Cathodic-Dehalogenation Polymerization Strategy for Fabricating Aromatic Polymer Thin Films

Abstract

The formation of carbon-carbon (C-C) bonds is crucial in organic and polymer chemistry. Although chemists have developed various methods, the effort to seek the construction of C-C bonds under mild and neutral conditions has never stopped. The organic chemical reactions are essentially about the interactions between electron-rich and electron-deficient species, typically involving electron transfer and recombination. It is reasonable to believe that the electron can directly serve as the catalyst and achieve the C-C bond construction under mild conditions via the electrochemical method. Despite a series of electrochemical C-C coupling reactions being proposed, there is still great space remaining for cathodic C-C coupling. Additionally, the film form of the materials is more conducive to their applications. Therefore, integrating the C-C coupling under mild conditions with the corresponding thin film fabrication will be valuable and attractive, yet challenging.

In this thesis, a cathodic-dehalogenation polymerization strategy has been developed, achieving the C-C bond formation between repeating units with the electron serving directly as the catalyst. Notably, this strategy can harmoniously integrate polymerization and thin film fabrication. It is applicable to polymerizing a series of halogen-substituted benzene/polycyclic aromatic hydrocarbons (PAHs) under neutral and mild conditions onto various conductive substrates, including 1,4-diiodo(or dibromo)benzene, 1,4-dibromonaphthalene, 1,5-dibromonaphthalene, 9,10-dibromoanthracene, 2,6-dibromoanthracene, 2,7-dibromophenanthrene, 2,7-dibromopyrene, 1,3,5-tribromobenzene, and 1,3,5-tris(4-bromophenyl)benzene.

Next, this cathodic-dehalogenation method was further optimized to lower the onset reductive potential without compromising the quality of the thin film. The dimethylsulfoxide/acetonitrile mixture system was proven to be an effective medium for this polymerization. Furthermore, the metal-free additive – triphenylphosphine can further reduce the onset reductive potential of halogen-substituted aromatic compounds, facilitating the cathodic-dehalogenation polymerization process and the formation of high-quality thin films.

Finally, the optimized cathodic-polymerization method was extended to halogen-substituted aromatic compounds containing heteroatoms (i.e., nitrogen). The poly(2,5-pyridine) and poly(2,5-pyrazine) thin films were successfully fabricated using this strategy on nickel foams and exhibited good performance in the alkaline hydrogen evolution reaction (HER) with the overpotentials of 212.8 and 180.7 mV and the Tafel slope of 157.0 and 122.4 mV dec^-1, respectively.

Date of Award	3 Oct 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Qichun ZHANG (Supervisor)