Linear and hyperbranched poly(ether-ketone)s (PEKs) containing flexible oxyethylene spacers grafted multiwalled carbon nanotube (PEK-g-MWNT) nanocomposites were prepared by direct Friedel-Crafts acylation as the polymer forming and grafting reaction. To achieve the composites, in situ polycondensations of AB monomers 3-(2-phenoxyethoxy)benzoic acid (3-PEBA) and 4-(2-phenoxyethoxy)benzoic acid (4-PEBA), and AB(2) monomer 3,5-bis(2-phenoxyethoxy)benzoic acid (3,5-BPEBA) were carried out in the presence of multiwalled carbon nanotubes (MVVNTs). The reaction conditions, polyphosphoric acid (PPA) with additional phosphorous phentoxide (P2O5) in the temperature range of 110-120 degrees C, were previously optimized. The conditions were used as the polymerization and grafting medium that were indeed benign not to damage MWNTs but strong enough to promote the covalent attachment of PEKs onto the surface of the electron-deficient MWNTs. From scanning electron microscopy (SEM) and transmission electron microscopy studies, the polymers were uniformly grafted onto the MWNTs. The resultant nanocomposites are soluble in most strong acids such as trifluoroacetic acid, methanesulfonic acid, and sulfuric acid. Both isothermal and dynamic TGA studies in air showed that nanocomposites displayed improved thermo-oxidative stability when compared with those of corresponding PEK homopolymers. (C) 2008 Wiley Periodicals, Inc.