The First Example of Main-Chain Cyclic Azobenzene Polymers

A novel main-chain azobenzene cyclic polymer, cyclic-PEHPA, has been successfully synthesized by 'click' cyclization of the alpha-alkyne-omega-azido hetero-difunctional linear precursors (linear-PEHPA), which is synthesized by a step-growth polymerization of the 3'-ethynylphenyl[4-hexyl-(2-azido-2-methyl- propionate) phenyl] azobenzene (EHPA). Gel permeation chromatography, and H-1 NMR and FT-IR spectra confirmed the complete transformation of linear-PEHPA into cyclic-PEHPA. With the same molecular weights, the cyclic-PEHPAs are found to have higher glass transition temperatures than the linear-PEHPAs, but almost the same decomposition temperatures. In addition, the obtained cyclic azobenzene polymer with lower molar mass shows a slightly better trans-cis-trans photoisomerization ability than the corresponding linear-PEHPA.