Thermoresponsivity, Micelle Structure, and Thermal-Induced Structural Transition of an Amphiphilic Block Copolymer Tuned by Terminal Multiple H-Bonding Units

Responsive Supramolecular Polymers for Diagnosis and Treatment

Stimuli-responsive supramolecular polymers are ordered nanosized materials that are held together by non-covalent interactions (hydrogen-bonding, metal-ligand coordination, π-stacking and, host-guest interactions) and can reversibly undergo self-assembly. Their non-covalent nature endows supramolecular polymers with the ability to respond to external stimuli (temperature, light, ultrasound, electric/magnetic field) or environmental changes (temperature, pH, redox potential, enzyme activity), making them attractive candidates for a variety of biomedical applications. To date, supramolecular research has largely evolved in the development of smart water-soluble self-assemblies with the aim of mimicking the biological function of natural supramolecular systems. Indeed, there is a wide variety of synthetic biomaterials formulated with responsiveness to control and trigger, or not to trigger, aqueous self-assembly. The design of responsive supramolecular polymers ranges from the use of hydrophobic cores (i.e., benzene-1,3,5-tricarboxamide) to the introduction of macrocyclic hosts (i.e., cyclodextrins). In this review, we summarize the most relevant advances achieved in the design of stimuli-responsive supramolecular systems used to control transport and release of both diagnosis agents and therapeutic drugs in order to prevent, diagnose, and treat human diseases.

Probing the inner local density of complex macromolecules by pyrene excimer formation

The direct relationship existing between the average rate constant 〈k〉 for pyrene excimer formation and the local concentration [Py]loc of ground-state pyrenyl labels covalently attached to a macromolecule was established for 55 pyrene-labeled macromolecules (PyLM). These PyLM belonged to three different families of macromolecules with the first representing short monodisperse linear chains end-labeled with pyrene (polystyrene, poly(ethylene oxide), and poly(N-isopropyl acrylamide)), the second representing long polydisperse linear chains randomly labeled with pyrene (poly(methyl acrylate), poly(methyl methacrylate), polystyrene, poly(butyl methacrylate), poly(methoxyethyl methacrylate), and poly(N-isopropyl acrylamide)), and the third being comprised of two series of pyrene end-labeled low generation dendrimers with a bis(hydroxymethyl)propionic acid or a polyamidoamine backbone. The assumption, that the polymeric segments probed by an excited pyrenyl label covalently attached to one of these macromolecules obeyed Gaussian statistics, enabled the calculation of their square root average squared end-to-end distance (LPy), which was applied to calculate [Py]loc. The log-log plots of 〈k〉 as a function of [Py]loc yielded straight lines with a slope of unity for all families of macromolecules studied in four different organic solvents demonstrating the validity and generality of the 〈k〉-vs.-[Py]loc relationship. Since an experimentalist knows how the the pyrenyl labels are covalently attached onto a macromolecule, [Py]loc offers a means to probe the local density of a macromolecule, which can be employed to characterize its conformation in solution. Consequently, the 〈k〉-vs.-[Py]loc relationship provides a novel experimental means to probe the conformation of macromolecules which should establish pyrene excimer formation as an appealing method for conformational studies of macromolecules in solution, which should nicely complement scattering techniques.

Structural Evolution of a Polystyrene-Block-Poly(Ethylene Oxide) Block Copolymer in Tetrahydrofuran/Water Cosolvents

This study aims to quantitatively investigate the effect of water content on the self-assembly behavior of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) in tetrahydrofuran/water cosolvents by small-angle X-ray scattering. PS-b-PEO chains preferentially form fractal aggregates at a dilute concentration in neat tetrahydrofuran (THF). By adding a small amount of water into THF, PS-b-PEO forms gelled networks. The gelled networks have correlated inhomogeneities, which were generated through mesophase separation. These gelled networks are not present when PS-b-PEO is dissolved in THF/methanol and THF/ethanol cosolvents. The substitution of water with 12 M HCl reduces the viscosity of the gelled networks. Those results indicate that the gelled networks of PS-b-PEO need hydrogen bonds formed from surrounding water molecules to be bridging agents, which connect different PEO block chains together. Upon increasing the water content in THF/water cosolvents, dispersed micelles with a core-shell conformation or aggregated micelles preferentially coexist with fractal aggregates.

Molecular bases for temperature sensitivity in supramolecular assemblies and their applications as thermoresponsive soft materials

Thermoresponsive supramolecular assemblies have been extensively explored in diverse formats, from injectable hydrogels to nanoscale carriers, for a variety of applications including drug delivery, tissue engineering and thermo-controlled catalysis. Understanding the molecular bases behind thermal sensitivity of materials is fundamentally important for the rational design of assemblies with optimal combination of properties and predictable tunability for specific applications. In this review, we summarize the recent advances in this area with a specific focus on the parameters and factors that influence thermoresponsive properties of soft materials. We summarize and analyze the effects of structures and architectures of molecules, hydrophilic and lipophilic balance, concentration, components and external additives upon the thermoresponsiveness of the corresponding molecular assemblies.

