Effect of pH on the Micellar Morphology of Semicrystalline PCL‐b‐PEO Block Copolymers in Aqueous Solution

The effect of pH on the micellar morphology of a semicrystalline poly(?‐caprolactone)‐block‐poly(ethylene oxide) block copolymer (PCL₆₆‐b‐PEO₄₄) in aqueous solution is investigated. Spherical micelles are formed in neutral and acidic solutions. However, addition of alkali to the neutral micellar solution triggers a sphere‐to‐cylinder transformation of the micellar morphology. The micelles are stable in both neutral and acidic solutions, but the size of the micelles becomes gradually larger in the alkali solution. This phenomenon is interpreted in terms of the effect of pH value on the reduced tethering density of the corona in the semicrystalline micelles.     

Electrochromic Response of Nanostructured Poly(3,4‐ethylenedioxythiophene) Films Grown in an Aqueous Micellar Solution

Poly(3,4‐ethylenedioxythiophene) (PEDOT) films have been electropolymerized from an aqueous micellar solution encompassing the monomer (EDOT) and the moieties sodium dodecyl sulfate (SDS) and lithium triflate. The presence of these anionic dopants in the polymer matrix and a doping level of 0.26 have been confirmed by X‐ray photoemission and electron paramagnetic resonance (EPR) spectroscopy. The hydrophobic micellar core encompassing the monomer orchestrates the growth of a uniform homogeneous polymer deposit as electron microscopy and atomic force microscopy studies reveal the film to be composed of a continuous interlinked network of quasi‐spherical grains (50–150 nm in dimensions) and pores alongwith a low surface roughness. The film exhibits a large coloration efficiency of 153 cm² · C^?1 and a transmission modulation of 62% (λ = 632.8 nm), which are manifestations of the open ion‐permeable morphology. The Q_{(inserted/extracted)} ratio ranges between 1.2 and 1.4 when cycled back and forth between the clear and blue states 2 500 times, thereby affirming the suitability of these films for practical electrochromic smart windows.

     

Synthesis of Functional Sugar‐Based Polyurethanes

Three new polyurethanes (PUs) with multiple pendant allyl groups were synthesized from L ‐arabinitol as versatile materials for the preparation of tailor‐made PUs with varying degrees of functionalization. Their allyl functional groups can take part in thiol‐ene reactions to obtain greatly diverse materials. Thus, six new highly functional PUs were prepared by click chemistry to give polymers with NHBoc [NH‐CO‐OC(CH₃)₃], amino, carboxylic, and 1,2‐dihydroxyethyl side groups. The thermal stability of the novel PUs was investigated by thermogravimetric analysis and differential scanning calorimetry. This strategy provides a simple and versatile platform for the design of new materials whose functionality can be easily modified to anchor diverse biologically active molecules.     

Synthesis,Characterization and Behavior in Aqueous Solution of Star‐Shaped Poly(acrylic acid)

Summary: We report the synthesis of star‐shaped poly(acrylic acid) (PAA), with 5, 8, and 21 arms, by atom transfer radical polymerization of tert‐butyl acrylate. We employ the core‐first approach using glucose‐, saccharose‐ and cyclodextrin‐based initiators. Subsequent acidic treatment of poly(tert‐butyl acrylate) (PtBA) leads to star‐shaped poly(acrylic acid) (PAA). Alkaline cleavage of the arms enabled us to determine the initiation site efficiency. The PAA stars and arms were esterified to poly(methyl acrylate) (PMA). Molecular weight determination by means of GPC/viscosity, MALDI‐TOF MS and NMR end‐group determination showed that the initiation site efficiency is close to unity. Results from potentiometric titration of PAA arms and stars show that the apparent pK_a values increase with increasing arm number, which is a direct result of increasing segment density. Osmometry measurements of aqueous solutions of the PAA stars result in osmotic coefficients between 0.05 and 0.38, indicating that most of the counterions are confined within the star. The confinement increases with arm number.

Potentiometric titration curves for stars (PAA₁₀₀)₂₁, (PAA₁₀₀)₈, (PAA₁₀₀)₅, and linear PAA₁₀₀.     

