Quantitative Kinetic Modelling of the Pyrolysis of Polyisobutylene

Above 300 °C, high MW polyisobutylene degrades to more than 60 volatile products. This paper sets out to predict their molecular weights, precise structures and relative yields by quantitatively modelling the detailed pyrolysis mechanisms. A preliminary statistical analysis of the observed oligomeric species shows that random scission is the predominant mechanism producing oligomers and an initial model is set up on this basis. Monomer, dimer, and trimer are however being supplemented by depropagation plus intramolecular transfer (backbiting) mechanisms and the relative yields and structures of the products from these sources are then predicted. All predictions are then combined to obtain a complete model for the structures and the relative yields of all products from the pyrolysis. The model leads to the prediction of 55 pyrolysis products, compared with the 63 products detected experimentally. Also, the predicted yields are similar to those in the experimental chromatogram. Finally, the predicted weight fractions of all 55 products compare well with those observed experimentally. The procedure is useful in that it assesses the relative importance of possible degradation mechanisms and helps to elucidate which structural features contribute towards the thermal instability of the polymer. The approach may also be developed to predict the more complex thermal degradation patterns of oils, which are more difficult to study experimentally.