Streamlining the Generation of Advanced Polymer Materials through the Marriage of Automation and Multiblock Copolymer Synthesis in Emulsion

Synthetic polymers are of paramount importance - an incredibly wide range of polymeric materials possessing an impressive variety of properties have been developed to date. The recent emergence of artificial intelligence and automation presents a great opportunity to significantly speed up discovery and development of the next generation of advanced polymeric materials. We have focused on the high-throughput automated synthesis of multiblock copolymers, which are defined as three or more distinct polymer segments comprising different monomer compositions covalently bonded together in linear sequence. The present work has exploited automation to prepare high molecular weight multiblock copolymers (typically > 100,000 g mol-1) using RAFT polymerization in aqueous emulsions. A variety of original multiblock copolymers have been synthesised via a Chemspeed robot, exemplified by a high molecular weight multiblock copolymer comprising thirteen blocks. Libraries of copolymers of randomized monomer compositions, block orders, and block lengths were generated. One multiblock copolymer contained all four monomer families listed in the pool, which is unprecedented in the literature. The present work demonstrates that automation has the power to render complex and laborious syntheses of such unprecedented materials not just possible, but facile and straightforward, representing the way forward to the next generation of macromolecular architectures.

Piercing of the Human Parainfluenza Virus by Nanostructured Surfaces

This paper presents a comprehensive experimental and theoretical investigation into the antiviral properties of nanostructured surfaces and explains the underlying virucidal mechanism. We used reactive ion etching to fabricate silicon (Si) surfaces featuring an array of sharp nanospikes with an approximate tip diameter of 2 nm and a height of 290 nm. The nanospike surfaces exhibited a 1.5 log reduction in infectivity of human parainfluenza virus type 3 (hPIV-3) after 6 h, a substantially enhanced efficiency, compared to that of smooth Si. Theoretical modeling of the virus-nanospike interactions determined the virucidal action of the nanostructured substrata to be associated with the ability of the sharp nanofeatures to effectively penetrate the viral envelope, resulting in the loss of viral infectivity. Our research highlights the significance of the potential application of nanostructured surfaces in combating the spread of viruses and bacteria. Notably, our study provides valuable insights into the design and optimization of antiviral surfaces with a particular emphasis on the crucial role played by sharp nanofeatures in maximizing their effectiveness.

Multiblock copolymer synthesis via RAFT emulsion polymerization

A multiblock copolymer is a polymer of a specific structure that consists of multiple covalently linked segments, each comprising a different monomer type. The control of the monomer sequence has often been described as the "holy grail" of synthetic polymer chemistry, with the ultimate goal being synthetic access to polymers of a "perfect" structure, where each monomeric building block is placed at a desired position along the polymer chain. Given that polymer properties are intimately linked to the microstructure and monomer distribution along the constituent chains, it goes without saying that there exist seemingly endless opportunities in terms of fine-tuning the properties of such materials by careful consideration of the length of each block, the number and order of blocks, and the inclusion of monomers with specific functional groups. The area of multiblock copolymer synthesis remains relatively unexplored, in particular with regard to structure-property relationships, and there are currently significant opportunities for the design and synthesis of advanced materials. The present review focuses on the synthesis of multiblock copolymers via reversible addition-fragmentation chain transfer (RAFT) polymerization implemented as aqueous emulsion polymerization. RAFT emulsion polymerization offers intriguing opportunities not only for the advanced synthesis of multiblock copolymers, but also provides access to polymeric nanoparticles of specific morphologies. Precise multiblock copolymer synthesis coupled with self-assembly offers material morphology control on length scales ranging from a few nanometers to a micrometer. It is imperative that polymer chemists interact with physicists and material scientists to maximize the impact of these materials of the future.

