Toward the Rational Design of Organic Catalysts for Organocatalysed Atom Transfer Radical Polymerisation

Thanks to their diversity, organic photocatalysts (PCs) have been widely used in manufacturing polymeric products with well-defined molecular weights, block sequences, and architectures. Still, however, more universal property-performance relationships are needed to enable the rational design of such PCs. That is, a set of unique descriptors ought to be identified to represent key properties of the PCs relevant for polymerisation. Previously, the redox potentials of excited PCs (PC*) were used as a good descriptor for characterising very structurally similar PCs. However, it fails to elucidate PCs with diverse chromophore cores and ligands, among which those used for polymerisation are a good representative. As showcased by model systems of organocatalysed atom transfer radical polymerisation (O-ATRP), new universal descriptors accounting for additional factors, such as the binding and density overlap between the PC* and initiator, are proposed and proved to be successful in elucidating the experimental performances of PCs in polymerisation. While O-ATRP is exemplified here, the approach adopted is general for studying other photocatalytic systems.

Solvent-Free Chemical Recycling of Polymethacrylates made by ATRP and RAFT polymerization: High-Yielding Depolymerization at Low Temperatures

Although controlled radical polymerization is an excellent tool to make precision polymeric materials, reversal of the process to retrieve the starting monomer is far less explored despite the significance of chemical recycling. Here, we investigate the bulk depolymerization of RAFT and ATRP-synthesized polymers under identical conditions. RAFT-synthesized polymers undergo a relatively low-temperature solvent-free depolymerization back to monomer thanks to the partial in situ transformation of the RAFT end-group to macromonomer. Instead, ATRP-synthesized polymers can only depolymerize at significantly higher temperatures (>350 °C) through random backbone scission. To aid a more complete depolymerization at even lower temperatures, we performed a facile and quantitative end-group modification strategy in which both ATRP and RAFT end-groups were successfully converted to macromonomers. The macromonomers triggered a lower temperature bulk depolymerization with an onset at 150 °C yielding up to 90 % of monomer regeneration. The versatility of the methodology was demonstrated by a scalable depolymerization (≈10 g of starting polymer) retrieving 84 % of the starting monomer intact which could be subsequently used for further polymerization. This work presents a new low-energy approach for depolymerizing controlled radical polymers and creates many future opportunities as high-yielding, solvent-free and scalable depolymerization methods are sought.

Polymerisation-Induced Self-Assembly of Graft Copolymers

We report the polymerisation-induced self-assembly of poly(lauryl methacrylate)-graft-poly(benzyl methacrylate) copolymers during reversible addition-fragmentation chain transfer (RAFT) grafting from polymerisation in a backbone-selective solvent. Electron microscopy images suggest the phase separation of grafts to result in a network of spherical particles, due to the ability of the branched architecture to freeze chain entanglements and to bridge core domains. Small-angle X-ray scattering data suggest the architecture promotes the formation of multicore micelles, the core morphology of which transitions from spheres to worms, vesicles, and inverted micelles with increasing volume fraction of the grafts. A time-resolved SAXS study is presented to illustrate the formation of the inverted phase during a polymerisation. The grafted architecture gives access to unusual morphologies and provides exciting new handles for controlling the polymer structure and material properties.

Thermoresponsive Self-Assembly of Twofold Fluorescently Labeled Block Copolymers in Aqueous Solution and Microemulsions

