Polymer crystal engineering for control of stereochemical structure of polymers: Stereospecific monomer synthesis and stereospecific solid-state polymerization

We have successfully synthesized diisotactic and disyndiotactic polymers of (Z,Z). and (E,E)-muconates with various benzyl ester groups. Muconic acid was conveniently converted into its corresponding ester derivatives without EZ isomerization when reacted with benzyl bromides in the presence of potassium carbonate in hexamethylphosphoramide (HMPA) at room temperature. Several monomers undergo photopolymerization in the crystalline state to give stereoregular polymers according to the monomer configuration and the ester substituents. X-ray single crystal structure analysis of the monomer and polymer crystals has revealed the process of the topochemical polymerization. In the monomer crystals with 4-alkoxybenzyl groups as the ester substituents, a columnar structure is formed by the alternate stacking of monomer molecules with the aid of weak hydrogen bonds such as CH/pi and CH/O intermolecular interactions. The alternate stacking is appropriate for syndiotactic polymerization, being different from the crystal structures of many other ester monomers which provide a diisotactic polymer due to the translational monomer stacking in a column, as is seen in the crystals of the 4-chloro-, 4-bromo-, and 4-nitro-substituted benzyl esters. Weak and flexible intermolecular interactions provide a variety of crystal structures and different type of molecular stacking leading to the different tacticity of the polymers.