The photoluminesence (PL) quantum efficiency of poly(p-phenylene vinylene) (PPV) varies with preparation and processing. We report the achievement of nearly 100% PL quantum efficiency through design and synthesis of alternating rigid conjugated segments of dimethoxy -PV (three dimethoxy -phenyl and two vinyl units) and flexible nonconjugated segments along the polymer backbone. There is enhanced solubility in desired solvents and increased interchain separation through the incorporation of the methoxy sidegroups, forming poly [1,6-hexanedioxy -2,6-dimethoxy-1,4-phenylene]-1,2-ethenylene-[3,6-dimethoxy-1,4-phenylene]-1,2-ethenylene-[3,5-dimethoxy-1,4-phenylene], or GDBBC. The nearly temperature-independence of the PL decay at different emission wavelengths of the alternating block co-polymer (ABC) shows that the excitons are highly localized due to the blocking by non-conjugated segments as well as due to increased interchain separation due to the methoxy sidegroups. The contrasting temperature- and time- dependent results for the corresponding PV oligomer and PPV, supports the role of reduced exciton migration in achieving high quantum efficiency. The results are compared to the exciton confinement at the interface of light emitting electron and hole transporting polymers, forming an exciplex, for which the luminescence decay is also temperature independent. Electroluminescent devices using GDBBC as the lightemitting layer and incorporating poly[1-phenyl-2-(p-n-carbazolylphenyl) acetylene], (PDPA-Cz), as hole transporting layer, in the structure ITO/PDPACz/GDBBC/Alq/MgAg have ~1% external quantum efficiency with green emission from the GDBBC. EL devices based on exciplex emission are also discussed.