Nadia A. Rizk

Charge-transfer complexes as polymerization inhibitors: 2. Studies on the mechanism of inhibition of the radical polymerizations of acrylonitrile and vinyl acetate using the amine-chloranil complexes

Abstract The mechanism of inhibition of the bulk and solution polymerizations of acrylonitrile (AN) and vinyl acetate (VA) has been investigated using the charge-transfer complexes of triethylamine (TEA) and N,N-dimethylaniline (DMA) with chloranil. Complete inhibition is achieved by the complexes of both amines for the polymerizations of both monomers. The greater inhibiting efficiency of the TEA complexes is explained in terms of their greater stabilities. The results support the idea that inhibition by quinones involves electron-transfer from the polymeric radicals to the quinone forming molecular complexes of polymeric cations and semiquinone anions. The latter are the actual inhibitors so that the efficiency of inhibition depends on the feasibility of their formation which is determined by the stability of the complexes formed. The nature of the inhibition reaction products is determined by the extent to which the semiquinone anions are found as kinetically independent species in the polymerizing system. The mechanism suggested accounts for the great differences in the inhibiting powers of quinones for the polymerizations of various monomers.

Studies on the retardation and inhibition of polymerizations by quinones in polar and nonpolar solvents

Abstract The influences of solvent polarity on the mechanism and efficiency of retardation and inhibition have been investigated for the sensitized polymerizations of styrene(St), methylmethacrylate(MMA) and acrylonitrile(AN) in benzene and acetonitrile in the presence of p -benzoquinone(BQ). 2,5-dichlorobenzoquinone(dichl. BQ), chloranil and duroquinone(DQ). The inhibiting efficiency is higher in nonpolar than in polar solvents for St and AN, especially for chloranil and DQ. However. the effect is reversed for both quinones in the case of MMA for which they act as weak retarders in acetonitrile and as accelerators in benzene. A mechanism based on electron-transfer from the polymeric radicals to the quinones, forming ion-pairs, is suggested for the inhibition, the resulting semiquinone anions being the actual inhibiting species. The nature of the final products depends on the degree of separation of the ion-pair partners and their subsequent reactions which are determined by the polarity of the solvent. Thus, while an ether St-chloranil copolymer is formed in benzene. a quinonoid product is formed in acetonitrile. The formation of quinonoid polymeric products is favoured in the inhibition whenever stabilization of the growing chain-quinone adduct is possible.

The interaction between polymerization initiators and inhibitors in solutions—II. The reaction between benzoyl peroxide and p-benzoquinone in concentrated solutions; Some observations on the induced decomposition of the peroxide

Abstract The reaction between benzoyl peroxide and p -benzoquinone in concentrated solutions in a wide variety of solvents has been investigated by isolation and identification of the reaction products. Despite the high efficiency of p -benzoquinone as a trap for benzoyloxy radicals, partial decarboxylation to phenyl radicals usually occurs. Complete suppression of decarboxylation is achieved only when p -benzoquinone is present at such a high concentration that it is effectively the solvent for the reaction. The benzoyloxy- and phenyl semiquinones show marked differences in reactivity, the former tend to combine to form dibenzoyloxy dibenzoquinone while disproportionation is favoured by the latter to form quinhydrone of monophenylbenzoquinone. At lower quinone ratio, the peroxide undergoes induced decomposition by phenyl radicals both in “reactive” and “unreactive” solvents. The induced decomposition involves the formation of radical intermediates which undergo disproportionation, but not intramolecular rearrangement, to form p -phenylbenzoyloxy radicals. The latter can be captured, before undergoing decarboxylation, by the benzoyloxysemiquinones formed in the reaction. A correlation between the electron donating property of a radical and its capability to induce the decomposition of the peroxide was developed.

The interaction between polymerization initiators and inhibitors in solution—IV: The reaction of benzoyl peroxide with hydroquinone in solution

Abstract The reaction between benzoyl peroxide and hydroquinone in a wide variety of solvents has been investigated by isolation and identification of the products. Benzoic acid, p -benzoquinone and benzoyloxy derivatives of p -benzoquinone are obtained under all conditions. Their relative amounts are largely determined by the molar ratios of the reactants and the nature of the solvent. In strongly polar solvents of high solvation power, p -benzoquinone is formed in preference to its derivatives. Nevertheless, the yield of 2,5-dibenzoyloxy p -benzoquinone reaches a maximum in acrylonitrile. Only in this solvent, at equal molar ratios of reactants, is full substitution in the hydroquinone nucleus achieved with the formation of tetrabenzoyloxy hydroquinone. These facts, together with the partial polymerization of acrylonitrile at room temperature at slightly higher peroxide/hydroquinone ratios and the complete suppression of the polymerization when perchloric acid is added, could be explained by a heterolytic mechanism involving the formation and controlled separation of ion pairs derived from the reactants.

Some new initiators and inhibitors in vinyl polymerization

Abstract This work examines some discrepancies between claimed effects in polymerization of certain metal acetylacetonates. Cupric acetylacetonate inhibits the thermal polymerization of styrene but acts only as a retarder in the sensitized polymerization of styrene and in the thermal polymerization of methyl methacrylate. Aluminium acetylacetonate initiates weakly the polymerization of styrene, but ferric acetylacetonate does not influence the rates of polymerization of the above mentioned monomers. Tetramethylthiuram disulphide (TMTD) acts as a free radical initiator for styrene polymerization with a rate very nearly proportional to [TMTD]∗. A low molecular weight phenolic condensate inhibits efficiently styrene polymerization. A mixture of TMTD and this phenolic condensate greatly decreases the induction period and increases the rate of subsequent polymerization, indicating a complex formation between the two substances.