alpha-diimine, diamine, and diphosphine iron catalysts for the controlled radical polymerization of styrene and acrylate monomers

The synthesis and characterization of a family of iron complexes of the type R[N,N]FeX2 (X = halide; R[N,N] = RNCH−CHNR, R = alkyl, aryl) and their application as catalysts for the controlled polymerization of styrenyl and acrylate monomers is described. Polymerizations catalyzed by alkylimine iron complexes give rise to atom transfer radical polymerization (ATRP) of styrene and methyl methacrylate, while those catalyzed by arylimine iron complexes give rise to catalytic chain transfer polymerization. A study of the ketimine series, R,Me[N,N]FeCl2 (where R,Me[N,N] = RNC(Me)−C(Me)NR, R = Cy, Ph, DiPP), showed that electronic factors govern the mechanistic pathway. Controlled polymerizations were also observed for methyl acrylate and p-methoxystyrene monomers. Moderate control over the polymerization of 2-hydroxypropyl methacrylate was achieved in methanolic solution using methyl-α-bromophenylacetate as an initiator. The analogous diamine and diphosphine iron complexes, (Et2NCH2CH2NEt2)FeCl2 and (R2PCH2CH2PR2)FeCl2 (R = i-Pr, Ph, C6F5, C6H11, Et), were also prepared and screened for ATRP behavior. The phosphine complexes were found to be the most reducing in the series, followed by the amines and then the imines, but large peak-to-peak (ΔEp) separations, indicative of poor reversibility, resulted in their poor performance as ATRP catalysts.