The helix is a ubiquitous motif observed in nature from the molecular to the macroscopic levels. At the molecular level in biological systems, helical motifs include α-helical secondary structures of enzymes and proteins, double helical structure of DNA and RNA, and super coiled triple helixes of collagen. Through complexation, these helical motifs can create super coiled structures. Chemically, when aggregating two complementary helical polymeric chains this complexation is referred to as a stereocomplex. Synthetic polymer examples of stereocomplexes include mixtures of syndiotactic and isotactic PMMA, and mixtures of D- and L-polylactide. Stereocomplexes when formed can improve mechanical and thermal properties. Herein we report the chiral synthesis of poly (N-methyl-N’-(2-methyl-6-isopropylphenyl)carbodiimide, left- (M) and right-hand (P) helices and their self-assembly phenomena upon thermal annealing. These polymerizations are carried out using enantiomerically pure chiral catalysts. The resultant helical polymers show optical activity due to their single-handed structure. Annealing at high temperatures should introduce helix inversions that would racemize the sample. Surprisingly, when heated, their optical rotations never go to zero (racemize). Accompanying the changes in optical rotations, the polymers begin to gel and precipitation can be detected. Likewise, changes to their solid-state morphologies are observed by AFM and SEM imaging. We hypothesize that when racemization begins, stereocomplexation takes place between two complementary strands. To elucidate the properties and structure of this stereocomplex, solid-state 13C NMR, 1H-NMR, DSC, TGA, p-XRD, TM-AFM, SEM, and VCD will be reported.

Molecular screws of polycarbodiimides from helix sense selective polymerization and their complexation

251st National Meeting (2016)

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