Factors affecting the dimerization of persistent nitrogen-centered 1-phenyl urazole radicals to tetrazanes

Upcycling Polyolefin Blends into High-Performance Materials by Exploiting Azidotriazine Chemistry Using Reactive Extrusion

The revalorization of incompatible polymer blends is a key obstacle in realizing a circular economy in the plastics industry. Polyolefin waste is particularly challenging because it is difficult to sort into its constituent components. Untreated blends of polyethylene and polypropylene typically exhibit poor mechanical properties that are suitable only for low-value applications. Herein, we disclose a simple azidotriazine-based grafting agent that enables polyolefin blends to be directly upcycled into high-performance materials by using reactive extrusion at industrially relevant processing temperatures. Based on a series of model experiments, the azidotriazine thermally decomposes to form a triplet nitrene species, which subsequently undergoes a complex mixture of grafting, oligomerization, and cross-linking reactions; strikingly, the oligomerization and cross-linking reactions proceed through the formation of nitrogen-nitrogen bonds. When applied to polyolefin blends during reactive extrusion, this combination of reactions leads to the generation of amorphous, phase-separated nanostructures that tend to exist at polymer-polymer interfaces. These nanostructures act as multivalent cross-linkers that reinforce the resulting material, leading to dramatically improved ductility compared with the untreated blends, along with high dimensional stability at high temperatures and excellent mechanical recyclability. We propose that this unique behavior is derived from the thermomechanically activated reversibility of the nitrogen-nitrogen bonds that make up the cross-linking structures. Finally, the scope of this chemistry is demonstrated by applying it to ternary polyolefin blends as well as postconsumer polyolefin feedstocks.

Evaluating the Effectiveness of Tethered Bis(urazolyl) Diradicals as Molecular Building Blocks for Dynamic Covalent Chemistry

Dynamic covalent chemistry (DCvC) is a powerful means by which to rapidly prepare complex structures from simple molecular building blocks. Effective DCvC behavior is contingent upon the reversibility of covalent bond formation. Stabilized radical species, therefore, have been effectively used for these applications. In earlier work we demonstrated that properly substituted 1-arylurazolyl radicals showed promise as oxygen-insensitive heterocyclic N-centered radicals with a propensity for reversible bond formation. In this work we have synthesized several tethered bis(urazolyl) diradicals, varying by the type and length of connectivity between the urazole rings, and tested them for DCvC behavior. We have found that when the two aryl rings to which the urazolyl radical sites are attached are tethered by a chain of five or more carbons, equilibrium mixtures of monomeric and dimeric species are formed by N-N bond formation between two radical sites. DCvC behavior is observed that is sensitive to changes in temperature, concentration, and (to a lesser extent) solvent. In general, the dimer species is favored at lower temperatures and higher concentrations.

Unanticipated formation of a novel octaazacyclodecane ring upon oxidation of a 1,1-bis-urazole

Tetrahydrotetrazoles are a little-explored class of five-membered heterocycles with four contiguous singly-bonded N atoms. Recent work in our labs has demonstrated that urazole radicals are amenable to N—N bond formation via radical combination to form such a chain of four N atoms. Previously described 1,1-bis-urazole compounds appeared to be convenient precursors to the target tetrazoles via their oxidation to intermediate urazole diradicals, which upon N—N bond formation would complete the tetrazole framework. While oxidation proceeded smoothly, the novel 10-membered octaaza heterocycle 7,7,18,18-tetraacetyl-4,10,15,21-tetraphenyl-1,2,4,6,8,10,12,13,15,17,19,21-dodecaazapentacyclo[17.3.0.02,6.08,12.013,17]docosan-3,5,9,11,14,16,20,22-octone, C42H32N12O12, was obtained (36% yield) instead of the expected tetrazole product, as confirmed by X-ray crystallography. Calculations at the (U)B3LYP/6-311G(d,p) level of theory suggest that the desired tetrazoles have weak N—N bonds connecting the two urazole units.