Polyimide synthesis high-pressure

A variety of methods are known for the synthesis of polyimides and other condensation polymers, however, the application of high pressure has seldom appeared in the literature to date. Early in 1969 Morgan and Scott reported on the high-pressure polycondensation and simultaneous hot-pressing of intractable polybenzimidazopyrrolone, that is infusible and insoluble, directly from the combination of 3,3f,4,4f-tetraaminobiphenyl and pyromellitic dianhydride (Eq.6) [29,30]. 

Quite recently, we have investigated systematically the high-pressure polycondensation leading to the formation of a variety of polyimides, and polyben-zoxazoles as well [19,33]. Here we applied the salt monomer method to the high-pressure synthesis of aliphatic polyimides. 

Table 1. High-pressure synthesis and thermal behavior of aliphatic polyimides ...

Contrary to the high-pressure polycondensation, when the polycondensation of the salt monomers was conducted in a molten state under atmospheric or reduced pressure for the preparation of the polyimides having Tm below 300°C, this often led to the formation of crosslinked aliphatic polyimides that were insoluble even in concentrated sulfuric acid. Therefore, the high-pressure polycondensation process provides a simple and effective method for the synthesis of the linear polyimides with well-defined structures that caused high crystallinity, compared with the other synthetic methods. 

The high-pressure synthesis of wholly aromatic polyimides directly from salt monomers would be promising in view of the reactive processing involving po-... 

Furthermore, we have extended the high-pressure polycondensation to the synthesis of condensation polymers other than polyimides. A successful example was the high-pressure synthesis of polybenzoxazoles starting from the bis-o-aminophenol (or its hydrochloride) and aliphatic dinitriles (Eq. 8) [35]. The polycondensation readily proceeded under high pressure in a closed reaction vessel with the elimination of the by-product of ammonia (or ammonium chloride). 

The high-pressure polyimide synthesis from salt monomers was extended to the development of a new approach for hybrid materials composed of aliphatic polyimides and silica, wherein this method was combined with the sol-gel process [63,64]. The preparation process is outlined in Scheme 2, where aliphatic polyimide P-9PM was used as the polyimide component. 

In a typical experiment, a monomer or its salt (2 g) in a polar high boiling solvent (1-2 ml) was irradiated under nitrogen atmosphere. The microwave assisted polycondensation proceeded rapidly and was completed within 5 min (inherent viscosity around 0.5 dL/g) for the polyamide synthesis [68] and within 2 min for the polyimides (inherent viscosity above 0.5 dL/g) [69]. The rate of polycondensation of the salt monomers under various conditions decreased in the following series the microwave induced polycondensation>solid-state thermal polymeriza-tion>high-pressure thermal polycondensation [71]. 

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