Macromolecular engineering by atom transfer radical polymerization

Metal-free atom transfer radical polymerization with ppm catalyst loading under sunlight

Organocatalytic atom transfer radical polymerization (O-ATRP) is recently emerging as an appealing method for the synthesis of metal-free polymer materials with well-defined microstructures and architectures. However, the development of highly effective catalysts that can be employed at a practical low loading are still a challenging task. Herein, we introduce a catalyst design logic based on heteroatom-doping of polycyclic arenes, which leads to the discovery of oxygen-doped anthanthrene (ODA) as highly effective organic photoredox catalysts for O-ATRP. In comparison with known organocatalysts, ODAs feature strong visible-light absorption together with high molar extinction coefficient (ε455nm up to 23,950 M-1 cm-1), which allow for the establishment of a controlled polymerization under sunlight at low ppm levels of catalyst loading.

Durable and recyclable conjugated microporous polymer mediated controlled radical polymerization under white LED light irradiation

In this work, we report the first example of the use of a conjugated microporous polymer material (EI-CMP) as a heterogeneous catalyst in reversible complexation-mediated radical polymerization under white LED light irradiation.

Tuning dispersity of linear polymers and polymeric brushes grown from nanoparticles by atom transfer radical polymerization

Molecular weight distribution imposes considerable influence on the properties of polymers, making it an important parameter, impacting morphology and structural behavior of polymeric materials.

Synthesis of core-crosslinked star polymers via organocatalyzed living radical polymerization

Core-crosslinked star polymers synthesized via a grafting-through approach using RCMP.

Versatile Types of Cyclodextrin-Based Nucleic Acid Delivery Systems

Nowadays, nucleic acid therapy has become a promising way for the treatment of various malignant diseases. Cyclodextrin (CD)-based nucleic acid delivery systems have attracted widespread attention due to the favorable chemical structures and excellent biological properties of CD. Recently, a variety of CD-based nucleic acid delivery systems has been designed according to the different functions of CD for flexible gene therapies. In this review, the construction strategies and biomedical applications of CD-based nucleic acid delivery systems are mainly focused on. The review begins with an introduction to the synthesis and properties of simple CD-grafted polycations. Thereafter, CD-related supramolecular assemblies based on different guest components are discussed in detail. Finally, different CD-based organic/inorganic nanohybrids and their relevant functions are demonstrated. It is hoped that this brief review will motivate the delicate design of CD-based nucleic acid delivery systems for potential clinical applications.

Reversible-deactivation radical polymerization of vinyl acetate mediated by tralen, an organomediator

An organic compound, tralen, has been developed as a mediator to control the radical polymerization of vinyl acetate, methyl acrylate, and N-vinyl pyrrolidone via the reversible termination mechanism.

Reduction-responsive double hydrophilic block copolymer nano-capsule synthesized via RCMP-PISA

Double hydrophilic block copolymer vesicles synthesized via RCMP-PISA are degradable under a reductive conditions.

A covalent organic framework as a photocatalyst for atom transfer radical polymerization under white light irradiation

TFPPy-Td-COFs have been synthesized to serve as heterogeneous photocatalysts for mediating photo-induced ATRP with copper as a co-catalyst under white light irradiation.

First polyallene-based well-defined amphiphilic diblock copolymer via RAFT polymerization

We report the first well-defined amphiphilic diblock copolymer via allene and vinyl monomers.

Information-Based Design of Polymeric Drug Formulation Additives

Tailor-made copolymers are designed based on a peptide-poly(ethylene glycol) (QFFLFFQ-PEG) conjugate as a blueprint, to solubilize the photosensitizer meta-tetra(hydroxyphenyl)chlorin (m-THPC). The relevant functionalities of the parent peptide-PEG are mimicked by employing monomer pairs that copolymerize in a strictly alternating manner. While styrene (S) or 4-vinylbenzyl-phthalimide (VBP) provide aromatic moieties like Phe, the aliphatic isobutyl side chain of Leu4 is mimicked by maleic anhydride (MA) that reacts after polymerization with isobutylamine to give the isobutylamide-carboxyl functional unit (iBuMA). A set of copolymer-PEG solubilizers is synthesized by controlled radical polymerization, systematically altering the length of the functional segment (DP_n = 2, 4, 6) and the side chain functionalization (iBuMA, iPrMA, MeMA). The m-THPC hosting and release properties of P[S-alt-iBuMA]₆-PEG reached higher payload capacities and more favored release rates than the parent peptide-PEG conjugate. Interestingly, P[S-alt-RMA]_n-PEG mimics the sensitivity of the peptide-PEG solubilizer well, where the exchange of Leu4 residue by Val and Ala significantly reduces the drug loading by 92%. A similar trend is found with P[S-alt-RMA]_n-PEG as the exchange of iBu → iPr → Me reduces the payload capacity up to 78%.

