Optically active conjugated polymer from solvent chirality transfer polymerization in monoterpenes.

Area of Science:

• Polymer Chemistry
• Chiral Materials
• Spectroscopy

Background:

• Investigating asymmetric polymerization of acetylene monomers is crucial for developing novel chiral materials.
• Chiral solvents can induce optical activity in polymers, but efficiency varies significantly with monomer structure.
• Understanding the relationship between monomer structure, polymerization conditions, and polymer chirality is key.

Purpose of the Study:

• To explore asymmetric polymerization of disubstituted acetylene monomers in chiral monoterpene solvents.
• To compare the optical activity and chiroptical properties of polymers derived from different acetylene monomers.
• To elucidate the structural basis for induced chirality and its stability in the synthesized polymers.

Main Methods:

• Asymmetric polymerization of various disubstituted (DPAs, PHA-pC1) and monosubstituted (PA-pC1) acetylene monomers.
• Utilized chiral monoterpenes (e.g., (-)-α-pinene) as solvents with varying enantiomeric excess (ee).
• Characterized polymer optical activity using Circular Dichroism (CD) spectroscopy and measured gCD values.
• Employed theoretical calculations to model polymer conformation and time-correlated single photon counting for emission analysis.

Main Results:

• 1,2-diphenylacetylene (DPA-pC1) with a para-trimethylsilyl group yielded an optically active polymer with a large Cotton effect, despite lacking a stereogenic center.
• The polymer synthesized in 87% ee (-)-α-pinene showed the strongest CD signal (gCD value at 385 nm: ∼3.2 × 10⁻³).
• Cotton bands exhibited opposite signs in CD signals when using (-)- and (+)-α-pinenes, indicating enantioselective induction.
• Theoretical calculations supported a helical conformation for the cis-cisoid model.
• The chiral polymer displayed a stable CD signal over a wide temperature range (20–80 °C) and prolonged storage.
• Other tested polymers showed no CD signal, highlighting the specific efficacy of DPA-pC1.

Conclusions:

• Disubstituted acetylene monomers, particularly DPA-pC1, can be effectively polymerized asymmetrically in chiral monoterpenes to produce optically active polymers.
• The induced chirality and strong Cotton effect are attributed to the specific monomer structure and helical polymer conformation.
• The synthesized chiral polymer exhibits remarkable thermal and temporal stability of its chiroptical properties.