The molecular weight and its distribution of the resulting polymers were investigated during the crystalline-state photopolymerization of diethyl cis,cis-muconate (EMU). EMU crystals were prepared by several methods, recrystallization, milling, freeze drying, and precipitation, to obtain the crystals with various sizes of 10(-6) to 10(-2) m. After crystalline-state photopolymerization via a crystal-to-crystal process, polymer crystals were isolated and characterized by optical microphotography and scanning electron micrography. Molecular weight and its distribution were determined by gel permeation chromatography with 1,1,1,3,3,3-hexafluoro-2-propanol and by intrinsic viscosity in trifluoroacetic acid. It was revealed that the size of the EMU crystals depended on the method used for the crystal preparation, and that the molecular weight of the polymer decreased as the crystal size became small.