Thermal Behavior, Stability, and Decomposition Mechanism of Poly(N-vinylimidazole)

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Homo- and copolymers of N-vinylimidazole belong to a rapidly emerging class of polymeric materials. Because of the fact that these materials can be utilized in several high-temperature processes and applications, such as catalysis, fuel cells, polymeric ionic liquids (PIL), precursors for new materials by thermolysis, etc., and because fundamental details on the thermal behavior of such polymers are lacking, systematic investigations have been carried out to reveal the stability and the mechanism of thermal decomposition of poly(N-vinylimidazole) (PVIm) by using a variety of techniques, such as differential scanning calorimetry (DSC), thermogravimetry (TG), thermogravimetry–mass spectrometry (TG-MS), and pyrolysis–gas chromatography/mass spectrometry (Py-GC/MS). The investigated PVIm was obtained by free radical polymerization initiated by AIBN in benzene at 70 °C. By the unique combination of the applied methods to investigate the thermal decomposition mechanism of PVIm, it was found that the thermal decomposition of PVIm takes place in one main step in the temperature range 340–500 °C. An initial mass loss of 4% occurs before the main endothermic decomposition step due to the evaporation of water and acetone physically bound to the polymer during purification. The major products of the thermal decomposition of PVIm are 1H-imidazole and 1-vinylimidazole accompanied by several minor products, such as benzene and several alkyl aromatics. The relative ratios between imidazoles and aromatics, i.e., the 2 orders of higher amounts of imidazoles, indicate that in contrast to other polymers with heteroatom pendant groups, e.g., poly(vinyl chloride) (PVC), poly(vinyl acetate), (PVAc) and poly(vinyl alcohol) (PVA), not zip-elimination of 1H-imidazole but homolytic scission of the carbon–nitrogen bond is the main reaction of its formation. 1-Vinylimidazole is formed by main chain scission followed by depolymerization. Both 1H-imidazole and 1-vinylimidazole formation lead in part to macroradicals and short conjugated double bond sequences (polyenes) in the chain, the thermolytic cyclization, isomerization, and aromatization of which result in the low amounts of aromatics. These findings served for the basis of formulating the mechanism of the thermal decomposition of PVIm, which can be utilized in the course of further investigations with this unique polymer.