Mechanically Mediated Atom Transfer Radical Polymerization: Exploring Its Potential at High Conversions

Mechanoluminescent Materials Enable Mechanochemically Controlled Atom Transfer Radical Polymerization and Polymer Mechanotransduction

Organic mechanophores have been widely adopted for polymer mechanotransduction. However, most examples of polymer mechanotransduction inevitably experience macromolecular chain rupture, and few of them mimic mussel's mechanochemical regeneration, a mechanically mediated process from functional units to functional materials in a controlled manner. In this paper, inorganic mechanoluminescent (ML) materials composed of CaZnOS-ZnS-SrZnOS: Mn2+ were used as a mechanotransducer since it features both piezoelectricity and mechanolunimescence. The utilization of ML materials in polymerization enables both mechanochemically controlled radical polymerization and the synthesis of ML polymer composites. This procedure features a mechanochemically controlled manner for the design and synthesis of diverse mechanoresponsive polymer composites.

Tribochemically Controlled Atom Transfer Radical Polymerization Enabled by Contact Electrification

Traditional mechanochemically controlled reversible-deactivation radical polymerization (RDRP) utilizes ultrasound or ball milling to regenerate activators, which induce side reactions because of the high-energy and high-frequency stimuli. Here, we propose a facile approach for tribochemically controlled atom transfer radical polymerization (tribo-ATRP) that relies on contact-electro-catalysis (CEC) between titanium oxide (TiO2 ) particles and CuBr2 /tris(2-pyridylmethylamine (TPMA), without any high-energy input. Under the friction induced by stirring, the TiO2 particles are electrified, continuously reducing CuBr2 /TPMA into CuBr/TPMA, thereby conversing alkyl halides into active radicals to start ATRP. In addition, the effect of friction on the reaction was elucidated by theoretical simulation. The results indicated that increasing the frequency could reduce the energy barrier for the electron transfer from TiO2 particles to CuBr2 /TPMA. In this study, the design of tribo-ATRP was successfully achieved, enabling CEC (ca. 10 Hz) access to a variety of polymers with predetermined molecular weights, low dispersity, and high chain-end fidelity.

Process intensification of continuous-flow seATRP by a sonicated multi-reactor setup

Simplified electrochemically mediated atom transfer radical polymerization (seATRP) is a versatile technique for synthesizing polymers with precise control and complex architecture. Continuous-flow seATRP has recently been realized by using a sonicated microreactor but still faces limitations such as relatively low conversion and difficulties in synthesizing polymers with high molecular weight. Herein, a novel multi-reactor setup is demonstrated. By tuning the currents applied to different reaction stages in the setup, 90% conversion can be achieved while maintaining relatively low dispersity (<1.35). Meanwhile, the unique design enables a wider processing window for sonication due to greater viscous attenuation in the second reactor, thus largely addressing the problem associated with high viscosity during the synthesis of high molecular weight polymers. The developed setup also offers an alternative strategy for future scale-up of continuous-flow seATRP.

Enhancing Ultrasound-Assisted Iodine-Mediated Reversible-Deactivation Radical Polymerization by Piezoelectric Nanoparticles

The development of mechanochemical tools for regulating the polymerization process has received an increasing amount of attention in recent years. Herein, we report the example of the mechanically controlled iodine-mediated reversible-deactivation radical polymerization (mechano-RDRP) using piezoelectric tetragonal BaTiO3 nanoparticles (T-BTO) as mechanoredox catalyst and alkyl iodide as the initiator. We demonstrated a more efficient mechanochemical initiation and reversible deactivation process than sonochemical activation via a mechanoredox-mediated alkyl iodide cleavage reaction. The mechanochemical activation of the C-I bond was verified by density functional theory (DFT) calculations. Theoretical calculations together with experimental results confirmed the more efficient initiation and polymerization than the traditional sonochemical approach. The influence of BaTiO3, initiator, and solvent was further examined to reveal the mechanism of the mechano-RDRP. The results showed good controllability over molecular weight and capacity for a one-pot chain extension. This work expands the scope of mechanically controlled polymerization and shows good potential in the construction of adaptive materials.

Atom Transfer Radical Polymerization: A Mechanistic Perspective

Since its inception, atom transfer radical polymerization (ATRP) has seen continuous evolution in terms of the design of the catalyst and reaction conditions; today, it is one of the most useful techniques to prepare well-defined polymers as well as one of the most notable examples of catalysis in polymer chemistry. This Perspective highlights fundamental advances in the design of ATRP reactions and catalysts, focusing on the crucial role that mechanistic studies play in understanding, rationalizing, and predicting polymerization outcomes. A critical summary of traditional ATRP systems is provided first; we then focus on the most recent developments to improve catalyst selectivity, control polymerizations via external stimuli, and employ new photochemical or dual catalytic systems with an outlook to future research directions and open challenges.

Dimensional Optimization for ZnO-Based Mechano-ATRP with Extraordinary Activity

Various piezoelectric nanomaterials were utilized in ultrasound-mediated atom transfer radical polymerization (ATRP), owing to their outstanding piezoelectric effect. However, the relationship between the morphology of those piezocatalysts and polymerization has not been clearly established. Herein, we employed different piezoelectric zinc oxide (ZnO) nanomaterials to achieve novel mechano-induced ATRP (mechano-ATRP). Based on the synergistic effect of piezoelectric properties and specific surface area, the catalytic activity of 1D ZnO nanorods (1D-ZnO NRs) with increased aspect ratio outperformed that of 0D ZnO nanoparticles (0D-ZnO NPs). Compared to the conventional ATRP system, this system exhibited extraordinary activity toward the less activated monomer acrylonitrile (67% conversion after 6 h), with a narrow molecular weight distribution (polydispersity index ∼ 1.19). Furthermore, implications of ZnO loading, copper salt amount, degree of polymerization, monomer, and solvent were also studied for the highly efficient mechano-ATRP.

