Reversible-deactivation radical polymerization (Controlled/living radical polymerization): From discovery to materials design and applications

Recent Developments and Future Challenges in Controlled Radical Polymerization: A 2020 Update

Careful simulations of conversion vs. time plots and full molecular weight distributions have been performed using the PREDICI® program package in conjunction with the kinetic scheme suggested by the CSIRO group for the reversible addition fragmentation chain transfer (RAFT) process to probe RAFT agent mediated polymerizations. In particular, conditions leading to inhibition and rate retardation have been examined to act as a guide to optimum living polymerization behavior. It is demonstrated that an inhibition period of considerable length is induced by either slow fragmentation of the intermediate RAFT radicals appearing in the pre-equilibrium or by slow re-initiation of the leaving group radical of the initial RAFT agent. The absolute values of the rate coefficients governing the core equilibrium of the RAFT process - at a fixed value of the equilibrium constant - are confirmed to be crucial in controlling the polydispersity of the resulting molecular weight distributions: A higher interchange frequency effects narrower distributions. It is further demonstrated that the size of the rate coefficient controlling the addition reaction of propagating radicals to polyRAFT agent, κβ, is mainly responsible for optimizing the control of the polymerization. The fragmentation rate coefficient, κ-β, of the macroRAFT intermediate radical, on the other hand, may be varied over orders of magnitude without affecting the amount of control exerted over the polymerization. On the basis of the basic RAFT mechanism, its value mainly governs the extent of rate retardation in RAFT polymerizations.

Kinetic analysis of reversible addition fragmentation chain transfer (RAFT) polymerizations: Conditions for inhibition, retardation, and optimum living polymerization

A Review of MOFs and Their Composites-Based Photocatalysts: Synthesis and Applications

The development of advanced materials based on well-defined polymeric architectures is proving to be a highly prosperous research direction across both industry and academia. Controlled radical polymerization techniques are receiving unprecedented attention, with reversible-deactivation chain growth procedures now routinely leveraged to prepare exquisitely precise polymer products. Reversible addition-fragmentation chain transfer (RAFT) polymerization is a powerful protocol within this domain, where the unique chemistry of thiocarbonylthio (TCT) compounds can be harnessed to control radical chain growth of vinyl polymers. With the intense recent focus on RAFT, new strategies for initiation and external control have emerged that are paving the way for preparing well-defined polymers for demanding applications. In this work, the cutting-edge innovations in RAFT that are opening up this technique to a broader suite of materials researchers are explored. Emerging strategies for activating TCTs are surveyed, which are providing access into traditionally challenging environments for reversible-deactivation radical polymerization. The latest advances and future perspectives in applying RAFT-derived polymers are also shared, with the goal to convey the rich potential of RAFT for an ever-expanding range of high-performance applications.

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Progress and Perspectives Beyond Traditional RAFT Polymerization

Reversible addition-fragmentation chain-transfer (RAFT) polymerization is a valuable tool for synthesizing macromolecules with controlled topologies and diverse chemical functionalities. However, the application of RAFT polymerization to additive-manufacturing processes has been prevented due to the slow polymerization rates of typical systems. In this work, we developed and optimized a rapid visible (green) light mediated RAFT polymerization process and applied it to an open-air 3D printing system. The reaction components are non-toxic, metal free and environmentally friendly, which tailors these systems toward biomaterial fabrication. The inclusion of RAFT agent in the photosensitive resin provided control over the mechanical properties of 3D printed materials and allowed these materials to be post-functionalized after 3D printing. Additionally, photoinduced spatiotemporal control of the network structure provided a one-pass approach to 4D printed materials. This RAFT-mediated 3D and 4D printing process should provide access to a range of new functional and stimuli-responsive materials.

A Versatile 3D and 4D Printing System through Photocontrolled RAFT Polymerization

In this study, porphyrinic zirconium (Zr) MOFs were investigated as heterogeneous photocatalysts for photoinduced electron transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization of various monomers under a broad range of wavelengths, producing polymers with high monomer conversions, narrow molecular weight distributions, low dispersity and good chain-end fidelity. Screening of various porphyrinic Zr-MOFs (Zn) containing Zn-metalled porphyrinic ligands demonstrated that MOF-525 (Zn) with the smallest size had the best photocatalytic activity in PET-RAFT polymerization, due to enhanced dispersion and light penetration. Oxygen tolerance and temporal control were also demonstrated during MOF catalysed PET-RAFT. Results suggested that the polymerization rates were significantly affected by changing the size and surface area of MOFs, and the heterogeneous MOF photocatalysts could be easily separated and recycled for up to five independent PET-RAFT polymerizations without an obvious decrease in efficiency. Finally, the MOF photocatalysts were utilized to create three-dimensional polymeric objects with high resolution via visible light mediated stereolithography in an open-air environment.

Porphyrinic Zirconium Metal–Organic Frameworks (MOFs) as Heterogeneous Photocatalysts for PET-RAFT Polymerization and Stereolithography

Photoinduced electron/energy transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization has been successfully employed in three-dimensional (3D) printing to synthesize materials with controlled architecture and diverse functions However, there is a lack of understanding of the impact of various RAFT agents on the 3D printing process, as only trithiocarbonate RAFT agents were used in the previous systems In this work, RAFT agents with different activating Z groups and leaving R groups are incorporated in resin formulations and applied to open-air 3D printing systems under mild green light irradiation (λₘₐₓ = 525 nm, I₀ = 032 mW/cm²) The Z and R groups of RAFT agents influence the polymerization kinetics and 3D printing process (cure time) and affect the mechanical properties of 3D-printed materials The impact of the concentration of trithiocarbonates on mechanical properties is also investigated The 3D-printed materials containing RAFT agents are easily postmodified after printing via one-pot in situ aminolysis and thiol-Michael additions The RAFT-mediated photoinduced polymerization appears a promising method for producing novel and functional materials

Zhiheng Zhang

Papers

A Versatile 3D and 4D Printing System through Photocontrolled RAFT Polymerization

Porphyrinic Zirconium Metal–Organic Frameworks (MOFs) as Heterogeneous Photocatalysts for PET-RAFT Polymerization and Stereolithography

Effect of Thiocarbonylthio Compounds on Visible-Light-Mediated 3D Printing

Porphyrinic Zirconium MOFs as Heterogeneous Photocatalysts for PET‐RAFT Polymerization and Stereolithography

Free-standing sulfonated graphene-polypyrrole-polyethylene glycol foam for highly flexible supercapacitors