Photoinduced ATRP, (π-ATRP)
(in the absence of Photoinitiators)(1)
Prior work on photoinduced initiation of an ATRP employed reverse ATRP or SR&NI procedures in conjunction with a standard photoinitiator. Yagci et al. (2-3) discussed the photochemical generation of the activator from the added CuIIX/L complex in the presence of methanol and subsequent reaction of the activator with the added alkyl halide. They employed a high concentrations of catalyst while Mosnacek used UV light to activate a reaction with lower concentrations of catalyst.(4)
Our interest in photoinitiation of an ATRP arose out of a desire to expand the possibility of photoinduced reduction of CuII halide complexes to visible light and determine if it was possible that excited state CuII species could undergo redox processes that are inaccessible from the ground state. The microscopic control of these photochemical reactions, due to the simple triggering by visible light, was envisioned to be an attractive feature for building useful materials with complex molecular architectures.
The mechanism of the photoinduced ATRP was studied using narrow bandwidth light emitting diodes (LEDs). This avoids complications due to absorption at two or more different wavelengths. The photoinduced ATRP was also used to make block copolymers, and the reaction was performed in water. The reaction was also conducted in the presence of sunlight.(1) LEDs were selected as the light source since they are; inexpensive, efficient, exhibit a long lifetime and are available in various wavelengths with narrow emission range. Photo-reactors were created by running a 5 m strip of LEDs inside a circular chamber.
The lights emitted in: violet (392 ± 7 nm), blue (450 ± 10 nm) and red (631 ± 9 nm)
The reactors were used for the polymerization of methyl methacrylate, oligo(ethylene oxide) monomethyl ether methacrylate (molecular weight 300) (OEOMA), ethyl acrylate (EA) and methyl acrylate (MA) with ethyl
a--bromoisobutyrate (EBiB) as the initiator for acrylates and ethyl α-bromophenylacetate (EBPA) for methacrylates with 100 ppm of CuBr2 complexes with tris(2-pyridylmethyl) amine (TPMA), N,N,N’,N”,N”-pentamethyldiethylenetriamine (PMDETA) and with tris((4-methoxy-3,5-dimethylpyridin-2-yl)methyl)amine (TPMA*) as ligands. UV/Vis/NIR spectroscopy was used to characterize the complexes. The CuII complexes absorb very strongly in the UV with some absorption in the violet region, weak absorption in the blue region, and weak absorption in the red region.
Absorbance measured for reactionwith initial concentration of reagents [M]0:[I]0:[CuBr2]0:[L]0 = 300:1:0.03:0.135, [M] = 4.7/5.5 M in DMF
When using different wavelength for the LEDs, the rates of polymerization of MMA followed the absorbance. The reaction was fastest with violet light, followed by blue light and no polymerization occurred with red light, and the control was good with both violet and blue light. The rate of polymerization did not change significantly in solvents of differing polarity. Blank experiments conducted in the absence of initiator or catalyst progressed at a lower rate, generating polymers with high molecular weight and broad dispersity, which provided insight into the mechanism.
The results reported in the paper are best explained by the photoreduction of the X-CuII/L deactivator complex to generate CuI/L and a halogen radical. The halogen radical can react with monomer to initiate a growing polymer chain that is subject to the ATRP equilibrium. When alkyl halide initiators are added these new chain represent a very small fraction of growing chains. The procedure can be considered a hybrid of ICAR (5) and ARGET ATRP.
The reaction does not progress in the absence of light and reactions with intermittent on-off switching of light progress in a well controlled manner with molecular weight close to the theoretical values and narrow dispersity.
One key advantage of ATRP is that this technique can be used to form well defined block copolymers, as seen by the chain extension of a PMMA macroinitiator formed using this photoinduced activation procedure (π-ATRP), which itself was chain extended with ethyl acrylate providing polymers of higher MW with minimal tailing in the GPC traces.
The procedure was extended to polymerize oligo(ethylene oxide) monomethyl ether methacrylate in water (67%), in the presence of an added halide salt to promote the formation of the deactivator.(1) The polymerization was conducted at room temperature in the presence of violet radiation. Conversion reached 60% in 11.5 h forming a polymer of 65,000 MW with Mw/Mn = 1.34
Theoretically the least expensive light source is the sun, and the photoinduced ATRP of MA and MMA with sunlight is fast, yet still well controlled.
[M]0:[I]0:[CuBr2]0:[L]0 = 300:1:0.03:0.135, [M] = 5.5 M in DMF.
In summary photochemical ATRP is possible under mild reaction conditions with LED irradiation or sunlight, and importantly no unnecessary byproducts are formed. The procedure can be employed to create block copolymers, can be performed in water and the reaction can be modulated by controlling light intensity and wavelength.