Design of a Bioinert Interface Using an Amphiphilic Block Copolymer Containing a Bottlebrush Unit of Oligo(oxazoline)

We designed an amphiphilic block copolymer, poly(methyl methacrylate)-block-poly[oligo(2-ethyl-2-oxazoline) methacrylate] (PMMA-b-P[O(Ox)MA]), suitable for bioinert coating. Angular-dependent X-ray photoelectron spectroscopy and neutron reflectivity measurements revealed that the outermost surface of a dried film of PMMA-b-P[O(Ox)MA] was covered with the PMMA block-rich layer. Once the film came into contact with water, the P[O(Ox)MA] bottlebrush block was segregated toward the water interface. This structural rearrangement in the outermost region of the film resulted in the formation of the swollen oligo(oxazoline) layer with excellent bioinertness in terms of the suppression of serum protein adsorption and NIH3T3 fibroblast adhesion.

Solvent Effects and Side Reactions in Organocatalyzed Atom Transfer Radical Polymerization for Enabling the Controlled Polymerization of Acrylates Catalyzed by Diaryl Dihydrophenazines

Investigation of the effects of a solvent on the photophysical and redox properties of the photoredox catalyst (PC), N,N-di(2-naphthyl)-5,10-dihydrophenazine (PC 1), revealed the opportunity to use tetrahydrofuran (THF) to modulate the reactivity of PC 1 toward achieving a controlled organocatalyzed atom transfer radial polymerization (O-ATRP) of acrylates. Compared with dimethylacetamide (DMAc), in tetrahydrofuran (THF), PC 1 exhibits a higher quantum yield of intersystem crossing (ΦISC = 0.02 in DMAc, 0.30 in THF), a longer singlet excited-state lifetime (τ Singlet = 3.81 ns in DMAc, 21.5 ns in THF), and a longer triplet excited-state lifetime (τ Triplet = 4.3 μs in DMAc, 15.2 μs in THF). Destabilization of 1 •+, the proposed polymerization deactivator, in THF leads to an increase in the oxidation potential of this species by 120 mV (E 1/2 0 = 0.22 V vs SCE in DMAc, 0.34 V vs SCE in THF). The O-ATRP of n-butyl acrylate (n-BA) catalyzed by PC 1 proceeds in a more controlled fashion in THF than in DMAc, producing P(n-BA) with low dispersity, Đ (Đ < 1.2). Model reactions and spectroscopic experiments revealed that two initiator-derived alkyl radicals add to the core of PC 1 to form an alkyl-substituted photocatalyst (2) during the polymerization. PC 2 accesses a polar CT excited state that is ~40 meV higher in energy than PC 1 and forms a slightly more oxidizing radical cation (E 1/2 0 = 0.22 V for 1 •+ and 0.25 V for 2 •+ in DMAc). A new O-ATRP procedure was developed wherein PC 1 is converted to 2 in situ. The application of this method enabled the O-ATRP of a number of acrylates to proceed with moderate to good control (Đ = 1.15-1.45 and I* = 83-127%).

Hydrophilic Surface Functionalization of Electrospun Nanofibrous Scaffolds in Tissue Engineering

Electrospun polymer nanofibers have received much attention in tissue engineering due to their valuable properties such as biocompatibility, biodegradation ability, appropriate mechanical properties, and, most importantly, fibrous structure, which resembles the morphology of extracellular matrix (ECM) proteins. However, they are usually hydrophobic and suffer from a lack of bioactive molecules, which provide good cell adhesion to the scaffold surface. Post-electrospinning surface functionalization allows overcoming these limitations through polar groups covalent incorporation to the fibers surface, with subsequent functionalization with biologically active molecules or direct deposition of the biomolecule solution. Hydrophilic surface functionalization methods are classified into chemical approaches, including wet chemical functionalization and covalent grafting, a physiochemical approach with the use of a plasma treatment, and a physical approach that might be divided into physical adsorption and layer-by-layer assembly. This review discusses the state-of-the-art of hydrophilic surface functionalization strategies of electrospun nanofibers for tissue engineering applications. We highlighted the major advantages and drawbacks of each method, at the same time, pointing out future perspectives and solutions in the hydrophilic functionalization strategies.

