Crystalline-State Polymerization of Diethyl (Z,Z)-2,4-Hexadienedioate via a Radical Chain Reaction Mechanism To Yield an Ultrahigh-Molecular-Weight and Stereoregular Polymer

We have investigated the reaction mechanism of the radical polymerization of diethyl (Z,Z) -2,4-hexadienedioate (EMU) in the crystalline state under UV irradiation to yield a polymer of ultrahigh molecular weight with a highly regulated stereochemical structure. The molecular weight of poly(EMU) was estimated to be much higher than 10 6 by gel permeation chromatography and viscometry, irrespective of the polymerization temperature, in contrast to the polymerization rate, which significantly depended on the temperature. The crystal structural change monitored by X-ray diffraction during the polymerization has evidenced the crystal-to-crystal reaction process yielding the poly(EMU) crystals. The results of X-ray diffraction and NMR spectroscopy confirmed that the propagation proceeded under a crystal-lattice control, leading to the meso -diisotactic- trans -2,5-polymer as a completely regulated structure. The ESR measurement has revealed the radical chain mechanism and the formation of a long-lived propagating radical during the polymerization. In the initiation step of the polymerization, it is supposed that the radical formation is induced not only photochemically but also via a spontaneous process because of the crystal-lattice strain.