Aggregation Behavior of Polystyrene/Poly(acrylic acid) Heteroarm Star Copolymers in 1,4‐Dioxane and Aqueous Media

Ionic heteroarm star copolymers bearing polystyrene (PS) and poly(acrylic acid) (PAA) arms (PS_nPAA_n) were prepared by quantitative hydrolysis of the poly(tert‐butyl acrylate) (PtBA) arms of the corresponding PS_nPtBA_n star copolymer. The aggregation properties of these copolymers were studied in various solvents. In 1,4‐dioxane PS₁₂PAA₁₂ (with nearly symmetrical PS and PAA arms) forms reverse micelles of low aggregation number (N_agg) and spherical morphology. In an 80 : 20 (v/v) 1,4‐dioxane/water mixture these micelles are transformed to regular micelles with an unexpectedly high N_agg and an elongated rod‐like structure. An abnormal behavior was observed in aqueous solutions of charged PS₂₄PANa₂₄ (with asymmetrical PS and PAA arms, W_PS = 19 wt.‐%) at low concentrations. A non‐equilibrium physical gel is formed, characterized by a very high viscosity and an elastic response upon oscillatory shearing.     

Mesomorphic Sugar‐Coated Polydiacetylene Polymers

The derivatization of a polydiacetylene (PDA) polymer with free hydroxyl groups containing sugars leads to amphiphilicity of the polymer and the emergence of thermotropic liquid crystalline behavior, in addition to the thermochromatic properties. The diacetylene monomers, incorporated with mono‐ and disaccharides, exhibit mono‐ and/or enantiotropic phase behavior in a wide temperature range. Structural characterization and modeling studies reveal a lamellar arrangement and interdigitization of the molecules in the mesophase. The importance of the amphiphilicity as a source to induce mesomorphic behavior in PDA polymers is emphasized.     

Kinetic Investigations of Cu‐Mediated ATRP in Aqueous Solution

Cu‐mediated atom‐transfer radical polymerization (ATRP) is studied via on‐line vis/NIR spectroscopy in an aqueous solution of a monomer‐free model system, with CuBr/2,2′‐bipyridine acting as the catalyst, and 2‐hydroxyethyl 2‐bromoisobutyrate as the initiator, at a pressure of up to 2000 bar. Excess NaBr is added to avoid the water‐assisted dissociation of the Br–Cu(II)/L bond. The activation–deactivation equilibrium constant, K_model, is measured at different compositions of the water–poly(ethylene glycol) dimethylether (PEO) solvent mixture, in which PEO mimics a water‐soluble monomer. K_model increases by about three orders of magnitude in passing from a PEO to a water environment. The change in K_model is essentially due to the effect on the activation rate coefficient. Kinetic analysis of the model system upon variation of NaBr concentration in conjunction with predici simulations shows that the NaBr content has no significant impact on the activation and deactivation rate coefficients and thus on K_model, but on dispersity and on the degree of chain‐end functionality.

     

Double Hydrophilic Block Copolymer Self‐Assembly in Aqueous Solution

Self‐assembly of double hydrophilic block copolymers (DHBCs) in water is an emerging area of research. The self‐assembly process can be derived from aqueous two‐phase systems that are composed of hydrophilic homopolymers at elevated concentration. Consecutively, DHBCs form self‐assembled structures like micelles, vesicles, or particles at high concentrations in water and without the use of external triggers that would change solubility of individual blocks. Careful choice of the two hydrophilic blocks and design of the polymer structure allows formation of self‐assembled structures with high efficiency. The present contribution highlights recent research in the area of DHBC self‐assembly, including the polymer types employed and strategies for crosslinking of the self‐assembled structures. Moreover, an overview of aqueous multiphase systems and theoretical considerations of DHBC self‐assembly are presented, as well as an outlook regarding potential future applications in areas such as the biomedical field.     

Free‐Radical Polymerization of N‐Vinylimidazole and Quaternized Vinylimidazole in Aqueous Solution

Aqueous‐phase free‐radical batch polymerizations of N‐vinylimidazole (NVI) and quaternized N‐vinylimidazole (QVI) are conducted with varying initial monomer and initiator concentrations at 70 and 85 °C. The polymerization rate of NVI is very slow at the natural pH of 9 due to degradative radical addition to monomer. The rates are increased by lowering the pH, wherein the degradative addition to NVI monomer is partially (at pH 4) and completely (at pH 1) hindered, with the polymerization rate matching that of QVI at pH 1. The initial rates of polymerization for both NVI and QVI are independent of temperature. A kinetic model developed in Predici that includes the pH‐dependent side reactions can reasonably represent both QVI and NVI polymerization.

     

Phase Behavior of Vesicle‐Forming Block Copolymers in Aqueous Solutions

The binary phase diagram of amphiphilic poly(ethylene oxide)‐block‐poly(γ‐methyl‐ε‐caprolactone) block copolymers in water is examined for four polymers having the same hydrophilic block length but different hydrophobic block lengths across the whole concentration range. The bulk polymers show no ordered morphology. With increasing water concentration the polymers undergo transitions from lamellar phases to packed vesicles and subsequently all polymers self‐assemble into vesicles in dilute aqueous solutions. Additionally, the largest polymer forms an inverse hexagonal phase, and the smallest polymer self‐aggregates into rod‐like micelles and showed a hexagonal phase.