Aluminum Tralen Complex Meditated Reversible-Deactivation Radical Polymerization of Vinyl Acetate

The Al^III(tralen)Cl complex (tralenH₂ = N,N'-di(cyclohepta-2,4,6-trien-1-one-2-yl)-1,2-diaminobenzene) has been synthesized and applied to mediate the reversible-deactivation radical polymerization (RDRP) of vinyl monomers. The polymerization of unconjugated monomers such as vinyl acetate (VAc) and N-vinylpyrrolidone (NVP) with Al^III(tralen)Cl showed the living characters of linearly increased molecular weight with conversion and formation of block copolymer. However, the control manners in the polymerization of conjugated monomers like acrylates and styrene were limited. The electron paramagnetic resonance (EPR) spectrum indicated that Al^III(tralen)BArF (BArF = tetrakis(3,5-trifluormethylphenyl)borate) and propagating radicals formed a paramagnetic dormant species, possibly PVAc-Al^III(tralen)BArF, via the single-electron transfer to the tralen ligand.

Towards lignin derived thermoplastic polymers

Lignin is the second most abundant biobased material found on earth. It is produced mainly as a byproduct of pulp and paper industry and biorefineries. Despite its abundance, lignin valorization is not achieved on a large scale. Recently, there has been a growing demand for using the renewable and biodegradable raw materials in the commodity polymers. Potential use of lignin as a component in thermoplastic polymers is a promising approach for its value-added utilization. Given the vast applications of thermoplastic materials, there is lack of comprehensive review on lignin based thermoplastic polymers in literature. This review focuses on the utilization of lignin as functional and structural component of the thermoplastic polymers which requires structural modifications of lignin pertaining to the polymeric system. First, various lignin modifications were discussed in view of controlling the homogeneity, reactivity, processability and compatibility of lignin for successful thermoplastic copolymer synthesis and blend processing. Then, various copolymerization methodologies of lignin applicable for thermoplastic monomers are reviewed. Lastly, the lignin based thermoplastic blends are discussed which covers the lignin blends with various thermoplastic polymers and the chemical modifications required to improve its compatibility in polymer matrix. Some of the promising potential applications and future perspectives to achieve the goal of lignin-based commercial thermoplastics polymers are addressed.

Polyene-Free Photoluminescent Polymers via Hydrothermal Hydrolysis of Polyacrylonitrile in Neutral Water

We report the hydrothermally enhanced hydrolysis of polyacrylonitrile (PAN) in neutral water, which generates photoluminescent polymers with low unsaturation degrees. Despite the hydrophobic nature of PAN, the product can be dissolved in water at a high concentration (≥100 g/L). The product exhibits complete absence of alkenes or aromatic structures, and photoluminescence originates from newly formed N- and O-containing groups. The presence of both n to π* and π to π* transitions is confirmed by time-dependent density functional theory (TD-DFT) calculations. The efficient transformation of PAN benefits from the enhanced hydrolysis of nitrile groups. While similar reactions have been reported previously under alkaline environments, we demonstrate that efficient hydrolysis can also occur in neutral water under the hydrothermal condition. Two additional methods based on different mechanisms are discussed to demonstrate the simplicity and efficiency of the hydrothermal reaction.

Metal-Free ATRP Catalyzed by Visible Light in Continuous Flow

ATRP of methyl methacrylate catalyzed by Eosin Y, an inexpensive and an environmental benign dye, was performed in a continuous flow reactor made of FEP tubing and irradiated by visible light green LEDs. The reaction under flow conditions was significantly more rapid and controlled compared to that in batch giving 90% of polymerization after only 3 h of irradiation. The formed polymers in flow have M n measured by GPC and DOSY NMR in accordance with the theoretical values and show low dispersities (Ð < 1.5). The livingness of the polymers has been confirmed by LED on and LED off experiments and by the synthesis of block copolymers. The protocol described herein serves as a "proof of concept" of using Eosin Y as a photocatalyst for controlled polymerization and of using 1D and 2D NMR for polymer characterization. The protocol could be replicated in the future for other reversible-deactivation radical polymerizations.

