MECHANISM OF LIVING FREE-RADICAL POLYMERIZATIONS WITH NARROW POLYDISPERSITY - ELECTRON-SPIN-RESONANCE AND KINETIC-STUDIES

This work focuses on the mechanism of the pseudoliving free radical bu lk polymerization of styrene, which produces narrow polydispersity res in, using nitroxide radicals to reversibly terminate the growing polym er chain. The polymerization kinetics and the rate of breaking of the nitroxide to polymer NO-C bond were studied. The polymerization gives an increase in molecular weight that is linear with conversion to high conversion, gives polydispersities that are narrow (<1.3) and relativ ely insensitive to conversion, and shows first-order kinetics in monom er concentration. Calculations demonstrate that the kinetics of polyme rization greatly reduce the termination of the polymerization by the r eaction of two growing chains, which is the predominant mechanism in c onventional free radical polymerization. It is concluded that this nit roxide-mediated free radical polymerization is a living polymerization . To study the stability of the NO-C bond, model compounds, comprising a styrene unit terminated at either end with a benzoyloxy moiety and a nitroxide moiety, were prepared and characterized. These compounds w ere heated in situ in the ESR cavity to generate nitroxide radicals. R adical concentrations were followed as a function of time and temperat ure, and an activation energy for cleavage of the NO-C bond was calcul ated. A proposed mechanism fits the available kinetic data well, and m easured activation energies for nitroxide compounds correlate with rat es of polymerization.