ARGET ATRP and ICAR ATRP
Activator ReGenerated by Electron Transfer (ARGET) ATRP
In many ways Activators ReGenerated by Electron Transfer (ARGET) is not just another way to initiate an ATRP but is a new way to run a CRP. A "green" procedure that uses a much lower concentration of catalyst present in the system.
When we considered the implications of the convenient AGET procedure for initiating an ATRP, described in the previous section of this page, where the activators are generated by electron transfer we realized that it should be possible to use the reducing agents to constantly regenerate the ATRP activator, the CuI species, from CuII species irreversibly formed during termination processes, without directly or indirectly producing initiating species that generate new chains.(1) The amount of Cu-based catalysts in atom transfer radical polymerization (ATRP) of styrene could therefore be reduced to a few ppm in the presence of the appropriate reducing agents such as FDA approved tinII 2-ethylhexanoate (Sn(EH)2), glucose,(1,2) or ascorbic acid,(3) hydrazine and phenyl hydrazine.(4) Since the reducing agents allow starting an ATRP with the oxidatively stable CuII species and the reducing/reactivating cycle can be employed to eliminate air or radical traps in the system it is included in the procedures for initiating an ATRP. For example, styrene was polymerized by the addition of 5 ppm of CuCl2/Me6TREN and 500 ppm of Sn(EH)2 to the reaction resulting in preparation of a polystyrene with Mn = 12,500 (Mn,th = 12,600) and Mw/Mn = 1.28.(5)
ARGET ATRP has also been applied to polymerization from surfaces, even in the presence of limited amounts of air.(6) This is shown in the following schematic. The images show two examples, one conducted in a a sealed vial and the other, in the middle, in a 50 ml reaction flask.
Generally, in an ARGET system it is desirable to add an excess of the ligand compared to the amount required to form the transition metal complex in order to compensate for competitive complexation of the low amount of added transition metal with monomer/solvent/reducing agent that are all present in significant molar excess.
Indeed it has been determined that the ARGET procedure can be driven based solely on addition of excess ligand or ligand substitute(7,8) and more recently by a nitrogen containing monomer, 2-(dimethylamino) ethyl methacrylate (DMAEMA)(9) indicating that nitrogen containing compounds such as DMAEMA and triethylamine are actually acting more as reducing agents than ligand substitutes.(10)
Another advantage of ARGET ATRP is that catalyst induced side reactions are also reduced to a significant degree and it is now possible to drive an ATRP reaction to much higher conversion and prepare copolymers with much higher molecular weight(6,11) while retaining chain end functionality. This has been confirmed by successful chain extension of macromolecules formed using this initiation/continuous reactivation system.(12)
The concept of Initiators for Continuous Activator Regeneration (ICAR) could simplistically be considered a "reverse" ARGET ATRP. In ICAR ATRP a source of organic free radicals is employed to continuously regenerate the CuI activator which is otherwise consumed in termination reactions when catalysts are used at very low concentrations. With this technique, controlled synthesis of polystyrene and poly(meth)acrylates (Mw/Mn < 1.2) can be conducted with catalyst concentrations between 10-50 ppm, where removal or recycling of the catalyst complex would be unwarranted for many applications. The reaction is driven to completion with low concentrations of added standard free radical initiators.(11) A procedure that could be conducted in existing industrial polymerization equipment.
Four ATRP catalysts with a broad range of KATRP values were examined in ICAR ATRP of styrene. They included the CuCl2 complexes with tris[2-(dimethylamino)ethyl]amine (Me6TREN), tris[(2-pyridyl)methyl]amine (TPMA) as ligands, both available from ATRP Solutions [http://atrpsolutions.com/products.html] N,N,N',N",N"-pentamethyldiethylenetriamine (PMDETA), and 4,4'-di-(5-nonyl)-2,2'-bipyridine (dNbpy).
