Post‐Polymerization Thiol Substitutions Facilitated by Mechanochemistry

Liquid-assisted grinding enables a direct mechanochemical functionalization of polystyrene waste

The plastic waste crisis has grave consequences for our environment, as most single-use commodity polymers remain in landfills and oceans long after their commercial lifetimes. Utilizing modern synthetic techniques to chemically modify the structure of these post-consumer plastics (e.g., upcycling) can impart new properties and added value for commercial applications. To expand beyond the abilities of current solution-state chemical processes, we demonstrate post-polymerization modification of polystyrene via solid-state mechanochemistry enabled by liquid-assisted grinding (LAG). Importantly, this emblematic trifluoromethylation study modifies discarded plastic, including dyed materials, using minimal exogenous solvent and plasticizers for improved sustainability. Ultimately, this work serves as a proof-of-concept for the direct mechanochemical post-polymerization modification of commodity polymers, and we expect future remediation of plastic waste via similar mechanochemical reactions.

TM-Free and TM-Catalyzed Mechanosynthesis of Functional Polymers

Mechanochemically induced methods are commonly used for the depolymerization of polymers, including plastic and agricultural wastes. So far, these methods have rarely been used for polymer synthesis. Compared to conventional polymerization in solutions, mechanochemical polymerization offers numerous advantages such as less or no solvent consumption, the accessibility of novel structures, the inclusion of co-polymers and post-modified polymers, and, most importantly, the avoidance of problems posed by low monomer/oligomer solubility and fast precipitation during polymerization. Consequently, the development of new functional polymers and materials, including those based on mechanochemically synthesized polymers, has drawn much interest, particularly from the perspective of green chemistry. In this review, we tried to highlight the most representative examples of transition-metal (TM)-free and TM-catalyzed mechanosynthesis of some functional polymers, such as semiconductive polymers, porous polymeric materials, sensory materials, materials for photovoltaics, etc.

Solvent-Free Mechanochemical Post-Polymerization Modification of Ionic Polymers

Despite their superior stability and facile handling, ionic polymers have limited solubility in most organic solvents, restricting the range of substrates and reaction conditions to which they can be applied. To overcome this solubility issue, the present study presents a solvent-free mechanochemical reaction. Specifically, a post-polymerization modification of ammonium-functionalized polyether was demonstrated using a solvent-free vibrational ball-milling technique. The formation of imine bonds between the ionic polymer and an aromatic aldehyde led to the complete conversion to imine within 1 h without any bond breakage on the polymer backbone. The viability of this approach for a wide range of aldehydes was also evaluated, highlighting the potential of the mechanochemical post-polymerization modification of polymers that are inaccessible by conventional solution approaches.

Mechanochemistry for sustainable and efficient dehydrogenation/hydrogenation

Hydrogenation and dehydrogenation reactions are one of the pillars of the chemical industry, with applications from bulk chemicals to pharmaceuticals manufacturing. The ability to selectively add hydrogen across double and (or) triple bonds is key in the chemist’s toolbox and the enabling component in the development of sustainable processes. Traditional solution-based approaches to these reactions are tainted by significant consumption of energy and production of solvent waste. This review highlights the development and applications of recently emerged solvent-free approaches to conduct the hydrogenation of organic molecules using mechanochemistry, i.e., chemical transformations induced or sustained by mechanical force. In particular, we will show mechanochemical techniques such as ball-milling enabled catalytic or stoichiometric metal-mediated hydrogenation and dehydrogenation reactions that are simple, fast, and conducted under significantly milder conditions compared with traditional solution routes. Importantly, we highlight the current challenges and opportunities in this field, while also identifying exciting cases in which mechanochemical hydrogenation strategies lead to new, unique targets and reactivity.

Post-Polymerization Modification of Ring Opening Metathesis Polymerization (ROMP)-Derived Materials Using Wittig Reactions

This communication describes our recent efforts to utilize Wittig olefination reactions for the post-polymerization modification of polynorbornene derivatives prepared through ring opening metathesis polymerization (ROMP). Polymerizing α-bromo ester-containing norbornenes provides polymers that can undergo facile substitution with triphenylphosphine. The resulting polymeric phosphonium salt is then deprotonated to form an ylide that undergoes reaction with various aryl aldehydes in a one-pot fashion to yield the respective cinnamates. These materials can undergo further modification through photo-induced [2 + 2] cycloaddition cross-linking reactions.