Steric hindrance effect on thermo‐responsive behaviors of well‐defined water‐soluble semi‐rigid polymers

Three series of water‐soluble semi‐rigid thermo‐responsive polymers with well‐defined molecular weights based on mesogen‐jacketed liquid crystal polymers, polybis(N‐(2‐hydroxypropyl) pyrrolidone) 2‐vinylterephthalate] P(2‐HPPVTA)], polybis(N‐(1‐methyl‐2‐hydroxyethyl) pyrrolidone) 2‐vinylterephthalate] P(1‐M‐2‐HEPVTA)] and polybis(N‐hydroxypropyl pyrrolidone) 2‐vinylterephthalate] (PHPPVTA) have been synthesized via reversible addition‐fragmentation chain transfer polymerization. The steric hindrance effects on liquid crystalline property and thermo‐responsive behaviors of semi‐rigid water‐soluble polymers (P(2‐HPPVTA), P(1‐M‐2‐HEPVTA), and PHPPVTA) were carefully investigated. From molecular structure, the steric hindrance of P(1‐M‐2‐HEPVTA) is stronger than that of P(2‐HPPVTA). Polarized light microscope and one‐dimensional wide‐angle X‐ray diffraction revealed that both the P(2‐HPPVTA) and P(1‐M‐2‐HEPVTA) display a columnar nematic phase, indicating that the steric hindrance effect do not affect liquid crystalline behavior of the polymers. The dynamic light scattering results demonstrated that P(1‐M‐2‐HEPVTA) exhibited lower cloud point compared with that of P(2‐HPPVTA) at the same mass concentration and the same molecular weight. The more significant molecular weight and concentration dependence on cloud point have been observed in P(2‐HPPVTA) solution than in P(1‐M‐2‐HEPVTA) solution. We also discovered that the cloud points of both P(2‐HPPVTA) and P(1‐M‐2‐HEPVTA) solution are lower in D₂O than in H₂O. It is noted that the cloud point of PM‐2 is 9.9 °C lower in D₂O than in H₂O, much less pronounced than the cloud point difference of PH‐2. The differences of thermo‐responsive behaviors between P(2‐HPPVTA) and P(1‐M‐2‐HEPVTA) were resulted from the steric hindrance effect existed in their side groups. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 3429–3438