The thermal degradation of poly(oxycarbonylethylene) (poly‐β‐propiolactone)

Abstract

AbstractThe 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: 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)). 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 fragmentations (Eq. (2)). Autocatalysed acidic cleavage of the polyester fragments into an α,β‐unsaturated ester and an acid as shown with low molecular model compounds (Eq. (8)). Substitution of the labile α‐hydrogen atoms in poly(oxycarbonylethylene) by methyl groups—such as in poly(oxycarbonyl‐1,1‐dimethylethylene)—reduces the rate of degradation drastically. This observation together with the results obtained in the pyrolysis of the low‐molecular model compounds, ethyl β‐acetoxypropionate and β‐acetoxypropionic acid, support the postulated mechanism.