The non-oxidative thermal degradation mechanism of poly(di-n-propyl-) (PDnPI) and poly(di-iso-propyl itaconate) (PDnPI) were investigated and compared to the structurally similar polymethacrylates. Analogous to the corresponding polymethacrylates, PDnPI thermally decomposes predominantly by depolymerisation, whereas a considerable amount of side chain scission occurs during the thermal degradation of PDiPI. The thus formed carboxyl groups decarboxylate at elevated temperatures forming a carbonaceous residue.