Determination of ATRP Equilibrium Constants under Polymerization Conditions

Atom transfer radical polymerization (ATRP) equilibrium constants (K_ATRP) were measured during polymerization of methyl acrylate (MA) with Cu^IBr/Cu^IIBr₂ in either dimethyl sulfoxide (DMSO) or acetonitrile (MeCN) in the presence of either tris(2-pyridylmethyl)amine (TPMA) or tris[2-(dimethylamino)ethyl]amine (Me₆TREN) as the ligand and with ethyl 2-bromopropionate as the initiator. The ln(K_ATRP) values changed linearly with the volume fraction of solvents in the reaction medium, allowing extrapolation of the values for K_ATRP to bulk conditions, which were 2 × 10^-9 and 3 × 10^-8 for TPMA and Me₆TREN ligands at 25 °C, respectively. The temperature effect on K_ATRP values was studied in MA/MeCN = 1/1 (v/v) with TPMA as the ligand in the temperature range from 0 to 60 °C. The K_ATRP values increased with temperature providing ΔH = 36 kJ mol^-1 in MeCN.