Atom transfer radical polymerization
Like its counterpart, ATRA, or atom transfer radical addition, ATRP is a means of forming a carbon-carbon bond with a transition metal catalyst.
As the name implies, the atom transfer step is crucial in the reaction responsible for uniform polymer chain growth.
ATRP (or transition metal-mediated living radical polymerization) was independently discovered by Mitsuo Sawamoto[1] and by Krzysztof Matyjaszewski and Jin-Shan Wang in 1995.
Various transition metal complexes, namely those of Cu, Fe, Ru, Ni, and Os, have been employed as catalysts for ATRP.
This reversible process rapidly establishes an equilibrium that is predominately shifted to the side with very low radical concentrations.
[5] 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).
Therefore, it is important that the other components of the polymerization (initiator, catalyst, ligand, and solvent) are optimized in order for the concentration of the dormant species to be greater than that of the propagating radical while being low enough as to prevent slowing down or halting the reaction.
For example, initiators with multiple alkyl halide groups on a single core can lead to a star-like polymer shape.
Toluene, 1,4-dioxane, xylene, anisole, DMF, DMSO, water, methanol, acetonitrile, or even the monomer itself (described as a bulk polymerization) are commonly used.
The KATRP value depends on the homo-cleavage energy of the alkyl halide and the redox potential of the Cu catalyst with different ligands.
If we know three of these four combinations, the fourth one can be calculated as: The KATRP values for different alkyl halides and different Cu catalysts can be found in literature.
However, the determination of the loss of chain end functionality based on 1H NMR and mass spectroscopy methods cannot provide precise values.
The halogen remaining at the end of the polymer chain after polymerization allows for facile post-polymerization chain-end modification into different reactive functional groups.
External visible light stimulation ATRP has a high responding speed and excellent functional group tolerance.
However, techniques such as Activator Generated by Electron Transfer (AGET) ATRP provide potential alternatives which are not air-sensitive.
A good reducing agent (e.g. hydrazine, phenols, sugars, ascorbic acid) should only react with CuII and not with radicals or other reagents in the reaction mixture.
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.
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.
[24][25] 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%) Trace metal catalyst remaining in the final product has limited the application of ATRP in biomedical and electronic fields.
[26] This technique was later expanded to polymerization of acrylonitrile by Matyjaszewski et al.[27] Mechano/sono-ATRP uses mechanical forces, typically ultrasonic agitation, as an external stimulus to induce the (re)generation of activators in ATRP.
Esser-Kahn, et al. demonstrated the first example of mechanoATRP using the piezoelectricity of barium titanate to reduce Cu(II) species.
Mechochemically homolyzed water molecules undergoes radical addition to monomers, which in turn reduces Cu(II) species.
[32] Metalloenzymes have been used for the first time as ATRP catalysts, in parallel and independently, by the research teams of Fabio Di Lena[33] and Nico Bruns.
[34] This pioneering work has paved the way to the emerging field of biocatalytic reversible-deactivation radical polymerization.