A nonionic double hydrophilic block copolymer with a long permanently hydrophilic and a small thermoresponsive block is synthesized by reversible addition-fragmentation chain-transfer polymerization (RAFT). By employing a specifically designed chain-transfer agent, the polymer is functionalized with complementary end groups which are suited for Förster resonance energy transfer (FRET). The end group attached to the permanently hydrophilic block of poly(N,N-dimethylacrylamide) pDMAm is designed as a permanently hydrophobic segment ("sticker") comprising a long alkyl chain and the 4-aminonaphthalimide fluorophore. The other end attached to the thermoresponsive block of poly(N-isopropylacrylamide) pNiPAm incorporates a coumarin fluorophore. The temperature-dependent self-assembly of the twofold fluorescently labeled copolymer is studied in pure aqueous solution as well as in an o/w microemulsion by several techniques including turbidimetry, dynamic light scattering (DLS), and fluorescence spectroscopy. It is compared to the behaviors of the analogous twofold-labeled pDMAm and pNiPAm homopolymer references. The findings indicate that the block copolymer behaves as a polymeric surfactant at low temperatures, with one relatively small hydrophobic end block and an extended hydrophilic chain forming "hairy micelles". At elevated temperatures above the LCST phase transition of the pNiPAm block, however, the copolymer behaves as an associative telechelic polymer with two nonsymmetrical hydrophobic end blocks, which do not mix. Thus, instead of a network of bridged "flower micelles", large dynamic aggregates are formed. These are connected alternatingly by the original micellar cores as well as by clusters of the collapsed pNiPAm blocks. This type of structure is even more favored in the o/w microemulsion than in pure aqueous solution, as the microemulsion droplets constitute an attractive anchoring point for the hydrophobic dodecyl sticker but not for the collapsed pNiPAm chains.

RAFT-Based Polymers for Click Reactions

The parallel development of reversible deactivation radical polymerization and click reaction concepts significantly enriches the toolbox of synthetic polymer chemistry. The synergistic effect of combining these approaches manifests itself in a growth of interest to the design of well-defined functional polymers and their controlled conjugation with biomolecules, drugs, and inorganic surfaces. In this review, we discuss the results obtained with reversible addition-fragmentation chain transfer (RAFT) polymerization and different types of click reactions on low- and high-molar-mass reactants. Our classification of literature sources is based on the typical structure of macromolecules produced by the RAFT technique. The review addresses click reactions, immediate or preceded by a modification of another type, on the leaving and stabilizing groups inherited by a growing macromolecule from the chain transfer agent, as well as on the side groups coming from monomers entering the polymerization process. Architecture and self-assembling properties of the resulting polymers are briefly discussed with regard to their potential functional applications, which include drug delivery, protein recognition, anti-fouling and anti-corrosion coatings, the compatibilization of polymer blends, the modification of fillers to increase their dispersibility in polymer matrices, etc.

Shape-Controlled Nanoparticles from a Low-Energy Nanoemulsion

Nanoemulsion technology enables the production of uniform nanoparticles for a wide range of applications. However, existing nanoemulsion strategies are limited to the production of spherical nanoparticles. Here, we describe a low-energy nanoemulsion method to produce nanoparticles with various morphologies. By selecting a macro-RAFT agent (poly(di(ethylene glycol) ethyl ether methacrylate-co-N-(2-hydroxypropyl) methacrylamide) (P(DEGMA-co-HPMA))) that dramatically lowers the interfacial tension between monomer droplets and water, we can easily produce nanoemulsions at room temperature by manual shaking for a few seconds. With the addition of a common ionic surfactant (SDS), these nanoscale droplets are robustly stabilized at both the formation and elevated temperatures. Upon polymerization, we produce well-defined block copolymers forming nanoparticles with a wide range of controlled morphologies, including spheres, worm balls, worms, and vesicles. Our nanoemulsion polymerization is robust and well-controlled even without stirring or external deoxygenation. This method significantly expands the toolbox and availability of nanoemulsions and their tailor-made polymeric nanomaterials.

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.

Dual Roles of Amino-Functionalized Silicon Quantum Dots (SiQDs) for Visible-Light-Induced Surface-Initiated PET-RAFT Polymerization on Substrates

Silicon quantum dots (SiQDs) are explored for the first time as an efficient photocatalyst for visible-light-regulated reversible addition-fragmentation chain transfer (RAFT) polymerization. The fluorescence quenching confirmed the photoinduced electron transfer (PET) between SiQDs and RAFT reagents. Besides all features of controlled radical polymerization, the SiQDs catalyzed PET-RAFT polymerization also exhibit good temporal control, high chain-end fidelity, and versatility with diverse monomers. Moreover, amino-functionalized SiQDs can be easily coated on the surface of substrates (silicon wafer) owing to the electrostatic interaction, and play a dual role of polymer-substrate connector and photocatalyst for the surface-initiated PET-RAFT polymerization. The SiQD-coated wafer was also proved to be an efficient recycle photocatalyst for PET-RAFT polymerization.

