Atom transfer radical polymerization (ATRP) is an example of a living polymerization or a controlled/living radical polymerization (CRP). Like its counterpart, ATRA or atom transfer radical addition, it is a means of forming a carbon-carbon bond through a transition metal catalyst. As the name implies, the atom transfer step is the key step in the reaction responsible for uniform polymer chain growth. ATRP (or transition metal-mediated living radical polymerization) was independently discovered by Mitsuo Sawamoto et al. and by Jin-Shan Wang and Krzysztof Matyjaszewski in 1995.
- The following scheme presents a typical ATRP reaction:
The uniformed polymer chain growth, which leads to low dispersity, 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, only a few (theoretically one) monomer unit is added at a time. This slow rate of propagation is responsible for the low polydispersity due to the fact that the chains polymerized are of a uniform length, as well as the fact that termination is avoided until almost 100 percent conversion.
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 initiator. 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).
Components of ATRP
There are five important variable components of Atom Transfer Radical Polymerizations. They are the monomer, initiator, catalyst, solvent and temperature. The following section breaks down the contributions of each component to the overall polymerization.
Monomers that are typically used in ATRP are molecules with substituents that can stabilize the propagating radicals; 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.
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. Most initiators for ATRP are alkyl halides. Alkyl halides such as alkyl bromides are more reactive than alkyl chlorides and both have good molecular weight control. The shape or structure of your initiator can determine the architecture of your polymer. For example, initiators with multiple alkyl halide groups on a single core can lead to a star-like polymer shape.
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:
- there needs to be two accessible oxidation states that are separated by one electron
- the metal center needs to have a reasonable affinity for halogens
- the coordination sphere of the metal needs to be expandable when its oxidized so to be able to accommodate the halogen
- a strong ligand complexation.
The most studied catalysts are those that include copper, which has shown the most versatility, with successful polymerizations for a wide selection of monomers.
Toluene,1,4-dioxane, xylene, anisole, DMF, DMSO, water, methanol, acetonitrile, chloroform, and other solvents are used.
In reverse ATRP, the catalyst is added in its higher oxidation state. Chains are activated by conventional radical initiators (e.g. AIBN) and deactivated by the transition metal. The source of transferrable halogen is the copper salt, so this must be present in concentrations comparable to the transition metal. A mixture of radical initiator and active (lower oxidation state) catalyst allows for the creation of block copolymers (contaminated with homopolymer) which is impossible using standard reverse ATRP. This is called SR&NI (simultaneous reverse and normal initiation ATRP).
Activators generated by electron transfer uses a reducing agent unable to initiate new chains (instead of organic radicals) as regenerator for the low-valent metal. Examples are metallic Cu, tin(II), ascorbic acid, or triethylamine. It allows for lower concentrations of transition metals, and may also be possible in aqueous or dispersed media.
Hybrid and bimetallic systems
This technique uses a variety of different metals/oxidation states, possibly on solid supports, to act as activators/deactivators, possibly with reduced toxicity or sensitivity. Iron salts can, for example, efficiently activate alkyl halides but requires an efficient Cu(II) deactivator which can be present in much lower concentrations (3–5 mol%)
Initiators for continuous activator regeneration is a technique that uses large excesses of initiator to continuously regenerate the activator, lowering its required concentration from thousands of ppm to around 1 ppm; making it an industrially relevant technique. Styrene is especially interesting because it generates radicals when sufficiently heated.
Activators regenerated by electron transfer can be used to make block copolymers using a method similar to AGET but requiring strongly reduced amounts of metal, since the activator is regenerated from the deactivator by a large excess of reducing agent (e.g. hydrazine, phenoles, sugars, ascorbic acid, etc...) It differs from AGET ATRP in that AGET uses reducing agents to generate the active catalyst (in quasi stoichiometric amounts) while in ARGET a large excess is used to continuously regenerate the activator allowing transition metal concentrations to drop to ~1 ppm without loss of control.
Polymers Made by ATRP
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