Counterion-Induced UCST for Polycations

Quantifying Hydrophilicity in Polyelectrolytes and Polyzwitterions

The affinity of charged polymers for water is of central interest in polyelectrolyte science. Hydration controls the solution properties of polyelectrolytes as well as their performance in materials having a balance of positive and negative repeat units, such as polyelectrolyte complexes, PECs, and polyzwitterions, PZs. As with neutral polymers, a ranking of water affinity, loosely termed hydrophilicity, is often sought. Apart from the solubility in water, there are few methods for determining relative hydrophilicity. The scaling exponent of size with molecular weight provides, for polymers in general, a classical measure of solvent quality. In this work, using aqueous size exclusion chromatography coupled with static light scattering, the radius of gyration scaling with molecular weight was determined for a range of cationic and anionic polyelectrolytes and for some polyzwitterions. For a more definitive comparison of hydrophilicity, solution calorimetry was used to measure the enthalpy of solution, ΔH sol, when rigorously dried samples of these polymers were dissolved in aqueous 0.1 M NaCl. All polymers yielded strongly exothermic ΔH sol, which provided a ranking of hydrophilicity. The first four molecules of water appear to generate almost all of the heat. Methacryl versions of polymers were more hydrophilic, as ΔH sol was 3-5 kJ mol-1 more exothermic than the nonmethacryl polymer. Polyzwitterions were shown to be strongly hydrated, consistent with the proposed mechanisms for their antifouling properties, although water is not necessarily more strongly held for PZs compared to PEs.

Pre- and post-functionalization of thermoresponsive cationic microgels with ionic liquid moieties carrying different counterions

We investigate the effect of different anions on the temperature-dependent solution properties of poly(N-vinylcaprolactam) microgels carrying alkylated ionic liquid vinylimidazolium moieties synthesized by a pre- and post-functionalization approach.

Morphological transitions of cationic PISA particles by salt, triflate ions and temperature; comparison of three polycations

Three strong polycation stabilizers, poly((vinylbenzyl) trimethylammonium chloride), PVBTMAC, poly((2-(methacryloyloxy)ethyl)trimethylammonium chloride), PMOTAC, and poly((3-acrylamidopropyl) trimethylammonium chloride), PAMPTMAC have been synthesized with reversible addition-fragmentation chain transfer, RAFT, reactions. Solubilities of the polycations...

Effects of temperature on chaotropic anion-induced shape transitions of star molecular bottlebrushes with heterografted poly(ethylene oxide) and poly(N,N-dialkylaminoethyl methacrylate) side chains in acidic water

This article reports a study of the effects of temperature on chaotropic anion (CA)-induced star-globule shape transitions in acidic water of three-arm star bottlebrushes composed of heterografted poly(ethylene oxide) (PEO) and either poly(2-(N,N-dimethylamino)ethyl methacrylate) (PDMAEMA) or poly(2-(N,N-diethylamino)ethyl methacrylate) (PDEAEMA) (the brushes denoted as SMB-11 and -22, respectively). The brush polymers were synthesized by grafting alkyne-end-functionalized PEO and PDMAEMA or PDEAEMA onto an azide-bearing three-arm star backbone polymer using the copper(i)-catalyzed alkyne-azide cycloaddition reaction. Six anions were studied for their effects on the conformations of SMB-11 and -22 in acidic water: super CAs [Fe(CN)6]3- and [Fe(CN)6]4-, moderate CAs PF6- and ClO4-, weak CA I-, and for comparison, kosmotropic anion SO42-. At 25 °C, the addition of super and moderate CAs induced shape transitions of SMB-11 and -22 in pH 4.50 water from a starlike to a collapsed globular state stabilized by PEO side chains, which was driven by the ion pairing of protonated tertiary amine groups with CAs and the chaotropic effect. The shape changes occurred at much lower salt concentrations for super CAs than moderate CAs. Upon heating from near room temperature to 70 °C, the super CA-collapsed brushes remained in the globular state, whereas the moderate CA-collapsed brushes underwent reversible globule-to-star shape transitions. The transition temperature increased with increasing salt concentration and was found to be higher for SMB-22 at the same salt concentration, presumably caused by the chaotropic effect. In contrast, I- and SO42- had small effects on the conformations of SMB-11 and -22 at 25 °C in the studied salt concentration range, and only small and gradual size variations were observed upon heating to 70 °C. The results reported here may have potential uses in the design of stimuli-responsive systems for substance encapsulation and release.