Electrochemically-Initiated RAFT Synthesis of Low Dispersity Multiblock Copolymers by Seeded Emulsion Polymerization

We describe electrochemically initiated emulsion polymerization with reversible addition-fragmentation chain transfer (eRAFT) to form well-defined multiblock copolymers with low molar mass dispersity. We demonstrate the utility of our emulsion eRAFT process with the synthesis of low dispersity multiblock copolymers by seeded RAFT emulsion polymerization at ambient temperature (∼30 °C). Thus, a triblock, poly(butyl methacrylate)-block-polystyrene-block-poly(4-methylstyrene) [PBMA-b-PSt-b-PMS], and a tetrablock, poly(butyl methacrylate)-block-polystyrene-block-poly(styrene-stat-butyl acrylate)-block-polystyrene [PBMA-b-PSt-b-P(BA-stat-St)-b-PSt], were synthesized as free-flowing, colloidally stable latexes commencing with a surfactant-free poly(butyl methacrylate) macroRAFT agent seed latex. A straightforward sequential addition strategy with no intermediate purification steps was able to be employed due to the high monomer conversions achieved in each step. The method takes full advantage of compartmentalization phenomena and the nanoreactor concept described in previous work to achieve the predicted molar mass, low molar mass dispersity (Đ ∼ 1.1-1.2), incrementing particle size (Z_av = 100-115 nm), and low particle size dispersity (PDI ∼ 0.02) for each generation of the multiblocks.

Terminology for chain polymerization (IUPAC Recommendations 2021)

Chain polymerizations are defined as chain reactions where the propagation steps occur by reaction between monomer(s) and active site(s) on the polymer chains with regeneration of the active site(s) at each step. Many forms of chain polymerization can be distinguished according to the mechanism of the propagation step (e.g., cyclopolymerization – when rings are formed, condensative chain polymerization – when propagation is a condensation reaction, group-transfer polymerization, polyinsertion, ring-opening polymerization – when rings are opened), whether they involve a termination step or not (e.g., living polymerization – when termination is absent, reversible-deactivation polymerization), whether a transfer step is involved (e.g., degenerative-transfer polymerization), and the type of chain carrier or active site (e.g., radical, ion, electrophile, nucleophile, coordination complex). The objective of this document is to provide a language for describing chain polymerizations that is both readily understandable and self-consistent, and which covers recent developments in this rapidly evolving field.

A brief guide to polymerization terminology (IUPAC Technical Report)

The use of self-consistent terminology to describe polymerizations is important for litigation, patents, research and education. Imprecision in these areas can be both costly and confusing. To address this situation the International Union of Pure and Applied Chemistry (IUPAC) has made recommendations, which are summarized below. In the version shown as the supplementary material, references and hyperlinks lead to source documents; screen tips contain definitions published in IUPAC recommendations. More details can also be found in the IUPAC Purple Book. This guide is one of a series on terminology and nomenclature. Refer to the supplementary material for the complete and interactive version of this brief guide.

Reconsidering terms for mechanisms of polymer growth: the “step-growth” and “chain-growth” dilemma

Terms used for mechanisms of polymer growth are varied and problematic; we detail here our concerns with the terms “step-growth” and “chain-growth.” Ultimately, we seek terms that are simple, accurate, and attractive to the polymer community.

Synthesis of Multicompositional Onion-like Nanoparticles via RAFT Emulsion Polymerization

Synthesis of multicompositional polymeric nanoparticles of diameters 100-150 nm comprising well-defined multiblock copolymers reaching from the particle surface to the particle core was conducted using surfactant-free aqueous macroRAFT emulsion polymerization. The imposed constraints on chain mobility as well as chemical incompatibility between the blocks result in microphase separation, leading to formation of an onion-like multilayered particle morphology with individual layer thicknesses of approximately 20 nm. The approach provides considerable versatility in particle morphology design as the composition of individual layers as well as the number of layers can be tailored as desired, offering more complex particle design compared to approaches relying on self-assembly of preformed diblock copolymers within particles. Microphase separation can occur in these systems under conditions where the corresponding bulk system would not theoretically result in microphase separation.

Selective Bond Cleavage in RAFT Agents Promoted by Low-Energy Electron Attachment

Radical polymerization with reversible addition-fragmentation chain transfer (RAFT polymerization) has been successfully applied to generate polymers of well-defined architecture. For RAFT polymerization a source of radicals is required. Recent work has demonstrated that for minimal side-reactions and high spatio-temporal control these should be formed directly from the RAFT agent or macroRAFT agent (usually carbonothiosulfanyl compounds) thermally, photochemically or by electrochemical reduction. In this work, we investigated low-energy electron attachment to a common RAFT agent (cyanomethyl benzodithioate), and, for comparison, a simple carbonothioylsulfanyl compound (dimethyl trithiocarbonate, DMTTC) in the gas phase by means of mass spectrometry as well as quantum chemical calculations. We observe for both compounds that specific cleavage of the C-S bond is induced upon low-energy electron attachment at electron energies close to zero eV. This applies even in the case of a poor homolytic leaving group (. CH3 in DMTTC). All other dissociation reactions found at higher electron energies are much less abundant. The present results show a high control of the chemical reactions induced by electron attachment.

