Under thermal effect, the end of polymer chain departs, and forms low free radical which has low activity. Then according to the chain reaction mechanism, the polymer loses the monomer one by one. However, the molecular chain doesn't change a lot in a short time. The reaction is shown below. This process is common for polymethymethacrylate (perspex).
CH-C(CH)COOCH-CH-C*(CH)COOCH→CH-C*(CH)COOCH + CH=C(CH)COOCH