Effect of End Groups on the Cloud Point Temperature of Aqueous Solutions of Thermoresponsive Polymers: An Inside View by Flory-Huggins Theory

In an effort to gain insight into the origin of the effects of end groups on the cloud point temperature (Tcp) as a function of the polymer molar mass of thermoresponsive polymers with lower critical solution behavior in dilute aqueous solutions, we use the Flory-Huggins (FH) theory amended for end groups. The theory was applied to available experimental data sets of poly(N-isopropylacrylamide) (PNIPAM), poly(4-vinylbenzyl methoxytris(oxyethylene) ether) (PTEGSt), and poly(α-hydro-ω-(4-vinylbenzyl)tetrakis(oxyethylene) ether) (PHTrEGSt). The theory relates the variations in TcpM,ϕcp for different end groups to the effective FH χ parameter of the end groups and explains the qualitative notion that the influence of the end groups is related to the hydrophobicity/hydrophilicity of the end groups relative to that of the so called intrinsic TcpM,ϕcp response of a polymer without end groups. The limits to the applicability of the FH theory are established, and a set of possible theoretical improvements is considered. The ultimate scrutiny of the simple FH theory and suggested improved theories must await the measurement of truly thermodynamic cloud points; the available cloud points are merely estimations of the thermodynamic cloud point, for which the deviation to the true cloud point cannot be established with sufficient accuracy.

Effect of anions on the phase transition temperature of two structurally isomeric polymers: poly(N-isopropylacrylamide) and poly(2-isopropyl-2-oxazoline)

In chaotropic solution, the different lower critical solution temperature (LCST) increments of two structural isomers, namely, poly(N-isopropylacrylamide) (PNIPAAm) and poly(2-isopropyl-2-oxazoline) (PiPOx), is studied.

Precision synthesis for well-defined linear and/or architecturally controlled thermoresponsive poly(N-substituted acrylamide)s

We describe the progress in precision polymerizations of specific kinds of N-alkylacrylamides and N,N-dialkylacrylamides to produce polymers showing thermoresponsive properties in aqueous media, which representatively include the reversible-deactivation radical polymerizations...

Thermoresponsive properties of poly(N-isopropyl,N-methylacrylamide) and its statistical and block copolymers with poly(N,N-dimethylacrylamide) prepared by B(C6F5)3-catalyzed group transfer polymerization

B(C₆F₅)₃-catalyzed GTP synthesis of poly(N-isopropyl,N-methylacrylamide) as a new thermoresponsive material.

Poly( N-isopropylacrylamide)-Based Polymer-Inducing Isothermal Hydrophilic-to-Hydrophobic Phase Transition via Detachment of Hydrophilic Acid-Labile Moiety

The polymerization of N-isopropylacrylamide (NIPAAm) with ionizable monomers results in pH-responsive lower critical solution temperature (LCST) polymer which works in an ionization-dependent manner. However, gradual ionization of the comonomer occurs at a broad pH range due to the electrostatic field generated by the polymers, limiting the extent of LCST shift in response to pH change. Furthermore, excess introduction of comonomer may dull phase transition behavior. Here, we report the development of an ionization-independent LCST polymer that exerts a sharp isothermal hydrophilic-to-hydrophobic phase transition in response to slight pH change. Our polymer has a poly(NIPAAm/2-aminoisoprpylacrylamide (AIPAAm)) (P(NIPAAm/AIPAAm)) backbone that retains the continuous structural similarity of N-alkyl groups for preserving phase transition sensitivity, and primary amine for forming hydrophilic acid-labile 2-propionic-3-methylmaleic (PMM) amide linkage. The PMM moiety improves the polymer's hydrophilicity and drastically increases the LCST. Detachment of the PMM moiety in response to mild acidic condition (pH < 6.8) lowers the LCST to that of original P(NIPAAm/AIPAAm), permitting isothermal pH-responsive phase transition. Utilizing this mechanism, P(NIPAAm/AIPAAm) modified with PMM amide linkage exhibits a sharp hydrophilic-to-hydrophobic transition at a physiological temperature (37 °C) and, strikingly, facilitates interaction with cultured cells. Most importantly, our polymer showed significantly higher accumulation within a solid tumor after systemic injection compared to conventional PNIPAAm, which may be due to its phase transition responding to slightly acidic tumor microenvironment. Thus, this study provides a novel polymer that offers delicate control of LCST and pH-responsiveness suitable for use in even fuzzy biological environments.

Living Radical Polymerization by the RAFT Process—A First Update

This paper provides a first update to the review of living radical polymerization achieved with thiocarbonylthio compounds (ZC(=S)SR) by a mechanism of Reversible Addition–Fragmentation chain Transfer (RAFT) published in June 2005. The time since that publication has witnessed an increased rate of publication on the topic with the appearance of well over 200 papers covering various aspects of RAFT polymerization ranging over reagent synthesis and properties, kinetics, and mechanism of polymerization, novel polymer syntheses, and diverse applications.