Subsequent work sought to determine “How are Radicals (Re)generated in Photochemical ATRP”. (6) The polymerization mechanism of photochemical mediated Cu-based ATRP was extensively investigated using both experimental and kinetic modeling techniques. As shown in the following schematic there are several distinct pathways that can lead to photochemical (re)generation of CuI activator species or formation of radicals,
Proposed activator (re)generation pathways in PhotoATRP.
The upper pathway is direct reduction of XCuII/L, while the middle three are the generation of radicals by the reactions of an alkyl halide and/or a ligand, and bottom is the photochemical reduction of CuII by an electron donor.
These potential (re)generation pathways include direct photochemical reduction of the CuII complexes by excess free amine moieties and unimolecular reduction of the CuII complex, which could be considered to be a hybrid between activators regenerated by electron-transfer (ARGET) or initiator for continuous activator regeneration ICAR) ATRP processes, since CuII is reduced in the presence of light by electron transfer as in ARGET while giving a halogen radical that can initiate new chain, as in ICAR ATRP. An alternative radical (re)generation mechanism has the photo-excited alkyl halide, ligand, or their combined interaction, generating a radical species which can react with monomer in a photochemical mechanism akin to ICAR ATRP, as shown in lines 2-4 of the schematic. These photochemical radical generation processes are similar to initiators for continuous activator regeneration (ICAR) ATRP processes.
In the case of the alkyl halide, homolytic cleavage of the carbon-halogen bond is anticipated as shown in line 2 of the schematic. (7) In the case of the photochemical interaction involving the ligand, the nitrogen centered radical cation is expected to be generated (8) and in order conserve both charge and spin, a second molecule must accept this electron. This alternative molecule can be an electron poor alkene, such as a (meth)acrylate moiety as shown in the third line of the scheme. (8) Alternatively, the ligand and alkyl halide can photochemically generate the nitrogen centered radical cation, an alkyl radical and a halide anion, as shown in line 4. A final possibility is that the CuII complexes in the excited state can react with electron donating species, e.g. an amine based ligand, reducing the CuII species and generating a radical cation species from the ligand, as shown in line 5. In all cases where the nitrogen centered radical cation is generated, it rapidly undergoes proton transfer, giving a protonated amine and a carbon centered radical. It should be noted that all of these photochemical radical (re)generation pathways are parallel pathways.
A series of model experiments, ATRP reactions and kinetic simulations, were performed to evaluate the contribution of these reactions to the photochemical ATRP process (6) using 392 nm radiation. The results of these studies clearly indicated that the dominant radical (re)generation reaction is the photochemical reduction of CuII complexes by free amines moieties, from amine containing ligands. The unimolecular reduction of the CuII deactivator complex is not significant and virtually no polymerization occurs in the absence of free ligand. (9) Escentially this is an ARGET ATRP process with the amine being oxidized to a radical cation which can initiate a new chain after proton transfer. However, there is some contribution from ICAR ATRP reactions involving the interaction of alkyl halides and ligand, ligand with monomer, and the photochemical cleavage of the alkyl halide. Nevertheless the predominant mechanism of photochemical mediated ATRP is consistent with a photochemical ARGET ATRP reaction dominating the radical (re)generation process
In the above summary of all potential contributing reactions to a photochemical ATRP, reactions with the highest rate are in bold arrows while reactions that dictate the rate are thin solid lines and reactions with low contributions to the mechanism are dashed lines.
(1) Konkolewicz, D.; Schroder, K.; Buback, J.; Bernhard, S.; Matyjaszewski, K. ACS Macro Letters 2012, 1, 1219-1223.
(2) Tasdelen, M. A.; Uygun, M.; Yagci, Y. Macromolecular Chemistry and Physics 2011, 212, 2036–2042.
(3) Tasdelen, M. A.; Ciftci, M.; Uygun, M.; Yagci, Y. ACS Symp. Ser. 2012, 1100, 59-72.
(4) Mosnacek, J.; Ilcikova, M. Macromolecules 2012, 45, 5859-5865.
(5) Konkolewicz, D.; Magenau, A. J. D.; Averick, S. E.; Simakova, A.; He, H.; Matyjaszewski, K. Macromolecules 2012, 45, 4461-4468.
(6) Ribelli, T. G.; Konkolewicz, D.; Bernhard, S.; Matyjaszewski, K. J. Am. Chem. Soc. 2014, 136, 13303-13312.
(7) Klán, P.; Wirz, J. Photochemistry of Organic Componds: From Concepts to Practice; John Wiley & Sons, 2009.
(8) Lewis, F. D.; Crompton, E. M. SET Addition of Amines to Alkene, 2nd ed.: CRC Press: Boca Raton, 2010.
(9) Anastasaki, A.; Nikolaou, V.; Zhang, Q.; Burns, J.; Samanta, S. R.; Waldron, C.; Haddleton, A. J.; McHale, R.; Fox, D.; Percec, V.; Wilson, P.; Haddleton, D. M. J. Am. Chem. Soc. 2014, 136, 1141-1149.