Correction: Polymerization-induced self-assembly via RAFT in emulsion: effect of Z-group on the nucleation step

Correction for ‘Polymerization-induced self-assembly via RAFT in emulsion: effect of Z-group on the nucleation step’ by Thiago R. Guimarães et al., Polym. Chem., 2021, 12, 122–133, DOI: 10.1039/D0PY01311K.

Initiation of RAFT Polymerization: Electrochemically Initiated RAFT Polymerization in Emulsion (Emulsion eRAFT), and Direct PhotoRAFT Polymerization of Liquid Crystalline Monomers

We report on two important advances in radical polymerization with reversible addition–fragmentation chain transfer (RAFT polymerization). (1) Electrochemically initiated emulsion RAFT (eRAFT) polymerization provides rapid polymerization of styrene at ambient temperature. The electrolytes and mediators required for eRAFT are located in the aqueous continuous phase separate from the low-molar-mass-dispersity macroRAFT agent mediator and product in the dispersed phase. Use of a poly(N,N-dimethylacrylamide)-block-poly(butyl acrylate) amphiphilic macroRAFT agent composition means that no added surfactant is required for colloidal stability. (2) Direct photoinitiated (visible light) RAFT polymerization provides an effective route to high-purity, low-molar-mass-dispersity, side chain liquid-crystalline polymers (specifically, poly(4-biphenyl acrylate)) at high monomer conversion. Photoinitiation gives a product free from low-molar-mass initiator-derived by-products and with minimal termination. The process is compared with thermal dialkyldiazene initiation in various solvents. Numerical simulation was found to be an important tool in discriminating between the processes and in selecting optimal polymerization conditions.

Polymerization-induced self-assembly via RAFT in emulsion: effect of Z-group on the nucleation step

It is demonstrated that the nature of the Z-group of trithiocarbonate RAFT agents can have a major effect on the nucleation step of aqueous RAFT PISA performed as emulsion polymerization.

Definitions and notations relating to tactic polymers (IUPAC Recommendations 2020)

This document summarizes and extends definitions and notations for the description of tactic polymers and the diad structures of which they are composed. It formally recognizes and resolves apparent inconsistencies between terminology used in the polymer field to describe tactic polymers and terminology in more common use in organic chemistry. Specifically, the terms m and r diads are recommended to replace the terms meso and racemo diads. The definitions are also updated from those in the existing Stereochemistry Document to use the term ‘stereogenic centre’, rather than ‘chiral or prochiral atoms’. Further, the terms relating to tacticity have been defined for the constituent macromolecules, rather than for the polymers composed of those macromolecules. Therefore, this document also forms an addendum and corrigendum to the 1981 document, ‘Stereochemical definitions and notations relating to polymers’.

Polymerization-Induced Phase Segregation and Self-Assembly of Siloxane Additives to Provide Thermoset Coatings with a Defined Surface Topology and Biocidal and Self-Cleaning Properties

In this work, we report on the incorporation of a siloxane copolymer additive, poly((2-phenylethyl) methylsiloxane)-co(1-phenylethyl) methylsiloxane)-co-dimethylsiloxane), which is fully soluble at room temperature, in a rapid-cure thermoset polyester coating formulation. The additive undergoes polymerization-induced phase segregation (PIPS) to self-assemble on the coating surface as discrete discoid nanofeatures during the resin cure process. Moreover, the copolymer facilitates surface co-segregation of titanium dioxide pigment microparticulate present in the coating. Depending on the composition, the coatings can display persistent superhydrophobicity and self-cleaning properties and, surprisingly, the titanium dioxide pigmented coatings that include the siloxane copolymer additive display high levels of antibacterial performance against Gram-positive (Staphylococcus aureus) and Gram-negative (Pseudomonas aeruginosa) bacteria. This antibacterial performance is believed to be associated with the unique surface topology of these coatings, which comprise stimuli-responsive discoid nanofeatures. This paper provides details of the surface morphology of the coatings and how these relates to the antimicrobial properties of the coating.

