Joint atom transfer and organometallic mediated control over radical polymerizations by iron amine-bisphenolate complexes

Chloro-substituted amine-bis(phenolate) iron(III) halides are highly efficient mediators of controlled radical polymerization. Both styrene and methyl methacrylate polymerizations proceed rapidly, affording excellent control over molecular weights and PDIs. Kinetic and mechanistic studies illustrate the controlled nature of the polymerizations and the significant effect of both catalyst and initiator concentration, where a monomer-dependence of the mechanism of control is implied. While styrene polymerizations are most effective in bulk, with 0.6 eq. of AIBN, optimal results for MMA polymerizations are obtained when reactions are carried out in solution with excess radical initiator (3 or 6 eq.). We propose that the difference in mechanism between styrene and MMA is due to the stronger iron-carbon bond formed during MMA polymerization. These data, alongside polymer end-group analysis, suggest that control is imparted by cooperation between ATRP and OMRP mechanisms.