Most addition polymers are thought to he formed by head-to-tail addition of monomer units. Previous studies of PVC have confirmed that the structure is head-to-tail, although the possible presence of a small number of head-to-head units cannot be discounted, It has been suggested that it is these head-to-head structures which are responsible for the relative instability of PVC compared to model compounds. By chlorination of polybutadiene under carefully controlled conditions, a model PVC has been obtained which is made up entirely of head-to-head units. A comprehensive study of the thermal degradation of this model head-to-head PVC has been carried out using mainly the techniques of thermal volatilisation analysis.