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Oxidative changes in poly[methyl(phenyl)silanediyl] (PMPSi) films exposed in a Weather‐O‐Meter ATLAS Ci 3000+ were monitored by Fourier transform infrared spectroscopy. Increases in absorption in the siloxane, carbonyl, and hydroxyl regions, decreases in absorption in the SiSi region, and changes in the aromatic CH bond region were monitored. Experiments in argon and air indicated the key role of oxygen in the process. The photooxidation of PMPSi was retarded by triazine‐ and oxalanilide‐based ultraviolet absorbers [2‐(4,6‐diphenyl‐1,3,5‐triazin‐2‐yl)‐5‐(hexyloxy)phenol and N‐(4‐tert‐butyl‐2‐ethoxyphenyl)‐N′‐(2‐ethylphenyl)oxalanilide] protecting the polymer by the excited‐state intramolecular proton‐transfer mechanism. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 714–721, 2004

Poly[methyl(phenyl)silanediyl] films were irradiated in ATLAS Weather‐O‐Meter Ci 3000+ for ca 3 h. Changes in the region of siloxane, carbonyl, hydroxyl, and C‐H (aromatic) bands were monitored by FTIR. Main attention was paid to changes in siloxane formation, which could be reduced in the presence of phenolic UV absorber 1 and oxalanilide 3.