Eiichiro Yoshikawa

Determination of absolute rate constants for radical polymerization of diisopropyl fumarate based on a quantitative scavenge of propagating radical

Abstract During polymerization of diisopropyl fumarate (DiPF), the propagating poly(DiPF) radical was scavenged quantitatively by a stable free radical, because of its long-lived nature and high steady-state concentration. By addition of a known amount of 1,3,5-triphenylverdazyl to the polymerization mixture, the concentration of poly(DiPF) radical was determined to be 10 −5 −10 −4 mol dm −3 , which allowed the evaluation of the propagation rate constant ( k p ) from the overall rate of polymerization: k p = 0.31 ± 0.07 dm 3 mol −1 s −1 at 30°C. The lifetime of the polymer radical was too long for a rotating sector experiment. The rate constant for bimolecular termination of the radical ( k t ) was calculated from the decay of poly(DiPF) radical followed by the scavenger method: k t = 0.84 dm 3 mol −1 s −1 . These rate constants, which are exceptionally small, are accounted for by the steric factor of the ester alkyl group accumulated along the polymer chain and extremely slow diffusion of the polymer radical. Because the termination rate determined is too slow to interpret the molecular weight of the polymer formed, primary radical termination is to be considered as an additional termination process in the actual polymerization.

Observation of steric effect in radical polymerization of dialkyl fumarates by spin trapping with 2,4,6-tri-tert-butylnitrosobenzene

Abstract Polymer radicals derived from diethyl, diisopropyl, dicyclohexyl and di-tert-butyl fumarates were spin-trapped with 2,4,6-tri-tert-butylnitrosobenzene (BNB), which yields an anilino radical and a nitroxide depending on the steric environment of the radical centre. The spin adduct was prepared by fumarate polymerization initiated with di-tert-butyl hyponitrite in the presence of BNB and by the reaction with active poly(fumarate) radical. According to the e.s.r. spectra of the spin adducts, increases in the bulkiness of the ester alkyl group and the polymer chain length brought about a larger quantity of the anilino radical. This finding indicates the tendency that the reactivity of poly(fumarate) radical toward BNB is differentiated by the steric hindrance caused by the polymer chain and the ester alkyl group.

Contribution of primary radical termination to radical polymerization of diethyl fumarate estimated from absolute rate constants

SummaryDiethyl fumarate was radically polymerized under UV irradiation and concentration of the propagating radical was determined to be of the order of 10-5 mol/L by scavenge with a stable free radical. The absolute rate constant for propagation (kp) was evaluated from the overall rate of polymerization at 30°C: Kp =(2.9 ± 0.3) × 10-2 L/mol · s. The rate constant for mutual termination of the polymer radical (kt) was calculated from the decreasing rate of the radical concentration in the dark: kt=8.0 L/mol·s. The kt value determined is one twentieth of that evaluated previously by a rotating sector method. This discrepancy is accounted for by contribution of much faster primary radical termination.