite_note-7"><a href="#cite_ref-7">^</a> <cite id="CITEREFFaustFehérváriKennedy1982" class="citation journal cs1">Faust, R.; Fehérvári, A.; Kennedy, J. P. (1982). "Quasiliving Carbocationic Polymerization. II. The Discovery: the α-Methylstyrene System". Journal of Macromolecular Science, Part A. 18 (9): 1209. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1080%2F00222338208077219">10.1080/00222338208077219</a>.</cite>

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<li id="cite_note-8"><a href="#cite_ref-8">^</a> <cite id="CITEREFFaustKennedy1986" class="citation journal cs1">Faust, R.; Kennedy, J.P. (1986). "Living carbocationic polymerization". Polymer Bulletin. 15 (4). <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1007%2FBF00254850">10.1007/BF00254850</a>. <a href="S2CID_(identifier)" class="mw-redirect" title="S2CID (identifier)">S2CID</a> <a rel="nofollow" class="external text" href="https://api.semanticscholar.org/CorpusID:103321146">103321146</a>.</cite>

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<li id="cite_note-9"><a href="#cite_ref-9">^</a> <cite id="CITEREFFaustKennedy1987" class="citation journal cs1">Faust, R.; Kennedy, J. P. (1987). "Living carbocationic polymerization. IV. Living polymerization of isobutylene". Journal of Polymer Science Part A: Polymer Chemistry. 25 (7): 1847. <a href="Bibcode_(identifier)" class="mw-redirect" title="Bibcode (identifier)">Bibcode</a>:<a rel="nofollow" class="external text" href="https://ui.adsabs.harvard.edu/abs/1987JPoSA..25.1847F">1987JPoSA..25.1847F</a>. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1002%2Fpola.1987.080250712">10.1002/pola.1987.080250712</a>.</cite>

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<li id="cite_note-10"><a href="#cite_ref-10">^</a> Mishra, Munmaya K.; Kennedy, Joseph P. (1987). "Living carbocationic polymerization. VII. Living Polymerization of Isobutylene by Tertiary Alkyl (or Aryl) Methyl Ether/Boron Trichloride Complexes". Journal of Macromolecular Science Part A - Chemistry. 24 (8): 933]

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<li id="cite_note-11"><a href="#cite_ref-11">^</a> E.J. Goethals , Beatrice Verdonck in Living and controlled polymerization Joseph Jagur-Grodzinski, ed. (2005)

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