Kinetics of the Epoxidation of Butyl Rubber; Development of a High Precision Analytical Method for Unsaturation Measurement

Document Type


Publication Date



A precise, simple and inexpensive method was developed for measuring the unsaturation (U) in butyl rubbers. The method, based on the epoxidation of the double bonds using 3-chloroperoxybenzoic acid, was suggested first by Dreyfuss and Kennedy, Their test was refined based on rate measurements. The accuracy and precision of the new, refined version was found to be very good; 44 data points yielded a standard deviation of 0.0128 mol % around the sample mean U=1.78 mol %. The results agreed very well with 500 MHz 1H NMR data, and were shown to be superior to the traditional iodine index test. The epoxidation was shown to be a second order reaction by the kinetic measurements. The rate constant of the epoxidation was found to be k=287 L/mol · s at 25°C. Rate measurements in the 25–40°C temperature range yielded ΔH*=32 kJ/mol (7.66 kcal/mol) for the activation enthalpy of the epoxidation. The reaction was simulated by a fifth order Runge-Kutta numerical integration with adaptive step-size control, using measured rate constants. Measured values under different conditions showed very good agreement with the simulated rate plots. The traditional qualitative analytical approach was shown to require varying reaction times for varying levels of unsaturation due to a minor side reaction. Understanding the kinetics allowed the determination of unsaturation based on ‘kinetic’ approaches, eliminating the problems associated with the traditional approach. One kinetic approach developed uses two rate measurements at t1/t2=2, and does not require the knowledge of the rate constant. Unsaturation values obtained by this method agreed well with 500 MHz NMR data, but its precision was not satisfactory. The other kinetic approach predicts unsaturation from a single rate measurement. Data generated by this method agreed very well with 500 MHz NMR data, and the precision of this method was found to be equal to that of the ‘traditional’ analytical method discussed in this paper.

Publication Title

Rubber Chemistry and Technology





First Page


Last Page