Abstract
Solid poly(4,4′-oxydiphenylenepyromellitamic acid) was treated in benzene by a mixture of pyridine and acetic and/or trifluoroacetic anhydride. The kinetics of transformation of polyamic acid into a copolymer containing imide and isoimide rings was studied by IR spectroscopy. Imide groups are predominantly formed in the presence of acetic anhydride whereas isoimide structures are favoured in the presence of trifluoroacetic anhydride. The effects of a different content of pyridine and of mixtures consisting of acetic and trifluoroacetic anhydride in the reaction medium on the composition of the resulting copolymers was investigated. The experimental results have been explained using quantum chemical calculations of reaction pathways in the cyclization of amic acids. The preferred formation of either imide or isoimide rings has been explained by the ability of different reaction media to favour deprotonation of the amide group or protonation of carboxyls in polyamic acids.
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