Comprehensive study of polymerization of pyrrole: A theoretical approach

Abstract

Polypyrrole is a popular conducting polymer. The pyrrole polymerization involving radical cations are very fast; therefore, it is difficult for instruments to probe the reaction progress at a molecular level, leading to different mechanisms proposed previously. As such, density functional theory calculations were performed to investigate how pyrroles polymerize, and provide insights into polypyrrole electrosynthesis. From the results, the CC couplings between two radical cations of pyrrole py-Hα+, and between a radical cation of an oligomer (py)x-Hα+ and py-Hα+ generating the dimeric doubly charged product py-Hα-αα-py-Hα2+ and the oligomeric doubly charged product (py)x-Hα-αα-py-Hα2+ respectively dominate the polymerization mechanism. It is plausible that the CC coupling between py-Hα+ and a neutral pyrrole molecule py-Hα forms the dimeric singly charged product py-Hα-αα-py-Hα+, subsequently being oxidized to py-Hα-αα-py-Hα2+. Unless the singly charged product py-Hα-αα-py-Hα+ can be stabilized, it cannot participate in later deprotonation. The CC coupling between two (py)x-Hα+ to form another oligomeric doubly charged product (py)x-Hα-αα-(py)x-Hα2+ is a minor pathway. The formation of oligomeric singly charged products (py)x-Hα-αα-py-Hα+ and (py)x-Hα-αα-(py)x-Hα+ via the CC couplings between (py)x-Hα+ and py-Hα, between py-Hα+ and (py)x-Hα, and between (py)x-Hα+ and (py)x-Hα unlikely occurs. Therefore, the formation of each polypyrrole linkage involves the loss of two electrons in the main. Weak bases such as Cl− and H2O are required to take the H+ in the α position away in the two successive deprotonations of (py)x-Hα-αα-py-Hα2+ to form a neutral oligomer (py)x-αα-py. Resonance-induced stability determines how pyrrole polymerizes.