Abstract
Nonionic-zwitterionic diblock copolymers are designed to feature a coil-to-globule collapse transition with an upper critical solution temperature (UCST) in aqueous media, including physiological saline solution. The block copolymers that combine presumably highly biocompatible blocks are synthesized by chain extension of a poly(ethylene glycol) (PEG) macroinitiator via atom transfer radical polymerization (ATRP) of sulfobetaine and sulfabetaine methacrylates. Their thermoresponsive behavior is studied by variable temperature turbidimetry and 1H NMR spectroscopy. While the polymers with polysulfobetaine blocks exhibit phase transitions in the physiologically interesting window of 30–50 °C only in pure aqueous solution, the polymers bearing polysulfabetaine blocks enabled phase transitions only in physiological saline solution. By copolymerizing a pair of structurally closely related sulfo- and sulfabetaine monomers, thermoresponsive behavior can be implemented in aqueous solutions of both low and high salinity. Surprisingly, the presence of the PEG blocks can affect the UCST-transitions of the polyzwitterions notably. In specific cases, this results in “schizophrenic” thermoresponsive behavior displaying simultaneously an UCST and an LCST (lower critical solution temperature) transition. Exploratory experiments on the UCST-transition triggered the encapsulation and release of various solvatochromic fluorescent dyes as model “cargos” failed, apparently due to the poor affinity even of charged organic compounds to the collapsed state of the polyzwitterions.
Highlights
Amphiphilic block copolymers are generally considered to behave as “macrosurfactants” [1,2,3,4,5].Despite their structural similarity to standard surfactants, they feature a distinct property profile with characteristic differences to their low molar mass counterparts
The nonionic-zwitterionic block copolymers were prepared by ARGET atom transfer radical polymerization (ATRP) polymerization of zwitterionic methacrylates (Figure S1), and characterized by standard methods such as 1 H NMR and Size exclusion chromatography (SEC)
NMR spectra, which show the presence of both constitutional repeat units in the product (Figure 2b–d). This is corroborated by the SEC elugrams, which show that the molar mass distributions shifted to longer elution times compared to the macroinitiator employed, without an indication of any remaining unreacted macroinitiator
Summary
Amphiphilic block copolymers are generally considered to behave as “macrosurfactants” [1,2,3,4,5].Despite their structural similarity to standard surfactants, they feature a distinct property profile with characteristic differences to their low molar mass counterparts. Polymers 2018, 10, x FOR PEER REVIEW copolymer is dissolved in a non-selective organic solvent that is water-miscible, the polymer with an excess of water, and the solvent is removed, e.g., by evaporation or by dialysis [1,4,6,11,12] To circumvent such a cumbersome procedure, “smart” macrosurfactants may be used, in which the hydrophobic block is designed to be stimulus-responsive and can such a cumbersome procedure, “smart” macrosurfactants may be used, in which the hydrophobic reversibly change its character from hydrophilic to hydrophobic upon exposure to a trigger block is designed to be stimulus-responsive and can reversibly change its character from hydrophilic [8,13,14,15]. Enables control of the system’s viscosity [16,17,18,19], as well as the transport and controlled delivery of such a responsive aggregation behavior enables control of the system’s viscosity [16,17,18,19], as well as the poorly water-soluble active agents [20,21,22] (Scheme 1)
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