The polymer community has recently developed a great deal of research interests in establishing and exploiting novel synthetic approaches for constructing well-defined, complex polymers via photo-organocatalysis techniques. Conventional photoinduced polymerization systems have been based on the use of metallic (photo)catalysts to conduct polymerization of various systems.
These techniques are based on the use of light as a green reactant for activating (photo)catalytic species and driving forward the polymerization reaction and for this purpose copper(II) complexes or novel transitional metal photocatalysts have been efficiently used. However, photoredox organocatalysis offers a unique opportunity to utilize metal-free photocatalysts as highly stable, relatively nontoxic compounds that can be availed from renewable resources with low-cost processes. Moreover, some disadvantages associated with the use of metallic catalysts, which impede their further utilization in some specific areas, can be avoided by using organocatalysts.
Phtoredox organocatalysis has been successfully investigated and implemented in, for example, various photoinduced atom transfer radical polymerization (ATRP) or reversible addition fragmentation chain transfer (RAFT) systems (see here for a recent post on photoinduced RAFT). Understanding the mechanism of these reactions is a key factor for developing successful strategies.
A group of researchers led by Professor Matyjaszewski in Carnegie Mellon University has recently reported a comprehensive mechanistic study elucidating the mechanism of the photoinduced metal-free ATRP. Phenothiazine was already proved as efficient ATRP photocatalyst owing to its highly reducing properties in the photoexcited state which can be used to activate the alkyl halide initiator giving rise to the formation of initiating radicals and radical cation photocatalyst and halide anion complex. A key aspect of successful control over the polymerization reaction is the ability of the generated radical cation to undergo deactivation processes with the propagating radicals and thus regenerating the initial ground state photocatalyst. Combining a series of polymerization, kinetics, cyclic voltammetry, laser flash photolysis, dissociative electron transfer, Marcus and DFT calculations, the authors drew an accurate and comprehensive picture of the pathways involved during the catalytic process leading to a better control over the polymerization and synthesis of well-defined polymers.
Read more on:
Mechanism of Photoinduced Metal-Free Atom Transfer Radical Polymerization: Experimental and Computational Studies, J. Am. Chem. Soc., 10.1021/jacs.5b13455
To be continued…