Helix Inversion of Poly(propiolic esters)

Abstract

The effects of temperature and solvents on the helical conformation of stereoregular cis−transoidal poly(propiolic esters) [(CHCCO2R*)n] with various chiral side chains, which were prepared with [(nbd)RhCl]2, were investigated using circular dichromism spectroscopy. CD effects of the polymers with short side chains, where R* = (S)-(CH2)nCH(CH3)C2H5 (n = 1−4), were only slightly amplified with decreasing temperature from +20 to −50 °C, and this behavior was independent of the solvents examined (CHCl3, THF, and n-hexane). In contrast, although the temperature dependence of CD effects for polymers with long alkyl pendants [5 and 6, where R* = (S)-(CH2)5CH(CH3)C2H5 and (S)-(CH2)2CH(CH3)(CH2)3CH(CH3)2, respectively] was similar in n-hexane to those for the other polymers, the Cotton effects of 5 and 6 in CHCl3 decreased in intensity with decreasing temperature and inverted in sign between −30 and −40 °C. This CD inversion process was proved to originate from the thermally induced screw-sense inversion, and computational semiempirical calculation suggested that the transitions from M (left-handed) to P (right-handed) and from P to M helices took place for polymers 5 and 6, respectively, with decreasing temperature. The screw-sense inversion driven by the change in solvent composition between n-hexane and CHCl3 was also achieved at +20 °C for a copolymer of hexyl with (S)-2-methylbutyl propiolates.