Angew.Chem.Int.Ed.http://doi.org/fz5hsn(2012)

The properties of any given polymeric material are influenced significantly by the molecular weights of the individual polymer chains and also the distribution of those values. Methods that offer very good control over these parameters are therefore prized in polymer synthesis. In particular, controlled living radical polymerization has attracted much attention and acronyms such as ATRP (atom transfer radical polymerization) and RAFT (reversible addition fragmentation chain transfer polymerization) are now commonplace in the chemical literature. Some relatively recent developments in this area have focused on how to control these reactions with an external stimulus, such as light or an applied electrical current.

Credit: © 2012 WILEY

Previous attempts to photoregulate living radical polymerization processes have been somewhat limited, either giving rise to poor molecular weight control or only enabling the initiation step — rather than the subsequent chain-growth steps — to be controlled. Now, Brett Fors and Craig Hawker from the University of California, Santa Barbara, have shown that the living radical polymerization of methacrylates can be turned on and off at the flick of a (light) switch by using a commercially available iridium-based photredox catalyst. The Ir(III) catalyst absorbs visible light to give an excited-state Ir(III)* species that can react with an alkyl bromide initiator to produce an alkyl radical and an Ir(IV)Br complex. The alkyl radical initiates the polymerization of a methacrylate monomer to give a propagating polymer radical, which can then react with the Ir(IV)Br species to give a dormant polymer chain with a bromide end group, at the same time as regenerating the original Ir(III) complex.

In the presence of light, this process (pictured) continues to cycle in this fashion, although the alkyl bromide is no longer a small-molecule initiator but a polymer chain. When the light is turned off, however, the propagation reaction stops almost immediately — importantly, Fors and Hawker show that no chain termination occurs during this resting period. When the light is turned back on an hour later, the polymerization reaction resumes and continues until the light source is removed once more. Further evidence for the living nature of the system is provided through the efficient formation of a diblock copolymer.