Let There Be Light: Photo-Cross-Linked Block Copolymer Nanoparticles

Linear Cyclobutane-Containing Polymer Synthesis via [2 + 2] Photopolymerization in an Unconfined Environment under Visible Light

The [2 + 2] photopolymerization of diolefinic monomers is an appealing approach for the construction of polymeric materials. Herein, we demonstrate that the establishment of an effective donor-acceptor conjugation by introducing electron-donating alkoxy groups at appropriate positions of the benzene ring could activate p-phenylenediacrylate (PDA), thus enabling the development of the first solution [2 + 2] photopolymerization of such monomers under the irradiation of visible light. Variation on the alkoxy groups and the ester parts could allow access to a series of linear cyclobutane-containing polymer products with high molecular weight (up to 140 kDa) and good solubility in common solvents. Further, temporal control and postpolymerization modification with preinstalled pendant C═C bonds via thiol-ene click reaction are also demonstrated with this [2 + 2] photopolymerization system.

Amino acid derived biopolymers: Recent advances and biomedical applications

Over the past few years, amino acids (AA) have emerged as promising biomaterials for the synthesis of functional polymers. Owing to the diversity of functional groups in amino acids, various polymerization methods may be used to make a wide range of well-defined functional amino-acid/peptide-based optically active polymers with varying polymer lengths, compositions, and designs. When incorporated with chirality and self-assembly, they offer a wide range of applications and are particularly appealing in the field of drug delivery, tissue engineering, and biosensing. There are several classes of these polymers that include polyamides (PA), polyesters (PE), poly(ester-amide)s (PEA)s, polyurethanes (PU)s, poly(depsipeptide)s (PDP)s, etc. They offer the ability to control functionality, conjugation, crosslinking, stimuli responsiveness, and tuneable mechanical/thermal properties. In this review, we present the recent advancements in the synthesis strategies for obtaining these amino acid-derived bio-macromolecules, their self-assembly properties, and the wealth of prevalent applications.

Interactions and Dynamics in Aqueous Solutions of pH-Responsive Polymers: A Combined Fluorescence and Dielectric Relaxation Study

Interaction and dynamics of aqueous solutions of pH-responsive smart polymers are investigated via steady-state, time-resolved fluorescence emission spectroscopy with the help of external local reporter coumarin 153 (C153), while MHz to GHz dielectric relaxation spectroscopic (DRS) measurement reports the intrinsic medium relaxation features. A series of pH-responsive random copolymers (DPL-DP60) comprising of a pH-responsive moiety 2-((leucinyl)oxy)ethyl methacrylate (l-Leu-HEMA) and hydrophobic methyl methacrylate (MMA) are synthesized and characterized. A balance between the pH-responsive (l-Leu-HEMA) and the hydrophobic (MMA) content dictates the phase transition pH, which is found to be ∼5-7 for these aqueous copolymer solutions (1 mg/mL). Dynamic light scattering measurements in aqueous solutions of these polymers reflect a small particle size (∼2-8 nm) at solution pH below their individual phase transition pH, while a large particle size (∼140-340 nm) forms beyond their phase transition pH. No signature of a phase transition pH-driven abrupt change in static and dynamic properties of aqueous polymer solutions has been registered from pH-dependent dielectric relaxation as well as solute (C153)-centric fluorescence measurements. A significant impact of varying the l-Leu-HEMA/MMA segment ratio on steady-state fluorescence emission and rotational anisotropy decay of the fluorophore solute (C153) has been observed. MHz to GHz DRS in aqueous solutions of these pH-responsive polymers reflects bulk water-like dielectric features.

Cloud Point Driven Dynamics in Aqueous Solutions of Thermoresponsive Copolymers: Are They Akin to Criticality Driven Solution Dynamics?

