High-temperature infrared spectroscopy to 1808C identifies irreversible chemical changes that occur in solid state complexes of 1,2polybutadiene and palladium chloride, prior to the onset of oxidation. The decrease in infrared absorption intensities at 910, 994, 1418 and 1640 cm 21 signifies the consumption of carbon–carbon double bonds in the polymer whose microstructure is 80% 1,2-vinyl. The loss of CyC functionality in the sidegroup at elevated temperatures can be explained by palladium-catalyzed dimerization reactions. Transient experiments between 1008C and 1408C reveal that the reduction in the CyC infrared signal at 1640 cm 21 follows an approximate 1st-order rate law which is consistent with a previous calorimetric study of exothermic kinetics over the same temperature range. Characteristic nthorder chemical reaction time constants for solid films of 1,2-polybutadiene with 4 mol% palladium chloride have been calculated at 1008C, 1258C and 1408C via infrared spectroscopy. These reaction time constants decrease at higher temperature with an apparent Arrhenius activation energy of < 43 kJ/mol. Palladium complexes with cis-polybutadiene reveal that p back-donation of electron density from the metal center into the antibonding orbitals of the alkene group in the main chain shifts the CyC absorption from 1653 to 1543 cm 21. The uncomplexed signal at 1653 cm 21 is insensitive to high-temperature annealing whereas the palladium-p -complexed signal at 1543 cm 21 is severely attenuated after annealing for a few minutes at 1508C. This suggests that the formation of a p -complex is required before hightemperature irreversible chemical reactions. Infrared-determined characteristic nth-order chemical reaction time constants for solid films of cis-polybutadiene with 4 mol% palladium chloride at 1008C, 1258C and 1408C are five-fold longer than the corresponding reaction time constants for 1,2-polybutadiene with 4 mol% PdCl2. This kinetic mismatch is consistent with the trend in reaction rates for mono-substituted vs. di-substituted small-molecule alkenes, and suggests that a mixing strategy is required to compatibilize 1,2-polybutadiene and cispolybutadiene via PdCl2 at high temperatures.
Посилання на статтю:
High-temperature infrared kinetics of transition-metal-catalyzed chemical reactions in solid state complexes of polybutadienes with palladium chloride / Laurence A. Belfiore, Xinzhi Sun, Pronab Das, Joon Y. Lee // Polymer. – 1999. – N 40. – P. 5583–5599.