Recent spectroscopic investigations and model calculations concerning the short chain reaction intermediate (oligodiacetylenes) of the solid state polymerization in diacetylene crystals are discussed. The oligodiacetylenes are best described by diradical (DR), asymetric carbene (AC), dicarbene (DC) and stable oligomer (SO) molecules of different chain lengths. Their structures and properties are investigated in detail by electron spin resonance (ESR) and optical absorption and emission spectroscopy. Theoretical model calculations are performed to describe the spectroscopic features of the oligodiacetylenes. The transition from diradicals to dicarbenes observed at a critical chain length of six repeat units is quantitatively explained using a configuration model. Triplet excition-polaron exitations are shown to be closely related to charged polarons and to dipole and quadrupole excitations relevant for nonlinear optical effects.