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= ATRP (Chemistry) =

ATRP or Atom Transfer Radical Polymerization is an example of a living polymerization or a controlled/living radical polymerization (CRP). Like it’s counter part, ATRA or Atom Transfer Radical Addition, it is a means of forming carbon-carbon bond through transition metal catalyst. As the name implies, the atom transfer step is the key step in the reaction responsible for uniform polymer chain growth. Krzysztof Matyjaszewski developed ATRP in 1995. This is a typical ATRP reaction:

thumb|600px|center|General ATRP Reaction. A. Initiation. B. Equilibrium with dormant specie. C. Propagation

The uniformed polymer chain growth leading to low polydispersity stems from the transition metal based catalyst. This catalyst provides an equilibrium between active, and therefore propagating, polymer and an inactive form of the polymer; known as the dormant form. Since the dormant state of the polymer is vastly preferred in this equilibrium, side reactions are suppressed.

This equilibrium in turn lowers the concentration of propagating radicals, therefore suppressing unintentional termination and controlling molecular weights.

ATRP reactions are very robust in that they are tolerant of many functional groups like allyl, amino, epoxy, hydroxy and vinyl groups present in either the monomer or the initator. ATRP methods are also advantageous due to the ease of preparation, commercially available and inexpensive catalysts (copper complexes), pyridine based ligands and initiators (alkyl halides).

thumb|center|477px|In this scheme, the ATRP with styrene is depicted as an example of ATRP. If all the styrene is reacted (the conversion is 100%) the polymer will have 100 units of styrene built into it. PMDETA stands for N,N,N',N,N pentamethyldiethylenetriamine.

Components of ATRP
There are five important variable components of Atom Transfer Radicaal Polymerizations. They are the monomer, initiator, catalyst, solvent and temperature. The following section breaks down the contributions of each component to the overall polymerization.

Monomer
Monomers that are typically used in ATRP are molecules with substituents that can stabilize the propagating radica ls; for example, styrenes, (meth)acrylates, (meth)acrylamides, and acrylonitrile. ATRP are successful at leading to polymers of high number average molecular weight and a narrow polydispersity index when the concentration of the propagating radical balances the rate of radical termination. Yet, the propagating rate is unique to each individual monomer. Therefore, it is important that the other components of the polymerization (initiator, catalysts, ligands and solvents) are optimized in order for the concentration of the dormant species to be greater than the concentration of the propagating radical and yet not too great to slow down or halt the reaction. ,

Initiator
The number of growing polymer chains is determined by the initiator. The faster the initiation, the fewer terminations and transfers, the more consistent the number of propagating chains leading to narrow molecular weight distributions. Organic halides that are similar in the organic framework as the propagating radical are often chosen as initiators. Alkyl halides such as alkyl bromides are more reactive than alkyl chlorides and both have good molecular weight control. ,

Catalytic
The catalyst is the most important component of ATRP because it determines the equilibrium constant between the active and dormant species. This equilibrium determines the polymerization rate and an equilibrium constant too small may inhibit or slow the polymerization while an equilibrium constant too large leads to a high distribution of chain lengths.

There are several requirements for the metal catalyst:


 * 1) there needs to be two accessible oxidation states that are separated by one electron
 * 2) the metal center needs to have a reasonable affinity for halogens
 * 3) the coordination sphere of the metal needs to be expandable when its oxidized so to be able to accommodate the halogen
 * 4) a strong ligand complexation.

The most studied catalysts are those that polymerizations involving copper, which has shown the most versatility, showing successful polymerizations regardless of the monomer.