Chiral Coalition in Helical Sense Enhancement of Copolymers: The Role of the Absolute Configuration of Comonomers.

Abstract

Both the role of the absolute configuration and the tendency of a chiral monomer to promote a certain helical scaffold in a poly(phenylacetylene) (PPA) have been evaluated to study the communication between two chiral monomers within a copolymer chain. Nineteen different PPA copolymer series-47 helical copolymers altogether-were prepared to explore the existence of a chiral-to-chiral communication mechanism. From the data obtained, we found that communication between two different chiral monomers emerges when both exhibit two special features: (a) a different conformational composition-one must exist in a single low-energy conformation ("chiral Sergeant") and the other must present two conformers ("chiral Soldier")-and (b) the induction of a similar scaffold in the PPA, either cis-cisoidal or cis-transoidal. In the selected systems, the chiral Soldier includes the 4-ethynylanilide (para position) of either (R)- or (S)-2-methoxy-2-phenylacetic acid pendants characterized by their conformational flexibility (equilibrium between synperiplanar and antiperiplanar conformations). The chiral Sergeant contains the same chiral groups but linked to the backbone in meta position [3-ethynylanilide of (R)- or (S)-2-methoxy-2-phenylacetic acid] and is selected on the basis of its restricted antiperiplanar conformation. Incorporation of a very small amount (1%) of the Sergeant into a chain composed of just the Soldier transforms the originally axially racemic chain into a helix with strong sense preference (either M or P) that is determined by the absolute configuration of the Soldier.