8 research outputs found

    ABC Triblock Terpolymers with Orthogonally Deprotectable Blocks: Synthesis, Characterization, and Deprotection

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    The development and availability of new protecting groups are vital for all branches of synthetic chemistry. Equally vital is the facile and selective removal of these groups, leaving intact other protecting groups which are to be cleaved later. Herein we demonstrate this concept of orthogonal deprotection by combining three different forms of protected methacrylic acid (MAA) in the same terpolymers. One of these protected monomers was 2-(pyridin-2-yl)­ethyl methacrylate (PyEMA), recently developed by us, whose units can be readily converted to MAA units either under alkaline hydrolysis conditions or thermally. The second monomer was tetrahydro-2<i>H</i>-pyran-2-yl methacrylate (THPMA) which can be cleaved either via acidic hydrolysis or by thermolysis. The third form of protected MAA was benzyl methacrylate (BzMA) which can be subjected to hydrogenolysis. Two ABC triblock terpolymers based on all three of these monomers were successfully synthesizedone by reversible addition–fragmentation chain transfer (RAFT) polymerization and the other by group transfer polymerization (GTP)thus proving the feasibility of these demanding three-stage syntheses via both polymerization methods. The molecular weight characteristics and compositions of the terpolymers were determined by gel permeation chromatography and proton nuclear magnetic resonance spectroscopy (<sup>1</sup>H NMR), respectively. Subsequently, the ABC triblock terpolymers were sequentially subjected to conditions of alkaline hydrolysis, acidic hydrolysis, and hydrogenolysis, leading to the selective cleavage of the PyEMA, the THPMA and the BzMA units, respectively, without affecting the remaining types of protecting groups, according to analysis using <sup>1</sup>H NMR spectroscopy. Similarly selective was the acidic hydrolysis followed by alkaline hydrolysis. Thermal treatment at 130 °C of the terpolymers led to the conversion of both the PyEMA and the THPMA units to MAA units, without affecting the BzMA units, yielding amphiphilic diblock copolymers whose self-assembly properties in water were investigated

    Dynamic Covalent Star Poly(ethylene glycol) Model Hydrogels: A New Platform for Mechanically Robust, Multifunctional Materials

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    We develop a new platform of dynamic covalent model polymer networks comprising two types of four-armed star poly­(ethylene glycol)­s (tetraPEG), one end-functionalized with benzaldehyde groups and the other with benzaacyl­hydrazides, resulting in hydrazone cross-links. These materials, henceforth to be called tetraPEG DYNAgels, display remarkable mechanical properties, much superior to those based on randomly cross-linked analogues. Fast aqueous gel formation takes place both at acidic and, unexpectedly, at alkaline conditions, with gel formation times covering 4 orders of magnitude in the pH range from 2.0 to 12.5 and a maximum gelation time appearing at pH 8.5. Frequency-dependent oscillatory rheology indicates a finite lifetime of the cross-links in tetraPEG DYNAgels at acidic conditions. Furthermore, these materials exhibit self-healing ability and reversibility under acidic and moderately acidic conditions; however, these properties can be canceled by chemical reduction of the cross-links. The system is highly modular, allowing the facile incorporation of other functionalities, e.g., hydrophobicity or amphiphilicity, introduced via polymers bearing terminal benzaldehyde groups

