The chiral induction of polycarbodiimides through diastereomer formation

• Main Author: Schlitzer, David Scott
• Language: ENG
• Published: ScholarWorks@UMass Amherst 1998
• Subjects: Polymers|Organic chemistry
• Online Access: https://scholarworks.umass.edu/dissertations/AAI9841921

The cooperativity of polycarbodiimides was probed at several length scales. Each experiment relies on formation of a predominance of a single-handed helix due to diastereomer formation. Polymerization of N-(R)-2,6-dimethylheptyl-$N\sp\prime$-hexyl carbodiimide, 1 ( ($\alpha$) $\sb{365}$ = +7.6, c = 1.67, 20$\sp\circ$C, chloroform), gives poly-1. Poly-1 adopts a kinetically controlled conformation during the polymerization ( ($\alpha$) $\sb{365}$ = 7.5, toluene, 20$\sp\circ$C); this polymer adopts a thermodynamic conformation upon annealing (65$\sp\circ$C, 24h ( ($\alpha$) $\sb{365}$ = $-$159, toluene, 20$\sp\circ$C)) or through the addition of benzoic acid (5 eq. acid per polymer repeat unit) at ambient temperature (ca. 1 h ( ($\alpha$) $\sb{365}$ = $-$144, chloroform, 20$\sp\circ$C)). The polymerization of 1 with its enantiomer (60% ee of 1) displays majority rules behavior; this polymer displays the same optical and conformational properties as a poly-1. The protonation of poly-1 with a chiral acid, camphorsulfonic acid (CSA), shows a diastereomeric interaction between the acid and the polymer. The (R)-CSA shows a more favorable interaction with poly-1 ( ($\alpha$) $\sb{365}$ = $-$300, chloroform, 20$\sp\circ$C) than the (S)-enantiomer ( ($\alpha$) $\sb{365}$ = 24, chloroform, 20$\sp\circ$C). Both acids protonate the polymer to the same extent as evidenced by similar pHs for the two samples (pH = $-$0.8 for both samples in THF). Polycarbodiimides are basic due to the presence of a guanidine-like repeat unit in the polymer backbone. Optically inactive poly(di-n-hexyl carbodiimide) was induced to adopt a predominance of one handedness through chiral protonation of the basic guanidine repeat units with (S)-CSA. At a ratio of 8:1 polymeric repeats:(S)-CSA a strong divergence from linearity occurred giving the polymer a specific rotation greater than 250$\sp\circ$ (365 nm, 20$\sp\circ$C, chloroform). Varying the amount of chiral anion present while holding the total acid concentration fixed demonstrates that the chiral anion concentration is independently responsible for the chiral induction.