Copper ATRP Catalysts with Quadridentate Amine Ligands: The Effects of Steric and Electronic Tuning on the Polymerization of Methyl Methacrylate

A series of nine linear quadridentate ligands with amine, pyridine, and quinoline donor groups of different steric and electronic demand were prepared for complexation with CuX and CuX 2 halide salts. Select Cu(II) complexes were reacted with either AgPF 6 or NaBPh 4 to generate dicationic di-PF 6 salts and monocationic halo-BPh 4 complexes, respectively. These Cu complexes with different oxidation states, counterions, and chelate ligands were screened as catalysts for the atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) in toluene or anisole solution at 80 °C. Catalysts were generated in situ or were isolated prior to subsequent introduction to the reaction media. Copper metal was added to all polymerizations of precomplexed catalyst systems (i.e., all Cu(II) complexes and those prepared ahead of time from CuBr). Molecular weight vs percent monomer conversion and kinetics plots are provided to illustrate the level of control that was achieved under various conditions. PDIs of poly(methyl methacrylate) (PMMA) products were generally low (∼1.1−1.4). Reaction rates exhibited the following trend: [Cu(ligand)X]BPh 4 > Cu(ligand)X > Cu(ligand)X 2, with the last of these showing longer induction periods. In general, reactions were faster and more controlled in anisole than in toluene. Polymerizations were slower with catalysts made from ligands bearing fluorine substituents than with nonfluorinated analogues. Other Cu systems with bulky quinoline or methylpyridine groups in the terminal positions tended to exhibit poor reactivity and/or molecular weight control. Consistent with the accepted mechanism of ATRP, analysis of tacticity indicates no special stereoselectivity was observed in polymerizations run with catalysts bearing chiral quadridentate ligands.