The synthesis of poly(phenylacetylene)s with bulky chiral silyl groups and the thermal stability of their helical conformation

The polymerization of (−)-p-[(tert-butylmethylphenyl)silyl]phenylacetylene (t-BuMePhSi*PA) and (+)-p-[{methyl(α-naphthyl)phenyl}silyl]phenylacetylene (MeNpPhSi*PA) with the [(nbd)RhCl]2Et3N catalyst yielded polymers with very high molecular weights over 2 × 106 in high yields. The optical rotations of the formed poly(t-BuMePhSi*PA) and poly(MeNpPhSi*PA) were as high as −356 and −150° (c = 0.11 g/dL in CHCl3), respectively. The circular dichroism (CD) spectrum of poly(t-BuMePhSi*PA) in CHCl3 exhibited very large molar ellipticities ([θ]) in the UV region: [θ]max = 9.2 × 104 ° · cm2 · dmol−1 at 330 nm and −8.0 × 104 ° · cm2 · dmol−1 at 370 nm. The [θ]max values of poly(MeNpPhSi*PA) were also fairly large: [θ]max = 7.1 × 104 ° · cm2 · dmol−1 at 330 nm and −5.3 × 104 ° · cm2 · dmol−1 at 370 nm. The optical rotations of poly(t-BuMePhSi*PA) and poly(MeNpPhSi*PA), measured in tetrahydrofuran, chloroform, and toluene solutions, were hardly dependent on temperature in the range 22–65 °C. The CD effects of these polymers hardly changed in the temperature range 28–80 °C, either. These results indicate that the helical structures of these polymers are thermally appreciably stable.