A quantum-chemical study of conformations and electronic structures of poly(m-phenylene) [PMP] and the related polyphenylenes was performed to elucidate the origin of the broken conjugation found in m-phenylene linked conjugated polymers. Potential energy curves of the polymers as a function of both torsion and helical angles were constructed through semiempirical Hartree-Fock band calculations at the Austin model I (AM1) level. It is found that two helical conformations of PMP are possible: one with a helical angle (alpha) of 72 degrees and the other with alpha = 144 degrees. The former is identical with the conformation of an oligomer in the solid state, m-deciphenyl structure. Our calculations predict that both helices are more stable by 2.5 kcal/mol per phenyl ring than the anti-coplanar conformation and that they exhibit absorption peaks at 5.8 eV. The electronic structure of PMP is, however, not affected significantly by increasing the planarity of a PMP chain but affected by copolymerization with other conjugated units. This implies that localization occurs in the m-phenylene ring itself. We examined the electronic structures of PMP and the related copolymers and found that the weak conjugation along the m-phenylene linked conjugated backbone is related to the inherent nodal nature of the frontier molecular orbitals of the unit even in the planar conformation.