How are Radicals (Re)Generated in Photochemical ATRP?

• Published in: Journal of the American Chemical Society Vol. 136; no. 38; pp. 13303 - 13312
• Main Authors: Ribelli, Thomas G, Konkolewicz, Dominik, Bernhard, Stefan, Matyjaszewski, Krzysztof
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 24.09.2014
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja506379s

The polymerization mechanism of photochemically mediated Cu-based atom-transfer radical polymerization (ATRP) was investigated using both experimental and kinetic modeling techniques. There are several distinct pathways that can lead to photochemical (re)­generation of CuI activator species or formation of radicals. These (re)­generation pathways include direct photochemical reduction of the CuII complexes by excess free amine moieties and unimolecular reduction of the CuII complex, similar to activators regenerated by electron-transfer (ARGET) ATRP processes. Another pathway is photochemical radical generation either directly from the alkyl halide, ligand, or via interaction of ligand with either monomer or with alkyl halides. These photochemical radical generation processes are similar to initiators for continuous activator regeneration (ICAR) ATRP processes. A series of model experiments, ATRP reactions, and kinetic simulations were performed to evaluate the contribution of these reactions to the photochemical ATRP process. The results of these studies indicate 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, 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. Therefore, the mechanism of photochemically mediated ATRP is consistent with a photochemical ARGET ATRP reaction dominating the radical (re)­generation.