Divergent Synthesis of Graft and Branched Copolymers through Spatially Controlled Photopolymerization in Flow Reactors

Using RAFT Polymerization Methodologies to Create Branched and Nanogel-Type Copolymers

This review aims to highlight the most recent advances in the field of the synthesis of branched copolymers and nanogels using reversible addition-fragmentation chain transfer (RAFT) polymerization. RAFT polymerization is a reversible deactivation radical polymerization technique (RDRP) that has gained tremendous attention due to its versatility, compatibility with a plethora of functional monomers, and mild polymerization conditions. These parameters lead to final polymers with good control over the molar mass and narrow molar mass distributions. Branched polymers can be defined as the incorporation of secondary polymer chains to a primary backbone, resulting in a wide range of complex macromolecular architectures, like star-shaped, graft, and hyperbranched polymers and nanogels. These subcategories will be discussed in detail in this review in terms of synthesis routes and properties, mainly in solutions.

Unleashing the Power of PET-RAFT Polymerization: Journey from Porphyrin-Based Photocatalysts to Combinatorial Technologies and Advanced Bioapplications

The emergence of photoinduced energy/electron transfer-reversible addition-fragmentation chain transfer polymerization (PET-RAFT) not only revolutionized the field of photopolymerization but also accelerated the development of porphyrin-based photocatalysts and their analogues. The continual expansion of the monomer family compatible with PET-RAFT polymerization enhances the range of light radiation that can be harnessed, providing increased flexibility in polymerization processes. Furthermore, the versatility of PET-RAFT polymerization extends beyond its inherent capabilities, enabling its integration with various technologies in diverse fields. This integration holds considerable promise for the advancement of biomaterials with satisfactory bioapplications. As researchers delve deeper into the possibilities afforded by PET-RAFT polymerization, the collaborative efforts of individuals from diverse disciplines will prove invaluable in unleashing its full potential. This Review presents a concise introduction to the fundamental principles of PET-RAFT, outlines the progress in photocatalyst development, highlights its primary applications, and offers insights for future advancements in this technique, paving the way for exciting innovations and applications.

Amide-Containing Bottlebrushes via Continuous-Flow Photoiniferter Reversible Addition-Fragmentation Chain Transfer Polymerization: Micellization Behavior

Herein, a series of ternary amphiphilic amide-containing bottlebrushes were synthesized by photoiniferter (PI-RAFT) polymerization of macromonomers in continuous-flow mode using trithiocarbonate as a chain transfer agent. Visible light-mediated polymerization of macromonomers under mild conditions enabled the preparation of thermoresponsive copolymers with low dispersity and high yields in a very short time, which is not typical for the classical reversible addition-fragmentation chain transfer process. Methoxy oligo(ethylene glycol) methacrylate and alkoxy(C12-C14) oligo(ethylene glycol) methacrylate were used as the basic monomers providing amphiphilic and thermoresponsive properties. The study investigated how modifying comonomers, acrylamide (AAm), methacrylamide (MAAm), and N-methylacrylamide (-MeAAm) affect the features of bottlebrush micelle formation, their critical micelle concentration, and loading capacity for pyrene, a hydrophobic drug model. The results showed that the process is scalable and can produce tens of grams of pure copolymer per day. The unmodified copolymer formed unimolecular micelles at temperatures below the LCST in aqueous solutions, as revealed by DLS and SLS data. The incorporation of AAm, MAAm, and N-MeAAm units resulted in an increase in micelle aggregation numbers. The resulting bottlebrushes formed uni- or bimolecular micelles at extremely low concentrations. These micelles possess a high capacity for loading pyrene, making them a promising choice for targeted drug delivery.

Emulsion-Directed Synthesis of Poly-Porphyrin Nanoparticles as Heterogeneous Photocatalysts for PET-RAFT Polymerization

Heterogeneous photocatalysts have attracted extensive attention in photo-induced electron transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization due to their remarkable advantages such as easy preparation, tunable photoelectric properties, and recyclability. In this study, zinc (II) 5,10,15,20-tetrakis(4-aminophenyl)porphyrin (ZnTAPP)-based poly-porphyrin nanoparticles (PTAPP-Zn) are constructed by an emulsion-directed approach. It is investigated as a heterogeneous photocatalyst for PET-RAFT polymerization of various methacrylate monomers under visible light exposure, and the reactions show refined polymerization control with high monomer conversions. Furthermore, it is demonstrated that the PTAPP-Zn nanoparticles with the larger pore size enhance photocatalytic activity in PET-RAFT polymerization. In addition, the capabilities of oxygen tolerance and temporal control are demonstrated and PTAPP-Zn particles can be easily recycled and reused without an obvious decrease in catalytic efficiency.