Cloud point driven interaction and relaxation dynamics of aqueous solutions of amphiphilic thermoresponsive copolymers were explored through picosecond resolved and steady state fluorescence measurements employing hydrophilic (coumarin 343, C343) and hydrophobic (coumarin 153, C153) solute probes of comparable sizes. These thermoresponsive random copolymers, with tunable cloud point temperatures (T_cp's) between 298 and 323 K, were rationally designed first and then synthesized via reversible addition-fragmentation chain transfer (RAFT) copolymerization of methyl methacrylate (MMA) and poly(ethylene glycol) monomethyl ether methacrylate (PEGMA). Subsequently, copolymers were characterized by NMR spectroscopy and size exclusion chromatography (SEC). A balance between the hydrophilic (PEGMA) and the hydrophobic (MMA) content dictates the critical aggregation concentration (CAC), with CAC ∼ 2-14 mg/L for these copolymers in aqueous media. No abrupt changes in the steady state spectral features of both C153 and C343 in the aqueous solutions of these polymers near but below the cloud point temperatures were observed. Interestingly, spectral properties of C153 in these solutions show the impact of hydrophobic/hydrophilic interaction balance but not by those of C343. More specifically, C153 reported a blue shift (relative to that in neat water) and heterogeneity in its local environment. This suggested different locations for the hydrophilic (C343) and the hydrophobic (C153) probes. In addition, the excited state fluorescence lifetime (⟨τ_life⟩) of C153 increased with the increase of hydrophobic (MMA) content in these copolymers. However, C343 reported no such variations, although fluorescence anisotropy decays for both solutes were significantly slowed down in these aqueous solutions compared to neat water. Anisotropy decays indicated bimodal time-dependent friction for these solutes in aqueous solutions of these copolymers but monomodal in neat water. A linear dependence of the average rotational relaxation rates (⟨k_rot⟩ = ⟨τ_rot⟩^-1) of the type ⟨k_rot⟩ ∝ (|T - T_cp|/T_cp)^γ with negative values for the exponent γ was observed for both solutes. No slowing down of the solute rotation with temperature approaching the T_cp was detected; rather, rotation became faster upon increasing the solution temperature, suggesting domination of the local friction.

Maximizing the symbiosis of static and dynamic bonds in self-healing boronic ester networks

Networks that contain boronic ester crosslinks undergo dynamic bond exchange that enables self-healing behavior and reprocessing.

Amino acid-derived stimuli-responsive polymers and their applications

The recent advances achieved in the study of various stimuli-responsive polymers derived from natural amino acids have been reviewed.

A new class of dual responsive self-healable hydrogels based on a core crosslinked ionic block copolymer micelle prepared via RAFT polymerization and Diels-Alder "click" chemistry

Amphiphilic diblock copolymers of poly(furfuryl methacrylate) (PFMA) with cationic poly(2-(methacryloyloxy)ethyltrimethyl ammonium chloride) (PFMA-b-PMTAC) and anionic poly(sodium 4-vinylbenzenesulfonate) (PFMA-b-PSS) were prepared via reversible addition fragmentation chain-transfer (RAFT) polymerization by using PFMA as a macro-RAFT agent. The formation of the block copolymer was confirmed by FTIR and ¹H NMR analyses. In water, the amphiphilic diblock copolymers, (PFMA-b-PMTAC) and (PFMA-b-PSS), formed micelles with PFMA in the core and the rest of the hydrophilic polymers like PMTAC and PSS in the corona. The PFMA core was crosslinked by using Diels-Alder (DA) "Click" chemistry in water at 60 °C where bismaleimide acted as a crosslinker. Afterwards, both the core crosslinked micelles were mixed at an almost equal charge ratio which was determined by zeta potential analysis to prepare the self-assembled hydrogel. The de-crosslinking of the hydrophobic PFMA core in the self-assembled hydrogel via rDA reaction took place at 165 °C as determined from DSC analysis. This hydrogel showed self-healing behavior using ionic interaction (in the presence of water) and DA chemistry (in the presence of heat).

