We have developed schemes to construct and characterize the microstructure and macroscopic properties of individual grain boundary defects in extended-chain, conjugated polymers. Our approach has been to take [1,6-di(N-carbazolyl)-2,4-hexadiyne] (DCHD) diacetylene monomer crystals and introduce a single defect under specified boundary conditions. Two monomer seed crystals are cut from a precursor single crystal and then brought into close proximity with one another. Monomer bicrystals are created by a recrystallization step involving slow evaporation of a DCHD solution. The monomer bicrystals are then converted into polymer bicrystals through thermal energy or by exposure to high-energy radiation. We have found that the ability to retain a cohesive interface between the crystals after the solid-state reaction is a sensitive function of their relative misorientation and the method of polymerization. In general, small-angle grain boundaries remain intact, while large-angle grain boundaries are broken after polymerization. The geometrical conditions required to obtain a coherent interface are more stringent for radiation than thermal polymerization. The macroscopic properties of the polymer bicrystals are particularly sensitive to the geometry of the interface. The efficiency of photoconductive charge carrier transport across the grain boundary decreases systematically with increasing misorientation between crystals, with the fracture localized to the engineered interface. Our results are consistent with decreasing covalent bond connectivity of the polymer chains across the interface with increasing misorientation angle.