Hyperbranched Multiarm Copolymers with a UCST Phase Transition: Topological Effect and the Mechanism

Abstract

A novel thermoresponsive hyperbranched multiarm copolymer with a hydrophobic hyperbranched poly[3-ethyl-3-(hydroxymethyl)oxetane] core and many poly(acrylamide- co-acrylonitrile) (P(AAm- co-AN)) arms was for the first time synthesized through a reversible addition-fragmentation chain-transfer polymerization. These copolymers show reversible, sharp, and controlled temperature-responsive phase transitions at the upper critical solution temperature (UCST) in water and electrolyte solution. It is the first report on the hyperbranched copolymers with a UCST transition. Two series copolymers with variable AN content (series A) and variable arm length (series B) were synthesized to study the influence of molecular structure on the UCST transition. It was found that the UCST of copolymers could be raised by increasing the AN content or decreasing the arm length. Most interestingly, the amplification effect of the hyperbranched topological structure leads to a broad change of the UCST from 33.2 to 65.2 °C with the little change of AN content (5.9%). On the basis of variable temperature nuclear magnetic resonance, dynamic light scattering, and transmission electron microscopy, a UCST transition mechanism, in combination with hydrophilic/hydrophobic balance and multimicelle aggregate (MMA), was proposed. This work enriches the UCST copolymer topology and may extend the knowledge on the structure-activity relationship as well as the mechanism of the UCST polymers.

Advances in thermo-responsive polymers exhibiting upper critical solution temperature (UCST)

“…RAFT polymerization was then performed using Aam and AN as monomer, AIBN as initiator and HBPO-DTBA as RAFT agent (Figure 7). In contrast to the report of Asadujjaman et al [34], the UCST of this novel hyperbranched polymer can be increased by decreasing the arm (chain) length due to its unique molecular topology [39]. Synthesis of an ABA type triblock copolymers using P(AAm-co-AN) as outer blocks or middle block and hydrophilic poly(poly(ethylene glycol) methyl ether methacrylate) (PPEGMMA) and poly (N,N-dimethylacrylamide-co-7-acryloyl-4-methylcoumarin) as middle block and outer block, respectively, have also been reported [40,41].…”

“…Copolymerization of P(AAm-co-AN) with PVP was also explored in order to generate a LbL temperature-responsive films, which assembled in well-defined block copolymer micelles below UCST [38]. Qi et al [39] reported synthesis of first hyperbranched UCST polymer using poly [3-ethyl-3-(hydroxymethyl) oxetane] (HBPO) as core and P(AAm-co-AN) as arm. Hydroxyl end groups of HBPO was first converted into carboxylic acid by attaching maleic anhydride followed by generation of RAFT agent HBPO-DTBA (poly[3-ethyl-3-(hydroxymethyl)oxetane]-dithiobenzoic acid).…”

“…(EHO-[3-ethyl-3-(hydroxymethyl)oxetane], monomer for synthesis of hyperbranched HBPO. Reproduced with permission from[39], Copyright © 2018 American Chemical Society.…”

UCST‐Type Copolymer through the Combination of Water‐Soluble Polyacrylamide and Polycaprolactone‐Like Polyester

“…However, by copolymerizing with hydrophobic polymers, the UCST AAm‐based copolymer can be obtained. It is important to note that copolymers of AAm and non‐polar vinyl monomer such as poly(acrylamide‐ co ‐acrylonitrile) [ 19,28–33 ] and poly(acrylamide‐ co ‐styrene) [ 34 ] were reported to display the UCST behavior in water. Upon cooling below UCST, its primary amide groups which are responsible for the thermally reversible hydrogen bonding tend to be switched from the interaction with water to intramolecular formation leading to self‐aggregation and phase separation from water molecules.…”

Synthesis of block cationic polyacrylamide precursors using an aqueous RAFT dispersion polymerization

“…A similar approach was used for preparation of hyperbranched copolymers with AM and acrylonitrile. 41 Wang et al 14,15 synthesized hyperbranched PAMs using a semi-batch strategy by controlling crosslinker addition. The PAM-based nanogels were obtained via a RAFT dispersion polymerization.…”