Chemical Imidization Study by Spectroscopic Techniques. 2. Polyamic Acids

Abstract

Chemical imidization of polyamic acid based on 1,5-diaminonaphthalene and a dianhydride was studied by UV−visible and FT-IR ATR techniques under various reaction conditions, such as polyamic acid concentration, solvent condition (dry vs humidity-exposed NMP), catalysts of tertiary amines with different degrees of steric crowding and base strength, and the concentration of the acetic anhydride and pyridine. With acetic anhydride and pyridine, the reaction was very similar to that of the model amic acids, with the simultaneous formation of bisimide groups and bisisoimide groups, followed by isomerization of bisisoimde groups to bisimide groups, which is very sensitive to the dryness of the solvent. The reaction with polymer films supported a similar trend. Chemical imidization seems to proceed by nucleophilic catalysis by tertiary amines, depending on the steric crowding as well as the base strength. Reducing steric crowding in the catalyst facilitates the formation of an acylammonium (or pyridinium) cation. The adjacent positive charge on the nitrogen makes the acyl group more electrophilic, making it easier for the amic acid anion to attack and form the mixed anhydride intermediate. The catalyst can further accelerate the reaction by deprotonating the mixed anhydride if the tertiary amine is not sterically crowded. The anionic mixed anhydride quickly cyclizes to give an imide group and an isoimide group. N-Methylpyrrolidine and triethylenediamine were found to make the reaction at least 50 times faster than pyridine, which is the most commonly used industrial catalyst, without changing the molecular weight and its distribution. This investigation shows that the acidity of the original amic acid and the acid produced during the reaction have a profound impact on the products and their ratio, which can be understood based on the mechanistic scheme proposed for the model amic acid. It is suggested that a careful choice of polyamic acid concentration, dehydrating agent, and catalyst may lead to greater control over the reaction and the polymer properties.

Characterization of Polyimides by Combining Mass Spectrometry and Selective Chemical Reaction

“…It is also observed that, despite its relatively high basicity, TEA does not promote imidization to any significant extent. A similar observation was reported by Kailani et al 25 for TEA-catalyzed imidization of a polyamic acid in N-methylpyrollidone solution. In contrast to the undegraded PAA ( Figure 5B), the MALDI spectrum for degraded PAA ( Figure 5A) clearly proves the identity of the poly(pyromellitic dianhydride-co-4,4′-oxydianiline) amic acid.…”

Kinetic study of low‐temperature imidization of poly(amic acid)s and preparation of colorless, transparent polyimide films

“…32 In another report, E a for chemical imidization of a poly(amic acid) of 4,4 0 -(hexafluoroisopropylidene)diphthalic anhydride and 1,5-diaminonaphthalene was presented to be 12.3 kJ/mol when an approximately 13:1 molar ratio of amine catalyst to PAA repeating unit was used. 28 …”

“…There have been a few reports on low-temperature imidization, but they have mainly focused on the imidization kinetics of fully aromatic poly(amic acid)s. 27,28 In the present work, we prepared transparent polyimide films from partially aliphatic poly(amic acid)s by means of low-temperature imidization and also studied the imidization kinetics.…”

Nucleophilic reactivities of tertiary alkylamines

“…Tertiary alkylamines are generally used as Brønsted bases in organic synthesis 1–2. On the other hand, many reactions are known, where tertiary amines act as nucleophilic catalysts 3–32. 1,4‐Diazabicyclo[2.2.2]octane (DABCO, 1e ) and quinuclidine ( 1f ), for example, are common catalysts in Baylis–Hillman reactions7 and in cyclopropanations of Michael acceptors 8 .…”