Impact of Organometallic Intermediates on Copper-Catalyzed Atom Transfer Radical Polymerization

In atom transfer radical polymerization (ATRP), radicals (R•) can react with CuI/L catalysts forming organometallic complexes, R−CuII/L (L = Nbased ligand). R−CuII/L favors additional catalyzed radical termination (CRT) pathways, which should be understood and harnessed to tune the polymerization outcome. Therefore, the preparation of precise polymer architectures by ATRP depends on the stability and on the role of R−CuII/L intermediates. Herein, spectroscopic and electrochemical techniques were used to quantify the thermodynamic and kinetic parameters of the interactions between radicals and Cu catalysts. The effects of radical structure, catalyst structure and solvent nature were investigated. The stability of R−CuII/L depends on the radical-stabilizing group in the following order: cyano > ester > phenyl. Primary radicals form the most stable R−CuII/L species. Overall, the stability of R−CuII/L does not significantly depend on the electronic properties of the ligand, contrary to the ATRP activity. Under typical ATRP conditions, the R−CuII/L build-up and the CRT contribution may be suppressed by using more ATRP-active catalysts or solvents that promote a higher ATRP activity.