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

International audience; 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-methoxy3,5-dimethylpyrid-2-ylmethyl)amine (BPMA*(Pr)) as well as tridentate BPMA(Me) were used as ligands. L/(CuX2)-X-II (X = Cl or OTf) complexs 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, kappa(app)(CRT) showed larger values for more reducing L/Cu-I complexes, which correlated with the L/Cu-II-R (R = CH(CH3)(COOCH3)) 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.