Thomas G. McKenzie

Beyond Traditional RAFT: Alternative Activation of Thiocarbonylthio Compounds for Controlled Polymerization

Recent developments in polymerization reactions utilizing thiocarbonylthio compounds have highlighted the surprising versatility of these unique molecules. The increasing popularity of reversible addition–fragmentation chain transfer (RAFT) radical polymerization as a means of producing well‐defined, ‘controlled’ synthetic polymers is largely due to its simplicity of implementation and the availability of a wide range of compatible reagents. However, novel modes of thiocarbonylthio activation can expand the technique beyond the traditional system (i.e., employing a free radical initiator) pushing the applicability and use of thiocarbonylthio compounds even further than previously assumed. The primary advances seen in recent years are a revival in the direct photoactivation of thiocarbonylthio compounds, their activation via photoredox catalysis, and their use in cationic polymerizations. These synthetic approaches and their implications for the synthesis of controlled polymers represent a significant advance in polymer science, with potentially unforeseen benefits and possibilities for further developments still ahead. This Research News aims to highlight key works in this area while also clarifying the differences and similarities of each system.

Tertiary amine catalyzed photo-induced controlled radical polymerization of methacrylates

A novel tertiary amine catalyst (TAC) and trithiocarbonate (TTC) synergistic photo-induced controlled radical polymerization of methacrylates in the absence of conventional photo-initiators, metal-catalysts, or dye sensitizers, has been realized under mild UV irradiation (λmax ≈ 365 nm), yielding polymethacrylates with low molecular weight distributions and excellent end-group fidelity.

Investigation into the photolytic stability of RAFT agents and the implications for photopolymerization reactions

The photolytic stability of various RAFT agents (i.e., thiocarbonylthio-containing compounds) under irradiation from a blue LED light source has been investigated. The effect and implications of efficient photo-fragmentation and potential photo-degradation with regard to their performance in photopolymerization reactions is reported. The stability is found to depend strongly on the structure of the fragmenting (R-) group and the reactivity of the carbon-centered radical formed following photolytic cleavage. This is proposed to be due to the competitive rates of radical recombination and thiyl radical degradation, and has implications on the choice of monomer (as monomer propagation requires re-initiation of the oligomeric/polymeric RAFT agent). These findings can provide guidelines and increase understanding when conducting a photopolymerization employing thiocarbonylthio RAFT agents.

Trithiocarbonates as intrinsic photoredox catalysts and RAFT agents for oxygen tolerant controlled radical polymerization

This study reports the discovery that trithiocarbonate RAFT agents can significantly reduce the amount of dissolved oxygen when irradiated with visible light (λmax ≈ 460 nm) in the presence of a sacrificial tertiary amine. By taking advantage of this effect, we conducted a series of photo-CRPs of acrylates without the requirement of pre-degassing the reaction mixtures. In these systems, the trithiocarbonate plays a triple role of photocatalyst for oxygen removal, initiator, and RAFT agent for polymerization control. We believe this robust and facile synthetic method will be beneficial for cost-effective industrial applications as well as the laboratory-scale synthesis of functional polymeric materials.

A novel solid state photocatalyst for living radical polymerization under UV irradiation

This study presents the development of a novel solid state photocatalyst for the photoinduced controlled radical polymerization of methacrylates under mild UV irradiation (λmax ≈ 365 nm) in the absence of conventional photoinitiators, metal-catalysts or dye sensitizers. The photocatalyst design was based on our previous finding that organic amines can act in a synergistic photochemical reaction with thiocarbonylthio compounds to afford well controlled polymethacrylates under UV irradiation. Therefore, in the current contribution an amine-rich polymer was covalently grafted onto a solid substrate, thus creating a heterogeneous catalyst that would allow for facile removal, recovery and recyclability when employed for such photopolymerization reactions. Importantly, the polymethacrylates synthesized using the solid state photocatalyst (ssPC) show similarly excellent chemical and structural integrity as those catalysed by free amines. Moreover, the ssPC could be readily recovered and re-used, with multiple cycles of polymerization showing minimal effect on the integrity of the catalyst. Finally, the ssPC was employed in various photo-“click” reactions, permitting high yielding conjugations under photochemical control.

Fenton-RAFT Polymerization: An “On-Demand” Chain-Growth Method

Fine control over the architecture and/or microstructure of synthetic polymers is fast becoming a reality owing to the development of efficient and versatile polymerization techniques and conjugation reactions. However, the transition of these syntheses to automated, programmable, and high-throughput operating systems is a challenging step needed to translate the vast potential of precision polymers into machine-programmable polymers for biological and functional applications. Chain-growth polymerizations are particularly appealing for their ability to form structurally and chemically well-defined macromolecules through living/controlled polymerization techniques. Even using the latest polymerization technologies, the macromolecular engineering of complex functional materials often requires multi-step syntheses and purification of intermediates, and results in sub-optimal yields. To develop a proof-of-concept of a framework polymerization technique that is readily amenable to automation requires several key characteristics. In this study, a new approach is described that is believed to meet these requirements, thus opening avenues toward automated polymer synthesis.

