High-temperature infrared kinetics of transition-metal-catalyzed chemical reactions in solid state complexes of polybutadienes with palladium chloride

High-temperature infrared spectroscopy to 180 degrees C identifies irreversible chemical changes that-occur in solid state complexes of 1,2-polybutadiene and palladium chloride, prior to the onset of oxidation. The decrease in infrared absorption intensities at 910, 994, 1418 and 1640 cm(-1) signifies the consumption of carbon-carbon double bonds in the polymer whose microstructure is 80% 1,2-vinyl. The loss of C=C functionality in the sidegroup at elevated temperatures can be explained by palladium-catalyzed dimerization reactions. Transient experiments between 100 degrees C and 140 degrees C reveal that the reduction in the C=C infrared signal at 1640 cm(-1) follows an approximate Ist-order rate law which is consistent with a previous calorimetric study of exothermic kinetics over the same temperature range. Characteristic nth-order chemical reaction time constants for solid films of 1,2-polybutadiene with 4 mol% palladium chloride have been calculated at 100 degrees C, 125 degrees C and 140 degrees C via infrared spectroscopy. These reaction time constants decrease at higher temperature with an apparent Arrhenius activation energy of approximate to 43 kJ/mol. Palladium complexes with cis-polybutadiene reveal that pi back-donation of electron density from the metal center into the antibonding orbitals of the alkene group in the main chain shifts the C=C absorption from 1653 to 1543 cm(-1). The uncomplexed signal at 1653 cm(-1) is insensitive to high-temperature annealing whereas the palladium-pi-complexed signal at 1543 cm(-1) is severely attenuated after annealing for a few minutes at 150 degrees C. This suggests that the formation of a pi-complex is required before high-temperature irreversible chemical reactions. Infrared-determined characteristic nth-order chemical reaction time constants for solid films of cis-polybutadiene with 4 mol% palladium chloride at 100 degrees C, 125 degrees C and 140 degrees C 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 cis-polybutadiene via PdCl2 at high temperatures. (C) 1999 Elsevier Science Ltd. All rights reserved.