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Chain transfer

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</style><div role="note" class="hatnote navigation-not-searchable">See also: <a href="Chain-growth_polymerization" title="Chain-growth polymerization">Chain-growth polymerization</a> and <a href="Chain_shuttling_polymerization" title="Chain shuttling polymerization">Chain shuttling polymerization</a></div>

In <a href="Polymer_chemistry" title="Polymer chemistry">polymer chemistry</a>, chain transfer is a <a href="Polymerization" title="Polymerization">polymerization</a> reaction by which the activity of a growing <a href="Polymer" title="Polymer">polymer</a> chain is transferred to another <a href="Molecule" title="Molecule">molecule</a>:<a href="#cite_note-1">[1]</a><a href="#cite_note-2">[2]</a>

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle {\ce {P}}^{\bullet }+{\ce {XR -> PX + R}}^{\bullet }}">

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<annotation encoding="application/x-tex">{\displaystyle {\ce {P}}^{\bullet }+{\ce {XR -> PX + R}}^{\bullet }}</annotation>

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where • is the <a href="Active_center_(polymer_science)" title="Active center (polymer science)">active center</a>, P is the initial polymer chain, X is the <a href="End_group" title="End group">end group</a>, and R is the <a href="Substituent" title="Substituent">substituent</a> to which the active center is transferred.

Chain transfer reactions reduce the average <a href="Molecular_weight" class="mw-redirect" title="Molecular weight">molecular weight</a> of the final polymer. Chain transfer can be either introduced deliberately into a polymerization (by use of a chain transfer agent) or it may be an unavoidable side-reaction with various components of the polymerization. Chain transfer reactions occur in most forms of <a href="Addition_polymerization" class="mw-redirect" title="Addition polymerization">addition polymerization</a> including <a href="Radical_polymerization" title="Radical polymerization">radical polymerization</a>, <a href="Ring-opening_polymerization" title="Ring-opening polymerization">ring-opening polymerization</a>, <a href="Coordination_polymerization" title="Coordination polymerization">coordination polymerization</a>, and <a href="Cationic_polymerization" title="Cationic polymerization">cationic polymerization</a>, as well as <a href="Anionic_polymerization" class="mw-redirect" title="Anionic polymerization">anionic polymerization</a>.

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<div class="quotebox-title" style=""><a href="International_Union_of_Pure_and_Applied_Chemistry" title="International Union of Pure and Applied Chemistry">IUPAC</a> definitions</div>

Chain transfer (in a <a href="Chain-growth_polymerization" title="Chain-growth polymerization">chain polymerization</a>): Chemical reaction occurring during a chain polymerization in which an <a href="Active_center_(polymer_science)" title="Active center (polymer science)">active center</a> is transferred from a growing macromolecule or oligomer molecule to another molecule or to another site on the same molecule.<a href="#cite_note-PAC-REP-08-04-03-3">[3]</a>

Chain-transfer agent: Substance able to react with a chain carrier by a reaction in which the original chain carrier is deactivated and a new chain carrier is generated.<a href="#cite_note-PAC-REP-08-04-03-3">[3]</a>

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

Chain transfer reactions are usually categorized by the nature of the molecule that reacts with the growing chain.<a href="#cite_note-4">[4]</a>

<ul><li>Transfer to chain transfer agent. Chain transfer agents have at least one weak <a href="Chemical_bond" title="Chemical bond">chemical bond</a>, which therefore facilitates the chain transfer reaction. Common chain transfer agents include <a href="Thiols" class="mw-redirect" title="Thiols">thiols</a>, especially dodecyl mercaptan (DDM), and <a href="Halocarbons" class="mw-redirect" title="Halocarbons">halocarbons</a> such as <a href="Carbon_tetrachloride" title="Carbon tetrachloride">carbon tetrachloride</a>. Chain transfer agents are sometimes called modifiers or regulators.</li>

<li>Transfer to monomer. Chain transfer to <a href="Monomer" title="Monomer">monomer</a> may take place in which the growing polymer chain abstracts an atom from unreacted monomer existing in the reaction medium. Because, by definition, polymerization reactions only take place in the presence of monomer, chain transfer to monomer determines the theoretical maximum molecular weight that can be achieved by a given monomer. Chain transfer to monomer is especially significant in cationic addition polymerization and ring-opening polymerization. </li>

<li>Transfer to polymer. Chain transfer may take place with an already existing polymer chain, especially under conditions in which much polymer is present. This often occurs at the end of a radical polymerization when almost all <a href="Monomer" title="Monomer">monomer</a> has been consumed. <a href="Branching_(polymer_chemistry)" title="Branching (polymer chemistry)">Branched</a> polymers are formed as monomer adds to the new radical site which is located along the polymer backbone. The properties of <a href="Low-density_polyethylene" title="Low-density polyethylene">low-density polyethylene</a> are critically determined by the amount of chain transfer to polymer that takes place. </li>

