The thermal degradation of poly(oxycarbonylethylene) (poly-β-propiolactone) has been investigated within a temperature range of 180–220°C and under various pressures. The degradation begins with a considerable decrease in molecular weight, followed by an accelerating formation of acrylic acid. The reaction involves the following steps: 1 Random chain scission of polymer chains by typical ester pyrolysis with the formation of fragments one of which containing a carboxylic group the other one a CC-double bond at the chain end (Eq. (1)). 2 Formation of the volatile acrylic acid by scissions at the chain end, the amount of acrylic acid or the weight-loss increasing with the number of fragmentat…