Rapidly self-deoxygenating controlled radical polymerization in water via in situ disproportionation of Cu(i)

Rapidly self-deoxygenating Cu-RDRP in aqueous media is investigated. The disproportionation of Cu(i)/Me6Tren in water towards Cu(ii) and highly reactive Cu(0) leads to O2-free reaction environments within the first seconds of the reaction, even when the reaction takes place in the open-air. By leveraging this significantly fast O2-reducing activity of the disproportionation reaction, a range of well-defined water-soluble polymers with narrow dispersity are attained in a few minutes or less. This methodology provides the ability to prepare block copolymers via sequential monomer addition with little evidence for chain termination over the lifetime of the polymerization and allows for the synthesis of star-shaped polymers with the use of multi-functional initiators. The mechanism of self-deoxygenation is elucidated with the use of various characterization tools, and the species that participate in the rapid oxygen consumption is identified and discussed in detail.

Generators of Maximal Subgroups of Harada-Norton and some Linear Groups

Group theory, the ultimate theory for symmetry, is a powerful tool that has a direct impact on research in robotics, computer vision, computer graphics and medical image analysis. Symmetry is very important in chemistry research and group theory is the tool that is used to determine symmetry. Usually, it is not only the symmetry of molecule but also the symmetries of some local atoms, molecular orbitals, rotations and vibrations of bonds, etc. that are important. Harada-Norton group is an example of a sporadic simple group. There are 14 maximal subgroups of Harada-Norton group. Generators (also known as words) of 11 maximal subgroups are already known. The aim of this note is to give generators of the remaining 3 maximal subgroups, which is an open problem mentioned on A World-wide-web Atlas of Group Representations (http://brauer.maths.qmul.ac.uk/Atlas) [1]. In this report we compute the generators of A6 × A6.D8, 23+2+6.(3 × L3(2)) and 34 : 2.(A4 × A4).4. Moreover we also compute the generators for the Maximal subgroups of some linear groups.

Synthesis of Polypropylene via Catalytic Deoxygenation of Poly(methyl acrylate)

We propose the defunctionalization of vinyl polymers as a strategy to access previously inaccessible polyolefin materials. By utilizing B(C₆F₅)₃-catalyzed deoxygenation in the presence of silane, we demonstrate that eliminating the pendent ester in poly(methyl acrylate) effectively leaves a linear hydrocarbon polymer with methyl pendants, which is polypropylene. We further show that a polypropylene-b-polystyrene diblock copolymer and a polystyrene-b-polypropylene-b-polystyrene triblock copolymer can be successfully derived from the poly(methyl acrylate)-containing block polymer precursors and exhibit quite distinct materials properties due to their chemical transformation. This unique postpolymerization modification methodology, which goes beyond the typical functional group conversion, can offer access to a diverse range of unprecedented polyolefin block polymers with a variable degree of functional groups.

Synthesis of block cationic polyacrylamide precursors using an aqueous RAFT dispersion polymerization

Synthesis of cationic polyacrylamides (CPAMs) by introducing cationic polymer precursors followed by chain extension of acrylamide (AM) homopolymer blocks via RAFT polymerization is a promising approach for engineering high-performance CPAMs. However, the aqueous solution polymerization of AM usually leads to high viscosity, thus limiting the solid content in the polymerization system. Herein a novel approach is introduced that uses a random copolymer of AM and methacryloxyethyltrimethyl ammonium chloride (DMC) as a macro RAFT chain transfer agent (mCTA) and stabilizer for aqueous RAFT dispersion polymerization of AM. The AM/DMC random copolymers synthesized by RAFT solution polymerization, having narrow dispersities (Đ _s) at different molecular weights and cationic degrees (C _s), could serve as the mCTA, which was confirmed by mCTA chain extension in aqueous solution polymerization of AM under different C _s, solid contents, AM addition contents, extended PAM block lengths, and mCTA chain lengths. The block CPAMs had a Đ value of less than 1.2. A model was developed using the method of moments with consideration of the diffusion control effect, for further understanding the chain extension kinetics. Predicted polymerization kinetics provided an accurate fit of the experimental data. The AM/DMC random copolymers were further used for aqueous RAFT dispersion polymerization of AM under different polymerization temperatures, C _s, and mCTA chain lengths. The resulting products had a milky appearance, and the block copolymers had Đ _s of less than 1.3. Higher C _s and longer chain lengths on mCTAs were beneficial for stabilizing the polymerization systems and produced smaller particle sizes and less particle aggregation. The products remained stable at room temperature storage for more than a month. The results indicate that aqueous RAFT dispersion polymerization using random copolymers of AM and DMC at moderate cationic degrees as a stabilizer and mCTA is a suitable approach for synthesizing CPAM block precursors at an elevated solid content.