Inverse Miniemulsion Enables the Continuous-Flow Synthesis of Controlled Ultra-High Molecular Weight Polymers

We report the controlled synthesis of ultra-high molecular weight (UHMW) polymers (Mn ≥ 106 g/mol) via continuous flow in a tubular reactor. At high monomer conversion, UHMW polymers in homogeneous batch polymerization exhibit high viscosities that pose challenges for employing continuous flow reactors. However, under heterogeneous inverse miniemulsion (IME) conditions, UHMW polymers can be produced within the dispersed phase, while the viscosity of the heterogeneous mixture remains approximately the same as the viscosity of the continuous phase. Conducting such IME polymerizations in flow results in a faster rate of polymerization compared to batch IME polymerizations while still providing excellent control over molecular weight up to 106 g/mol. Crucial emulsion parameters, such as particle size and stability under continuous flow conditions, were examined using dynamic light scattering. A range of poly(N,N-dimethylacrylamide) and poly(4-acryloylmorpholine) polymers with molecular weights of 104-106 g/mol (Đ ≤ 1.31) were produced by this method using water-soluble trithiocarbonates as photoiniferters.

Photo-reactive polymers for the immobilisation of epidermal growth factors

Photo-reactive polymers are important for biomaterials, including devices with a 3D-structure. Here, different types of photo-reactive polymers were prepared and utilised for immobilisation of growth factors. They were synthesised by conjugation of gelatin with the azidophenyl group or by copolymerisation of the azidophenyl group-coupled methacrylate with poly(ethylene glycol) methacrylate. The azidophenyl content and the zeta potential of the prepared polymers were measured. After spin coating of polymers, the thickness and the water contact angle of coated layers were measured. The amount of the immobilised epidermal growth factor (EGF) was determined using fluorescence labelling. Cell adhesion responded to the nature of photo-reactive polymers but did not depend on the immobilised EGF. However, cell growth was dependent on the amount of immobilised EGF and was significantly affected by the nature of photo-reactive polymers. The study shows that the properties of the photo-immobilisation matrix significantly influence the biological activity.

Controllable synthesis and structural design of novel all-organic polymers toward high energy storage dielectrics

As the core unit of energy storage equipment, high voltage pulse capacitor plays an indispensable role in the field of electric power system and electromagnetic energy related equipment. The mostly utilized polymer materials are metallized polymer thin films, which are represented by biaxially oriented polypropylene (BOPP) films, possessing the advantages including low cost, high breakdown strength, excellent processing ability, and self-healing performance. However, the low dielectric constant (εr < 3) of traditional BOPP films makes it impossible to meet the demand for increased high energy density. Controlled/living radical polymerization (CRP) and related techniques have become a powerful approach to tailor the chemical and physical properties of materials and have given rise to great advances in tuning the properties of polymer dielectrics. Although organic-inorganic composite dielectrics have received much attention in previous studies, all-organic polymer dielectrics have been proven to be the most promising choice because of its light weight and easy large-scale continuous processing. In this short review, we begin with some basic theory of polymer dielectrics and some theoretical considerations for the rational design of dielectric polymers with high performance. In the guidance of these theoretical considerations, we review recent progress toward all-organic polymer dielectrics based on two major approaches, one is to control the polymer chain structure, containing microscopic main-chain and side-chain structures, by the method of CRP and the other is macroscopic structure design of all-organic polymer dielectric films. And various chemistry and compositions are discussed within each approach.

Double thermoresponsive graft copolymers with different chain ends: feasible precursors for covalently crosslinked hydrogels

The tailored synthesis of graft copolymers from acrylic and methacrylic monomers can be accomplished solely through photoiniferter reversible addition-fragmentation chain transfer (RAFT) polymerization. Samples with poly[oligo(ethylene glycol) methacrylate] (POEGMA) backbones synthesized under green light irradiation and poly(N-isopropylacrylamide) (PNIPAM) side chains growing under blue light irradiation are presented. As monitored by temperature-dependent dynamic light scattering (DLS) measurements and temperature-variable nuclear magnetic resonance (NMR) spectroscopy, the architecture of the graft copolymers allows unique two-step lower critical solution temperature (LCST) transitions in aqueous solutions. Meanwhile, different end-groups introduced by the corresponding RAFT agents affect the detailed thermoresponsive behavior remarkably. This RAFT strategy shows more advantages when the multiple trithiocarbonate groups are converted into thiol reactive pyridyl disulfide (PDS) groups via a facile post-polymerization modification. The PDS-terminated graft copolymer can then be regarded as a usable precursor for various applications, such as thermoresponsive hydrogels.