Electrochemical Behavior of Thiocarbonylthio Chain Transfer Agents for RAFT Polymerization

Electrochemical activation of thiocarbonylthio reversible addition-fragmentation chain transfer (RAFT) agents (S=C(Z)S-R) is explored as a potential method for initiating RAFT polymerization under mild conditions without producing initiator-derived byproducts. Herein we apply cyclic voltammetry to establish a predominant reduction mechanism, where electrochemical reduction is coupled to an irreversible first-order chemical reaction. Structure-dependent trends in cyclic voltammograms (CVs), and comparison to absorption spectra, clarify the role of R- and Z-groups in determining reduction processes. The major reduction peak moves to more cathodic potentials in the series dithiobenzoates > trithiocarbonates > heteroaromatic dithiocarbamates > xanthates ∼ N-alkyl-N-aryldithiocarbamates, due to the Z-group influence on thiocarbonyl bond reactivity. More active (electron-withdrawing, radical stabilizing) R-groups shift the reduction peak anodically, in part due to their influence on the rate of the coupled chemical reaction. Analysis of CVs across a range of scan rates revealed that kinetic control over the reduction mechanism is influenced by both the charge transfer rate and chemical reaction rate.

Exploitation of the Nanoreactor Concept for Efficient Synthesis of Multiblock Copolymers via MacroRAFT-Mediated Emulsion Polymerization

Multiblock copolymers are a class of polymeric materials with a range of potential applications. We report here a strategy for the synthesis of multiblock copolymers based on methacrylates. Reversible addition-fragmentation chain transfer (RAFT) polymerization is implemented as an emulsion polymerization to generate seed particles as nanoreactors, which can subsequently be employed in sequential RAFT emulsion polymerizations. The segregation effect allowed the synthesis of a high molar mass (>100,000 g·mol^-1) decablock homopolymer at a high polymerization rate to an extent not previously achieved. A heptablock copolymer containing seven different 100 unit blocks was also successfully prepared, demonstrating how the strategy can be employed to precisely control the polymer composition at a level hitherto not accessible in environmentally friendly aqueous emulsion polymerization. Importantly, the methodology is a batch process without any intermediate purification steps, thus, rendering industrial scale up more feasible.

Ab initio RAFT emulsion polymerization mediated by small cationic RAFT agents to form polymers with low molar mass dispersity

We report on low molar mass cationic RAFT agents that provide predictable molar mass and low molar mass dispersities (Đ_m) in ab initio emulsion polymerization.

Kinetics and mechanism for thermal and photochemical decomposition of 4,4′-azobis(4-cyanopentanoic acid) in aqueous media

The two diastereoisomers of 4,4′-azobis(4-cyanopentanoic acid) decompose at different rates in aqueous media. The major products (>50%) are amides produced by trapping the ketenimines formed by C–N coupling.

Light-Induced RAFT Single Unit Monomer Insertion in Aqueous Solution-Toward Sequence-Controlled Polymers

First report on the sequential, visible light-initiated, single unit monomer insertion (SUMI) of N,N-dimethylacrylamide (DMAm) into the reversible addition fragmentation chain transfer (RAFT) agent, 4-((((2-carboxyethyl)thio)carbonothioyl)thio)-4-cyanopentanoic acid (CTA₁ ), in aqueous solution is provided. The specificity for SUMI over formation of higher oligomers and/or RAFT agent-derived by-products is higher for longer irradiation wavelengths. Red light provides the cleanest product (selective SUMI), showing a linear pseudo-first order kinetic profile to high (>80%) conversion, but also the slowest reaction rate. Blue light provides a relatively rapid reaction, but also gives some by-products (<2%) and the kinetic profile displays a conversion plateau at >65% conversion. Higher specificity with red light is attributed to CTA₁ absorbing at longer wavelengths than the SUMI product, which allows selective excitation of CTA₁ . The use of a higher reaction temperature (65 °C vs ambient) results in a higher reaction rate and a reduction in oligomer formation.

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.