Fully oxygen-tolerant atom transfer radical polymerization triggered by sodium pyruvate

ATRP (atom transfer radical polymerization) is one of the most robust reversible deactivation radical polymerization (RDRP) systems. However, the limited oxygen tolerance of conventional ATRP impedes its practical use in an ambient atmosphere. In this work, we developed a fully oxygen-tolerant PICAR (photoinduced initiators for continuous activator regeneration) ATRP process occurring in both water and organic solvents in an open reaction vessel. Continuous regeneration of the oxidized form of the copper catalyst with sodium pyruvate through UV excitation allowed the chemical removal of oxygen from the reaction mixture while maintaining a well-controlled polymerization of N-isopropylacrylamide (NIPAM) or methyl acrylate (MA) monomers. The polymerizations of NIPAM were conducted with 250 ppm (with respect to the monomer) or lower concentrations of CuBr2 and a tris[2-(dimethylamino)ethyl]amine ligand. The polymers were synthesized to nearly quantitative monomer conversions (>99%), high molecular weights (M n > 270 000), and low dispersities (1.16 < Đ < 1.44) in less than 30 min under biologically relevant conditions. The reported method provided a well-controlled ATRP (Đ = 1.16) of MA in dimethyl sulfoxide despite oxygen diffusion from the atmosphere into the reaction system.

Macromolecular Engineering by Applying Concurrent Reactions with ATRP

Modern polymeric material design often involves precise tailoring of molecular/supramolecular structures which is also called macromolecular engineering. The available tools for molecular structure tailoring are controlled/living polymerization methods, click chemistry, supramolecular polymerization, self-assembly, among others. When polymeric materials with complex molecular architectures are targeted, it usually takes several steps of reactions to obtain the aimed product. Concurrent polymerization methods, i.e., two or more reaction mechanisms, steps, or procedures take place simultaneously instead of sequentially, can significantly reduce the complexity of the reaction procedure or provide special molecular architectures that would be otherwise very difficult to synthesize. Atom transfer radical polymerization, ATRP, has been widely applied in concurrent polymerization reactions and resulted in improved efficiency in macromolecular engineering. This perspective summarizes reported studies employing concurrent polymerization methods with ATRP as one of the reaction components and highlights future research directions in this area.

Progress Toward Sustainable Reversible Deactivation Radical Polymerization

The recent focus of media and governments on renewability, green chemistry, and circular economy has led to a surge in the synthesis of renewable monomers and polymers. In this review, focussing on renewable monomers for reversible deactivation radical polymerizations (RDRP), it is highlighted that for the majority of the monomers and polymers reported, the claim to renewability is not always accurate. By closely examining the sustainability of synthetic routes and the renewability of starting materials, fully renewable monomers are identified and discussed in terms of sustainability, polymerization behavior, and properties obtained after polymerization. The holistic discussion considering the overall preparation process of polymers, that is, monomer syntheses, origin of starting materials, solvents used, the type of RDRP technique utilized, and the purification method, allows to highlight certain topics which need to be addressed in order to progress toward not only (partially) renewable, but sustainable monomers and polymers using RDRPs.

The rise of bio-inspired polymer compartments responding to pathology-related signals

Self-organized nano- and microscale polymer compartments such as polymersomes, giant unilamellar vesicles (GUVs), polyion complex vesicles (PICsomes) and layer-by-layer (LbL) capsules have increasing potential in many sensing applications. Besides modifying the physicochemical properties of the corresponding polymer building blocks, the versatility of these compartments can be markedly expanded by biomolecules that endow the nanomaterials with specific molecular and cellular functions. In this review, we focus on polymer-based compartments that preserve their structure, and highlight the key role they play in the field of medical diagnostics: first, the self-assembling abilities that result in preferred architectures are presented for a broad range of polymers. In the following, we describe different strategies for sensing disease-related signals (pH-change, reductive conditions, and presence of ions or biomolecules) by polymer compartments that exhibit stimuli-responsiveness. In particular, we distinguish between the stimulus-sensitivity contributed by the polymer itself or by additional compounds embedded in the compartments in different sensing systems. We then address necessary properties of sensing polymeric compartments, such as the enhancement of their stability and biocompatibility, or the targeting ability, that open up new perspectives for diagnostic applications.

Role of Polar Protic Solvents in the Dissociation and Reactivity of Photogenerated Radical Ion Pairs

The UV photolysis of bimolecular charge transfer complexes is employed to yield reactive radical ions in their solvent-equilibrated electronic ground state. In polar protic media, noncovalent complexes of 1,2,4,5-tetracyanobenzene and toluene undergo efficient, ultrafast dissociation to ion pairs and equilibrate with their solvent environment before the resulting radical ions engage in electron transfer and proton abstraction on subnanosecond time scales. Solvent molecules play a critical role in these reactive pathways and in the dissociation and relaxation processes that precede them. We report a clear separation of time scales for these relaxation and reactive processes, which implies that solvent-solute interactions can be used as a tool for tuning the reaction pathways of equilibrated radical ions in solution.