Thermoresponsive Molecular Brushes with a Rigid-Chain Aromatic Polyester Backbone and Poly-2-alkyl-2-oxazoline Side Chains

Abstract

A new polycondensation aromatic rigid-chain polyester macroinitiator was synthesized and used to graft linear poly-2-ethyl-2-oxazoline as well as poly-2-isopropyl-2-oxazoline by cationic polymerization. The prepared copolymers and the macroinitiator were characterized by NMR, GPC, AFM, turbidimetry, static, and dynamic light scattering. The molar masses of the polyester main chain and the grafted copolymers with poly-2-ethyl-2-oxazoline and poly-2-isopropyl-2-oxazoline side chains were 26,500, 208,000, and 67,900, respectively. The molar masses of the side chains of poly-2-ethyl-2-oxazoline and poly-2-isopropyl-2-oxazoline and their grafting densities were 7400 and 3400 and 0.53 and 0.27, respectively. In chloroform, the copolymers conformation can be considered as a cylinder wormlike chain, the diameter of which depends on the side chain length. In water at low temperatures, the macromolecules of the poly-2-ethyl-2-oxazoline copolymer assume a wormlike conformation because their backbones are well shielded by side chains, whereas the copolymer with short side chains and low grafting density strongly aggregates, which was visualized by AFM. The phase separation temperatures of the copolymers were lower than those of linear analogs of the side chains and decreased with the concentration for both samples. The LCST were estimated to be around 45 °C for the poly-2-ethyl-2-oxazoline graft copolymer, and below 20 °C for the poly-2-isopropyl-2-oxazoline graft copolymer.

Loose Semirigid Aromatic Polyester Bottle Brushes at Poly(2-isopropyl-2-oxazoline) Side Chains of Various Lengths: Behavior in Solutions and Thermoresponsiveness

“…As for brushes 2 and 3, despite the rather large scatter of experimental points, they fit into the general dependence T1(c), demonstrating a tendency for T1 to decrease from 35 to 24 °C with an increase in c within a more than 10fold concentration range. This trend is consistent with the previously obtained dependence for PiPrOx molecular brushes with flexible and rigid main chains [62,65]. In this case, in general, the region of the onset of phase separation T1 for the studied APEO-g-PiPrOx lies above the entire corresponding temperature range 20-25 °C for the phase separation of graft copolymer with a flexible backbone APE8-g-PiPrOx [62].…”

“…On the contrary, molecular brushes with PAlOx side chains and the rigid main chain of aromatic APE (APE r.ch. -g-PAlOx) at a low z and short side chains aggregate into large supramolecular structures, and they are molecularly dispersed only at sufficiently long side chains [65]. In this case, the LCST is affected by the chemical structure of the side chains.…”

“…It has been shown that, in a selective solvent, APE8-g-PiPrOx polymerization-polycondensation molecular brush with short PiPrOx chains and a backbone containing sufficiently long alkylene fragments -(CH2)8in the monomer unit forms unimolecular micelles of a star-like conformation at a low z ~0.5. On the contrary, molecular brushes with PAlOx side chains and the rigid main chain of aromatic APE (APEr.ch.-g-PAlOx) at a low z and short side chains aggregate into large supramolecular structures, and they are molecularly dispersed only at sufficiently long side chains [65]. In this case, the LCST is affected by the chemical structure of the side chains.…”