Switchable Self-Assembly of Elastin- and Resilin-Based Block Copolypeptides with Converse Phase Transition Behaviors

Self-assembly of thermally responsive polypeptides into unique nanostructures offers intriguing attributes including dynamic physical dimensions, biocompatibility, and biodegradability for the smart bio-nanomaterials. As elastin-based polypeptide (EBP) fusion proteins with lower critical solution temperature (LCST) are studied as drug delivery systems, EBP block copolypeptides with the resilin-based polypeptide (RBP) displaying an upper critical solution temperature (UCST) have been of great interest. In this study, we report thermally triggered, dynamic self-assembly of EBP- and RBP-based diblock copolypeptides into switched nanostructures with reversibility under physiological conditions. Molecular DNA clones encoding for the EBP-RBP diblocks at different block length ratios were biosynthesized via recursive directional ligation and overexpressed, followed by nonchromatographic purification by inverse transition cycling. Genetically engineered diblock copolypeptides composed of the EBP with an LCST and the RBP with a UCST showed converse phase transition behaviors with both a distinct LCST and a distinct UCST (LCST < UCST). As temperature increased, three phases of these EBP-RBP diblocks were observed: (1) self-assembled micelles or vesicles below both LCST and UCST, (2) whole aggregates above LCST and below UCST, and (3) reversed micelles above both LCST and UCST. In conclusion, these stimuli-triggered, dynamic protein-based nanostructures are promising for advanced drug delivery systems, regenerative medicine, and biomedical nanotechnology.

CE and asymmetrical flow-field flow fractionation studies of polymer interactions with surfaces and solutes reveal conformation changes of polymers

Amphiphilic diblock copolymers consisting of a hydrophobic core containing a polymerized ionic liquid and an outer shell composed of poly(N-isoprolylacrylamide) were investigated by capillary electrophoresis and asymmetrical flow-field flow fractionation. The polymerized ionic liquid comprised poly(2-(1-butylimidazolium-3-yl)ethyl methacrylate tetrafluoroborate) with a constant block length (n = 24), while the length of the poly(N-isoprolylacrylamide) block varied (n = 14; 26; 59; 88). Possible adsorption of the block copolymer on the fused silica capillary, due to alterations in the polymeric conformation upon a change in the temperature (25 and 45 °C), was initially studied. For comparison, the effect of temperature on the copolymer conformation/hydrodynamic size was determined with the aid of asymmetrical flow-field flow fractionation and light scattering. To get more information about the hydrophilic/hydrophobic properties of the synthesized block copolymers, they were used as a pseudostationary phase in electrokinetic chromatography for the separation of some model compounds, that is, benzoates and steroids. Of particular interest was to find out whether a change in the length or concentration of the poly(N-isoprolylacrylamide) block would affect the separation of the model compounds. Overall, our results show that capillary electrophoresis and asymmetrical flow-field flow fractionation are suitable methods for characterizing conformational changes of such diblock copolymers.

Unification of lower and upper critical solution temperature phase behavior of globular protein solutions in the presence of multivalent cations

In globular protein systems, upper critical solution temperature (UCST) behavior is common, but lower critical solution temperature (LCST) phase transitions are rare. In addition, the temperature sensitivity of such systems is usually difficult to tune. Here we demonstrate that the charge state of globular proteins in aqueous solutions can alter their temperature-dependent phase behavior. We show a universal way to tune the effective protein interactions and induce both UCST and LCST-type transitions in the system using trivalent salts. We provide a phase diagram identifying LCST and UCST regimes as a function of protein and salt concentrations. We further propose a model based on an entropy-driven cation binding mechanism to explain the experimental observations.

Thermoresponsive behavior of poly[trialkyl-(4-vinylbenzyl)ammonium] based polyelectrolytes in aqueous salt solutions

A systematic method to induce thermoresponsive behavior for polycations with salts from the reversed Hofmeister series is introduced.

