A new poly(ladder-type pentaphenylene) bearing aryl groups at all bridgehead positions has been prepared. In comparison to previously reported bridged poly(p-phenylene)-type polymers such as polyfluorene and polyindenofluorene, this polymer is optimized for maximum overlap of the blue emission with the sensitivity of the human eye. Furthermore, the introduction of aryl substituents at all the bridgeheads produces a material which shows remarkably stable blue emission in light-emitting diodes and is not subject to oxidative degradation-induced defect formation. In particular, a comparison of the novel fully aryl-substituted polymer with a partially aryl-substituted polymer reveals that the oxidative degradation-induced defect formation can be almost completely eliminated upon aryl substitution.