One-handed helical double stranded polybisnorbornenesw Hui-Chun Yang, a Sherng-Long Lee, a Chun-hsien Chen,* a Nai-Ti Lin, a Hsiao-Ching Yang, b Bih-Yaw Jin a and Tien-Yau Luh* a Received (in Cambridge, UK) 22nd August 2008, Accepted 18th September 2008 First published as an Advance Article on the web 16th October 2008 DOI: 10.1039/b814672a Helical double stranded polymers incorporated with a covalently bound chiral ferrocene linker are synthesised and characterized by CD spectra and STM images and molecular dynamics simulations. There have been ever burgeoning interests in artificial double stranded oligomers, polymers as well as supramolecular sys- tems to mimic DNA chemistry. 1–7 Chelation to metal ions, 1 complexation through hydrogen-bonding, 2 p–p stacking, 3 or inclusion complexes 4 are typical strategies for the construction of the helical double stranded oligomers. Ladder-like organic polymers with two strands connected by covalent, dative or ionic bonds have been briefly explored. 5–7 Only a handful of double stranded polymers has been identified unambiguously to be helical. 1j,7,8 Stereoselective controls of the chirality of polymers leading to one-handed helical morphology can read- ily be achieved by incorporating a chiral auxiliary. Coopera- tive phenomena with an amplification of chirality have been shown to offer a powerful platform for the formation of helical polymers or supramolecules of single chirality. 9 We recently reported the synthesis and characterisation of a double stranded bisnorbornene polymer 1 by ring opening metathesis polymerisation (ROMP) 10 of the corresponding monomer 2. 7 The characteristics of 1 includes a rapid equilibrium among helical, supercoil and ladder conformations. Preliminary molecular dynamics–molecular mechanics calculations suggest that the helical conformation in 1 might be more strained than the supercoil and ladder structures. The spacing between each monomeric unit in 1 is about 5.5 A ˚ . 7 Electrochemical oxida- tion of 1 and related polymers suggested that all neighbouring monomeric units in these polymers may strongly interact with each other. 11 It is envisaged that incorporation of substitu- ent(s) onto the covalently bound ferrocene linker in 1 may lead to a conformational change so that the morphology of the polymers may be defined due to cooperative interactions between neighbouring monomeric units. Indeed, we recently found that unsymmetrical double stranded polymer 3, where the two polymeric strands are complimentary to each other, has a ladder-like structure, neither a helical nor supercoil structure being observed. 6 Rational design and synthesis of double stranded polymers or supramolecular systems with predictable helicity remain to be a challenge. 12 In this paper, we wish to report a systematic tuning of the chiral substituents in the ferrocene linker leading to morphological changes in the behaviour of double stranded bisnorbornene polymers. In our previous work, we used an ester linkage to connect the ferrocene-carboxylic acid moiety to the 4-aminobenzyl group to construct the double stranded bis-polynorbornene 1. It is note- worthy that the 4-aminobenzyl ester is very labile towards nucleo- philes 13 so that the double stranded 1 can be easily transformed into two equivalents of the corresponding single stranded poly- mer. 7 In order to increase the stability of the double stranded polymer, we swapped the ester group by coupling aminobenzoic acid with ferrocene-methanol derivatives 4. Optically active diols 4 were prepared according to literature procedures. 14 Reactions of acid chloride 5b with 4 afforded the corresponding mono- meric bisnorbornene derivative 6 (eqn (1), [a] D 25 in CHCl 3 (c 0.01 g mL 1 ): R,R-6a 124.2, S,S-6a +123.2, R,R-6b 37.0, S,S-6b +39.0).w ROMP of 6 with Grubbs I catalyst afforded the corresponding double stranded polymers 7 (eqn (1), M n (PDI): R,R-7a 16 300 (1.4), S,S-7a 17 500 (1.5), R,R-7b 16 300 (1.4), S,S-7b 15 000 (1.3)).w The 13 C NMR spectra of 6 and 7 are compared in Fig. 1. It is noteworthy that the olefinic carbons in 6 at d 135 ppm shifted to ca. d 132 ppm as a shoulder in 7 and the weak broad signals at ca. d 37 ppm were characteristic of the absorption of C 7 in 7. 6,7 All other peaks for 7 matched nicely those for 6. Hydrolysis of 7b gave 8b and the corresponding isotactic single stranded polymer 9 which was converted into a methyl ester 10 (eqn (2), M n (PDI): 5 500 (1.2)). 6,15 These results were consistent with the double stranded structure for 7. a Department of Chemistry, National Taiwan University, Taipei, Taiwan 106. E-mail: tyluh@ntu.edu.tw; chhchen@ntu.edu.tw; Fax: +886 2 2364 4971 b Department of Chemistry, Fu Jen Catholic University, Hsinchuang, Taipie, Taiwan 242 w Electronic supplementary information (ESI) available: [DETAILS]. See DOI: 10.1039/b814672a 6158 | Chem. Commun., 2008, 6158–6160 This journal is  c The Royal Society of Chemistry 2008 COMMUNICATION www.rsc.org/chemcomm | ChemComm