Molecular Mass Affects the Phase Separation of Aqueous PEG-Polycation Block Copolymer

Mechanisms of the phase separation and remixing of cationic PEG-containing block copolymers have been investigated in aqueous lithium triflate solutions. The polycation was poly(vinylbenzyl trimethylammonium triflate). We have previously reported on one such block copolymer, which upon cooling of a hot clear solution first underwent phase separation into a turbid colloid and, later, partially cleared again with further cooling. To better understand the balance of various interactions in the solutions/dispersions, a series of polymers with varying DP of the cationic block was synthesized. From one of the polymers, the alkyl end group (a fragment of the chain transfer agent) was removed. The length of the cationic block affected critically the behavior, but the hydrophobic end group had a minimal effect. Polymers with a short cationic block turn cloudy and partially clear again during a temperature decrease, whereas those with a long cationic block phase separate and slowly precipitate and remix only when heated. Phase separation takes place via particle formation, and we suggest different mechanisms for colloidal stabilization of particles composed of short or long chains.

What will happen when thermoresponsive poly(N-isopropylacrylamide) is tethered on poly(ionic liquid)s?

The thermoresponsive ionic liquid diblock copolymer of poly[1-(4-vinylbenzyl)-3-methylimidazolium tetrafluoroborate]-block-poly(N-isopropylacrylamide) (P[VBMI][BF₄]-b-PNIPAM) containing a hydrophilic poly(ionic liquid) block of P[VBMI][BF₄] is prepared by sequential reversible addition-fragmentation chain transfer (RAFT) polymerization. This P[VBMI][BF₄]-b-PNIPAM exhibits an abnormal thermoresponsive phase transition at a temperature above the phase transition temperature (PTT) of the PNIPAM block. For P[VBMI][BF₄]-b-PNIPAM including a short P[VBMI][BF₄] block, its aqueous solution becomes turbid at a temperature above the PTT of the thermoresponsive PNIPAM block, whereas for P[VBMI][BF₄]-b-PNIPAM containing a relatively long P[VBMI][BF₄] block even in the case of a relatively long PNIPAM block, the aqueous solution remains transparent at a temperature far above the PTT of the PNIPAM block, although a soluble-to-insoluble phase transition of the PINIPAM block is confirmed by dynamic light scattering (DLS) analysis and variable temperature ¹H NMR analysis. The reason that P[VBMI][BF₄]-b-PNIPAM exhibits an abnormal thermoresponse is discussed and ascribed to the highly hydrophilic and charged poly(ionic liquid) block of P[VBMI][BF₄] leading to the formation of small-sized micelles at a temperature above the PTT.

Thermo- and oxidation-responsive homopolypeptide: synthesis, stimuli-responsive property and antimicrobial activity

We present the design and synthesis of a side-chain functional polypeptide with a thioether spacer that is readily decorated to produce thermo- and oxidation-responsive polymers with antimicrobial activity.

Poly[oligo(2-ethyl-2-oxazoline)acrylate]-Based Poly(ionic liquid) Random Copolymers with Coexistent and Tunable Lower Critical Solution Temperature- and Upper Critical Solution Temperature-Type Phase Transitions

The synthesis of a series of dual thermosensitive nonionic-ionic random copolymers with varying compositions by reversible addition-fragmentation chain transfer polymerization is described. These copolymers contain oligo(2-ethyl-2-oxazoline)acrylate (OEtOxA) and either triphenyl-4-vinylbenzylphosphonium chloride ([VBTP][Cl]) or 3- n-butyl-1-vinylimidazolium bromide ([VBuIm][Br]) ionic liquid (IL) units. The copolymers having low content of ionic poly(ionic liquid) (PIL) (P[VBTP][Cl]/P[VBuIm][Br]) segments show only lower critical solution temperature (LCST)-type phase transition with almost linear increase of their cloud points with increasing percentage of ionic PIL segments. Furthermore, LCST-type cloud points ( T_cLs) are found very sensitive and tunable with respect to the nature and concentration of halide ions (X^- = Cl^-, Br^-, and I^-) and copolymer compositions. However, copolymers with high content of ionic PIL segments show both LCST-type followed by upper critical solution temperature (UCST)-type phase transitions in the presence of halide ions. Dual LCST- and UCST-type phase behaviors are prominent and repeatable for many heating/cooling cycles. Both types of cloud points are found to be sensitive to copolymer compositions, concentration, and nature and concentration of the halide ions. The phase behaviors of both types of copolymers with a very high ionic content (>90%) are exactly similar to that of P[VBTP][Cl] or P[VBuIm][Br] homopolymers showing only UCST-type phase transition in the presence of halide ions. The inherent biocompatibility of the P(OEtOxA) segment along with the interesting dual thermoresponsiveness makes these copolymers highly suitable candidates for biomedical applications including drug delivery.