Sonochemistry-assisted photocontrolled atom transfer radical polymerization enabled by manganese carbonyl

Sonochemistry-assisted photocontrolled atom transfer radical polymerization (SAP-ATRP) is developed to circumvent the problem caused by the low penetration depth of light.

Electro-Mechanochemical Atom Transfer Radical Cyclizations using Piezoelectric BaTiO3

The formation and regeneration of active CuI species is a fundamental mechanistic step in copper-catalyzed atom transfer radical cyclizations (ATRC). Typically, the presence of the catalytically active CuI species in the reaction mixture is secured by using high CuI catalyst loadings or the addition of complementary reducing agents. In this study it is demonstrated how the piezoelectric properties of barium titanate (BaTiO3 ) can be harnessed by mechanical ball milling to induce electrical polarization in the strained piezomaterial. This strategy enables the conversion of mechanical energy into electrical energy, leading to the reduction of a CuII precatalyst into the active CuI species in copper-catalyzed mechanochemical solvent-free ATRC reactions.

Role of External Field in Polymerization: Mechanism and Kinetics

The past decades have witnessed an increasing interest in developing advanced polymerization techniques subjected to external fields. Various physical modulations, such as temperature, light, electricity, magnetic field, ultrasound, and microwave irradiation, are noninvasive means, having superb but distinct abilities to regulate polymerizations in terms of process intensification and spatial and temporal controls. Gas as an emerging regulator plays a distinctive role in controlling polymerization and resembles a physical regulator in some cases. This review provides a systematic overview of seven types of external-field-regulated polymerizations, ranging from chain-growth to step-growth polymerization. A detailed account of the relevant mechanism and kinetics is provided to better understand the role of each external field in polymerization. In addition, given the crucial role of modeling and simulation in mechanisms and kinetics investigation, an overview of model construction and typical numerical methods used in this field as well as highlights of the interaction between experiment and simulation toward kinetics in the existing systems are given. At the end, limitations and future perspectives for this field are critically discussed. This state-of-the-art research progress not only provides the fundamental principles underlying external-field-regulated polymerizations but also stimulates new development of advanced polymerization methods.

Effects of various Cu(0), Fe(0), and proanthocyanidin reducing agents on Fe(iii)-catalysed ATRP for the synthesis of PMMA block copolymers and their self-assembly behaviours

Well-defined PMMA, PMMA-b-PBzMA and PMMA-b-PBMA polymers were obtained via green Fe-ATRP with the aid of proanthocyanidins. Interestingly, microphase separation was observed in PMMA-b-PBMA polymer with upper critical ordering temperature behaviour.

Ultrasound-Mediated Atom Transfer Radical Polymerization (ATRP)

Ultrasonic agitation is an external stimulus, rapidly developed in recent years in the atom transfer radical polymerization (ATRP) approach. This review presents the current state-of-the-art in the application of ultrasound in ATRP, including an initially-developed, mechanically-initiated solution with the use of piezoelectric nanoparticles, that next goes to the ultrasonication-mediated method utilizing ultrasound as a factor for producing radicals through the homolytic cleavage of polymer chains, or the sonolysis of solvent or other small molecules. Future perspectives in the field of ultrasound in ATRP are presented, focusing on the preparation of more complex architectures with highly predictable molecular weights and versatile properties. The challenges also include biohybrid materials. Recent advances in the ultrasound-mediated ATRP point out this approach as an excellent tool for the synthesis of advanced materials with a wide range of potential industrial applications.

Emerging Trends in Polymerization-Induced Self-Assembly

In this Perspective, we summarize recent progress in polymerization-induced self-assembly (PISA) for the rational synthesis of block copolymer nanoparticles with various morphologies. Much of the PISA literature has been based on thermally initiated reversible addition-fragmentation chain transfer (RAFT) polymerization. Herein, we pay particular attention to alternative PISA protocols, which allow the preparation of nanoparticles with improved control over copolymer morphology and functionality. For example, initiation based on visible light, redox chemistry, or enzymes enables the incorporation of sensitive monomers and fragile biomolecules into block copolymer nanoparticles. Furthermore, PISA syntheses and postfunctionalization of the resulting nanoparticles (e.g., cross-linking) can be conducted sequentially without intermediate purification by using various external stimuli. Finally, PISA formulations have been optimized via high-throughput polymerization and recently evaluated within flow reactors for facile scale-up syntheses.

Mechanically Initiated Bulk-Scale Free-Radical Polymerization

Mechanical initiation of polymerization offers the chance to generate polymers in new environments using an energy source with unique capabilities. Recently, a renewed interest in mechanically controlled polymerization has yielded many techniques for controlled radical polymerization by ultrasound. However, other types of polymerizations induced by mechanical activation are rare, especially for generating high-molecular-weight polymers. Herein is an example of using piezoelectric ZnO nanoparticles to generate free-radical species that initiate chain-growth polymerization and polymer crosslinking. The fast generation of high amounts of reactive radicals enable the formation of polymer/gel by ultrasound activation. This chemistry can be used to harness mechanical energy for constructive purposes in polymeric materials and for controlled polymerizations for bulk-scale reactions.