    Regular and Inverse Polyampholyte Hydrogels: A Detailed Comparison

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    Two series of polyampholyte (PA) hydrogels were prepared via reversible addition–fragmentation chain transfer polymerization, followed by removal of the protecting group of the acidic monomer repeating units (methacrylic acid, MAA), which were common in both series. One series bore (pyridin-2-yl)­methyl methacrylate (2PyMMA) basic monomer repeating units, whereas the second series carried basic monomer repeating units of 2-(dimethylamino)­ethyl methacrylate (DMAEMA). The 2PyMMA-MAA combination in the first series had the peculiarity that the basic 2PyMMA units were more acidic than the MAA units, thus resulting in so-called “inverse PA” hydrogels. The DMAEMA-MAA pair in the other series led to “regular PA” hydrogels in which the basicity and acidity of the two types of units were in the conventional sense. The swelling and hydrogen ion equilibrium properties of the two series were explored and thoroughly compared to each other. The aqueous degrees of swelling of inverse PA hydrogels were found to be generally lower than those of the regular ones due to the more hydrophobic character of the 2PyMMA basic units employed in inverse compared to those (DMAEMA) in regular PA gels. The aqueous swelling pH-profiles in both series of PA hydrogels presented a minimum. However, this swelling minimum was deeper and wider in the case of inverse PA hydrogels, because of the greater hydrophobicity of the basic (2PyMMA) units and the greater difference in the effective p<i>K</i> values of the two types of units (2PyMMA-MAA) in inverse PA hydrogels, respectively. This larger separation of the p<i>K</i>’s in inverse PA gels was directly confirmed from the hydrogen ion titration curves of all the gels. Finally, the isoelectric points of inverse PA hydrogels possessed no detectable dependence on PA composition, which must be contrasted to the strong composition-dependence of the isoelectric points of regular PAs

    Amphiphilic Polymer Conetworks Studied by SANS: Effect of the Type of Solubilizate and Molecular Architecture on the Swollen Gel Structure

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    Amphiphilic polymer conetworks (APCNs) are hydrogels with hydrophobic regions synthesized by cross-linking well-defined copolymers. Due to their amphiphilicity, they have oil solubilization ability. In this paper, we present a small-angle neutron scattering (SANS) study of the oil solubilization at the mesoscopic level in APCNs swollen in D2O, where for better contrast conditions, the hydrophobic monomer (M) was deuterated. The study was carried out on a series of APCNs where we systematically varied the mol fraction of the hydrophobic methyl methacrylate (M) monomer repeating units (from 0.1 to 0.9) with respect to the hydrophilic 2-(dimethylamino)ethyl methacrylate (D) monomer repeating units as well as the general block copolymer architecture (MDM vs DMD). First, the structure of the D2O-swollen APCNs was characterized by means of SANS, which showed a well-defined structure with a repeat spacing of the domains, d, that scales directly with the architecture of the building blocks of the APCNs. In the second step, the solubilization of oils of different polarities (octane, toluene, eugenol, and 1-hexanol) was probed, and a clear correlation of oil solubilization with the oil polarity was observed. The most unpolar oil, octane, did not solubilize at all, while the much more polar toluene and 1-hexanol were incorporated very well but in a markedly different fashion. Toluene completely swelled the M part, while 1-hexanol appeared to be much more associated with the amphiphilic interface. This demonstrates that the studied APCNs are very selective with respect to their solubilization properties and efficient for distinguishing different types of oils

    Biosourced Amphiphilic Degradable Elastomers of Poly(glycerol sebacate): Synthesis and Network and Oligomer Characterization

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    Glycerol (G, a triol) and sebacic acid (S, an α,ω-dicarboxylic acid) were condensed in the bulk to obtain poly­(glycerol sebacate) (PGS) cross-linked elastomers which were characterized in terms of their swelling, thermal, and mechanical properties. The soluble precursors to the elastomers were characterized in terms of their size, size distribution, and composition. In particular, G–S mixtures of five different compositions (molar G:S ratio = 2:1, 2:2, 2:3, 2:4, and 2:5) were copolymerized in the bulk at 120 °C in a three-step strategy (first step under inert gas atmosphere, followed by two steps <i>in vacuo</i>). When the G:S molar ratio was equal to (2:3) or close to (2:4), the stoichiometrically matched, network formation took place from the second condensation step, whereas three reaction steps were necessary for network formation far from stoichiometry, at G:S molar ratios equal to 2:2 and 2:5; at a G:S molar ratio of 2:1, no network formation was observed at all. Network composition also proved to be an important structural property, directly influencing the swelling and thermomechanical behavior of the elastomers. In particular, at the stoichiometrically matched G:S ratio of 2:3, corresponding to the cross-linking density maximum, the sol fraction extracted from the elastomers and the elastomer degree of swelling in aqueous media and in organic solvents presented a minimum, whereas the storage moduli of PGS elastomeric membranes in the dry state, measured within the temperature range between 35 and 140 °C, exhibited a maximum. The molecular weights of all soluble network precursors were found to be below 5000 g mol<sup>–1</sup> (gel permeation chromatography), containing but traces of ring oligomers (electron-spray ionization mass spectrometry). <sup>1</sup>H NMR spectroscopy indicated that the precursor composition was close to that expected on the basis of the G:S feed ratio and that monomer-to-polymer conversion increased from the first to the second condensation step

