Thermodynamic Properties of Copper Complexes Used as Catalysts in Atom Transfer Radical Polymerization

The thermodynamic properties of some copper complexes, among those frequently used as catalysts in controlled/living radical polymerization, has been studied in CH3CN + 0.1 M (C2H5)4NBF4. A combination of different techniques, namely potentiometry, spectrophotometry and cyclic voltammetry, has been used to determine the stability constants of all possible complexes of CuI and CuII present in binary and ternary systems composed of CuI or CuII, a halide ion (X=Cl-, Br-) and a polyamine ligand (L = pentamethyldiethylenetriamine, tris(2-dimethylaminoethyl)amine). The binary Cu-X systems show only mononuclear CuXx complexes, where x=1, 2, 3, 4 for CuII, and x=1, 2 for CuI. Conversely, in the case of the binary Cu-L systems, besides the mononuclear complexes CuLl, where l=1 or 2, also dinuclear complexes Cu2L were found. The ternary systems give rise to a mixture of mononuclear and dinuclear complexes of general formula CumLlXx. Besides the 1:1:1 complex obtained in all combinations, the following species were found: CuIILX2, CuI2LX and CuI2LX2. The stability constants of all these species were determined and used to construct speciation diagrams for both CuI and CuII species. Such diagrams show that often conditions favoring the quantitative formation of CuIIL, CuIIL2, or CuIILX can be easily realized, whereas isolation of a single predominant CuI species can hardly be achieved. Speciation diagrams for CuI as a function of CX/CCuI show interesting results that may be helpful in rationalizing the role of termination reactions in atom transfer radical polymerization.