End Group Stability of Atom Transfer Radical Polymerization (ATRP)-Synthesized Poly(N-isopropylacrylamide): Perspectives for Diblock Copolymer Synthesis

Studies on the end group stability of poly(N-isopropylacrylamide) during the atom transfer radical polymerization (ATRP) process are presented. Polymerization of N-isopropylacrylamide was conducted in different solvents using a copper(I) chloride/Me₆Tren catalyst complex. The influence of the ATRP solvent as well as the polymer purification process on the end group stability was investigated. For the first time, mass spectrometry results clearly underline the loss of ω end groups via an intramolecular cyclization reaction. Furthermore, an ATRP system based on a copper(I) bromide/Me₆Tren catalyst complex was introduced, that showed not only good control over the polymerization process, but also provided the opportunity of block copolymerization of N-isopropylacrylamide with acrylates and other N-substituted acrylamides. The polymers were characterized using ¹H-NMR spectroscopy and size exclusion chromatography. Polymer end groups were determined via ESI-TOF mass spectrometry enhanced by ion mobility separation (IMS).

Getting into Shape: Reflections on a New Generation of Cylindrical Nanostructures' Self-Assembly Using Polymer Building Blocks

Cylinders are fascinating structures with uniquely high surface area, internal volume, and rigidity. On the nanoscale, a broad range of applications have demonstrated advantageous behavior of cylindrical micelles or bottlebrush polymers over traditional spherical nano-objects. In the past, obtaining pure samples of cylindrical nanostructures using polymer building blocks via conventional self-assembly strategies was challenging. However, in recent years, the development of advanced methods including polymerization-induced self-assembly, crystallization-driven self-assembly, and bottlebrush polymer synthesis has facilitated the easy synthesis of cylindrical nano-objects at industrially relevant scales. In this Perspective, we discuss these techniques in detail, highlighting the advantages and disadvantages of each strategy and considering how the cylindrical nanostructures that are obtained differ in their chemical structure, physical properties, colloidal stability, and reactivity. In addition, we propose future challenges to address in this rapidly expanding field.

Modeling of Miniemulsion Polymerization of Styrene with Macro-RAFT Agents to Theoretically Compare Slow Fragmentation, Ideal Exchange and Cross-Termination Cases

A 5-dimensional Smith-Ewart based model is developed to understand differences for reversible addition-fragmentation chain transfer (RAFT) miniemulsion polymerization with theoretical agents mimicking cases of slow fragmentation, cross-termination, and ideal exchange while accounting for chain length and monomer conversion dependencies due to diffusional limitations. The focus is on styrene as a monomer, a water soluble initiator, and a macro-RAFT agent to avoid exit/entry of the RAFT leaving group radical. It is shown that with a too low RAFT fragmentation rate coefficient it is generally not afforded to consider zero-one kinetics (for the related intermediate radical type) and that with significant RAFT cross-termination the dead polymer product is dominantly originating from the RAFT intermediate radical. To allow the identification of the nature of the RAFT retardation it is recommended to experimentally investigate in the future the impact of the average particle size (dp) on both the monomer conversion profile and the average polymer properties for a sufficiently broad dp range, ideally including the bulk limit. With decreasing particle size both a slow RAFT fragmentation and a fast RAFT cross-termination result in a stronger segregation and thus rate acceleration. The particle size dependency is different, allowing further differentiation based on the variation of the dispersity and end-group functionality. Significant RAFT cross-termination is specifically associated with a strong dispersity increase at higher average particle sizes. Only with an ideal exchange it is afforded in the modeling to avoid the explicit calculation of the RAFT intermediate concentration evolution.

