The polymerization of acrylonitrile (M) initiated by the Ce(IV)–acetophenone (AP) redox pair has been studied in acetic–sulfuric acid mixtures in a nitrogen atmosphere. The rate of polymerization is proportional to [M]3/2, [AP]1/2 and [Ce(IV)]1/2. The rate of disappearance of ceric ion,–RCe, is proportional to [AP], [M], and [Ce(IV)]. The effect of certain salts, solvent, acid and temperature on both the rates have been investigated. A suitable kinetic scheme has been proposed, and the composite rate constants kp 2(k/k/t) and k0/ki are reported.
The mechanism of the polymerization of acrylamide in aqueous medium initiated by the glycerol (R)/Ce(IV) redox system was studied. The rate of monomer disappearance was found to be directly proportional to [MI3'', [Ce(IV)]"*, and R1'2 at lower concentrations of Ce(1V). The rate of the disappearance of ceric ions was found to be inversely proportional to [MI and directly proportional to [Ce(IV)] and [R]. Consistent with the findings of earlier investigations, a complex formation between monomer, acrylamide, and ceric ion is indicated. The experimental results show the termination to be mutual. On the basis of these and other kinetic results, an appropriate mechanism is proposed. At higher concentration of Ce(IV), a different mechanism seems to operate. The rates of disappearance of both the monomer and ceric ions are retarded on addition of anions like HSO;, SO;-, or CIO;, but they are accelerated on the addition of Mn(I1) ions. Interpretations of the above observations are furnished.
The kinetics and mechanism of the inhibiting action of toluene on the Ce(IV) initiated polymerization of acrylonitrile were studied together with the effects of [M], [Ce(IV)], [Toluene], [HClO4], [NaClO4], and [Acetic acid] on the rate of polymerization. The values of composite rate constant kkt/kpkoki for toluene (and substituted toluenes) was calculated by plotting [M]/Rp versus [M]−1. A very significant observation in the present study was that the plot of [M]/Rp versus [M]−1 gave a negative intercept, which seems to be a general observation for all inhibiting substrates. On the basis of the experimental data, it could be concluded that the benzyl radical obtained by hydrogen abstraction, and not a chain transfer, is responsible for the inhibition.
SynopsisThe present investigation describes a new approach for the determination of radical substituent parameters u.. The technique has been developed by an ingenious combination of two equations, one proposed by Yamamoto and Otsu and the other by Streitwieser and Perriri. The u.values for different substituents were first evaluated by application of this newly developed technique to the Ce(1V)-substituted toluenes-acrylonitrile system. The validity of these values was then tested by application to another system, the Ce( 1V)-substituted acetophenones-acrylonitrile system, which is the subject of the present study.Several attempts for the determination of the stabilizing ability of substituents in radical reactions (radical substituent parameters) have been made by different workers.Alfrey and Price' postulated the now famous Q-e equation, which related the reactivity ratios of monomers in binary copolymerization to two fundamental parameters Q and e for a given monomer and its corresponding radical.A general expression for the rate constants for reactions between a radical and a substrate has been developed by Bamford and Jenkim2 log k = log k3,T + a a + p The parameter a defines the polarity of the radical, and the parameters a and p are characteristics of the substrate.As a result of a number of studies, Timberlake et al.3-5 have reported their findings that no single substituent parameter like a, u+, u-, a~, and (TR, etc., can give the desired linear Hammett plot and have, therefore, recommended the use of multiparameter equations for Hammett p-a correlation.The use of multiparameter equations was first reported by Yukawa and Tsuno,G who have shown that a four-parameter modified form of the standard Hammett equation involving u+ and r improved the correlation obtained in aromatic electrophilic substitutions: log -= pa + r(u+a) (k", ) r(proportiona1ity constant) gives the contribution of the enhanced resonance effect for R substituent.
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