Mechanistic interpretation of base-catalyzed depolymerization of polystyrene

The effect of a base catalyst, MgO, on the decomposition of polystyrene was studied through degradation of both a monodisperse polymer (number average molecular weight =50,500 g/mol) and a polystyrene mimic, 1,3,5-triphenylhexane (TPH), to determine the potential of applying base catalysts as an effective means of polymer recycling. The presence of the catalyst increased the decomposition rate of the model compound but decreased the degradation rate of polystyrene as measured by evolution of low molecular weight products. Although the model compound results suggest that the rate of initiation was enhanced in both cases by the addition of the catalyst, this effect is overshadowed for the polymer by a decrease in the 'zip length' during depropagation due to termination reactions facilitated by the catalyst. Due to the small size of the model compound, this effect does not impact its observed conversion since premature termination still affords a quantifiable low molecular weight product. A decrease in the selectivity to styrene monomer in the presence of MgO was observed for both polystyrene and TPH. Reconciliation of our results with those of Zhang et al. [Z. Zhang, T. Hirose, S. Nishio, Y. Morioka, N. Azuma, A. Ueno, H. Ohkita and M. Okada, Industrial and Engineering Chemistry Research, 34 (1995) 4514.] based on differences in the reactor configuration used is discussed.