Basic information on the molecular weights and structures of poly(p-phenylene) PPP, poly(m-phenylene) PMP, poly(thiophene-2,5-diyl) PTh, and related copolymers has been obtained. PPPs prepared by a Ni-catalyzed dehalogenative polycondensation of p-C6H4Br2 with Mg (PPP(Ni)) and an oxidative polymerization of benzene using CuCl2 (PPP(Cu)) became soluble in DMF after nitration. The nitration proceeded essentially without crosslinking, and GPC analysis gave M-n values of 8300 and 6100 for nitrated PPP(Ni) and PPP(Cu), respectively. Light scattering analysis indicated that nitrated PPP(Ni) and PPP(Cu) assumed a rather stiff structure in DMF with rho(v) (degree of depolarization) in the range of 0.12-0.13. PMP and PTh also became soluble in DMF after nitration, and GPC analysis of nitrated PMP and PTh gave M-n values of 5500 and 8500, respectively. Light scattering analysis of nitrated PTh gave a rho(v) of 0.33, which indicated that the polymer had a stiff structure in DMF. Copolymers of p-phenylene (PP) and m-phenylene (MP) were soluble in organic solvents when the content of the PP unit was about 20%, and a soluble fraction with an M-n of 12 100 was obtained. Light scattering analysis indicated that the copolymer assumed an essentially random coil-like structure in THF. To the contrary, copolymers of 2,5-thienylene (2,5-Th) and 2,4-thienylene (2,4-Th) were insoluble in organic solvents presumably due to their essentially linear and stiff structure.