Synthesis of novel boronic acid-decorated poly(2-oxazoline)s showing triple-stimuli responsive behavior

Boronic acid-functionalized (co)polymers have gained increasing attention in the field of responsive polymers and polymeric materials due to their unique characteristics and responsiveness towards both changes in pH and sugar concentrations. This makes these (co)polymers excellently suited for various applications including responsive membranes, drug delivery applications and sensor materials. Unfortunately, boronic acid-based polymer research is also notorious for its challenging monomer synthesis and polymerization and its overall difficult polymer purification and manipulation. In light of this, many research groups have focused their attention on the optimization of various polymerization techniques in order to expand the field of BA-research including previously unexplored monomers and polymerization techniques. In this paper, a new post-polymerization modification methodology was developed allowing for the synthesis of novel boronic acid-decorated poly(2-alkyl-2-oxazoline) (PAOx) copolymers, utilizing the recently published PAOx methyl ester reaction platform. The developed synthetic pathway provides a straightforward method for the introduction of pH- and glucose-responsiveness, adding this to the already wide variety of possible responsive PAOx-based systems. The synthesized BA-decorated PAOx are based on the thermoresponsive poly(2-n-propyl-2-oxazoline) (PnPropOx). This introduces a pH and glucose dependence on both cloud and clearance point temperatures of the copolymer in aqueous and pH-buffered conditions, yielding a triply-responsive (co)polymer that highlights the wide variety of obtainable properties using this pathway

Hyperbranched Bisphosphonate‐Functional Polymers via Self‐Condensing Vinyl Polymerization and Postpolymerization Multicomponent Reactions

The synthesis of hyperbranched aminobisphosphonic acid polymers via reversible addition‐fragmentation chain transfer (RAFT) self‐condensing vinyl polymerization is reported. A novel acrylamide‐functional chain transfer monomer is synthesized and characterized by 1H and 13C NMR spectroscopy, elemental analysis, and mass spectrometry. The monomer is subsequently copolymerized with an acrylamide monomer bearing a pendent amine group to create hyperbranched amine‐functional polymers with degrees of branching dictated by changing the reaction stoichiometry. The aminobisphosphonate functional group is introduced via a 3‐component Kabachnik‐Fields reaction. An alternate functionalization of the amine polymers to create acid‐degradable imine hydrogels is also employed. This work demonstrates the application of multicomponent reactions to RAFT‐derived hyperbranched polymers and provides a new route to previously inaccessible polymers

Polymerization-induced thermal self-assembly (PITSA)

Polymerization-induced self-assembly (PISA) is a versatile technique to achieve a wide range of polymeric nanoparticle morphologies. Most previous examples of self-assembled soft nanoparticle synthesis by PISA rely on a growing solvophobic polymer block that leads to changes in nanoparticle architecture during polymerization in a selective solvent. However, synthesis of block copolymers with a growing stimuli-responsive block to form various nanoparticle shapes has yet to be reported. This new concept using thermoresponsive polymers is termed polymerization-induced thermal self-assembly (PITSA). A reversible addition-fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide from a hydrophilic chain transfer agent composed of N,N-dimethylacrylamide and acrylic acid was carried out in water above the known lower critical solution temperature (LCST) of poly(N-isopropylacrylamide) (PNIPAm). After reaching a certain chain length, the growing PNIPAm self-assembled, as induced by the LCST, into block copolymer aggregates within which dispersion polymerization continued. To characterize the nanoparticles at ambient temperatures without their dissolution, the particles were crosslinked immediately following polymerization at elevated temperatures via the reaction of the acid groups with a diamine in the presence of a carbodiimide. Size exclusion chromatography was used to evaluate the unimer molecular weight distributions and reaction kinetics. Dynamic light scattering and transmission electron microscopy provided insight into the size and morphologies of the nanoparticles. The resulting block copolymers formed polymeric nanoparticles with a range of morphologies (e.g., micelles, worms, and vesicles), which were a function of the PNIPAm block length

Synthesis of poly(1-vinylimidazole)-block-poly(9-vinylcarbazole) copolymers via RAFT and their use in chemically responsive graphitic composites

This study reports the synthesis of novel poly(1-vinylimidazole)-b-poly(9-vinylcarbazole) (PVI-b-PVK) block copolymers with varying monomer ratios using reversible addition-fragmentation chain-transfer (RAFT) polymerization and their incorporation in responsive composite materials. Specifically, non-covalent exfoliation of two different conductive fillers, multi-walled carbon nanotubes (MWCNTs) or reduced graphene oxide (rGO), was studied. The percolation threshold of the synthesized nanocomposites was dependent on the polymer used for dispersion, showing a better affinity of the fillers for block copolymers with higher relative carbazole content. Resistivity measurements showed selective variation in the resistance signal when the materials were exposed to various organic solvents and acids, providing a good basis for the design of sensing devices

Effect of Polymer Chemistry on the Linear Viscoelasticity of Complex Coacervates

Complex coacervates can form through the electrostatic complexation of oppositely charged polymers. The material properties of the resulting coacervates can change based on the polymer chemistry and the complex interplay between electrostatic interactions and water structure, controlled by salt. We examined the effect of varying the polymer backbone chemistry using methacryloyl- and acryloyl-based complex coacervates over a range of polymer chain lengths and salt conditions. We simultaneously quantified the coacervate phase behavior and the linear viscoelasticity of the resulting coacervates to understand the interplay between polymer chain length, backbone chemistry, polymer concentration, and salt concentration. Time-salt superposition analysis was used to facilitate a broader characterization and comparison of the stress relaxation behavior between different coacervate samples. Samples with mismatched polymer chain lengths highlighted the ways in which the shortest polymer chain can dominate the resulting coacervate properties. A comparison between coacervates formed from methacryloyl vs acryloyl polymers demonstrated that the presence of a backbone methyl group affects the phase behavior, and thus the rheology in such a way that coacervates formed from methacryloyl polymers have a similar phase behavior to those of acryloyl polymers with ∼10× longer polymer chains

Glucose-Sensitivity of Boronic Acid Block Copolymers at Physiological pH

Well-defined boronic acid block copolymers were demonstrated to exhibit glucose-responsive disassembly at physiological pH. A boronic acid-containing acrylamide monomer with an electron-withdrawing substituent on the pendant phenylboronic acid moiety was polymerized by reversible addition–fragmentation chain transfer (RAFT) polymerization to yield a polymer with a boronic acid p<i>K</i>_a = 8.2. Below this value, a block copolymer of this monomer with poly­(<i>N</i>,<i>N</i>-dimethylacrylamide) self-assembled into aggregates. Addition of base to yield a pH > p<i>K</i>_a or addition of glucose at pH = 7.4 resulted in aggregate dissociation that may prove promising for controlled delivery applications under physiological relevant conditions