Temperature induced helical contraction and expansion in branched polycarbodiimides and their solvent vapor sensing properties

Abstract

Helical polycarbodiimides with either naphthalene or anthracene side chains display chiroptical switching properties in solution and the solid state. This interesting chiroptical switching occurs due to the expansion and contraction of the helical backbone and the simultaneous reorientation of the aromatic side chains. Herein, we modified the aliphatic side-chain structure to induce the switching behavior in simple phenyl substituted polycarbodiimides. The polymerization of 1-hexynyl-5-phenylcarbodiimide introduced a terminal alkyne appended to the polymer backbone. By using the CLICK reaction, we were able to attach steric bulky branched alkyl groups to poly(N-phenyl)-N′-(n-branched alkyl) carbodiimide (P-1B). To increase the solubility and modulate the aliphatic sidechain packing of the polymer, 1-hexynyl-5-phenylcarbodiimide was copolymerized with phenyl octadecyl carbodiimide in different mole ratios. IR spectra and Specific Optical Rotation (SOR) studies showed these simple phenyl polymers show temperature driven solution state switching between two and sometimes three conformational states akin to what was only previously observed in polymers with polyaromatic side chains. In addition, one of the copolymers, P-2B, synthesized from the 50/50 mole feed ratio of the branched monomer and phenyl octadecyl carbodiimide showed solid-state sensing of various volatile organic compounds (VOCs) and solvent vapor.