Conformational Analysis of Poly(propionaldehyde azine) in Dilute Solution

The conformation of a photoresponsive polymer bearing azo units in the main chain, poly(propionaldehyde azine) (PrAz), in tetrahydrofuran was investigated by viscosity measurements before and after trans-to-cis photoisomerization and also by the rotational isomeric state (RIS) model. Before photoisomerization (i.e., in the case of trans-pPrAz), viscosity data indicated that pPrAz took a more extended conformation than did a diene polymer analog poly(butadiene). The RIS model analysis revealed that the extended conformation of pPrAz arose mainly from the difficulty of gauche conformation around the carbon-carbon bond in the main chain. After trans-to-cis photoisomerization (i.e., in the case of pPrAz consisting of trans and cis units), viscosity data indicated that the photoisomerization led to a slight extension of the pPrAz chain. The RIS model analysis explained that the photoisomerization formed preferentially the anti-cis isomer, resulting in a larger characteristic ratio to avoid severe steric hindrance between the ethyl groups on neighboring main-chain carbon atoms.