In
polymer chemistry
Polymer chemistry is a sub-discipline of chemistry that focuses on the structures, chemical synthesis, and chemical and physical properties of polymers and macromolecules. The principles and methods used within polymer chemistry are also applic ...
, chain transfer is a
polymerization
In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many fo ...
reaction by which the activity of a growing
polymer
A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...
chain is transferred to another
molecule
A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...
:
where • is the
active center, P is the initial polymer chain, X is the
end group, and R is the
substituent
In organic chemistry, a substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule.
The suffix ''-yl'' is used when naming organic compounds that contain a single bond r ...
to which the active center is transferred.
Chain transfer reactions reduce the average
molecular weight
A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...
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
addition polymerization including
radical polymerization,
ring-opening polymerization,
coordination polymerization, and
cationic polymerization, as well as
anionic polymerization.
Types
Chain transfer reactions are usually categorized by the nature of the molecule that reacts with the growing chain.
* Transfer to chain transfer agent. Chain transfer agents have at least one weak
chemical bond
A chemical bond is the association of atoms or ions to form molecules, crystals, and other structures. The bond may result from the electrostatic force between oppositely charged ions as in ionic bonds or through the sharing of electrons a ...
, which therefore facilitates the chain transfer reaction. Common chain transfer agents include
thiols, especially dodecyl mercaptan (DDM), and
halocarbons such as
carbon tetrachloride
Carbon tetrachloride, also known by many other names (such as carbon tet for short and tetrachloromethane, also IUPAC nomenclature of inorganic chemistry, recognised by the IUPAC), is a chemical compound with the chemical formula CCl4. It is a n ...
. Chain transfer agents are sometimes called ''modifiers'' or ''regulators''.
* Transfer to monomer. Chain transfer to
monomer
A monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or two- or three-dimensional network in a process called polymerization.
Classification
Chemis ...
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.

* 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
monomer
A monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or two- or three-dimensional network in a process called polymerization.
Classification
Chemis ...
has been consumed.
Branched polymers are formed as monomer adds to the new radical site which is located along the polymer backbone. The properties of
low-density polyethylene are critically determined by the amount of chain transfer to polymer that takes place.

* Transfer to solvent. In
solution polymerization, the solvent can act as a chain transfer agent. Unless the solvent is chosen to be
inert, very low molecular weight polymers (
oligomers) can result.
Historical development
Chain transfer was first proposed by
Hugh Stott Taylor and William H. Jones in 1930. They were studying the production of
polyethylene
Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging (plastic bags, plastic films, geomembranes and containers including bott ...
()''n''">/nowiki>()''n''/nowiki> from ethylene
Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon–carbon bond, carbon–carbon doub ...
">/nowiki>/nowiki> and hydrogen
Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
">/nowiki>/nowiki> in the presence of ethyl radicals that had been generated by the thermal decomposition of (Et)2Hg and (Et)4Pb. The observed product mixture could be best explained by postulating "transfer" of radical character from one reactant to another.
Flory incorporated the radical transfer concept in his mathematical treatment of vinyl polymerization in 1937. 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 World War II
World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...
in the US Rubber Reserve Company. The "Mutual" recipe for styrene-butadiene
Styrene-butadiene or styrene-butadiene rubber (SBR) describe families of synthetic rubbers derived from styrene and butadiene (the version developed by Goodyear is called Neolite). These materials have good abrasion resistance and good aging ...
rubber was based on the Buna-S recipe, developed by I. G. Farben 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 U. S. Rubber Company 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. 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, 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 emulsion polymerization. A series of papers from Frank R. Mayo (at the U.S. Rubber Co.) laid the foundation for determining the rates of chain transfer reactions.
In the early 1950s, workers at DuPont
Dupont, DuPont, Du Pont, duPont, or du Pont may refer to:
People
* Dupont (surname) Dupont, also spelled as DuPont, duPont, Du Pont, or du Pont is a French surname meaning "of the bridge", historically indicating that the holder of the surname re ...
conclusively demonstrated that short and long branching in polyethylene was due to two different mechanisms of chain transfer to polymer. Around the same time, the presence of chain transfer in cationic polymerizations was firmly established.[{{cite journal , last = Overberger , first = C. G. , author2=G. F. Endres , date=April 1955 , title = Ionic polymerization. VI. The mechanism of molecular termination by aromatic compounds in cationic polymerization of styrene , journal = Journal of Polymer Science , volume = 16 , issue = 82 , pages = 283–298 , bibcode =1955JPoSc..16..283O , doi = 10.1002/pol.1955.120168218]
Current activity
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 free radical living polymerizations including catalytic chain transfer polymerization, RAFT
A raft is any flat structure for support or transportation over water. It is usually of basic design, characterized by the absence of a hull. Rafts are usually kept afloat by using any combination of buoyant materials such as wood, sealed barre ...
, and iodine transfer polymerization (ITP). 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.
Notes
Polymerization reactions