Theory of chain growth in chemical oxidative polymerization of aniline derivatives
Graphical abstract
Introduction
Although polymers of aromatic amines are ones among the first synthetic macromolecules known to the mankind [1,2], a significant progress in understanding the mechanism of oxidative polymerization reaction leading to the formation of their chains has only been achieved relatively recently [[3], [4], [5]]. Radicals [6] and cations [[7], [8], [9]] have been previously considered as the active centers in the oxidative polymerization of aromatic amines, but now, there are evidences of the intermediate formation of cation radicals of aromatic amines [[10], [11], [12]]. An increased activity of aromatic amines bearing electron-donating substituents in oxidative copolymerization [13], as compared to monomers containing electron-withdrawing substituents [[14], [15], [16], [17], [18], [19]], indicates the formation of positively charged intermediates. A decrease in the rate of oxidative polymerization of aniline in the presence of scavengers of cation radicals indicates the intermediate formation of cation radicals [12]. Kinetic dependencies of oxidative polymerization are also consistent with the generation of cation radicals of aromatic amines [20].
There are facts indicating that the chain growth proceeds due to the recombination of cation radicals of insoluble oligomers of aromatic amines only with cation radicals of the monomer, while the rate of monomer oxidation resulting in the formation of cation radical limits the overall rate of oxidative polymerization [[20], [21], [22]]. The presence of autocatalitic effect is characteristic of the oxidative polymerization of aromatic amines [8,[21], [22], [23]] and has been confirmed by an increase in the reaction rate upon the addition of polyaniline obtained previously [23] or upon a dispersion of inert particles [[23], [24], [25]] that are the centers of precipitation of the resulting polymer. Insights into the mechanism of catalysis of the oxidative polymerization of aromatic amines by the resulting polymer are different (Scheme 1, mechanisms A [21,23] and B [20]), but in all the cases, it is assumed that the reaction between monomer and polymer determines the rate of catalytic step [20,21,23]. Therefore, the conclusions drawn in the present work are independent from the opinions about the detailed mechanism of catalytic action of the produced polymer.
Molecular weights of polymers of aromatic amines are usually determined by the GPC method, but at the same time, some complications arise due to the pronounced association of chains in solutions [[26], [27], [28]]. The consideration of tendencies in the influence of reagent concentrations, monomer conversion, and temperature on the molecular weight of polymer is of interest for the prediction of properties of the products of oxidative polymerization of aromatic amines [[29], [30], [31], [32], [33]] and for the refinement of mechanism for this reaction. However, the lack of any theoretical approach linking the molecular mass of polymer with the parameters listed above makes it difficult to directly control the properties of polymers of aromatic amines and hinders the clarification of mechanism for the oxidative polymerization.
This work has for the first time proposed a theory that considers the quantitative aspect of the formation of polymers produced in the reaction of chemical oxidative polymerization of aromatic amines (aniline derivatives).
Section snippets
Derivation of the key equation
Since the oxidation rate of oligomers of aromatic amines, whose chains form a new phase, is greater than that of the starting aromatic amine, the rate of oxidative polymerization is limited by the rate of oxidation of the monomer to its cation radical in the non-catalytic ( and catalytic ( steps and is determined by Eq. (1) [3,7,8,[20], [21], [22], [23]]:wherein CM is the concentration of salt form of the monomer (aromatic amine); and t is time.
It is known that only the basic
Conclusions
The equation that is suitable for the estimation of effect of the conditions of oxidative polymerization of aromatic amines (aniline derivatives) on the number-average molecular weight of resulting polymers has been derived. It was demonstrated that the molecular weight of products of oxidative polymerization of aromatic amines is linearly increasing upon increasing the monomer conversion. The decreased molecular weight of polymers upon an increase in temperature of the oxidative polymerization
Author contributions
Yaroslav O. Mezhuev – developed the theory and derived the basic equation.
Yuri V. Korshak – participated in the discussion, which served as the basis for the development of the theory and analyzed literature data.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The work was supported by Mendeleev University of Chemical Technology of Russia. Project Number K-2020-001.
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EPR monitoring of aniline polymerization: Kinetics and reaction mechanism
2021, Synthetic MetalsCitation Excerpt :The EPR signal of polarons during the induction period is a symmetric singlet of the Lorentzian line shape with a width of ∆Н = 7.4 ± 0.1 G and g = 2.0030 ± 0.0001, which, with the polymerization time, transforms into a singlet of the Lorentzian line shape with a width of ∆Н = 1.1 ± 0.1 G and g = 2.0030 ± 0.0001 (Fig. 3). The change in the signal width coincides with the onset of a sharp increase in the concentration of polarons, which is associated with an increase in the length of conjugated chains in proportion to the conversion of aniline [23]. A similar regularity in the change in the width of the EPR signal line during the polymerization of aniline was also noted in [50].
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