MOFs as Versatile Catalysts: Synthesis Strategies and Applications in Value-Added Compound Production

Catalysts played a crucial role in advancing modern human civilization, from ancient times to the industrial revolution. Due to high cost and limited availability of traditional catalysts, there is a need to develop cost-effective, high-activity, and nonprecious metal-based electrocatalysts. Metal-organic frameworks (MOFs) have emerged as an ideal candidate for heterogeneous catalysis due to their physicochemical properties, hybrid inorganic/organic structures, uncoordinated metal sites, and accessible organic sections. MOFs are high nanoporous crystalline materials that can be used as catalysts to facilitate polymerization reactions. Their chemical and structural diversity make them effective for various reactions compared to traditional catalysts. MOFs have been applied in gas storage and separation, ion-exchange, drug delivery, luminescence, sensing, nanofilters, water purification, and catalysis. The review focuses on MOF-enabled heterogeneous catalysis for value-added compound production, including alcohol oxidation, olefin oligomerization, and polymerization reactions. MOFs offer tunable porosity, high spatial density, and single-crystal XRD control over catalyst properties. In this review, MOFs were focused on reactions of CO2 fixation, CO2 reduction, and photoelectrochemical water splitting. Overall, MOFs have great potential as versatile catalysts for diverse applications in the future.

Double Stimuli-Responsive di- and Triblock Copolymers of Poly(N-isopropylacrylamide) and Poly(1-vinylimidazole): Synthesis and Self-Assembly

For the first time, double stimuli-responsive properties of poly(N-isopropylacrylamide) (PNIPA) and poly(1-vinylimidazole) (PVIM) block copolymers in aqueous solutions were studied. The synthesis of PNIPA₆₀-b-PVIM₉₀ and PNIPA₂₈-b-PVIM₆₂-b-PNIPA₂₉ was performed using reversible addition-fragmentation chain transfer (RAFT) polymerization. The polymers were characterized by size exclusion chromatography and ¹H NMR spectroscopy. The conformational behavior of the polymers was studied using dynamic light scattering (DLS) and fluorescence spectroscopy (FS). It was found that PNIPA and block copolymers conformation and ability for self-assembly in aqueous medium below and above cloud point temperature depend on the locus of hydrophobic groups derived from the RAFT agent within the chain. Additionally, the length of PVIM block, its locus in the chain and charge perform an important role in the stabilization of macromolecular micelles and aggregates below and above cloud point temperature. At 25 °C the average hydrodynamic radius (R_h) of the block copolymer particles at pH 3 is lower than at pH 9 implying the self-assembling of macromolecules in the latter case. Cloud points of PNIPA₆₀-b-PVIM₉₀ are ~43 °C and ~37 °C at a pH of 3 and 9 and of PNIPA₂₈-b-PVIM₆₂-b-PNIPA₂₉ they are ~35 °C and 31 °C at a pH of 3 and 9. Around cloud point independently of pH, the R_h value for triblock copolymer rises sharply, achieves the maximum value, then falls and reaches the constant value, while for diblock copolymer, it steadily grows after reaching cloud point. The information about polarity of microenvironment around polymer obtained by FS accords with DLS data.

Influence of Alkyl Acrylate Nature on Rheological Properties of Polyacrylonitrile Terpolymers Solutions, Spinnability and Mechanical Characteristics of Fibers

The influence of alkyl acrylate comonomers in the rank of methyl- (MA), butyl- (BA), ethylhexyl- (EGA), and lauryl- (LA) in ternary copolymers based on acrylonitrile, alkyl acrylate and acrylamide (PAN-alkyl acrylate) on their solutions rheological behavior in dimethyl sulfoxide (DMSO), and mechanical properties of the spun fibers have been investigated. To reveal the role of molecular weight, two series of copolymers with molecular weights of ~50 and 150 kg/mol have been studied. It was shown that the nature of the alkyl acrylate does not significantly affect the rheological behavior of their solutions regardless of the length of the alkyl substituent and the content of the alkyl acrylate in copolymers. An exception is the high-molecular PAN-LA, which is characterized by a non-Newtonian behavior at lower concentrations. Two series of fibers were spun from the characterized ranks of low and high-molecular-weight copolymer solutions. For all copolymers, a 2.5-5-fold increase in the strength and elastic modulus of the fiber was found with an increase in M_w. It has been shown that PAN-MA and PAN-LA fibers have a tensile strength of 800 MPa that is 1.5-3 times higher than that of other copolymers spun in the same conditions.

Synthesis of Lignosulfonate-Based Dispersants for Application in Concrete Formulations

Lignosulfonates (LS) are products from the sulfite pulping process that could be applied as renewable environmentally-friendly polymeric surfactants. Being widely used as plasticizers and water-reducing admixtures in concrete formulations LS compete in the market with petroleum-based superplasticizers, such as naphthalene sulfonate formaldehyde polycondensate (NSF) and copolymer polycarboxylate ethers (PCE). In this work, different chemical modification strategies were used to improve LS performance as dispersants for concrete formulations. One strategy consisted in increasing the molecular weight of LS through different approaches, such as laccase and polyoxometalate-mediated polymerization, glyoxalation, and reversible addition-fragmentation chain transfer (RAFT) polymerization. The other strategy consisted of preparing LS-based non-ionic polymeric dispersants using two different epoxidized oligomer derivatives of poly(ethylene glycol) (PEG) and poly(propylene glycol) (PPG). Modified LS were used to prepare cement pastes, which were examined for their fluidity. Results revealed that the most promising products are PPG-modified LS due to the introduction of PPG chains by reaction with phenolic moieties in LS. The enhanced dispersant efficiency of the ensuing products is probably related not only to electrostatic repulsion caused by the sulfonic ionizable groups in LS but also to steric hindrance phenomena due to the grafted bulky PPG chains.

