Search for: All records

Amidyl radicals mediate a diverse array of intermolecular aliphatic C(sp3)–H and decarboxylative functionalizations. Interestingly, we have observed that decarboxylative processes proceed with excellent chemoselectivity even with substrates containing weak C(sp3)–H bonds. Herein, we report a mechanistic basis for understanding this high chemoselectivity of amidyl radicals through divergent reaction pathways. A computational assessment of the transition state SOMOs and intrinsic bonding orbitals for amidyl radical hydrogen atom transfer (HAT) and concerted proton-electron transfer (CPET) processes support a shift in mechanism between aliphatic C(sp3)–H or carboxylic acid O–H abstraction, which is supported by experimental studies. These findings provide a rationale for the chemoselectivity of decarboxylative reactions mediated by amidyl radicals. 

C–H functionalization of commodity polyolefins affords functional materials derived from a high‐volume, low‐cost resource. However, current postpolymerization modification strategies result in randomly distributed functionalization along the length of the polymer backbone, which has a negative impact on the crystallinity of the resultant polymers, and thus the thermomechanical properties. Here, we demonstrate an amidyl radical mediated C–H functionalization of polyolefins to access blocky microstructures, which exhibit a higher crystalline fraction, larger crystallite size, and improved mechanical properties compared to their randomly functionalized analogues. Taking inspiration from the site‐selective C–H functionalization of small molecules, we leverage the steric protection provided by crystallites and target polymer functionalization to amorphous domains in a semicrystalline polyolefin gel. The beneficial outcomes of blocky functionalization are independent of the identity of the pendant functional group that is installed through functionalization. The decoupling of functional group incorporation and crystallinity highlights the promise in accessing nonrandom microstructures through selective functionalization to circumvent traditional tradeoffs in postpolymerization modification, with potential impact in advanced materials and upcycling plastic waste. 

Chemical upcycling of plastic waste into high-value materials has the potential to contribute to a more sustainable plastic economy. We report the synthesis of high-value ionomers directly from commodity polyolefins enabled by amidyl radical mediated C−H functionalization. The use of thiosulfonates as a linchpin functionality for the group transfer of a variety of heteroaryl groups provided tunable incorporation of ionizable functionality onto a variety of polyolefin substrates, including postconsumer polyethylene packaging waste. Synthetic, structural, and thermomechanical studies provided a comprehensive understanding of both structure−reactivity and structure−property relationships for polyolefin ionomers. X-ray scattering experiments conducted in the solid and melt states confirm the presence of ionic multiplets that serve as physical cross-links both below and above the melting temperature of polyolefin crystallites. The incorporation of ionic groups into the polyolefins yielded materials with significantly enhanced melt strength and tensile toughness. We anticipate that this approach to access performance-advantaged polyolefin ionomers from commodity substrates or plastic waste will enhance sustainability efforts and lead to new opportunities for this versatile class of thermoplastics. 

Developments in nanomolecular engineering in the last 20 years have led to the development of technology that uses ultrasonic irradiation in initiating the polymerization process for wider industrial and commercial applications. In this experimental study, ultrasound-assisted reversible addition chain-transfer (Sono-RAFT) polymerization was used to differentiate the effects of the bulk and continuous flow polymerization methods on three parameters─monomer conversion, polymer molar mass, and dispersity─using 2-hydroxyethyl acrylate, N-acryloyl morpholine, and N-dimethylacetamide as monomer substrates. Experimental results indicate that continuous flow polymerization demonstrated higher monomer conversion than polymerizations performed in batch under identical experimental conditions. Furthermore, the increased surface-to-volume ratio inherent to continuous flow reactors enabled Sono-RAFT at a higher monomer concentration than analogous batch reactions due to the higher cavitational intensity accessible in tubular microreactors. The key to continuous flow Sono-RAFT was the observation that stainless-steel microreactors result in increased cavitational intensity and decreased oxygen contamination compared to PFA tubing. We envision that these findings will further advance the field of mechanochemistry in polymer science and provide an approach to make sonochemically regulated polymerization more practical and sustainable. 

Upcycling plastic waste into reprocessable materials with performance-advantaged properties would contribute to the development of a circular plastics economy. Here, we modify branched polyolefins and postconsumer polyethylene through a versatile C−H functionalization approach using thiosulfonates as a privileged radical group transfer functionality. Cross-linking the functionalized polyolefins with polytopic amines provided dynamically cross-linked polyolefin networks enabled by associative bond exchange of diketoenamine functionality. A combination of resonant soft X-ray scattering and grazing incidence X-ray scattering revealed hierarchical phase morphology in which diketoenamine-rich microdomains phase-separate within amorphous regions between polyolefin crystallites. The combination of dynamic covalent cross-links and microphase separation results in useful and improved mechanical properties, including a ∼4.5-fold increase in toughness, a reduction in creep deformation at temperatures relevant to use, and high-temperature structural stability compared to the parent polyolefin. The dynamic nature of diketoenamine cross-links provides stress relaxation at elevated temperatures, which enabled iterative reprocessing of the dynamic covalent polymer network with little cycle-to-cycle property fade. The ability to convert polyolefin waste into a reprocessable thermoformable material with attractive thermomechanical properties provides additional optionality for upcycling to enable future circularity.