Macromolecules, Vol.34, No.5, 1496-1502, 2001
Helix inversion of poly(propiolic esters)
The effects of temperature and solvents on the helical conformation of stereoregular cis-transoidal poly(propiolic esters) [(CH=CCO2R*)(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)(n)CH(CH3)C2H5 (n = 1-4), were only slightly amplified with decreasing temperature from +20 to -50 degreesC, 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)(5)CH(CH3)C2H5 and (S)-(CH2)(2)CH(CH3)(CH2)(3)CH(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 degreesC. 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 degreesC for a copolymer of hexyl with (S)-2-methylbutyl propiolates.