Ultra-low volume oxygen tolerant photoinduced Cu-RDRP

We introduce the first oxygen tolerant ultra-low volume (as low as 5 μL) photoinduced Cu-RDRP of a range of hydrophobic, hydrophilic and semi-fluorinated monomers.

Photo-induced copper-RDRP in continuous flow without external deoxygenation

Photo-induced Cu-RDRP of acrylates in a continuous flow reactor without the need for deoxygenation or externally added reagents.

The effective treatment of multi-drug resistant tumors with self-assembling alginate copolymers

Alginates of two different chain lengths were alkyne functionalized on the hydroxyl group, leaving all carboxylic groups intact.

Recent development of brush polymers via polymerization of poly(ethylene glycol)-based macromonomers

Polymerization of poly(ethylene glycol)-based macromonomers is a facile and versatile synthetic method to generate well-defined brush polymers.

Reversibly Cross-linkable Bottlebrush Polymers as Pressure-Sensitive Adhesives

Dynamically cross-linkable bottlebrush polymer adhesives were synthesized by the grafting-from strategy through a combination of ring-opening metathesis polymerization (ROMP) and photoiniferter polymerization. A norbornene-containing trithiocarbonate was first polymerized by ROMP to form the bottlebrush polymer backbone; this was followed by blue-light-mediated photoiniferter polymerization of butyl acrylate initiated by the poly(trithiocarbonate) to form the bottlebrush polymer. This strategy afforded well-defined bottlebrush polymers with molar masses in excess of 11 000 kg/mol. For un-cross-linked bottlebrush polymers, 180° peel tests revealed a cohesive failure mode and showed similar peel strengths (∼30 g/mm) regardless of the backbone polymer degree of polymerization (DP). The bottlebrush polymers were then treated with butylamine to remove the trithiocarbonate, liberating thiols on each side-chain terminus. In the presence of oxygen, these thiols readily cross-linked via disulfide bond formation. The cross-linked bottlebrush polymers with a backbone DP of 400 showed a greater than sixfold improvement in peel strength, whereas those with a backbone DP of 100 exhibited a twofold enhancement compared with un-cross-linked samples along with a change to adhesive failure. Triphenylphosphine readily reduced the disulfide bonds, effectively removing all cross-links in the bottlebrush network and allowing for recasting of the adhesive, which showed similar adhesive and rheological properties to the original un-cross-linked samples.

Temperature and pH-Dual Responsive AIE-Active Core Crosslinked Polyethylene-Poly(methacrylic acid) Multimiktoarm Star Copolymers

A series of aggregation-induced emission (AIE) active core crosslinked miktoarm star copolymers, having multi polyethylene (PE) and poly(methacrylic acid) (PMAA) arms, were synthesized and their thermal/pH responsive properties were studied. The procedure involves (a) the synthesis of PE-Br by polyhomologation of dimethylsulfoxonium methylide with triethylborane as initiator, followed by oxidation-hydrolysis/esterification reactions and of poly(tert-butyl methacrylate) (PtBMA-Br) by atom transfer radical polymerization (ATRP) of tert-butyl methacrylate, (b) the synthesis of (PE)_n-(PtBMA)_m-P(TPE-2St) by ATRP of a double styrene-functionalized tetraphenylethene (TPE-2St) with PE-Br and PtBMA-Br macroinitiators, and (c) the hydrolysis of (PE)_n-(PtBMA)_m-P(TPE-2St) to afford the amphiphilic miktoarm star copolymers (PE)_n-(PMMA)_m-P(TPE-2St). Due to their spherical core-shell structure (temperature-responsive) and the presence of hydrophilic PMAA (pH-responsive) and TPE-2St (AIE), these miktoarm star copolymers are AIE materials with temperature/pH-dual responsivity. In addition, thanks to the coexistence of hydrophilic and hydrophobic arms, these materials promote stable water-in-oil emulsions.

