Atom transfer radical polymerisation (ATRP) was independently developed by Sawamoto and Matyjaszewski in 1995, utilising ruthenium(II) and copper(I) catalysts respectively.9-11 ATRP was developed from the atom transfer radical addition (ATRA) and the Kharasch reaction of the 1940s, which radically added halogens to olefins.12,13 ATRP relies on the reversible activation of a dormant halogen capped polymer to an active polymeric radical by transfer of the halogen atom to and from a transition metal complex.10,11 Said metal complex is in turn oxidised as it activates the dormant polymer chain and is reduced when deactivating an active polymer chain (Scheme 1.03).
9 Pn X Mtn Y/L Pn X Mtn+1 Y/L kact kdeact kp nM kt Termination
Scheme 1.03. Proposed mechanism for ATRP. Where Pn = polymeric molecule, X = halogen, Mtn =
transition metal with oxidation state of n, L = ligand, Y = ligand or counter ion and M = monomer.
Many transition metals have been utilised for ATRP (such as Re, Ru, Fe, Rh and Ni) but Cu(I) is still the most widely employed transition metal in ATRP, owing to its versatility and cost.14 Cu(I) is only one component essential for mediating ATRP, careful consideration of ligand and polymerisation initiator is also vital to achieve a controlled polymerisation. A vast array of Cu(I)/ligand complexes have been examined by Matyjaszewski and co-workers to determine the activity of each complex and the resulting ATRP equilibrium constant.15 It was shown that Cu(I)/ligand complex activity was highest when stabilised by ligands containing: alkyl amine ≈ pyridine > alkyl imine >> aryl imine > aryl amine.15 It was also shown that complex activity decreased from tetradentate ligands to tridentate ligands and finally bidentate ligands.15
Matyjaszewski and co-workers also examined the ATRP equilibrium constants for a number of alkyl halide initiators.15 Alkyl bromides are more active than the analogous alkyl chlorides.15 Furthermore, activity was found to increase with the level of alkyl halide substitution, tertiary alkyl halide esters are most active whereas primary alkyl halide esters are the least active.15 Finally, alternative initiator substituents for a given initiator type were examined and nitriles were found to increase activities more than benzyl derivatives.15 Common ATRP ligands and initiators are shown in Figure 1.02.
10 N N N N N N N N N N N O O Br Br Br O O Me6TREN PMDETA NPPMI EBiB MBP PEB BPY
Figure 1.02. Structures of common ATRP ligands and initiators.
ATRP is typically performed in non-polar solvents or bulk9-11 but polar-protic solvents have also been utilised as well as ionic liquids.16,17 Although there are many variables influencing the ATRP equilibrium position and thus the control of the polymerisation, ATRP has proven to be an extremely versatile technique and can polymerise a wide range of monomers; (meth)acrylates, styrenes, (meth)acrylamides and acrylonitrile.14 However, ATRP does require large quantities of copper complex catalyst and as such recent advances in ATRP have been focused on developing alternative ATRP techniques that reduce catalyst loading.
One such technique is initiators for continuous activator regeneration (ICAR) ATRP.18 ICAR ATRP utilises small quantities of free radical initiators, such as AIBN, to reduce accumulated Cu(II) back to Cu(I) which can subsequently activate dormant polymer chains again.18 ICAR ATRP has been limited in its application of synthesising block copolymers as some polymer chains will be initiated by the free radical initiator thus reducing the end group fidelity.18 However, ICAR ATRP has been utilised to synthesise block copolymers by chain extending ATRP
11 macroinitiators whilst only generating a negligible quantity of free radical initiated homopolymers.19
Another technique designed to reduce accumulated Cu(II) back to Cu(I) and thus lower the quantity of catalyst required is activators regenerated by electron transfer (ARGET) ATRP.20,21 ARGET ATRP regenerates Cu(I) by reduction of Cu(II) with a variety of compounds such as tin(II) 2-ethylhexanoate, glucose, ascorbic acid, phenols and hydrazine.18,20-22 For ARGET ATRP to be most efficient reducing agents utilised should not directly or indirectly produce radicals which can initiate a polymerisation.20,21 Deactivating species concentration should be suitably maintained so large concentrations of radicals are not generated and thus decreasing polymerisation control.22
UV/visible light irradiation has also been utilised to lower the quantity of copper catalyst, Cu(II) complexes are reduced to the active Cu(I) complexes in situ. This has been achieved with the presence of reducing agents such as methanol and without any reducing agents when irradiated at 350 nm at room temperature.23,24 This methodology has been highly optimised so that only 100 ppm loadings of Cu(II) catalyst is required and polymerisations can be irradiated with LEDs at 392/450 nm or with sunlight to generate the necessary active Cu(I) catalyst to afford a controlled polymerisation.25 Furthermore, monomer conversion could be stopped by removing the external light source, allowing the polymerisations to be switched on and off.25
Electrochemical potential has been exploited as an external stimulus to lower the quantity of copper catalyst required for ATRP.26 Electrochemically mediated ATRP (eATRP) reversibly activates the Cu catalyst by reduction of Cu(II) complex to the active Cu(I) complex.26 Controlled polymerisations could be maintained with
12 catalyst loadings as low as 50 ppm and polymerisation could be switched on and off by applying intermittent potentials.26 eATRP has been extended to aqueous systems by the eATRP of oligo(ethylene glycol) methyl ether methacrylate (OEGMA).27 Lately, Matyjaszewski and co-workers have aimed to broaden the appeal of eATRP by systematically optimising many electrochemical and polymerisation parameters such as, applied potential, catalyst concentration and choice of ligand as well as investigating Cu catalyst recycling.28
Zerovalent metals such as Fe(0) and Cu(0) have been added to ATRP containing Fe(II)Br2 and Cu(II)Br2, respectively, to (re)generate the active metal complexes.29
This technique is also designed to limit the quantity of catalyst required by preventing the accumulation of deactivated metal salts. However, the exact role of Cu(0) and Cu(II) in the polymerisation mechanism is still debated and two proposed mechanisms exist.30 Supplemental activator and reducing agent (SARA) ATRP describes Cu(0) as primarily a reducing agent for Cu(II) back to Cu(I) which is the principal activator of alkyl halides throughout polymerisation.30 An alternative mechanism, single electron transfer living radical polymerisation (SET-LRP), maintains that Cu(0) is the only activator of alkyl halides as all Cu(I) undergoes rapid disproportionation to Cu(0) and Cu(II).30 SET-LRP will now be introduced to highlight the differences from ATRP, particularly focusing on the alternative experimental conditions used to perform SET-LRP.