<li>Transfer to solvent. In <a href="Solution_polymerization" title="Solution polymerization">solution polymerization</a>, the solvent can act as a chain transfer agent. Unless the solvent is chosen to be <a href="Chemically_inert" title="Chemically inert">inert</a>, very low molecular weight polymers (<a href="Oligomers" class="mw-redirect" title="Oligomers">oligomers</a>) can result. </li></ul>

<div class="mw-heading mw-heading2"><h2 id="Historical_development">Historical development</h2></div>

Chain transfer was first proposed by <a href="Hugh_Stott_Taylor" title="Hugh Stott Taylor">Hugh Stott Taylor</a> and William H. Jones in 1930.<a href="#cite_note-5">[5]</a> They were studying the production of <a href="Polyethylene" title="Polyethylene">polyethylene</a> [(C 2H 4)n] from <a href="Ethylene" title="Ethylene">ethylene</a> [C 2H 4] and <a href="Hydrogen" title="Hydrogen">hydrogen</a> [H 2] in the presence of <a href="Ethyl_radical" class="mw-redirect" title="Ethyl radical">ethyl radicals</a> that had been generated by the thermal decomposition of <a href="Diethylmercury" title="Diethylmercury">(Et)2Hg</a> and <a href="Tetraethyllead" title="Tetraethyllead">(Et)4Pb</a>. The observed product mixture could be best explained by postulating "transfer" of radical character from one reactant to another.

<a href="Paul_Flory" title="Paul Flory">Flory</a> incorporated the radical transfer concept in his mathematical treatment of vinyl polymerization in 1937.<a href="#cite_note-6">[6]</a> He coined the term "chain transfer" to explain observations that, during polymerization, average polymer chain lengths were usually lower than predicted by rate considerations alone.

The first widespread use of chain transfer agents came during <a href="World_War_II" title="World War II">World War II</a> in the <a href="Reconstruction_Finance_Corporation" title="Reconstruction Finance Corporation">US Rubber Reserve Company</a>. The "Mutual" recipe for <a href="Styrene-butadiene" title="Styrene-butadiene">styrene-butadiene</a> rubber was based on the Buna-S recipe, developed by <a href="IG_Farben" title="IG Farben">I. G. Farben</a> in the 1930s. The Buna-S recipe, however, produced a very tough, high molecular weight rubber that required heat processing to break it down and make it processable on standard rubber mills. Researchers at Standard Oil Development Company and the <a href="United_States_Rubber_Company" title="United States Rubber Company">U. S. Rubber Company</a> discovered that addition of a mercaptan modifier to the recipe not only produced a lower molecular weight and more tractable rubber, but it also increased the polymerization rate.<a href="#cite_note-7">[7]</a> Use of a mercaptan modifier became standard in the Mutual recipe.

Although German scientists had become familiar with the actions of chain transfer agents in the 1930s,<a href="#cite_note-8">[8]</a> Germany continued to make unmodified rubber to the end of the war and did not fully exploit their knowledge.

Throughout the 1940s and 1950s, progress was made in the understanding of the chain transfer reaction and the behavior of chain transfer agents. Snyder et al. proved the sulfur from a mercaptan modifier did indeed become incorporated into a polymer chain under the conditions of bulk or <a href="Emulsion_polymerization" title="Emulsion polymerization">emulsion polymerization</a>.<a href="#cite_note-9">[9]</a> A series of papers from <a href="Frank_R._Mayo" title="Frank R. Mayo">Frank R. Mayo</a> (at the U.S. Rubber Co.) laid the foundation for determining the rates of chain transfer reactions.<a href="#cite_note-10">[10]</a><a href="#cite_note-11">[11]</a><a href="#cite_note-12">[12]</a>

In the early 1950s, workers at <a href="DuPont" title="DuPont">DuPont</a> conclusively demonstrated that short and long branching in polyethylene was due to two different mechanisms of chain transfer to polymer.<a href="#cite_note-13">[13]</a> Around the same time, the presence of chain transfer in cationic polymerizations was firmly established.<a href="#cite_note-14">[14]</a>

<div class="mw-heading mw-heading2"><h2 id="Current_activity">Current activity</h2></div>

The nature of chain transfer reactions is currently well understood and is given in standard polymerization textbooks. Since the 1980s, however, a particularly active area of research has been in the various forms of <a href="Living_free-radical_polymerization" title="Living free-radical polymerization">free radical living polymerizations</a> including <a href="Living_polymerization" title="Living polymerization">catalytic chain transfer polymerization</a>, <a href="RAFT_(chemistry)" class="mw-redirect" title="RAFT (chemistry)">RAFT</a>, and <a href="Living_free-radical_polymerization#Iodine-transfer_polymerization_(ITP)" title="Living free-radical polymerization">iodine transfer polymerization (ITP)</a>. In these processes, the chain transfer reaction produces a polymer chain with similar chain transfer activity to the original chain transfer agent. Therefore, there is no net loss of chain transfer activity.

