How to obtain a precipitated powder of Poly-Amic Acid (PAA) based on aromatic diamine and aromatic dianhydride?
It is well known that for several Aromatic Polyimides (PIs) in the course of synthesis of their precursors - Poly-Amic Acids (PAAs) - the product (it can be PAA or even partially or completely imidized PI) precipitates. It depends on chemical nature of initial monomers and the solvent employed for synthesis.
We would greatly appreciate if you could give us several concrete examples of conditions of PAA and or PI synthesis (solvent, Temperature, time) - which result in precipitation of PAA/PI in the course of synthesis.
Dear Sir. Concerning your issue about how to obtain a precipitated powder of Poly-Amic Acid based on aromatic diamine and aromatic dianhydride. The most important method for polyimide synthesis has been the “two-step” method involving a polyamic acid (PAA) precursor, which is subsequently cyclized to form the imide linkage. This route is also known as the “classical” method. Many of the mechanistic pathways involved have been elucidated by a number of researchers over the past forty years. Generally, the polyamic acid is formed when a difunctional amine and a difunctional anhydride mutually react in a polar aprotic solvent. This proceeds by a reversible nucleophilic substitution reaction when the amine attacks one of the carbonyl carbons in the anhydride moiety and displaces a carboxylate functionality, followed by proton transfer (Scheme 2.2.1.1.1). The reactivity and concentration of each of the monomeric units and the nature of the solvent will determine the rate of PAA formation and its stability with regard to other reagents. Some typical dianhydrides and diamines are shown in Figure 2.2.1.1.1, along with their common acronyms. Dianhydride reactivity was found to best correlate with electron affinity (Ea) (Table 2.2.1.1.1),10 and thus, the dianhydride with the greatest electrophilicity, i.e. the highest electron affinity, will react first with a given nucleophile. As shown by Ea values in Table 2.2.1.1.1, PMDA is clearly much more reactive than the other anhydrides. The reason for the relatively high reactivity of PMDA and the range of dianhydride Eas observed is the differences in electronic environment of the anhydride carbonyl groups which can easily be illustrated with BTDA, BPDA, and ODPA. BTDA is bridged by an electron withdrawing carbonyl group. By removing electron density through the pi-orbitals, the anhydride carbons experience a greater positive environment, which facilitate attack by a nucleophile. ODPA on the other hand is linked by an oxygen atom which is able to donate electrons into the ring thereby reducing the anhydride carbons’ affinity for incoming electrons from an attacking nucleophile. In the case of BPDA, where no bridging functionality is present to either withdraw or donate electron density, an intermediate Ea value between BTDA and ODPA is observed. For more details, I think the following below link may help you in your analysis:
https://vtechworks.lib.vt.edu/bitstream/handle/10919/28590/07chapter_2.pdf
Thanks
Dear Prof. Isam Eldin Hussein Elgailani :
Thank you very much for your excellent comments!
Yours very sincerely,
Leonid Vladimirov
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