Thermolysis of polyethylene hydroperoxides in the melt: 1. Experimental kinetics of hydroperoxide decomposition

Abstract

Polyethylene hydroperoxides have been generated in an open mixer at 150°C. Their thermolysis has been examined in the temperature range 150 to 200°C. The overall kinetics of decomposition could be easily explained by primary thermolysis reactions combined with auto-accelerated decomposition involving the alcohol groups resulting from hydroperoxide decomposition. However, these kinetics do not allow distinguishing between primary first-order and second-order decomposition. Both mechanisms yield the same good fit for the experimental data. The difficulties encountered with the integral approach showed the necessity for different attempts to thermolysis data analysis. The methods consisted in the determination of the rate constants through use of the data for the first stages only of hydroperoxide decomposition. One of these methods consisted in plotting the reciprocal polyethylene hydroperoxide concentration as a function of thermolysis time. Another method consisted in the determination of the first-order decomposition rate constant for the initial stages of hydroperoxide decomposition. These procedures allowed eliminating to a large extent the contribution from auto-accelerated hydroperoxide decomposition. Nonetheless, even with these precautions, the calculated first-order rate constant was increasing with the initial hydroperoxide concentration. Furthermore, a similar increase with the initial hydroperoxide concentration, although on a lower level, occurred if the experiments were carried out in the presence of phenolic antioxidants. This behavior is attributed to hydroperoxide decomposition induced by the free radicals arising from the primary hydroperoxide decomposition reaction. Linear extrapolation of the initial first-order rate constants to zero initial hydroperoxide concentration yields the true first-order rate constant for hydroperoxide thermolysis. The activation energy deduced from the variation of the extrapolated rate constants with temperature is close to 109 kJ mol −1. This value is much too small for monomolecular hydroperoxide decomposition. Hence, primary thermolysis of polyethylene hydroperoxides in the polymer melt is a pseudo-first-order reaction involving the interaction of the hydroperoxide group with a segment of the polymer. Although the data have been generated in the temperature range 150 to 200°C, the reaction is estimated to be dominant over true first-order decomposition up to 300°C. The rate constants for monomolecular decomposition of polyethylene hydroperoxide found in the literature are discussed.