Optically active polyurethane based on tyrosine: synthesis, characterization and study of hydrogen bonding

Optically active polyurethane (OPU) and racemic polyurethane (RPU) derived from chiral and racemic tyrosine were synthesized using a hydrogen transfer addition polymerization procedure. The structural and optical properties of the polyurethanes were systematically investigated using Fourier transform infrared spectroscopy, 1 H NMR, gel permeation chromatography (GPC), UV-vis spectroscopy, circular dichroism spectroscopy, thermogravimetric analysis (TGA) and X-ray diffraction techniques. In contrast to RPU, OPU possesses a single-handed helical conformation and optical activity owing to the induced asymmetric force field in the chiral monomer. This regular secondary structure facilitates the formation of numerous interchain hydrogen bonds, which increases the crystallinity and thermal stability of OPU. The high-temperature infrared spectroscopy results indicate that the hydrogen bonding collapses before thermal decomposition. In contrast to the intense infrared emissivity property of random coiled RPU, both helical stereostructures and hydrogen bonding contribute to decreasing this emissivity for OPU.