Polymerization thermal, reactive polyimide

The thermal polymerization of reactive polyimide oligomers is a critical part of a number of currently important polymers. Both the system in which we are interested, PMR-15, and others like it (LARC-13, HR-600), are useful high temperature resins. They also share the feature that, while the basic structure and chemistry of their imide portions is well defined, the mode of reaction and ultimately the structures that result from their thermally activated end-groups is not clear. Since an understanding of this thermal cure would be an important step towards the improvement of both the cure process and the properties of such systems, we have approached our study of PMR-15 with a focus only on this higher temperature thermal curing process. To this end, we have used small molecule model compounds with pre-formed imide moieties and have concentrated on the chemistry of the norbornenyl end-cap (1). 

Polymerization by Transimidization Reaction. Exchange polymerization via equihbrium reactions is commonly practiced for the preparation of polyesters and polycarbonates. The two-step transimidization polymerization of polyimides was described in an early patent (65). The reaction of pyromellitic diimide with diamines in dipolar solvents resulted in poly(amic amide)s that were thermally converted to the polyimides. High molecular weight polyimides were obtained by employing a more reactive bisimide system (66). The intermediate poly(amic ethylcarboamide) was converted to the polyimide at 240°C. 

Chemical vapor deposition (CVD) is a process whereby a thin solid film is synthesized from the gaseous phase by a chemical reaction. It is this reactive process that distinguishes CVD from physical deposition processes, such as evaporation, sputtering, and sublimation.8 This process is well known and is used to generate inorganic thin films of high purity and quality as well as form polyimides by a step-polymerization process.9-11 Vapor deposition polymerization (VDP) is the method in which the chemical reaction in question is the polymerization of a reactive species generated in the gas phase by thermal (or radiative) activation. 

Cyclotrimerization of polyfunctional aryl acetylenes offers a unique route to a class of highly aromatic polymers of potential value to the micro-electronics industry. These polymers have high thermal stability and improved melt planarization as well as decreased water absorption and dielectric constant, relative to polyimides. Copolymerization of two or more monomers is often necessary to achieve the proper combination of polymer properties. Use of this type of condensation polymerization reaction with monomers of different reactivity can lead to a heterogeneous polymer. Accordingly, the relative rates of cyclotrimerization of six para-substituted aryl acetylenes were determined. These relative rates were found to closely follow both the Hammett values and the spectroscopic constants A h and AfiCp for the para substituents. With this information, production of such heterogeneous materials can be either avoided or controlled. 

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