    Stimuli-Responsive Amphiphilic Polyelectrolyte Heptablock Copolymer Physical Hydrogels: An Unusual pH-Response

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    An amphiphilic cationic polyelectrolyte based on poly­[2-(dimethylamino)­ethyl methacrylate] (polyDMA) and poly­(<i>n</i>-butyl methacrylate) (polyBuMA) with a BuMA–DMA–BuMA–DMA–BuMA–DMA–BuMA heptablock copolymer architecture was studied in aqueous media. This copolymer was found to form a physical hydrogel via the intermolecular hydrophobic association (physical cross-linking) of the BuMA blocks. The rheological properties of the heptablock hydrogels were investigated as a function of copolymer concentration, and pH. The results showed a peculiar pH-dependence of the rheological properties, remarkably different from those observed with associative telechelic polyelectrolytes. Aqueous solutions of this copolymer were free-flowing sols at low pH (below 2) and high pH (above 8), whereas they turned into gels at intermediate pH values. The rheological properties studied as a function of pH showed two additional stiff–soft–stiff gel transitions at pH 4.5 and 6.5. Small-angle neutron scattering revealed the formation of a 3D transient network of bridged flower-like micelles whose structural characteristics, i.e., micellar radius, hard-sphere radius and hard-sphere volume fraction, were smoothly evolving with the pD

    Thermoresponsive Hydrogels Based on Telechelic Polyelectrolytes: From Dynamic to “Frozen” Networks

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    A novel thermoresponsive gelator of (B-<i>co</i>-C)-<i>b</i>-A-<i>b</i>-(B-<i>co</i>-C) topology, comprising a poly­(2-(dimethyl­amino)­ethyl methacrylate) (PDMAEMA) weak polyelectrolyte as central block, end-capped by thermosensitive poly­(triethylene glycol methyl ether methacrylate/<i>n</i>-butyl methacrylate) [P­(TEGMA-<i>co</i>-<i>n</i>BuMA)] random copolymers, was designed and explored in aqueous media. The main target of this design was to control the dynamics of the stickers by temperature as to create an injectable hydrogel that behaves as a weak gel at low temperature and as a strong gel at physiological temperature. Indeed, at low temperatures, the system behaves like a viscoelastic complex fluid (dynamic network), while at higher temperatures, an elastic hydrogel is formed (“frozen” network). The viscosity increases exponentially upon heating, about 5 orders of magnitude from 5 to 45 °C, which is attributed to the exponential increase of the lifetime of the self-assembled stickers. The integration of thermo- and shear responsive properties in the gelator endows the gel with injectability. Moreover, the gel can be rapidly recovered upon cessation of the applied stress at 37 °C, simulating conditions similar to those of injection through a 28-gauge syringe needle. All these hydrogel properties render it a good candidate for potential applications in cell transplantation through injection strategies

    Amphiphilic Polymer Conetworks Based on End-Linked “Core-First” Star Block Copolymers: Structure Formation with Long-Range Order

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    Amphiphilic polymer conetworks are cross-linked polymers that swell both in water and in organic solvents and can phase separate on the nanoscale in the bulk or in selective solvents. To date, however, this phase separation has only been reported with short-range order, characterized by disordered morphologies. We now report the first example of amphiphilic polymer conetworks, based on end-linked “core-first” star block copolymers, that form a lamellar phase with long-range order. These mesoscopically ordered systems can be produced in a simple fashion and exhibit significantly improved mechanical properties
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