Macromolecules, Vol.43, No.20, 8353-8362, 2010
Three Mechanisms of Asymmetric Polymerization of Phenylacetylenes Having an L-Amino Ether Residue and Two Hydroxy Groups
Three novel chiral phenylacetylenes having in octyloxyethanolamine residue derived from a L-aminoalcohol and two hydroxymethyl groups were synthesized and polymerized by two achiral catalysts ((nbd)Rh+[eta(6)-(C6H5)B-(C6H5)(3)] and [Rh(nbd)Cl](2)/triethylamine (TEA)) and a chiral catalytic system ([Rh(nbc)Cl](2)/(S)- or (R)-phenylethylamine ((S)- or (R-)-PEA)). All of the resulting polymers showed Cotton effects at wavelengths around 430 nm. This observation indicated that they had an excess of one-handed helical backbones. Positive and negative Cotton effects were observed for the polymers having an L-valinol residue produced by using (S)- and (R)-PEA as it cocatalyst, respectively, although the monomers had the same chirality. The two polymers having in L-aliminol or L-phenylalaninol residues obtained by using (S)- and (R)-PEA as a cocatalyst showed CD absorptions with identical signs. Therefore, we found that the chiral monomer having an L-valinol residue was suitable for both modes of asymmetric polymerization, that is, helix-sense-selective polymerization (HSSP) with the chiral catalytic system and asymmetric-induced polymerization (AIP) with the achiral catalysts. However, the other two monomers having an L-alaninol or L-phenylalaninol residue were not suitable for HSSP because the helix sense could not be controlled by the chirality of PEA. To explain the unexpected behaviors in the asymmetric polymerizations of the two chiral monomers having a chiral bidentate ligand, a novel third mechanism of asymmetric polymerization, that is, self-helix-sense-selective polymerization (SHSSP), is proposed in this Article. This Article discusses the contribution of the three mechanisms (AIP, HSSP, and SHSSP) in asymmetric polymerizations of the three monomers.