Microcrystalline films of the diacetylenes 4BCMU and 3BCMU were prepared by vacuum evaporation and polymerized by UV irradiation. A method for determining the absolute polymer content X in films has been developed. In 4BCMU the reaction rate is strongly decreased beyond X similar to 0.3. Existing models fail to fit the kinetics. A numerical model based on quenching of the monomer excited state, the initiation precursor, by energy transfer to a nearby existing chain is developed and quantitatively fits the kinetics. Such quenching may occur in any diacetylene solid, so that one should not assume that an apparent saturation of the absorbance of a film corresponds to complete polymerization. 3BCMU is less reactive and briefly compared to 4BCMU.