Mechanisms of thermal oxidation of poly(bisphenol A carbonate)

In the attempt to find evidence on the structure of the species produced in the thermal oxidative degradation of bisphenol A-polycarbonate (PC), two polycarbonate samples, one capped (incompletely) with phenyl groups at both ends (PC I) and the other one capped (incompletely) with tertbutyl phenyl groups at both ends (PC2), were used. The two PC samples were heated at 300 and 350degreesC under atmospheric air for up to 180 min, producing a THF-insoluble gel at the longer heating times. The oxidative process was followed as a function of the exposure time by SEC, H-1 NMR, MALDI-TOF, and SEC/MALDI-TOF techniques. The SEC curves showed extensive degradation, up to the formation of very low molar mass oligomers. Highly valuable structural information on the thermally oxidized PC species was obtained by using MALDI-TOF mass spectrometry. The MALDI-TOF spectra of the thermally oxidized PC 1 and PC2 samples showed the presence of polymer chains containing acetophenone, phenyl-substituted acetone, phenols, benzyl alcohol, and biphenyl terminal groups. Formation of acetophenone and phenol end groups was confirmed by H-1 NMR analysis. The mechanisms accounting for the formation of thermal oxidation products of PC involve the operation of several simultaneous reactions: (i) hydrolysis of carbonate groups of PC to form free bisphenol A end groups; (ii) oxidation of the isopropenyl groups of PC; (iii) oxidative coupling of phenols end groups to form biphenyl groups. The presence of biphenyl units among the thermal oxidation products confirmed the occurrence of cross-linking processes, which is responsible for the formation of the insoluble gel fraction. The MALDI-TOF analysis of the oxidation products of PC2 sample, capped with tert-butyl phenyl groups at both ends, unveiled a specific antioxidant action of these terminal groups, which are able to slow the rate of thermal oxidation of PC2 compared to PC1 sample.