Tuning the properties of hydrogen-bonded block copolymer worm gels prepared via polymerization-induced self-assembly

Polymerization-induced self-assembly (PISA) is exploited to design hydrogen-bonded poly(stearyl methacrylate)-poly(benzyl methacrylate) [PSMA-PBzMA] worm gels in n-dodecane. Using a carboxylic acid-based RAFT agent facilitates hydrogen bonding between neighboring worms to produce much stronger physical gels than those prepared using the analogous methyl ester-based RAFT agent. Moreover, tuning the proportion of these two types of end-groups on the PSMA chains enables the storage modulus (G') of a 20% w/w worm gel to be tuned from ∼4.5 kPa up to ∼114 kPa. This is achieved via two complementary routes: (i) an in situ approach using binary mixtures of acid- and ester-capped PSMA stabilizer chains during PISA or (ii) a post-polymerization processing strategy using a thermally-induced worm-to-sphere transition to mix acid- and ester-functionalized spheres at 110 °C that fuse to form worms on cooling to 20 °C. SAXS and rheology studies of these hydrogen-bonded worm gels provide detailed insights into their inter-worm interactions and physical behavior, respectively. In the case of the carboxylic acid-functionalized worms, SAXS provides direct evidence for additional inter-worm interactions, while rheological studies confirm both a significant reduction in critical gelation concentration (from approximately 10% w/w to 2-3% w/w) and a substantial increase in critical gelation temperature (from 41 °C to 92 °C). It is remarkable that a rather subtle change in the chemical structure results in such improvements in gel strength, gelation efficiency and gel cohesion.

Biological Stimuli-Induced Phase Transition of a Synthesized Block Copolymer: Preferential Interactions between PNIPAM-b-PNVCL and Heme Proteins

The beguiling world of functional polymers is dominated by thermoresponsive polymers with unique structural and molecular attributes. Limited work has been reported on the protein-induced conformational transition of block copolymers; furthermore, the literature lacks a clear understanding of the influence of proteins on the phase behavior of thermoresponsive copolymers. Herein, we have synthesized poly(N-isopropylacrylamide)-b-poly(N-vinylcaprolactam) (PNIPAM-b-PNVCL) by RAFT polymerization using N-isopropylacrylamide and N-vinylcaprolactam. Furthermore, using various biophysical techniques, we have explored the effect of cytochrome c (Cyt c), myoglobin (Mb), and hemoglobin (Hb) with varying concentrations on the aggregation behavior of PNIPAM-b-PNVCL. Absorption and steady-state fluorescence spectroscopy measurements were performed at room temperature to examine the copolymerization effect on fluorescent probe binding and biomolecular interactions between PNIPAM-b-PNVCL and proteins. Furthermore, temperature-dependent fluorescence spectroscopy and dynamic light scattering studies were performed to get deeper insights into the lower critical solution temperature (LCST) of PNIPAM-b-PNVCL. Small-angle neutron scattering (SANS) was also employed to understand the copolymer behavior in the presence of heme proteins. With the incorporation of proteins to PNIPAM-b-PNVCL aqueous solution, LCST has been varied to different extents owing to the preferential, molecular, and noncovalent interactions between PNIPAM-b-PNVCL and proteins. The present study can pave new insights between heme proteins and block copolymer interactions, which will help design biomimetic surfaces and aid in the strategic fabrication of copolymer-protein bioconjugates.

Novel Temperature/Reduction Dual-Stimulus Responsive Triblock Copolymer [P(MEO2MA-co- OEGMA)-b-PLLA-SS-PLLA-b-P(MEO2MA-co-OEGMA)] via a Combination of ROP and ATRP: Synthesis, Characterization and Application of Self-Assembled Micelles

Novel temperature/reduction dual stimulus-responsive triblock copolymers, poly [2-(2-methoxyethoxy) ethyl methacrylate-co-oligo (ethylene glycol) methacrylate]-b-(L-polylactic acid)-SS-b-(L-polylactic acid)-b-poly[2-(2-methoxyethoxy) ethyl methacrylate-co-oligo(ethylene glycol)methacrylate] [P(MEO₂MA-co-OEGMA)-b-PLLA-SS-PLLA-b-P(MEO₂MA-co-OEGMA)] (SPMO), were synthesized by ring opening polymerization (ROP) of L-lactide and 2,2’-dithio diethanol (SS-DOH), and random copolymerization of MEO₂MA and OEGMA monomers via atom transfer radical polymerization (ATRP) technology. The chemical structures and compositions of the novel copolymers were demonstrated by proton nuclear magnetic resonance (¹H NMR) and Fourier transform infrared spectroscopy (FTIR). The molecular weights of the novel copolymers were measured by size exclusive chromatography (SEC) and proved to have a relatively narrow molecular weight distribution coefficient (ÐM ≤ 1.50). The water solubility and transmittance of the novel copolymers were tested via visual observation and UV–Vis spectroscopy, which proved the SPMO had a good hydrophilicity and suitable low critical solution temperature (LCST). The critical micelle concentration (CMC) of the novel polymeric micelles were determined using surface tension method and fluorescent probe technology. The particle size and morphology of the novel polymeric micelles were characterized by dynamic light scattering (DLS) and transmission electron microscopy (TEM). The sol–gel transition behavior of the novel copolymers was studied via vial flip experiments. Finally, the hydrophobic anticancer drug doxorubicin (DOX) was used to study the in vitro release behavior of the novel drug-loaded micelles. The results show that the novel polymeric micelles are expected to become a favorable drug carrier. In addition, they exhibit reductive responsiveness to the small molecule reducing agent dithiothreitol (DTT) and temperature responsiveness with temperature changes.