From LCST to UCST: the phase separation behaviour of thermo-responsive polysiloxanes with the solubility parameters of solvents

Thermo-responsive polysiloxanes with tunable LCST- and UCST-type phase separation in mixed solvents were synthesised via a facile, highly efficient, catalyst-free aza-Michael addition of poly(aminopropylmethylsiloxane) to N-isopropylacrylamide.

Thermo and pH dual responsive polypeptides derived from “clickable” poly(γ-3-methylthiopropyl-l-glutamate)

Thermo and pH dual responsive polypeptides bearing sulfonium linkages and ammonium pendants were prepared from “clickable” thioether-containing polypeptides.

How to manipulate the upper critical solution temperature (UCST)?

In this mini-review, we discuss multi-stimuli-responsive polymers, which exhibit upper critical solution temperature (UCST) behavior mainly in aqueous solutions, and focus on examples where counter ions, electricity, light, or pH influence the thermoresponsiveness of these polymers.

Preparation and UCST-Type Phase Behaviours of Poly(γ-4-methylbenzyl-L-glutamate) Pyridinium Tetrafluoroborate Conjugates in Methanol or Water

Polymers with ionic liquid (IL) moieties can undergo thermally induced solution phase transitions by adjusting the balance of hydrophilicity and hydrophobicity between the cations and anions of the IL moieties, thus making them attractive candidates towards various applications such as separation, sensing, and biomedicine. In the present study, a series of poly(γ-4-methylbenzyl-l-glutamate) pyridinium tetrafluoroborate conjugates (P1–P4) containing various pyridinium moieties (i.e. pyridinium, 2-methylpyridinium, 3-methylpyridinium, and 4-methylpyridinium) were prepared by nucleophilic substitution between poly(γ-4-chloromethylbenzyl-l-glutamate) and pyridine or methylpyridines with different substituent positions, followed by ion-exchange reaction in the presence of NaBF4. 1H NMR spectroscopy and Fourier transform infrared spectroscopy analyses confirmed the molecular structures of P1–P4. 1H NMR analysis additionally revealed that P1–P4 showed high grafting efficiency in the range of 93–97 %. P1, P3, and P4 exhibited reversible UCST-type phase behaviours in both methanol and water, whereas P2 showed a reversible UCST-type phase behaviour in water only. Variable-temperature UV-visible spectroscopy was used to characterize the solution phase behaviours and UCST-type phase transition temperature (Tpt) values of P1–P4, which were in the range of 24.9–37.2°C in methanol (3 mg mL–1) and 40.9–55.7°C in water (10 mg mL–1). Tpt decreased significantly with decreasing polymer concentrations.

Tunable doubly responsive UCST-type phosphonium poly(ionic liquid): a thermosensitive dispersant for carbon nanotubes

Poly(triphenyl-4-vinylbenzylphosphonium chloride) synthesized via RAFT polymerization exhibits both tunable halide ion- and thermo-responsiveness (UCST-type) in aqueous solution and acts as a thermosensitive stabilizer for carbon nanotubes.

Synthesis and UCST-type phase behavior of α-helical polypeptides with Y-shaped and imidazolium pendants

UCST-type thermoresponsive polypeptides with Y-shaped and ionic liquid pendants were synthesized by a multi-step post-polymerization method.

Unexpected LCST-type phase behaviour of a poly(vinyl thiazolium) polymer in acetone

A poly(vinyl thiazolium) polymer in acetone solution exhibited an unexpected lower critical solution temperature (LCST)-type phase transition.

Poly(3-imidazolyl-2-hydroxypropyl methacrylate) – a new polymer with a tunable upper critical solution temperature in water

A novel imidazole-bearing polymer is synthesized and its solubility in water increases as the solution temperature rises or pH increases.