Copper-Mediated Polymerization without External Deoxygenation or Oxygen Scavengers

As a method for overcoming the challenge of rigorous deoxygenation in copper-mediated controlled radical polymerization processes [e.g., atom-transfer radical polymerization (ATRP)], reported here is a simple Cu0 -RDRP (RDRP=reversible deactivation radical polymerization) system in the absence of external additives (e.g., reducing agents, enzymes etc.). By simply adjusting the headspace of the reaction vessel, a wide range of monomers, namely acrylates, methacrylates, acrylamides, and styrene, can be polymerized in a controlled manner to yield polymers with low dispersities, near-quantitative conversions, and high end-group fidelity. Significantly, this approach is scalable (ca. 125 g), tolerant to elevated temperatures, compatible with both organic and aqueous media, and does not rely on external stimuli which may limit the monomer pool. The robustness and versatility of this methodology is further demonstrated by the applicability to other copper-mediated techniques, including conventional ATRP and light-mediated approaches.

Reporting pH-sensitive drug releaseviaunpaired spin fluorescence silencing

We pioneer an approach for the visualization of a self-reporting pH-controlled molecular release of a fluorescent drug from a nitroxide polymer scaffold.

Cu(0)-RDRP of styrene: balancing initiator efficiency and dispersity

The optimisation of all components within Cu(0)-wire mediated polymerisation of styrene is illustrated yielding well-defined polystyrene with enhanced initiator efficiency and dispersity at higher molecular weights.

Highly-ordered onion micelles made from amphiphilic highly-branched copolymers

Uniform onion micelles formed from up to ten nano-structured polymer layers were produced by the aqueous self-assembly of highly-branched copolymers.

Star-shaped Polymers through Simple Wavelength-Selective Free-Radical Photopolymerization

Star-shaped polymers represent highly desired materials in nanotechnology and life sciences, including biomedical applications (e.g., diagnostic imaging, tissue engineering, and targeted drug delivery). Herein, we report a straightforward synthesis of wavelength-selective multifunctional photoinitiators (PIs) that contain a bisacylphosphane oxide (BAPO) group and an α-hydroxy ketone moiety within one molecule. By using three different wavelengths, these photoactive groups can be selectively addressed and activated, thereby allowing the synthesis of ABC-type miktoarm star polymers through a simple, highly selective, and robust free-radical polymerization method. The photochemistry of these new initiators and the feasibility of this concept were investigated in unprecedented detail by using various spectroscopic techniques.

Alternative Route to Nanoscale Aggregates with a pH-Responsive Random Copolymer

A random copolymer, poly(methyl methacrylate-co-2-dimethylaminoethyl methacrylate) (poly(MMA-co-DMAEMA)) is shown to form nanoscale aggregates (NAs) (∼20 nm) at copolymer concentrations ≥10% w/w, directly from the preformed surfactant-stabilized latex (∼120 nm) in aqueous solution. The copolymer is prepared by conventional emulsion polymerization. Introducing a small mole fraction of DMAEMA (∼10%) allows the copolymer hydrophilicity to be adjusted by the pH and external temperature, generating NAs with tuneable sizes and a defined weight-average aggregation number, as observed by dynamic light scattering (DLS) and small-angle neutron scattering (SANS). These NAs are different from the so-called mesoglobular systems and are insensitive to temperature at fixed pH. The relatively broad chemical composition distribution of the copolymer and lumpy (or blocky but not diblock) incorporation of DMAEMA mean that the NAs cannot be simply thought of as conventional polymer micelles. In the acidic pH regime, the amphiphilic copolymer exhibits a defined critical assembly concentration (CAC) and a minimum air-water surface tension of 45.2 mN m^-1. This copolymer represents a convenient route to self-assembled NAs, which form directly in aqueous dispersions after pH and temperature triggers, rather than the typically applied (and time-consuming) water-induced micellization approach for common polymer micelles.

RAFT polymerization to form stimuli-responsive polymers

Stimuli-responsive polymers respond to a variety of external stimuli, which include optical, electrical, thermal, mechanical, redox, pH, chemical, environmental and biological signals. This paper is concerned with the process of forming such polymers by RAFT polymerization.

Factors affecting bottlebrush polymer synthesis by the transfer-to method using reversible addition–fragmentation chain transfer (RAFT) polymerization

The transfer-to method is a unique way to prepare bottlebrush polymers by reversible addition–fragmentation chain transfer (RAFT) polymerization.

Determination and correlation of regioselectivity and dead dormant species from head addition in acrylate RAFT polymerization

The dead dormant species from head addition in a RAFT process can be separated and quantified by combining chain-extension and GPEC.