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

Abstract

High-temperature infrared spectroscopy to 180°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°C and 140°C reveal that the reduction in the CC infrared signal at 1640 cm −1 follows an approximate 1st-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°C, 125°C and 140°C 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 π 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- π-complexed signal at 1543 cm −1 is severely attenuated after annealing for a few minutes at 150°C. This suggests that the formation of a π-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°C, 125°C and 140°C are five-fold longer than the corresponding reaction time constants for 1,2-polybutadiene with 4 mol% PdCl 2. 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 PdCl 2 at high temperatures.