Effect of Ligand Structure on the CuII–R OMRP Dormant Species and Its Consequences for Catalytic Radical Termination in ATRP

The kinetics and mechanism of catalytic radical termination (CRT) of n -butyl acrylate (BA) in MeCN in the presence of Cu complexes with tridentate and tetradentate ligands was investigated both theoretically and experimentally. The tetradentate TPMA, TPMA* 1, TPMA* 2, TPMA* 3, and the newly synthesized tridentate N -propyl- N, N -bis(4-methoxy-3,5-dimethylpyrid-2-ylmethyl)amine (BPMA* Pr ) as well as tridentate BPMA Me were used as ligands. L/Cu II X 2 (X = Cl or OTf) complexes were characterized by cyclic voltammetry (CV), UV–vis–NIR, and X-ray diffraction. Polymerization of BA initiated by azobis(isobutyronitrile) (AIBN) in MeCN in the presence of a L/Cu I complex showed higher rates of CRT for more reducing L/Cu I complexes. The ligand denticity (tri- vs tetradentate) had a minor effect on the relative polymerization kinetics but affected the molecular weights in a way specific for ligand denticity. Quantification of the apparent CRT rate coefficients, k CRT app, showed larger values for more reducing L/Cu I complexes, which correlated with the L/Cu II –R (R = CH(CH 3 )(COOCH 3 )) bond strength, according to DFT calculations. The bond strength is mostly affected by the complex reducing power and to a lesser degree by the ligand denticity. Analysis of kinetics and molecular weights for different systems indicates that depending on the ligand nature, the rate-determining step of CRT may be either the radical addition to L/Cu I to form the L/Cu II –R species or the reaction of the latter species with a second radical.