Assessment of the thermal degradation characteristics of isomeric poly(styrene)s using TG, TG/MS and TG/GC/MS

Atactic poly(styrene) is thermally less stable than would be expected for a purely head-to-tail polymer. To determine whether or not head-to-head units in the polymer might be responsible for this instability the degradation of both a typical atactic head-to-tail poly(styrene) and the corresponding fully head-to-head polymer has been examined using several thermal techniques—all involving thermogravimetry as a basic component. At low temperatures, e.g., 280 °C, the initial degradation is identical for the two polymers. However, while the degradation for the head-to-head polymer is well-behaved over long periods of time that for the head-to-tail polymer undergoes change relatively early in the reaction period. Further, the nature of the volatile fragments produced by degradation at 280 °C is strikingly different for the two polymers. The volume of volatiles formed is much smaller for the head-to-head polymer than for the head-to-tail polymer. Volatile products from decomposition of the head-to-head polymer reflect cleavage of the polymer main chain at head-to-head linkages while decomposition of the head-to-tail polymer smoothly generates only styrene monomer. At higher temperatures, 320 and 350 °C, degradation of the head-to-head polymer is still well-behaved while that for the head-to-tail polymer becomes much more complex. It can be concluded that degradation of the head-to-head polymer reflects scission of head-to-head linkages over a wide range of temperature. In contrast, the nature of the degradation of the head-to-tail polymer is strongly temperature dependent. At low temperatures (<300 °C) the initial degradation event is clearly scission of head-to-head linkages present as a consequence of polymerization termination by radical coupling. The macroradicals thus formed undergo unzipping to evolve styrene monomer. At higher temperatures, the degradation is more complex and involves random chain scission and subsequent transformations as well as head-to-head scission.