Chain transfer

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A chain transfer is a reaction in the course of a chain polymerization in which the activity of a growing polymer chain is transferred to another molecule :

P • + XR '→ PX + R' •

Chain transfer reactions reduce the average degree of polymerization of the finished polymer. Chain transfers can either be intentional (with the help of a chain transfer agent ) or they can be an unavoidable side reaction. Chain transfer reactions occur in most forms of chain polymerization, including free radical polymerization , ring-opening polymerization , coordination polymerization , and cationic polymerization .

Classification

Chain transfer reactions are usually categorized by the type of molecule that reacts with the growing chain.

Chain transfer reactions are usually classified according to the type of molecule to which the growing chain is transferred:

  • Transfer reactions to monomers : Chain transfer reactions to monomers take place when the growing polymer chain abstracts an atom from an unreacted monomer. Because polymerization reactions always take place in the presence of monomers, such chain transfer reactions determine the theoretical maximum molar mass of a polymer from the given monomer. Chain transfer reactions to monomers are particularly important in cationic and ring-opening polymerization.
  • Transfer reactions to polymers : Especially if there are many polymers in the reaction mixture, chain transfer to existing polymer chains can occur. This is often the case at the end of radical polymerization, when almost all of the monomers have been consumed. The formation of a new radical site along the main polymer chain results in branched polymers. The properties of PE-LD are largely determined by how many transfer reactions have taken place.
  • Transfer reactions to solvents: If solution polymerization takes place, the solvent can act as a chain transfer agent. If this happens (i.e. no inert solvent has been selected), polymers with a very low molar mass ( oligomers ) are formed.

history

The concept of the chain transfer reaction was first proposed by Taylor and Jones in 1930 . They investigated the generation of polyethene from ethene and hydrogen in the presence of ethyl radicals, which had been generated by the thermal decomposition of diethyl mercury [(Et) 2 Hg] and tetraethyl lead [(Et) 4 Pb]. The product mixture observed could best be explained if a "transfer" of the radical character from one reaction partner to another was postulated.

Flory adopted the concept of radical transfer in his mathematical modeling of vinyl polymerization in 1937. He introduced the term "chain transfer" to explain why the average chain length of polymers is shorter than a mere consideration of the rate of polymerization would predict.

Chain transfer agents first found widespread use in the US Rubber Reserve Company during World War II . The recipe used there at the time was based on the Buna-S recipe, which had been developed by IG Farben in the 1930s. However, the Buna-S formulation produced very hard, high molecular weight rubber. This had to be partially decomposed by heat treatment in order to make it processable in rubber rolling mills. Researchers at the Standard Oil Development Company and the US Rubber Company discovered that adding mercaptan as an additive not only made the molar mass lower and more controllable, but also increased the rate of polymerization. Mercaptan therefore became a standard part of the recipe. German scientists also recognized the effects of chain transfer agents in the 1930s. However, this knowledge was not used in practice; Until the end of the war, only unmodified rubber was produced.

During the 1940s and 1950s, chain transfer agents and their behavior became better understood. Snyder et al were able to show that the sulfur in the mercaptan added was actually incorporated into the polymer chain during bulk or emulsion polymerization. A series of work by Mayo (at US Rubber Co.) laid the groundwork for determining the reaction rates of chain transfer agents.

In the early 1950s, researchers at DuPont were able to conclusively show that short and long branches in polyethylene can be traced back to two different mechanisms of chain transfer to the polymer. At about the same time, the presence of chain transfers in cationic polymerization was firmly established.

Current research

Chain transfer reactions are now well understood and outlined in standard textbooks. Since the 1980s, research has been particularly active in the field of the various forms of free radical living polymerizations including catalytic chain transfer polymerizations , RAFT polymerizations and iodine transfer polymerizations. In these processes, the chain transfer reaction produces a polymer chain with chain transfer activity similar to that of the original chain transfer agent. Hence there is no net loss of chain transfer activity.

Individual evidence

  1. ↑ Entry on chain transfer . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.C00963 Version: 2.3.1.
  2. ^ PJ Flory: Principles of Polymer Chemistry. Cornell University Press, Ithaca, NY 1953, ISBN 0-8014-0134-8 , p. 136.
  3. ↑ Terminology for reversible-deactivation radical polymerization previously called "controlled" radical or "living" radical polymerization (IUPAC Recommendations 2010) . In: Pure and Applied Chemistry . tape 82 , no. 2 , 2010, p. 483-491 , doi : 10.1351 / PAC-REP-08-04-03 .
  4. ^ Hugh S. Taylor, William H. Jones: The thermal decomposition of metal alkyls in hydrogen-ethylene mixtures . In: J. Am. Chem. Soc. tape 52 , no. 3 , March 1930, p. 1111-1121 , doi : 10.1021 / ja01366a044 .
  5. ^ Paul J. Flory: The Mechanism of Vinyl Polymerizations . In: J. Am. Chem. Soc. No. 59 , February 1937, p. 241-253 , doi : 10.1021 / ja01281a007 .
  6. ^ GS Whitby (Ed.): Synthetic Rubber. John Wiley, New York 1954, p. 243.
  7. ↑ z. BK Meisenburg, I. Dennstedt, E. Zaucker, US Pat. 2,321,693 (assigned to IG Farben).
  8. ^ HR Snyder, John M. Stewart, RE Allen, RJ Dearborn: The Mechanism of Modifier Action in the GR-S Polymerization. In: Journal of the American Chemical Society . tape 68 , no. 8 , 1946, pp. 1422 , doi : 10.1021 / ja01212a007 .
  9. ^ FR Mayo: Journal of the American Chemical Society. 1943, 65, p. 2324.
  10. ^ RA Gregg, FR Mayo: Journal of the American Chemical Society. 1948, 70, p. 2372.
  11. ^ FR Mayo, RA Gregg, MS Matheson: Journal of the American Chemical Society. 1951, 73, p. 1691.
  12. ^ MJ Roedel: Journal of the American Chemical Society. 1953, 75, p. 6110 and following papers.
  13. ^ CG Overberger, GF Endres: Ionic Polymerization. VI. The mechanism of molecular termination by aromatic compounds in cationic polymerization of sytrene . In: Journal of Polymer Science . tape 16 , no. 82 , April 1955, p. 283–298 , doi : 10.1002 / pol . 1955.120168218 .