Rpn. Veregin et al., MECHANISM OF LIVING FREE-RADICAL POLYMERIZATIONS WITH NARROW POLYDISPERSITY - ELECTRON-SPIN-RESONANCE AND KINETIC-STUDIES, Macromolecules, 28(13), 1995, pp. 4391-4398
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.