This paper describes work on synthesis of chiral macromolecules as part of a materials chemistry study which seeks to establish links in these systems among molecular structure, three-dimensional molecular organization, and properties. The basic materials chemistry hypothesis driving this work is that chiral forces among stereogenic centers may serve as guiding fields to organize polymers in two or three dimensions. It follows that physical properties could be directly or indirectly controlled by chirality. In order to promote strong forces among chiral centers the chains were designed to have stereogenic carbons substituted by the strongly dipolar cyano group and spaced by 16 atoms along the backbone. The preparative chemistry of chiral macromolecules is challenging given the limited number of polymerization methodologies and the inherent translational periodicity of synthetic polymers. We have synthesized here low symmetry chiral macromolecules in which the only symmetry element retained is polar translation; achiral homologues lacking the nitrile function, the configurationally disordered polymer, and dimeric model compounds were prepared as well. The compounds exhibited crystalline and liquid crystalline phases, and significant differences were observed among homologues through differential scanning calorimetry and optical microscopy. The substitution of nitrile groups every 16 atoms along the polymer backbone, specially with configurational disorder, leads to glassy or less ordered condensed phases. In some polymers when the strongly dipolar stereogenic centers do not have preferred handedness, chains organize into mesophases rather than crystalline structures. This is surprising since the concentration of stereogenic centers is extremely dilute. Using dimeric model compounds, homochiral recognition among stereogenic centers with large dipole moments was identified as an important factor in the assembly of molecules into layered structures. Interestingly, catenation of the dipolar stereogenic centers in polymeric compounds apparently leads to layered structures even when configurational disorder exists along the polymer backbone. This type of structural control could be extremely useful as an approach to tune physical properties of polymeric materials.
ASJC Scopus subject areas
- Colloid and Surface Chemistry