polymer chemistry Polymer chemistry is a sub-discipline of chemistry that focuses on the structures of chemicals, chemical synthesis, and chemical and physical properties of polymers and macromolecules. The principles and methods used within polymer chemistry are ...

, degenerative chain transfer (also called degenerate chain transfer) is a process that can occur in a radical polymerization whereby reactivity of active centres are changed, hence significantly influencing the

molecular weight distribution The molar mass distribution (or molecular weight distribution) describes the relationship between the number of moles of each polymer species (Ni) and the molar mass (Mi) of that species. In linear polymers, the individual polymer chains rarely hav ...

of the resulting product. In chain polymerization processes it is observed that during the chemical reactions an active centre on a growing chain is transferred from a growing

macromolecule A macromolecule is a very large molecule important to biophysical processes, such as a protein or nucleic acid. It is composed of thousands of covalently bonded atoms. Many macromolecules are polymers of smaller molecules called monomers. ...

- P• - or

oligomer In chemistry and biochemistry, an oligomer () is a molecule that consists of a few repeating units which could be derived, actually or conceptually, from smaller molecules, monomers.Quote: ''Oligomer molecule: A molecule of intermediate relat ...

to another molecule (transfer agent XR) or to another site on the same molecule. :P• + XR → PX + R• This transfer involves termination of the initially growing chain to the completed macromolecule PX, where X denotes one end-group of the macromolecule. The example shows that the growing macromolecule as well as the transfer agent are consumed during this process. However, there are also chain transfer reactions that generate new chain carriers and new chain transfer agents at the same time with significant consequences for the distribution of the (average)

, the

dispersity In chemistry, the dispersity is a measure of the heterogeneity of sizes of molecules or particles in a mixture. A collection of objects is called uniform if the objects have the same size, shape, or mass. A sample of objects that have an incons ...

Đ and the (average)

degree of polymerization The degree of polymerization, or DP, is the number of monomeric units in a macromolecule or polymer or oligomer molecule. For a homopolymer, there is only one type of monomeric unit and the ''number-average'' degree of polymerization is given by D ...

of the product. These chain transfer reactions are called degenerative chain transfer reactions and are observed, for example in

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 barrel ...

-, ITRP-, or

TERP A ''terp'', also known as a ''wierde, woerd, warf, warft, werf, werve, wurt'' or ''værft'', is an artificial dwelling mound found on the North European Plain that has been created to provide safe ground during storm surges, high tides ...

- processes. RAFT = reversible addition-fragmentation chain transfer polymerization; ITRP = iodine-transfer polymerization; TERP = telluride-mediated polymerization. These polymerization techniques belong to the class of

reversible deactivation radical polymerization Reversible deactivation radical polymerizations (RDRPs) are members of the class of reversible deactivation polymerizations which exhibit much of the character of living polymerizations, but cannot be categorized as such as they are not without cha ...

s (RDRP) that show some characteristics of a

living polymerization In polymer chemistry, living polymerization is a form of chain growth polymerization where the ability of a growing polymer chain to terminate has been removed. This can be accomplished in a variety of ways. Chain termination and chain transfer ...

, however, they must not be addressed as living polymerizations because true living polymerizations are characterized by the absence of ''any'' termination reaction. In this sense, a chain-transfer agent RX is a substance that is able to react with a chain carrier by a reaction in which the original chain P• is deactivated and a new chain carrier R• is generated, as shown above.

References

{{Reflist, 2 Polymerization reactions