     

Termination Kinetics of 1‐Vinylpyrrolidin‐2‐one Radical Polymerization in Aqueous Solution

Termination kinetics of 1‐vinylpyrrolidin‐2‐one radical polymerization in aqueous solution has been studied at 40 °C between 20 and 100 wt.‐% VP. The <k_t>/k_p values from laser single‐pulse experiments with microsecond time‐resolved NIR detection of monomer conversion, in conjunction with k_p from literature, yield chain‐length‐averaged termination rate coefficients, <k_t>. Because of better signal‐to‐noise quality, experiments were carried out at 2 000 bar, but also at 1 500, 1 000, and 500 bar, thus allowing for estimates of <k_t> at ambient pressure. The dependence of <k_t> on monomer conversion indicates initial control by segmental diffusion followed by translational diffusion and finally reaction diffusion control. To assist the kinetic studies, viscosities of VP–water mixtures at ambient pressure have been determined.

     

Tunable Dual‐Thermoresponsive Phase Behavior of Zwitterionic Polysulfobetaine Copolymers Containing Poly(N,N‐dimethylaminoethyl methacrylate)‐Grafted Silica Nanoparticles in Aqueous Solution

Novel thermoresponsive copolymers of zwitterionic sulfobetaine methacrylate (SBMA)‐ and N,N‐dimethylaminoethyl methacrylate (DMAEMA)‐grafted silica nanoparticles are prepared via surface‐initiated atom transfer radical polymerization. The phase behavior of these nanoparticles is investigated. The hybrid nanoparticles exhibit tunable phase transition temperature between the upper critical solution temperature (UCST) and the lower critical solution temperature (LCST) in aqueous solution. A high PSBMA content in P(SBMA‐co‐DMAEMA) copolymer in the hybrid nanoparticles leads to aggregation at low temperatures because of dominant electrostatic interactions of ionic pairs, whereas a relatively low PSBMA content in the hybrid nanoparticles results in a phase transition at high temperatures as a result of predominant hydrophobic interactions of the PDMAEMA. Interestingly, hybrid nanoparticles with a suitable molar ratio of SBMA/DMAEMA exhibit both UCST and LCST in aqueous solution, where the water‐insoluble microdomains of hybrid nanoparticles are generated by the PSBMA or PDMAEMA region depending on the temperature. The above dual‐thermoresponsive phase transition is also controllable in salt solution. A facile change in the molar content of copolymer grafted on silica nanoparticles will tune the UCST and LCST readily.

     

Inorganic‐Salt‐Induced Morphological Transformation of Semicrystalline Micelles of PCL‐b‐PEO Block Copolymer in Aqueous Solution

The effect of inorganic salt at concentrations typical of a biological environment on the micellar morphology of semicrystalline PCL‐b‐PEO in aqueous solution is investigated. The salt is introduced either by dialysis of a THF solution against an aqueous solution of the salt or by adding it into an aqueous solution containing preformed micelles. The inorganic salt can induce sphere‐to‐rod or sphere‐to‐lamella transformations of the PCL‐b‐PEO micelles in aqueous solution, depending on the length of the PCL block. The inorganic salt induces “salting‐out” of the PEO block, leading to a decrease in the reduced tethering density of the corona in the micelles.

     

Micelle Behavior of Copolymers Composed of Linear and Hyperbranched Blocks in Aqueous Solution

Multi‐arm poly(THF)‐b‐polyglycerol linear‐hyperbranched copolymers consisting of a linear hydrophobic poly(THF) block and a hyperbranched hydrophilic polyglycerol block were synthesized by ring opening polymerization of tetrahydrofuran with silver trifluoromethane sulfonate as a catalyst and the consecutive polymerization of glycidol. Initiators with different geometries that propagated into different numbers of arms (e.g., 1, 2 or 4) were used to investigate the dependence of the micelle properties of the copolymers on the number of arms. The critical micelle concentration (CMC) first decreased and then increased with increasing numbers of arms showing a minimum CMC at an arm number of 2. A minimum CMC was difficult to obtain using multi‐arm block copolymers composed of linear polymers with the small number of arms investigated in this study. DLS showed that the 4‐arm poly(THF)‐b‐polyglycerol copolymer had a micelle size of approximately 83 nm under aqueous conditions, and the micelle size of the copolymers with different numbers of arms was similar. Unimolecular micelles could eventually be obtained using this system by increasing the number of arms, they are considered an ideal drug delivery carrier due to the concentration‐independent stability in the blood stream.

