A series of new high molecular weight poly( arylene ether)s containing the 1,2-dihydro-4-phenyl (2H)phthalazinone moiety have been synthesized. The inherent viscosities of these polymers are in the range of 0.33-0.64 dL/g. They are amorphous and readily soluble in chloroform, DMF, and DMAc. The glass transition temperatures of the polymers range from 241 to 320 degrees C and the 5% weight loss temperatures in nitrogen atmosphere range from 473 to 517 degrees C. The hydroxy group in the monomer 1,2-dihydro-4-( 4-hydroxyphenyl) (2H)phthalazin-1-one has been selectively transformed into the N,N＇-dimethylthiocarbamate group, which was then rearranged to give the S-(N,N＇-dimethylcarbamate) group via the Newman-Kwart rearrangement reaction. A series of poly( arylene thioether)s containing the 1,2-dihydro-4-phenyl (2H)phthalazinone moiety have also been synthesized via two types of reactions, a N-C coupling reaction and a one-pot reaction between the S-(N,N＇-dimethylcarbamate) and activated dihalo compounds, in diphenyl sulfone in the presence of a cesium carbonate and calcium carbonate mixture. These poly( arylene thioether)s also have high glass transition temperatures (ranging from 217-303 degrees C) and high thermal stabilities. Compared with their poly( ether) analogs, the poly(arylene thioether)s have glass transition temperatures several degrees lower, which is attributed to the more flexible C-S-C bonds.