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.