Utilizing the electron transfer mechanism of chlorophyll a under light for controlled radical polymerization

Efficient photoredox catalysts containing transition metals, such as iridium and ruthenium, to initiate organic reactions and polymerization under visible light have recently emerged. However, these catalysts are composed of rare metals, which limit their applications. In this study, we report an efficient photoinduced living radical polymerization process that involves the use of chlorophyll as the photoredox biocatalyst. We demonstrate that chlorophyll a (the most abundant chlorophyll in plants) can activate a photoinduced electron transfer (PET) process that initiates a reversible addition-fragmentation chain transfer (RAFT) polymerization under blue and red LED light (λmax = 461 and 635 nm, respectively). This process controls a wide range of functional and non-functional monomers, and offers excellent control over molecular weights and polydispersities. The end group fidelity was demonstrated by NMR, UV-vis spectroscopy, and successful chain extensions for the preparation of diblock copolymers.

Balancing the stability and drug release of polymer micelles by the coordination of dual-sensitive cleavable bonds in cross-linked core

The optimal structure design of nanocarriers to inhibit premature release of anticancer drugs from nanocarriers during blood circulation and improve drug release inside tumor cells is still a significant issue for polymer micelles applied to antitumor drug delivery. Herein, in order to balance the contradiction between polymer micellar stability and drug release, dual-sensitive cleavable cross-linkages of benzoic imine conjugated disulfide bonds were introduced into the core of the amphiphilic copolymer micelles to form core-cross-linked micelles. First, biodegradable poly(ethylene glycol)-b-(polycaprolactone-g-poly(methacrylic acid-p-hydroxy benzaldehyde-cystamine)), i.e. mPEG-b-(PCL-g-P(MAA-Hy-Cys)) (PECMHC) copolymers were synthesized and assembled into PECMHC micelles (PECMHC Ms). Then, simply by introducing H2O2 to the PECMHC Ms dispersions to oxidate the thiol groups of cystamine moieties in the core, core-cross-linked PECMHC micelles (cc-PECMHC Ms) ∼100 nm in size were readily obtained in water. In vitro studies of doxorubicin (DOX)-loaded cc-PECMHC Ms show that the cross-linked core impeded the drug release in the physical conditions, owing to the high stability of the micelles against both extensive dilution and salt concentration, while it greatly accelerated DOX release in mildly acidic (pH ∼5.0-6.0) medium with glutathione, owing to the coordination of the pH-sensitive cleaving of benzoic imine bonds and the reduction-sensitive cleaving of disulfide bonds. The in vivo tissue distribution and tumor accumulation of the DOX-loaded cc-PECMHC Ms were monitored via fluorescence images of DOX. DOX-loaded cc-PECMHC Ms exhibited enhanced tumor accumulation because of their high stability in blood circulation and less DOX premature release. Therefore, the cc-PECMHC Ms with dual-sensitive cleavable bonds in the cross-linked core were of excellent biocompatibility, high extracellular stability and had intelligent intracellular drug release properties, indicating promise as candidates for anticancer drug delivery.

Anthracene functionalized thermosensitive and UV-crosslinkable polymeric micelles

An anthracene-functionalized thermosensitive block copolymer was synthesized, which formed micelles by heating its aqueous solution above the lower critical solution temperature (LCST). The micelles were subsequently crosslinked by UV illumination at 365 nm with a normal handheld UV lamp.

Facile synthesis of drug-conjugated PHPMA core-crosslinked star polymers

Poly(N-(2-hydroxypropyl)methacrylamide) (PHPMA), a biocompatible and non-immunogenic polymer, was used to form core-crosslinked star polymers for potential drug delivery applications.

Photodimerization as an alternative to photocrosslinking of nanoparticles: proof of concept with amphiphilic linear polyoxazoline bearing coumarin unit

The photo-dimerization of the coumarinated inner compartment of the nanoparticles is investigated.