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

<li id="cite_note-1"><a href="#cite_ref-1">^</a> <a href="International_Union_of_Pure_and_Applied_Chemistry" title="International Union of Pure and Applied Chemistry">IUPAC</a>, <a href="IUPAC_books" class="mw-redirect" title="IUPAC books">Compendium of Chemical Terminology</a>, 5th ed. (the "Gold Book") (2025). Online version: (2006–) "<a rel="nofollow" class="external text" href="https://goldbook.iupac.org/terms/view/C00963.html">chain transfer</a>". <style data-mw-deduplicate="TemplateStyles:r1238218222">

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</style><a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1351%2Fgoldbook.C00963">10.1351/goldbook.C00963</a>

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<li id="cite_note-2"><a href="#cite_ref-2">^</a> Flory, P. J. Principles of Polymer Chemistry, Cornell University Press, Ithaca, NY, 1953, p. 136. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>0-8014-0134-8</bdi>

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<li id="cite_note-PAC-REP-08-04-03-3">^ <a href="#cite_ref-PAC-REP-08-04-03_3-0">a</a> <a href="#cite_ref-PAC-REP-08-04-03_3-1">b</a> <cite class="citation journal cs1"><a rel="nofollow" class="external text" href="https://www.degruyter.com/document/doi/10.1351/PAC-REP-08-04-03/pdf">"Terminology for reversible-deactivation radical polymerization previously called "controlled" radical or "living" radical polymerization (IUPAC Recommendations 2010)"</a>. <a href="Pure_and_Applied_Chemistry" title="Pure and Applied Chemistry">Pure and Applied Chemistry</a>. 82 (2): 483–491. 2010. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1351%2FPAC-REP-08-04-03">10.1351/PAC-REP-08-04-03</a>.</cite>

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<li id="cite_note-4"><a href="#cite_ref-4">^</a> <cite id="CITEREFCowie1991" class="citation book cs1">Cowie, J. M. G. (1991). Polymers: Chemistry & Physics of Modern Materials (2nd ed.). Blackie. pp. 63–64. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>0-216-92980-6</bdi>.</cite>

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<li id="cite_note-5"><a href="#cite_ref-5">^</a> <cite id="CITEREFTaylorWilliam_H._Jones1930" class="citation journal cs1">Taylor, Hugh S.; William H. Jones (March 1930). "The thermal decomposition of metal alkyls in hydrogen-ethylene mixtures". J. Am. Chem. Soc. 52 (3): 1111–1121. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1021%2Fja01366a044">10.1021/ja01366a044</a>.</cite>

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<li id="cite_note-6"><a href="#cite_ref-6">^</a> <cite id="CITEREFFlory1937" class="citation journal cs1">Flory, Paul J. (February 1937). "The Mechanism of Vinyl Polymerizations". J. Am. Chem. Soc. 59 (59): 241–253. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1021%2Fja01281a007">10.1021/ja01281a007</a>.</cite>

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<li id="cite_note-7"><a href="#cite_ref-7">^</a> Synthetic Rubber, Whitby, G. S., ed., John Wiley, NY 1954, p. 243.

</li>

<li id="cite_note-8"><a href="#cite_ref-8">^</a> For example, Meisenburg, K.; Dennstedt, I.; Zaucker, E. US Pat. 2,321,693 (assigned to I. G. Farben).

</li>

<li id="cite_note-9"><a href="#cite_ref-9">^</a> <cite id="CITEREFSnyderJohn_M._StewartR._E._AllenR._J._Dearborn1946" class="citation journal cs1">Snyder, H. R.; John M. Stewart; R. E. Allen; R. J. Dearborn (1946). "The Mechanism of Modifier Action in the GR-S Polymerization". Journal of the American Chemical Society. 68 (8): 1422. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1021%2Fja01212a007">10.1021/ja01212a007</a>.</cite>

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<li id="cite_note-10"><a href="#cite_ref-10">^</a> Mayo, F. R. J. Am. Chem. Soc., 1943, 65, 2324.

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<li id="cite_note-11"><a href="#cite_ref-11">^</a> Gregg, R. A.; Mayo, F. R. J. Am. Chem. Soc., 1948, 70, 2372.

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<li id="cite_note-12"><a href="#cite_ref-12">^</a> Mayo, F. R.; Gregg, R. A.; Matheson, M. S. J. Am. Chem. Soc., 1951, 73, 1691.

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<li id="cite_note-13"><a href="#cite_ref-13">^</a> see Roedel, M. J. J. Am. Chem. Soc., 1953, 75, 6110 and following papers.

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<li id="cite_note-14"><a href="#cite_ref-14">^</a> <cite id="CITEREFOverbergerG._F._Endres1955" class="citation journal cs1">Overberger, C. G.; G. F. Endres (April 1955). "Ionic polymerization. VI. The mechanism of molecular termination by aromatic compounds in cationic polymerization of styrene". Journal of Polymer Science. 16 (82): 283–298. <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/1955JPoSc..16..283O">1955JPoSc..16..283O</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%2Fpol.1955.120168218">10.1002/pol.1955.120168218</a>.</cite>

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