Synthesis of Cross-Linked Block Copolymer Nano-Assemblies and their Coating Application

Since the discovery of polymerization-induced self-assembly (PISA), convenient synthesis of concentrated block copolymer nano-assemblies dispersed in solvent has been achieved. Now, application of block copolymer nano-assemblies should be paid more attention. In this study, corona-cross-linked block copolymer nanoparticles of poly[dimethylacrylamide-co-(diacetone acrylamide)]-b-polystyrene [P(DMA-co-DAAM)-b-PS] containing the poly(DAAM) segment in the hydrophilic P(DMA-co-DAAM) block are synthesized initially by PISA following dispersion RAFT polymerization and then by covalent intraparticle cross-linking through the poly(DAAM) segment and adipic acid dihydrazide (ADH). Coating application of the corona-cross-linked block copolymer nano-assemblies is tried, and much higher water resistance of the corona-cross-linked block copolymer nano-assemblies than that of the linear block copolymer nano-assemblies is demonstrated. This article is protected by copyright. All rights reserved.

Selective Photoactivation of Trithiocarbonates Mediated by Metal Naphthalocyanines and Overcoming Activation Barriers Using Thermal Energy

Metal naphthalocyanines (MNcs) were demonstrated to be efficient photocatalysts to activate photoinduced electron-transfer reversible addition-fragmentation chain transfer (PET-RAFT) polymerization, enabling well-controlled polymerization of (meth)acrylates under near-infrared (λ = 780 nm) light. Owing to their lower redox potential compared to previously explored photocatalysts, the activation of trithiocarbonate RAFT agents exhibited a unique selectivity that was dependent on the nature of the R group. Specifically, MNcs were capable in activating tertiary R group trithiocarbonates, whereas no activation of the trithiocarbonate possessing a secondary R group was observed. The combination of density functional theory calculations and experimental studies have revealed new mechanistic insights into the factors governing a PET-RAFT mechanism and explained this unique selectivity of MNcs toward tertiary carbon trithiocarbonates. Interestingly, by increasing the reaction temperature moderately (i.e., ∼15 °C), the energy barrier prohibiting the photoactivation of the trithiocarbonate with a secondary R group was overcome, enabling their successful activation.

Random and Diblock Thermoresponsive Oligo(ethylene glycol)-Based Copolymers Synthesized via Photo-Induced RAFT Polymerization

Amphiphilic random and diblock thermoresponsive oligo(ethylene glycol)-based (co)polymers were synthesized via photoiniferter polymerization under visible light using trithiocarbonate as a chain transfer agent. The effect of solvent, light intensity and wavelength on the rate of the process was investigated. It was shown that blue and green LED light could initiate RAFT polymerization of macromonomers without an exogenous initiator at room temperature, giving bottlebrush polymers with low dispersity at sufficiently high conversions achieved in 1-2 h. The pseudo-living mechanism of polymerization and high chain-end fidelity were confirmed by successful chain extension. Thermoresponsive properties of the copolymers in aqueous solutions were studied via turbidimetry and laser light scattering. Random copolymers of methoxy- and alkoxy oligo(ethylene glycol) methacrylates of a specified length formed unimolecular micelles in water with a hydrophobic core consisting of a polymer backbone and alkyl groups and a hydrophilic oligo(ethylene glycol) shell. In contrast, the diblock copolymer formed huge multimolecular micelles.

A Photoinduced Dual-Wavelength Approach for 3D Printing and Self-Healing of Thermosetting Materials

Vat photopolymerization-based 3D printing techniques have been widely used to produce high-resolution 3D thermosetting materials. However, the lack of repairability of these thermosets leads to the production of waste. In this study, reversible addition fragmentation chain transfer (RAFT) agents are incorporated into resin formulations to allow visible light (405 nm) mediated 3D printing of materials with self-healing capabilities. The self-healing process is based on the reactivation of RAFT agent embedded in the thermosets under UV light (365 nm), which enables reformation of the polymeric network. The self-healing process can be performed at room temperature without prior deoxygenation. The impact of the type and concentration of RAFT agents in the polymer network on the healing efficiency is explored. Resins containing RAFT agents enable 3D printing of thermosets with self-healing properties, broadening the scope of future applications for polymeric thermosets in various fields.

Photo-Iniferter RAFT Polymerization

Light-mediated polymerization techniques offer distinct advantages over polymerization reactions fueled by thermal energy, such as high spatial and temporal control as well as the possibility to work under mild reaction conditions. Reversible addition-fragmentation chain-transfer (RAFT) polymerization is a highly versatile radical polymerization method that can be utilized to control a variety of monomers and produce a vast number of complex macromolecular structures. The use of light to drive a RAFT-polymerization is possible via multiple routes. Besides the use of photo-initiators, or photo-catalysts, the direct activation of the chain transfer agent controlling the RAFT process in a photo-iniferter (PI) process is an elegant way to initiate and control polymerization reactions. Within this review, PI-RAFT polymerization and its advantages over the conventional RAFT process are discussed in detail.

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

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