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The molecular attributes which govern the formation and structure of ordered phases in polymeric materials are poorly understood. This work studies the relationship between configurational order among dipolar functional groups bound to a semirigid polymer backbone and the nature of the condensed phases of these materials. The investigation first involved the synthesis of a family of macromolecules designed to address this relationship. The parent structures were polyesters possessing a periodic sequence and polar translational symmetry. Formal substitution of hydrogen by the nitrile functional group on the repeating unit of the parent backbone generates the chemical structure of the other members of this family. The extent of configurational order at the nitrile-bearing stereocenter was an important variable addressed in this work.
The synthesis of the polyesters was achieved by direct, self-condensation of their corresponding hydroxyacid monomers. A carbodiimide-based esterification reaction was developed involving mild conditions for bringing about this polymerization. It was found that high molecular weight polymers could be prepared at room temperature and near neutral pH by employing the catalysts synthesized from 4-dimethylaminopyridine and p-toluenesulfonic acid. These conditions were established by studies with small molecule model reactions. Nitrile containing monomers were synthesized with absolute stereocontrol by using Enders asymmetric alkylation chemistry and a method was developed for conversion of the chiral hydrazone to the corresponding nitrile functional group with minimal loss of stereochemical purity.
Physical characterization of phases formed by these new polymers involved thermal analysis, polarized light microscopy, as well as the synthesis and study of small-molecule model compounds. The model compounds had a structure which corresponded essentially to the dimer of the polymer repeat unit. The non-racemic model compound formed only a monotropic liquid crystal (LC) phase, while the racemate produced an enantiotropic LC phase. On cooling through the LC phase, the racemic model compound displayed hindered crystallization behavior relative to its non-racemic analogue.
The configurationally disordered polymer gave a single transition on heating and on cooling as measured by differential scanning calorimetry. The configurationally ordered polymer, on the other hand, showed a double endotherm on heating but only a single transition on cooling. Using evidence obtained from polarized light microscopy on these two polymers and evidence obtained from a molecular weight series of the configurationally disordered polymers, it is suggested that a liquid crystalline phase formed in the case of the disordered polymer while a crystalline phase formed in the case of the configurationally ordered polymer. These findings are consistent with the behavior of the small molecule model compounds and were confirmed by independently conducted electron diffraction studies. Thus, the ordered phases formed by these polymers are dictated by the chemical order and symmetry of their respective backbones.
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