High molecular weight poly(phthalazinone)s with high glass transition temperatures were prepared by a novel N-C coupling reaction. New bis(phthalazinone) monomers (5a-f), such as 7,7’-oxobis[4-phenylphthalazin- 1(2H)-one] (5c), were synthesized from the corresponding bis(phthalic anhydride)s (3a-f) in two steps. Other types of bis(phthalazinone) monomers (8, 9) were synthesized from phthalic anhydride and diphenyl sulfide or diphenyl ether. AB-type phthalazinone monomers (11a, 12-14) were also synthesized from phthalic anhydride. Poly(phthalazinone)s, having inherent viscosities of 0.3-0.4 dL/g, were prepared by the reaction of the bis(phthalazinone) monomers with an activated aryl halide such as bis(4-fluorophenyl) sulfone (22) in a dipolar aprotic solvent in the presence of potassium carbonate. AB-type phthalazinone monomers do not form high molecular weight polymers because of premature precipitation during the polymerization reaction. One of the AB-type phthalazinone monomers, 4-[4-((4-fluorophenyl)sulfonyl)phenyl]phthalazin-1(2H)-one, was successfully copolymerized with 4,4’-biphenol and bis(4-fluorophenyl) sulfone to form high molecular weight polymers. The copolymers showed higher glass transition temperatures (T(g)s) as the phthalazinone ratio increased. All phthalazinone homopolymers except the Bisphenol A phthalazinone polymers show T(g)s around 300 degrees C by differential scanning calorimetry. The decomposition temperatures for 5% weight losses in air and in nitrogen measured by thermogravimetric analysis are in the range of 480-530 degrees C. Polymers from the Bisphenol A phthalazinone monomer are soluble in chlorinated solvents such as chloroform. Other polymers are soluble in dipolar aprotic solvents such as N-methyl-2-pyrrolidinone. The soluble polymers can be cast into flexible films from solution.