ICAR ATRP of styrene was first conducted at low temperature (60°C) where organic radicals were produced solely by the slow decomposition of azobisisobutyronitrile (AIBN) (0.1 eq vs. ethyl 2-bromoisobutyrate (EtBrIB) initiator) in the presence of 50 ppm of CuCl2/L complexes. Interestingly, rates of polymerization differed by less than a factor of two amongst the reactions conducted with the different catalyst complexes. This was initially surprising, given that values of KATRP, which govern radical concentration and the rate of polymerization under normal and SR&NI ATRP conditions, differ by more than four orders of magnitude among these four complexes. Additional experiments and kinetic simulations explored the possibility that: (1) rates of polymerization and radical concentration under ICAR ATRP conditions are actually controlled by the rate of free radical initiator decomposition and, (2) the relative CuI and CuII concentrations conform accordingly as dictated by the KATRP value. Since a very small amount of Cu catalyst is employed in ICAR ATRP, catalysts with large values of KATRP (high concentration of CuII) and fast deactivation rate constants will minimize this ratio, allowing for a more even polymer chain growth and ultimately better control.
Cu complexes with TPMA have a large value of KATRP with the model polystyrene chain end 1-(bromoethyl)benzene (~7.9×10-6 at 60oC). While the analogous KATRP value of the Cu/PMDETA complex is much lower (~ 5.9×10-8), the deactivation rate constant (kda) for Cu/PMDETA is larger than that of TPMA, which can compensate for the product of kda[CuII]. Therefore Me6TREN and TPMA are more suitable ligands than PMDETA and dNbpy for ICAR ATRP at low Cu catalyst concentrations.
Simulations confirmed that the rate of polymerization in ICAR is governed by the rate of free radical initiator decomposition (as in RAFT) while control is ultimately determined by KATRP and the rate of deactivation (as in ATRP).(12)
As noted for AGET ATRP, ARGET ATRP and ICAR ATRP are also beginning to gain increasing attention because of the ease of setting up the reaction and the low amounts of catalyst employed in the reaction.(9,13-16) Furthermore ICAR ATRP can probably be conducted in existing industrial equipment using existing amounts of free radical initiators resulting in controlled radical polymerization being conducted with a similar rate of conversion to current uncontrolled polymerizations.
(1) Jakubowski, W.; Min, K.; Matyjaszewski, K. Macromolecules 2006, 39, 39-45.
(2) Jakubowski, W.; Matyjaszewski, K. Angewandte Chemie, International Edition 2006, 45, 4482-4486.
(3) Min, K.; Gao, H.; Matyjaszewski, K. Macromolecules (Washington, DC, United States) 2007, 40, 1789-1791.
(4) Matyjaszewski, K.; Jakubowski, W.; Min, K.; Tang, W.; Huang, J.; Braunecker, W. A.; Tsarevsky, N. V. Proceedings of the National Academy of Sciences of the United States of America 2006, 103, 15309-15314.
(5) Matyjaszewski, K.; Dong, H.; Jakubowski, W.; Pietrasik, J.; Kusumo, A. Langmuir 2007, 23, 4528-4531.
(6) Pietrasik, J.; Dong, H.; Matyjaszewski, K. Macromolecules 2006, 39, 6384-6390.
(7) Matyjaszewski, K.; Jakubowski, W.; Spanswick, J. In WIPO; (Carnegie Mellon University, USA). Application: WO 2007025310, 2007; p 95pp.
(8) Kwak, Y.; Matyjaszewski, K. Polymer International 2009, 58, 242-247.
(9) Dong, H.; Matyjaszewski, K. Macromolecules 2008, 41, 6868-6870.
(10) Tang, H.; Shen, Y.; Li, B.-G.; Radosz, M. Macromol. Rapid Commun. 2008, 29, 1834-1838.
(11) Dong, H.; Tang, W.; Matyjaszewski, K. Macromolecules 2007, 40, 2974-2977.
(12) Mueller, L.; Jakubowski, W.; Tang, W.; Matyjaszewski, K. Macromolecules 2007, 40, 6464-6472.
(13) Tanaka, K.; Matyjaszewski, K. Macromolecular Symposia 2008, 261, 1-9.
(14) Chan, N.; Cunningham, M. F.; Hutchinson, R. A. Macromol. Chem. Phys. 2008, 209, 1797-1805.
(15) Pintauer, T.; Matyjaszewski, K. Chemical Society Reviews 2008, 37, 1087-1097.
(16) Yamamoto, S.-i.; Matyjaszewski, K. Polymer Journal (Tokyo, Japan) 2008, 40, 496-497.