Control of Helix Sense by Composition of Chiral-Achiral Copolymers of N-Propargylbenzamides

• Published in: Chemistry : a European journal 2005-06, Vol.11 (12), p.3591-3598
• Main Authors: Tabei, Junichi, Shiotsuki, Masashi, Sato, Takahiro, Sanda, Fumio, Masuda, Toshio
• Format: Article
• Language: English
• Subjects: Benzamides - chemical synthesis; Catalysis; Cotton effect; helical structures; modified Ising model; Molecular Conformation; Molecular Weight; Organometallic Compounds - chemistry; polyacetylenes; polymers; Rhodium - chemistry; Temperature
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200401009

N‐Propargylbenzamides 1–7 were polymerized with (nbd)Rh+[η6‐C6H5B−(C6H5)3] to afford polymers with moderate molecular weights (Mn = 26 000–51 000) in good yields. The 1H NMR spectra demonstrated that the polymers have fairly stereoregular structures (81–88 % cis). The optically active polymers, poly(1) and poly(2), were proven by their intense CD signals and large optical rotations to adopt a stable helical conformation with an excess of one‐handed screw sense when heated in CHCl3 or toluene. The sign of Cotton effect could be controlled by varying the content in the copolymers of either chiral bulky 1 and achiral nonbulky 3, or chiral nonbulky 2 and achiral bulky 7. The smaller the pendant group in the copolymerization of achiral monomers with 1, the more easily did the preferential helical sense change with the copolymer composition. However, the copolymers of chiral nonbulky 2 and achiral nonbulky 3 did not change the helical sense, irrespective of the composition. The free energy differences between the plus and minus helical states, as well as the excess free energy of the helix reversal, of those chiral–achiral random copolymers were estimated by applying a modified Ising model. Controlling the screw direction of a helical copolymer, formed from acetylenic monomers as shown, may in certain cases be possible by altering its composition of monomers.