Synthesis and characterization of polyimides endcapped with phenylethynylphthalic anhydride

The synthesis of high glass transition temperature (Tg > 300°C), amorphous, soluble, poly‐imide oligomers of controlled molecular weight endcapped with 4‐phenylethynylphthalic anhydride endcapping agent is described. The 4‐phenylethynylphthalic anhydride was employed to afford a higher curing temperature (380–420°C) which widens the processing window compared to unsubstituted acetylene‐endcapped polyimides. The polyimides were synthesized via solution imidization techniques, using the ester‐acid of various dianhydrides and aromatic diamines. A “ one‐pot” procedure utilizing NMP as the solvent and o‐dichlo‐robenzene as the azeotroping agent reproducibly produced fully imidized, but yet soluble wholly aromatic polyimides. Thermally cured samples were prepared with gel contents of up to 98% that displayed good solvent resistance. Glass transition temperatures comparable to high molecular weight linear analogs were produced. These polyimides also show excellent thermal stability as judged by thermogravimetric analysis (TGA). Model phenylethynyl imide compounds were synthesized and used to follow and elucidate the nature of the products formed from the phenylethynyl curing by using high temperature magic‐angle 13C nuclear magnetic resonance (MAS NMR). Preliminary results indicate that the cure reaction can be followed by MAS NMR. However, the nature of the products being formed during the curing process is difficult to determine by the solid‐state MAS NMR alone. Differential scanning calorimetry (DSC) data clearly show that the model system does indeed melt and displays a wide window before the strong cure exotherm is observed. © 1995 John Wiley & Sons, Inc.