     

Novel Polymerization of Fluorinated 2‐Vinylcyclopropane in Aqueous Solution via Cyclodextrin Complexes

We report on the first examples of the free radical polymerization of a fluorinated 2‐vinylcyclopropane 1 and its copolymerization with an alkyl 2‐vinylcyclopropane 2 in aqueous solution via their host‐guest complexation with a random methylated β‐cyclodextrin (RAMEB) using a water‐soluble initiator (VA50). Upon polymerization, the dethreaded cyclodextrin remained in water solution, whereas the water‐insoluble polymer precipitated out and was isolated. The polymers prepared exhibited mesophase behavior over a wide range of temperature. Both T_g and T_i of the copolymers increased linearly with the amount of fluorinated co‐units.

Complexation of fluorinated vinylcyclopropane 1 with RAMEB and polymerization in water solution.     

The Effect of NaCl on the Conformational Behavior of Acenaphthylene Labeled Poly(N,N‐diethylacrylamide) in Dilute Aqueous Solution

Summary: The effect of salts on the conformational behavior of acenaphthylene labeled poly(N,N‐diethylacrylamide) (PDEA/ACE) has been investigated in dilute aqueous solution using UV‐vis and fluorescence spectroscopy. It is demonstrated here that the effectiveness of various salts in changing the lower critical solution temperature (T_LCS) of the PDEA/ACE solution followed the Hoffmeister series. The addition of NaCl linearly lowered the T_LCS. In dilute aqueous solution, the PDEA/ACE adopted a loose coil conformation below its T_LCS. The PDEA/ACE adopted a compact globule conformation above the temperature at which the coil to globule transition (CGT) was completed. A fluorescence anisotropy investigation suggested that a molten globule conformation existed during the CGT of the PDEA/ACE solution. In all of the above three conformations, the PDEA/ACE was more compact in the presence of NaCl than in the absence of any salt.

Temperature and NaCl concentration dependence of fluorescence anisotropy of a PDEA/ACE solution. Inset: thermodynamically stable states of PDEA/ACE.     

Chain Transfer to 2‐Mercaptoethanol in Methacrylic Acid Polymerization in Aqueous Solution

The chain‐transfer constant, C_S = k_tr/k_p, of 2‐mercaptoethanol (ME) for methacrylic acid (MAA) polymerization in aqueous solution has been measured at MAA concentrations between 5 and 30 wt% to be 0.12 ± 0.01 at 50 °C. Analysis has been carried out via both the Mayo and the chain‐length distribution (CLD) methods. No change of C_S with monomer concentration is observed. The chain‐transfer rate coefficient, k_tr, thus exhibits the same strong dependence on monomer concentration as the propagation rate coefficient, k_p.     

Rapid Phase Separation in Aqueous Solution of Temperature‐Sensitive Poly(N,N‐diethylacrylamide)

The dynamic phase separation behavior in aqueous solutions of poly(N,N‐diethylacrylamide) (PDEA), which is a representative thermoresponsive polymer, is studied using a laser temperature‐jump technique combined with transient photometry, with which the time constant for phase separation can be determined. Two components of the time constant (τ_fast and τ_slow) are clearly observed irrespective of the polymer concentration or molecular weight of the PDEA. Such a fast phase separation component (τ_fast) has never been observed in poly(N‐isopropylacrylamide) (PNIPAM) systems. Furthermore, the slow phase separation process of PDEA is faster than that in a PNIPAM system. It is shown that the fast phase separation process for PDEA originates from polymer aggregates formed even below the cloud point. The phase separation mechanism of PDEA is similar to that of poly (alkyl vinyl ether), rather than PNIPAM.

     

Structure of Metallo‐Supramolecular Micellar Gels

A small‐angle neutron scattering investigation of metallo‐supramolecular micellar gels built from polystyrene‐block‐poly(tert‐butylacrylate) PS‐b‐PtBA‐tpy (‐tpy: terpyridine) block copolymers and transition metal ions is presented. The influence of the copolymer concentration on the structure and spatial organization of the micelles, as well as the impact of the subsequent addition of metal ions (Fe(II), Ni(II), and Zn(II)) on these characteristics is extensively studied. The experimental scattering cross‐sections are fitted with an analytical formula based on the Percus–Yevick hard‐sphere model and on the Pedersen–Gerstenberg model for the structure and the form factor, respectively. The resulting structural information is correlated with previously performed light scattering and rheology experiments.