The thermooxidative degradation and SiOx film-formation mechanism of poly(vinylimidazole-co-vinyltrimethoxysilane)[poly(VT-co-VTS)] an copper was investigated with Fourier transform infrared reflection and absorption spectroscopy (FTIR-RAS). The spectral differences of the copolymers with different coating thicknesses were compared. Thermal degradation of the copolymer was catalyzed by copper in the copolymer film as well as on the copper surface. This catalytic effect of copper was observed regardless of coating thickness. Copper in the copolymer film participated in Cu-containing SiOx film formation during thermal degradation. In addition, the enhanced heat treatment causes the film defects in Cu-containing SiOx film as a result of thermal decomposition. Copper corrosion proceeded through these film defects. However, in a thick-coated sample, copper oxides were first formed through the crack in the barrier film as a result of the residual stress. Copper oxides in the him interacted with the SiOx film to form a Cu-rich phase in the vicinity of film defects and cracks.