Nano-scale clustering of integrin-binding ligands regulates endothelial cell adhesion, migration, and endothelialization rate: novel materials for small diameter vascular graft applications

Blood contacting devices are commonly used in todays medical landscape. However, such devices (including small diameter vascular grafts) are limited by poor blood compatibility and may fail due to thrombosis. An attractive strategy for improving the blood compatibility of such devices is to generate biomaterials that foster a confluent and functioning endothelial cell layer. Synthesizing materials that display integrin-binding peptide ligands is a common way to promote endothelialization. However, in addition to integrin-ligand binding, integrin clustering is necessary to achieve intracellular signaling events that influence cellular phenotype. In this study, we explored the impact of nano-scale clustering of integrin-binding ligands on endothelial cell functions by designing novel materials that promote the clustering of integrin receptors. RGD-functionalized copolymers were prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization and used for the preparation of random and nano-clustered surfaces spanning a range of global and local RGD densities (global densities 0.4-1.9 μg of peptide per mg of polymer and local densities of 1-2.4 ligands per nano-cluster). The adhesion and migration of endothelial cells was improved on nano-clustered surfaces compared to random surfaces. The highest adhesion and migration speed of endothelial cells and the rapid development of the endothelial monolayer were observed on surfaces with the highest local and global peptide density. These results indicate that the nano-clustering of peptide ligands is a promising strategy for next generation cardiovascular biomaterials, especially for small diameter vascular graft applications where the development of a confluent and functioning endothelium holds the potential to prevent device failure due to thrombosis.

Diverse approaches to star polymers via cationic and radical RAFT cross-linking reactions using mechanistic transformation

Core cross-linked star polymers were successfully synthesized via cationic reversible addition fragmentation chain-transfer (RAFT) polymerization, which produces macro RAFT agents as the arm polymers, followed by three different approaches used for the block copolymerization of divinyl monomers as the core and subsequent cross-linking reaction. The following three approaches were used: (i) one-pot cationic RAFT block polymerization and the simultaneous cross-linking reaction of divinyl ether; (ii) mechanistic transformation to radical RAFT block polymerization and the simultaneous cross-linking reaction of a hetero divinyl monomer, which possesses both vinyl ether and acrylate moieties; and (iii) one-pot cationic RAFT selective block copolymerization of the vinyl ether moiety in the hetero divinyl monomer, followed by mechanistic transformation to a radical cross-linking reaction of the acrylate moiety that remained in the pendant groups of the diblock polymer. All three methods were free from metal catalysts and produced core cross-linked star polymers with controlled molecular weights (Mw = 2–6 × 105), narrow molecular weight distributions (Mw/Mn = 1.1–1.4), and controlled arm numbers (Narm = 15–40) and sizes (Dn = 18–28 nm, Dw/Dn = 1.03–1.06) in relatively high yields (80–94%).

Controlled RAFT polymerization facilitated by a nanostructured enzyme mimic

Recent reports have revealed the potential of nanostructured materials to display enzyme-like activity for a broad range of applications. In this study, a glycine modified metal–organic framework (MOF) MIL-53(Fe) composite was utilized as an enzyme (e.g. peroxidase) mimic for the generation of reactive oxygen species (ROS) from hydrogen peroxide. The resultant hydroxyl radicals can act as initiators in the presence of chain transfer agents and monomers in aqueous or organic media, allowing for controlled polymerization via reversible addition–fragmentation chain transfer (RAFT). The polymer products present controllable molecular weights, narrow polymer dispersities and high ‘livingness’ as revealed by a chain extension experiment and MALDI-ToF analysis. By continuously supplying hydrogen peroxide to the MOF peroxidase mimic, ultrahigh molecular weight polyacrylamides (Mn > 1 MDa) of low dispersity (Đ < 1.25) were also obtained. By incorporating low cost, highly stable and easily isolated peroxidase-mimicking catalysts, glycine modified MIL-53(Fe) represents a versatile synthetic strategy to produce well-defined polymers from both hydrophilic and hydrophobic monomers.

High frequency sonoATRP of 2-hydroxyethyl acrylate in an aqueous medium

High frequency ultrasound (490 kHz, 40 W) was applied for the controlled polymerisation of 2-hydroxyethyl acrylate (HEA) via sonochemically induced atom transfer radical polymerisation (sonoATRP). The synthesis of poly(HEA) (DP 100–800) was found to reach high conversions (>90%) in short times (<60 min) with excellent molecular weight distribution (Đ < 1.1).