Atom Transfer Radical Polymerization

from Wikipedia, the free encyclopedia

Atom Transfer Radical Polymerization ( ATRP ) is a special form of controlled radical polymerization .

It is characterized by the fact that the concentration of free radicals is reduced by adding a transition metal complex and in combination with an atom transfer process with an organohalide to such an extent that chain termination reactions such as disproportionation or recombination are largely suppressed. The kinetic reason for this is the dependence of the reaction rates (both chain growth and termination reactions ) on the concentration of active chain ends. For growth, it follows a first-order rate law, and a second-order law for the recombinative termination reactions. Thus, the reaction rate of the termination reactions by a lowering of the chain end concentration (principle of all controlled radical polymerizations) is more affected than the reaction rate of the chain growth.

This allows polymers to be synthesized that are characterized by control over the molecular mass and a narrow distribution of the molar mass .

The ATRP was first discovered almost simultaneously in 1995 by Mitsuo Sawamoto and Krzysztof Matyjaszewski and described independently of one another.

Reaction course

The initiation takes place through the reaction of an alkyl halide R-X with a metal complex Mt n X n L m (Mt = metal center, n = positive charge of the metal center (e.g. 2+) and number of halides, X = halide, L = Ligand, m = number of ligands). This creates the radical R • , in the complex an oxidation of the metal atom takes place through the addition of X • (one-electron oxidation).

ATRP initiation

Chain start by the initiator radical R • and growth by the addition of a vinyl monomer takes place as in conventional radical polymerizations, R • and the growing chain are, however, in equilibrium with a halogenated, “dormant” form.

ATRP growth

Compounds such as carbon tetrachloride , halogenated esters , halogenated alkylbenzenes and sulfonyl halides are suitable as initiators . The metal complexes are based, for example, on Cu (I) , Ru (I) and Fe (II) . The ligands used include triphenylphosphine (PPh 3 ) and 2,2'-bipyridine (2,2'-bpy). Examples are the complexes CuCl / 2,2'-bpy, RuCl 2 (PPh 3 ) 3 and FeCl 2 (PPh 3 ) 3 . Numerous vinyl compounds such as acrylonitrile , acrylamide , methacrylamide , isoprene , 2-vinylpyridine , styrene and derivatives of styrene are suitable as monomers . The molecular weights obtained are between 1000 and 150,000 g / mol, depending on the reaction conditions, and the polydispersity is in the range from 1.1 to 1.3.

See also

literature

  • Sawamoto et al .: Macromolecules 28th 1995, p. 1721
  • J. Wang, K. Matyjaszewski: Journal of the American Chemical Society 117, 1995, pp. 5614-5615.

Individual evidence

  1. ↑ Bernd Tieke: Makromolekulare Chemie , 3rd edition, Wiley-VCH, Weinheim, 2014, p. 82f.