Thermal elimination of isobutene from meta and para tert‐di‐butyl pyromellitic acid

Abstract

AbstractAn investigation of the kinetics of the thermal elimination of isobutene from the meta and para isomers of the tert‐butyl diesters of pyromellitic acid has been carried out using Fourier transform‐Raman spectroscopy and mass spectroscopy. These studies indicate that the elimination of the tert‐butyl group occurs at a temperature 26°C lower for the meta isomer than for the para isomer; the maximum rate of elimination occurs at 184°C for the former and at 210°C for the latter. Analysis of the Raman spectra of the compounds indicates that this effect results from the better packing arrangement in the para monomer compared with the meta monomer. Formation of pyromellitic dianhydride in the tert‐butyl diesters of pyromellitic acid occurred only after formation of the pyromellitic acid; thus it occurred at lower temperatures for the meta isomer. When the meta and para tert‐butyl diesters of pyromellitic acid are dissolved at 1% concentration in poly (vinyl acetate), the elimination of isobutene occurs at 173°C for both isomers, indicating that it is the differences in crystal packing which give rise to the 26°C difference in the solid‐state samples. For the meta, para, and 50/50 mixed isomers of the tert‐butyl esters of oxydianiline/pyromellitic dianhydride polyamic acid, the elimination of the tert‐butyl group occurs at the same temperature (177°C). This result indicates that the packing arrangement of the tert‐butyl group is disrupted in the polymer chain, so that intermolecular bonding does not hinder thermal deprotection of the tert‐butyl group from the polymer. © 1992 John Wiley & Sons, Inc.