Self-Assembled Nanoparticles Based on Block-Copolymers of Poly(2-Deoxy-2-methacrylamido-d-glucose)/Poly(N-Vinyl Succinamic Acid) with Poly(O-Cholesteryl Methacrylate) for Delivery of Hydrophobic Drugs

The self-assembly of amphiphilic block-copolymers is a convenient way to obtain soft nanomaterials of different morphology and scale. In turn, the use of a biomimetic approach makes it possible to synthesize polymers with fragments similar to natural macromolecules but more resistant to biodegradation. In this study, we synthesized the novel bio-inspired amphiphilic block-copolymers consisting of poly(N-methacrylamido-d-glucose) or poly(N-vinyl succinamic acid) as a hydrophilic fragment and poly(O-cholesteryl methacrylate) as a hydrophobic fragment. Block-copolymers were synthesized by radical addition-fragmentation chain-transfer (RAFT) polymerization using dithiobenzoate or trithiocarbonate chain-transfer agent depending on the first monomer, further forming the hydrophilic block. Both homopolymers and copolymers were characterized by ¹H NMR and Fourier transform infrared spectroscopy, as well as thermogravimetric analysis. The obtained copolymers had low dispersity (1.05-1.37) and molecular weights in the range of ~13,000-32,000. The amphiphilic copolymers demonstrated enhanced thermal stability in comparison with hydrophilic precursors. According to dynamic light scattering and nanoparticle tracking analysis, the obtained amphiphilic copolymers were able to self-assemble in aqueous media into nanoparticles with a hydrodynamic diameter of approximately 200 nm. An investigation of nanoparticles by transmission electron microscopy revealed their spherical shape. The obtained nanoparticles did not demonstrate cytotoxicity against human embryonic kidney (HEK293) and bronchial epithelial (BEAS-2B) cells, and they were characterized by a low uptake by macrophages in vitro. Paclitaxel loaded into the developed polymer nanoparticles retained biological activity against lung adenocarcinoma epithelial cells (A549).

Control over the relative reactivities of monomers in RAFT copolymerization of styrene and acrylic acid

The relative monomer reactivities in the reversible addition–fragmentation chain transfer (RAFT) radical copolymerization of styrene (S) and acrylic acid (AA) in a solution of the polar solvent N,N-dimethylformamide are found to be dependent on the chemical nature of the RAFT agent. Polymeric RAFT agents based on polyacrylic acid enhance the difference in monomer reactivities (dithiobenzoate – r_AA = 0.09 ± 0.02, r_S = 3.5 ± 1.2, trithiocarbonate – r_AA = 0.08 ± 0.04, r_S = 3.03 ± 1.78) compared to low molecular weight RAFT agents (dibenzyl dithiobenzoate – r_AA = 0.14 ± 0.01, r_S = 1.00 ± 0.01, dibenzyl trithiocarbonate – r_AA = 0.08 ± 0.01, r_S = 0.85 ± 0.03). The opposite effect on the relative reactivity of acrylic acid is observed when polymeric RAFT agents based on polystyrene are used (dithiobenzoate – r_AA = 3.3 ± 0.4, r_S = 0.72 ± 0.05, trithiocarbonate – r_AA = 0.11 ± 0.01, r_S = 0.54 ± 0.03). In all the investigated systems the copolymers formed are characterized by narrow MWD due to the high efficiency of the chosen RAFT agents.

The tuning of the relative monomer reactivities of styrene and acrylic acid in a solution of N,N-dimethylformamide by using polymeric RAFT agents.

Responsive Hybrid (Poly)peptide-Polymer Conjugates

(Poly)peptide-polymer conjugates continue to garner significant interest in the production of functional materials given their composition of natural and synthetic building blocks that confer select and synergistic properties. Owing to opportunities to design predefined architectures and structures with different morphologies, these hybrid conjugates enable new approaches for producing micro- or nanomaterials. Their modular design enables the incorporation of multiple responsive properties into a single conjugate. This review presents recent advances in (poly)peptide-polymer conjugates for drug-delivery applications, with a specific focus on the utility of the (poly)peptide component in the assembly of particles and nanogels, as well as the role of the peptide in triggered drug release.

Thermoresponsive and Reducible Hyperbranched Polymers Synthesized by RAFT Polymerisation

Here, we report the synthesis of new thermoresponsive hyperbranched polymers (HBPs) via one-pot reversible addition-fragmentation chain transfer (RAFT) copolymerisation of poly(ethylene glycol)methyl ether methacrylate (PEGMEMA, Mn = 475 g/mol), poly(propylene glycol)methacrylate (PPGMA, Mn = 375 g/mol), and disulfide diacrylate (DSDA) using 2-cyanoprop-2-yl dithiobenzoate as a RAFT agent. DSDA was used as the branching agent and to afford the HBPs with reducible disulfide groups. The resulting HBPs were characterised by Nuclear Magnetic Resonance Spectroscopy (NMR) and Gel Permeation Chromatography (GPC). Differential Scanning Calorimetry (DSC) was used to determine lower critical solution temperatures (LCSTs) of these copolymers, which are in the range of 17⁻57 °C. Moreover, the studies on the reducibility of HBPs and swelling behaviours of hydrogels synthesized from these HBPs were conducted. The results demonstrated that we have successfully synthesized hyperbranched polymers with desired dual responsive (thermal and reducible) and crosslinkable (via thiol-ene click chemistry) properties. In addition, these new HBPs carry the multiplicity of reactive functionalities, such as RAFT agent moieties and multivinyl functional groups, which can afford them with the capacity for further bioconjugation and structure modifications.