Dynamic helical polymers such as poly(phenylacetylene)s (PPAs) are an important source of chiral nanomaterials because their helices can be tuned once they have been prepared, and therefore helical structures with either P or M helical sense or elongated/compressed helical scaffolds become available. In this way, novel materials with intrinsic properties (e.g., chiral sensors, asymmetric synthesis, chiral separations, etc.) connected to their helical structure can be generated.
Recently, we have developed a chiral helical poly(phenylacetylene) that although bears a chiral pendant —the anilide of (R)-a-methoxy-a-phenylacetic acid, (R)-MPA—, produces an equal population of P and M helices in equilibrium and therefore, it can be considered as axially racemic. Nevertheless, this racemic helix can be shifted to the left sense by addition of tiny amounts of monovalent metal ions such as Na+ or Ag+. However, if the metal ion is delivered in the presence of a donor cosolvent such as MeOH or MeCN, or these cosolvents are added once the helical polymer metal complex (HPMC) is formed, the polymer evolves to a right handed helical sense. Thus, using Ag+/MeCN or Na+/MeOH as external stimuli, the originally racemic helix is selectively converted into a P or an M helix. Moreover, when the polymer/metal ratio is appropriate, the metal ion acts also as a crosslinking agent of the polymer chains producing nano-aggregates with precise macroscopic P or M helicity.
In this paper we show that the combination of these concepts with the Sergeants and Soldiers effect allows to extend those results to different copolymers and therefore, to the preparation of a wide scope of macroscopically chiral nanostructures with different functionalities and P or M helical senses.
The mechanistic explanation is based on the addition of Ag+ or Na+ ions that are complexed by the scarce (R)-MPA units in the copolymer (minor component). The Sergeant character acquired by those units command the adoption of either the P or the M helix in the rest of the chain formed by the non-chiral monomers (i.e., the Soldiers), depending on the amount of MeOH or MeCN employed.
Arias S, Bergueiro J, Freire F, Quiñoá E, Riguera R.
Centre for Research in Biological Chemistry and Molecular Materials (CIQUS), University of Santiago de Compostela, Santiago de Compostela, Spain.
Poly(phenylacetylene) (PPA) copolymers containing (R)- or (S)-MPA as minor chiral pendant can be forced to selectively adopt the right- o left-handed helix, in the presence of small amounts of Na(+) or Ag(+) (“Sergeants and Soldiers Effect”) by addition of a donor cosolvent. The helical sense depends exclusively on the chiral monomer/donor cosolvent ratio, and this allows a perfect on/off tuning of the helicity of the copolymer. When the amount of the donor cosolvent is low, the metal ion complex is stabilized by a cation-π interaction, which is selectively cleaved when the amount of cosolvent is higher. Macroscopically chiral nanospheres and nanotubes composed by helical copolymers with P or M helical sense are also described. Our results demonstrate that it is possible to obtain the two enantiomeric helical structures (P and M helicities) and the corresponding nanospheres and nanotubes from a single helical copolymer, by controlled activation/deactivation of the Sergeant and Soldiers Effect with a donor cosolvent.
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