Electron structures of alkyl, alkoxy, and peroxy macroradicals for both polypropylene and polystyrene have been studied with quantum chemical methods. The polystyrene tertiary alkoxy macroradical has full delocalization of free spin on a neighboring aromatic ring. In addition, the spin is quite delocalized. on polystyrene tertiary alkoxy macroradicals and also on both secondary and tertiary peroxy macroradicals. Such strong delocalization does not occur with polypropylene macroradicals, and it is proposed that the inactivation of polystyrene macroradicals caused by spin delocalization is the reason that polystyrene photodegradation rates are slower than those of polypropylene. (C) 2003 Wiley Periodicals, Inc.