Chiral Coalition in Helical Sense Enhancement of Copolymers: The Role of the Absolute Configuration of Comonomers
Both the role of the absolute configuration and the tendency of a chiral monomer to promote a certain helical scaffold in a polyphenylacetylene (PPA) -that depends on the conformational composition at the pendant group-have been evaluated to study the communication between two chiral monomers within a copolymer chain. 19 different PPA copolymer series -47 copolymers altogether-were prepared to explore the existence of a chiral to chiral communication mechanism in helical copolymers. From the
... a obtained, we found that communication between two different chiral monomers within a copolymer chain emerge when the two monomers exhibit two special features: (a) a different conformational composition -one chiral monomer must exist in a single low energy conformation ("chiral Sergeant") and the other must present an equilibrium between two different conformers ("chiral Soldier")-, and (b) both chiral monomers must promote 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)-2methoxy-2-phenylacetic acid [(R)-or (S)-1], pendants characterized by their conformational flexibility once polymerized (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, (R)-or (S)-2] and is selected on the basis of its restricted antiperiplanar conformation. Incorporation of a very small amount (1%) of the Sergeant [either (R)-or (S)-2] to a chain composed just by 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. For instance, both poly-[(R)-1 0.9 -co-(R)-2 0.1 ] and poly-[(R)-1 0.9 -co-(S)-2 0.1 ] adopt M helical structures due to the absolute configuration of the soldier, although the extent of the chiral amplification induced by the Sergeant is different as a consequence of a different communication mechanism between the (R)-Sergeant/(R)-Soldier and the (S)-Sergeant/(R)-Soldier.