A racemic mixture
and two chiral monomers of 2-methyl-1-butyl propiolate,
i.e., <i><b>rac</b></i><b>1</b>, <i><b>R</b></i><b>1</b>, and <i><b>S</b></i><b>1</b>, were stereoregularly polymerized with a catalyst,
[Rh(norbornadiene)Cl]<sub>2</sub>, in methanol at 40 °C to obtain
the corresponding helical racemic and two chiral polymers, <b>P</b><i><b>rac</b></i><b>1</b>, <b>P</b><i><b>R</b></i><b>1</b>, and <b>P</b><i><b>S</b></i><b>1</b>, and a copolymer, <b>P</b><i><b>co</b></i>. The <sup>1</sup>H and <sup>13</sup>C NMR spectra of the racemic and chiral polymers differed, although
the NMR spectra of their monomers were the same. The structures of
the <b>P</b><i><b>co</b></i> copolymers with
different chiral monomer ratios were analyzed using 1D and 2D NMR,
optical rotation, circular dichroism (CD), UV–vis, and computational
methods to elucidate the stereochemical effect of the chiral monomers
together with the polymerization mechanism. The temperature dependence
of <sup>1</sup>H and <sup>13</sup>C NMR spectra in line shape and
intensity indicated that the helical main chain undergoes restricted
rotation around the ester methylene bonds −O–CH<sub>2</sub>– through a three-site jump exchange called an accordion-like
helix oscillation (HELIOS). The energetically preferred structures
of the helical-sense polymers <b>P</b><i><b>R</b></i><b>1</b> or <b>P</b><i><b>S</b></i><b>1</b> were simulated using the MMFF94 program. The dependence
of the NMR spectral line shapes, optical rotations, and calculated
structures on the monomer feed clearly indicated that the copolymers
alternatively incorporate <i><b>R</b></i><b>1</b> and <i><b>S</b></i><b>1</b> to generate one-handed
helical-sense chains. Based on these results, a polymerization mechanism
is proposed, explaining a strictly alternating copolymerization that
yields helical chains
