The solid-state polymerization of diacetylenes (MDA-PBT-PDA) is studied with a concerted reaction model and the calculation method of EHMO-ASED and EHCO-ASED, where MDA = crystalline molecular diacetylenes, PBT = polybutatrienes, and PDA = polydiacetylenes. As the reaction goes on, the symmetry of frontier orbitals inverts at state PBT, HOCO from C,-antisymmetry to C2-symmetry and LUCO from C2-symmetry to C,-antisymmetry, which means completion of the 1,4-addition. Two necessary conditions must be satisfied for the reaction to take place: 1) the geometric parameters must undergo a series of concerted changes to make the conformation suitable for the intermolecular 1 ,Caddition, which should overcome an energy barrier Eb; 2) the symmetry match between the frontier crystal orbitals of the reactant and the product must be satisfied-electrons of the reactant should be excited from HOCO (C,-antisymmetry) into LUCO (C2-symmetry), which faces an energy gap E,. At state MDA, there is E,(MDA) = 5.6 eV. If MDA and PDA are analyzed according to Woodward-Hoffmann's rules, this reaction would be considered photochemically allowed but thermochemically forbidden. It has been shown that the Eg gradually decreases along the reaction coordinate from state MDA to PBT. At state PBT there is E,(PBT) I 0.1 eV, and the electrons of the reactant can be easily excited 471