Chiral and achiral propiolic esters [HCtCCO 2 R, where R ) n-hexyl (HexPr), (CH 2 ) 4 Cl (CBPr), (R)-and (S)-CH 2 CHMeEt (MBPr), (1S,2S,5S)-myrtanyl (MyrtPr), and (1R,2S,5R)-menthyl (MentPr)] were copolymerized in the presence of [(nbd)RhCl] 2 in order to study the conformational properties of poly(propiolic esters). A clear cooperative effect on helical conformation was obtained in the copolymerization of CBPr with MyrtPr. A similar positive nonlinear relationship between the enantiomeric excess of MBPr and the observed chiroptical properties of copolymers was also recognized, which means a relatively long persistence length of the helix of poly(propiolic esters). In the copolymerization of HexPr with MentPr, on the other hand, randomly coiled (co)polymers were obtained when the MentPr content was around 60%. 1 H NMR spectra of poly-(propiolic esters) gave well-resolved two diastereotopic signals, attributed to the R-methylene protons in the side groups, owing to the slow helix-helix transformation on the NMR time scale. The energy barrier for the helix-helix transformation of poly(HexPr) was determined to be more than 18.5 kcal/mol by the variabletemperature NMR technique. The NMR study of the (co)polymers also enabled us to estimate the free energy difference between the helical and randomly coiled states (∆G r for poly(HexPr) ) 1.59 ( 0.16 kcal/mol at 22 °C).

Polymerization of (-)-menthyl propiolate (MtPr) by [(nbd)RhCl]2 and MoOCl4-n-Bu4Sn gave polymers in moderate yields. The 1 H NMR spectrum of the polymer formed with the Rh catalyst (PMtPr-Rh) showed a signal characteristic of the cis olefinic proton, meaning the high stereoregularity (cis) of PMtPr-Rh. In contrast, no clear signal attributable to the cis-olefinic proton was detected in the 1 H NMR spectrum of the polymer produced with the Mo catalyst (PMtPr-Mo), which indicates the geometrically irregular structure of PMtPr-Mo. PMtPr-Rh displayed much larger [R]D and molar ellipticity [θ] than those of PMtPr-Mo, MtPr, and (-)-menthyl acrylate. This leads to conclusions that the main chain of PMtPr-Rh exists in a helical conformation with an excess of one-handed screw sense and that induction of the helical conformation requires the cis-transoidal structure of the main chain. No serious decrease in magnitude of CD effects of PMtPr-Rh was observed upon heating the polymer solution at 110 °C, indicating the high thermal stability of the helix of PMtPr-Rh in solution.

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 °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.

Propiolic esters having various chiral substituents, [HCtCCO2R*, R* ) (S)-(CH2)nCHMeEt (n ) 0-5), (1S,2R,5R)-isomenthyl, and (1S,2S,5S)-myrtanyl], were polymerized with [(nbd)RhCl]2 or MoOCl4-n-Bu4Sn for the purposes of establishing the relationship between the helical conformations of the polymers and the structures of pendant chiral groups. In contrast to the poor stereoregularities of the polymers prepared with MoOCl4-n-Bu4Sn, [(nbd)RhCl]2-catalyzed polymerizations resulted in polymers with high cis contents. These stereoregular polymers with 1-4 alkylene spacers displayed intense CD effects and large optical rotations as well as large band gap energy (ca. 3.2 eV), indicating that they possess restricted main-chain conjugation and exist in helical conformations with an excess of a onehanded screw sense. In contrast, the polymers without alkylene spacers possessed more extended coplanarity of the main chains and exhibited poorer chiroptical properties than those bearing alkylene spacers. The introduction of bulkier substituents to the polymers enhanced the persistence length of the one-handed helix, resulting in an increase in the magnitude of Cotton effects and optical rotations. Polymer stereoregularity remarkably influenced the polymer conformation, and no distinct CD signals were detected for polymers with alkylene spacers if polymerizations were performed with the Mo catalyst.