Kinetics of Heterogeneous Atom Transfer Radical Polymerization of Methyl Methacrylate

A new kinetic equation for copper-mediated ATRP is introduced based on the experimental observation that the deactivator concentration ([CuII]) remains constant throughout the polymerization. This constant deactivator concentration can originate from the use of a heterogeneous transition metal/ligand system, resulting in the precipitation of deactivating species when the concentration exceeds a ceiling concentration. Equations are derived in which the decrease of the radical concentration in time is assigned to the decrease of the concentration of the activator and the dormant chains. When the equilibrium constant is low and/or the polymerization rate high, the radical concentration remains constant up to high conversions indicating little termination. In the case where a monomer is used with a high equilibrium constant and a relatively low polymerization rate, the radical concentration is continuously decreasing in time, as the concentration of both the activator and the dormant species is decreasing throughout the polymerization. To validate the kinetic equation, polymerization of methyl methacrylate was performed with an alkyl bromide initiator and a heterogeneous CuIBr/CuIIBr2/PMDETA system. Electron spin resonance spectroscopy was used to demonstrate that the deactivator concentration remains constant throughout the polymerization. Simulations and experimental results are in very good agreement.