Chain transfer
Encyclopedia
Chain transfer is a polymerization
reaction
by which the activity of a growing polymer
chain is transferred to another molecule
.
Chain transfer reactions reduce the average molecular weight 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 addition polymerization.
Chain transfer reactions are usually categorized by the nature of the molecule that reacts with the growing chain.
from ethylene
and hydrogen
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
in the US Rubber Reserve Company
. The "Mutual" recipe for styrene-butadiene
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.
Interestingly, 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 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
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.
, and iodine transfer polymerization. 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.
Polymerization
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...
reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
by which the activity of a growing polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
chain is transferred to another molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
.
- P• + XR' → PX + R'•
Chain transfer reactions reduce the average molecular weight 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
Addition polymerization
Chain growth polymerization is a polymerization technique where unsaturated monomer molecules add on to a growing polymer chain one at a time...
including radical polymerization
Radical polymerization
Free radical polymerization is a method of polymerization by which a polymer forms by the successive addition of free radical building blocks. Free radicals can be formed via a number of different mechanisms usually involving separate initiator molecules...
, ring-opening polymerization
Ring-opening polymerization
In polymer chemistry, ring-opening polymerization is a form of chain-growth polymerization, in which the terminal end of a polymer acts as a reactive center, where further cyclic monomers join to form a larger polymer chain through ionic propagation...
, coordination polymerization
Coordination polymerization
Coordination polymerization is a form of addition polymerization in which monomer adds to a growing macromolecule through an organometallic active center...
, and cationic addition polymerization.
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 bondChemical bondA chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...
, which therefore facilitates the chain transfer reaction. Common chain transfer agents include thiols, especially DDM, and halocarbons such as carbon tetrachlorideCarbon tetrachlorideCarbon tetrachloride, also known by many other names is the organic compound with the formula CCl4. It was formerly widely used in fire extinguishers, as a precursor to refrigerants, and as a cleaning agent...
. Chain transfer agents are sometimes called modifiers or regulators.
- Transfer to monomer. Chain transfer to monomerMonomerA monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...
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 monomerMonomerA monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...
has been consumed. BranchedBranching (chemistry)In polymer chemistry, branching occurs by the replacement of a substituent, e.g., a hydrogen atom, on a monomer subunit, by another covalently bonded chain of that polymer; or, in the case of a graft copolymer, by a chain of another type...
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 polymerizationSolution polymerizationSolution polymerization is a method of industrial polymerization. In this procedure, a monomer is dissolved in a non-reactive solvent that contains a catalyst....
, the solvent can act as a chain transfer agent. Unless the solvent is chosen to be inertInert-Chemistry:In chemistry, the term inert is used to describe a substance that is not chemically reactive.The noble gases were previously known as inert gases because of their perceived lack of participation in any chemical reactions...
, very low molecular weight polymers (oligomers) can result.
Historical Development
Chain transfer was first proposed by Taylor and Jones in 1930. They were studying the production of polyethylenePolyethylene
Polyethylene or polythene is the most widely used plastic, with an annual production of approximately 80 million metric tons...
from ethylene
Ethylene
Ethylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...
and hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
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
Paul Flory
Paul John Flory was an American chemist and Nobel laureate who was known for his prodigious volume of work in the field of polymers, or macromolecules...
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
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
in the US Rubber Reserve Company
Reconstruction Finance Corporation
The Reconstruction Finance Corporation was an independent agency of the United States government, established and chartered by the US Congress in 1932, Act of January 22, 1932, c. 8, 47 Stat. 5, during the administration of President Herbert Hoover. It was modeled after the War Finance Corporation...
. The "Mutual" recipe for styrene-butadiene
Styrene-butadiene
Styrene-Butadiene or Styrene-Butadiene-Rubber is a synthetic rubber copolymer consisting of styrene and butadiene. It has good abrasion resistance and good aging stability when protected by additives, and is widely used in car tires, where it may be blended with natural rubber...
rubber was based on the Buna-S recipe, developed by I. G. Farben
IG Farben
I.G. Farbenindustrie AG was a German chemical industry conglomerate. Its name is taken from Interessen-Gemeinschaft Farbenindustrie AG . The company was formed in 1925 from a number of major companies that had been working together closely since World War I...
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
United States Rubber Company
The United States Rubber Company was founded in Naugatuck, Connecticut in 1892. It was one of the original 12 stocks in the Dow Jones Industrial Average, and became Uniroyal Inc...
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.
Interestingly, 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
Emulsion polymerization
Emulsion polymerization is a type of radical polymerization that usually starts with an emulsion incorporating water, monomer, and surfactant. The most common type of emulsion polymerization is an oil-in-water emulsion, in which droplets of monomer are emulsified in a continuous phase of water...
. A series of papers from 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
E. I. du Pont de Nemours and Company , commonly referred to as DuPont, is an American chemical company that was founded in July 1802 as a gunpowder mill by Eleuthère Irénée du Pont. DuPont was the world's third largest chemical company based on market capitalization and ninth based on revenue in 2009...
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.
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, RAFTRAFT (chemistry)
Reversible Addition-Fragmentation chain Transfer or RAFT polymerization is one kind of controlled radical polymerization. Discovered at the Commonwealth Scientific and Industrial Research Organisation in 1998, RAFT polymerization is a relatively new method for the synthesis of living radical...
, and iodine transfer polymerization. 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.