Lewis acid-mediated radical polymerization utilises Lewis acids to alter the reactivity of polar monomers, facilitating control of various aspects of polymer structure. The most exciting application of these additives is their potential to influence main-chain stereostructure and provide a convenient route to stereoregular polymers. Unfortunately, even with these novel additives the stereocontrol achieved in most systems is modest, especially compared to the high stereoregularity of polymers synthesised via ionic or coordination polymerisation. We have been using a combination of theory and experiment to better understand why existing agents fail to achieve high degrees of isotacticity with common monomers such as methyl methacrylate (MMA), and how this might be addressed through improved reagent design. As a case study, we have examined the impact of lithium bis(trifluromethane) sulfonamide (LiNTf2) on MMA polymerization. Our experiments show that it fails to control the stereochemistry of the polymer but does catalyse the propagation step. Theoretical calculations reveal that the Li+ binds preferentially to the terminal and penultimate side chains of the propagating radical in a manner consistent with stereocontrol. However, this binding mode over-stabilizes the propagating radical and reduces its propagation rate. The propagation step is instead dominated by a less favoured, non-stereocontrolling binding mode, in which the terminal unit of the radical and monomer are bound simultaneously, which activates the monomer to such an extent that it leads to the experimentally observed propagation catalysis.

Lewis acid-mediated radical polymerization � catalysis vs. stereocontrol

248th National Meeting (2014)

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