Expanding the ROMP Toolbox: Synthesis of Air-Stable Benzonorbornadiene Polymers by Aryne Chemistry

Effect of Self-Healing by Dicyclopentadiene Microcapsules on Tensile and Fatigue Properties of Epoxy Composites

Microcapsules of urea-formaldehyde (UF) containing dicyclopentadiene (DCPD) were synthesized by the in situ polymerization technique for self-healing of epoxy. The dispersion of microcapsules in the epoxy matrix was achieved using ultrasonication. Composites of epoxy, having 0.5, 1.0, 1.5, and 2.0 wt.% of microcapsules capable of self-healing, were prepared. The shape and size of the microcapsules were determined by field emission electron microscopy. Spherical capsules of DCPD, with an average diameter of 172 nm, were obtained. Investigation of tensile properties indicated a decrease in the tensile modulus with an increase in wt.% of microcapsules. There was a reduction of 22%, 27%, 39%, and 30% in the elastic modulus of composites for 0.5, 1.0, 1.5, and 2.0 wt.% of microcapsules, respectively. Tensile strength was found to increase with an increase in wt.% of microcapsules. The tensile strength of the composites increased by 33%, 20%, 8%, and 21% for 0.5, 1.0, 1.5, and 2.0 wt.% of microcapsules, respectively, in comparison with that of neat epoxy. The fatigue life of composites was investigated by conducting uniaxial tension-tension fatigue tests at constant stress amplitudes of 20, 25, 30, and 35 MPa, at a constant stress ratio (R = 0.1) and a frequency of 3 Hz. The fatigue life of composites increased with an increase in wt.% of microcapsules in comparison with that of neat epoxy. It was found that the fatigue life of the composites decreased with 1.5 and 2.0 wt.% of microcapsules in comparison with composites with 0.5 and 1.0 wt.% of microcapsules. The fracture surfaces of the tested samples were examined with the help of scanning electron microscopy (SEM) to understand the various mechanisms responsible for the change in modulus, strength, failure strain, and fatigue life of composites.

Expedient Synthesis and Ring-Opening Metathesis Polymerization of Pyridinonorbornenes

Pyridine-containing polymers are promising materials for a variety of applications from the capture of contaminants to the self-assembly of block copolymers. However, the innate Lewis basicity of the pyridine motif often hampers living polymerization catalyzed by transition-metal complexes. Herein, we report the expedient synthesis of pyridinonorbornene monomers via a [4+2] cycloaddition between 2,3-pyridynes and cyclopentadiene. Well-controlled ring-opening metathesis polymerization was enabled by careful structural design of the monomer. Polypyridinonorbornenes exhibited high Tg and Td, a promising feature for high-temperature applications. Investigation of the polymerization kinetics and of the reactivity of the chain ends shed light on the influence of nitrogen coordination on the chain-growth mechanism.

Bottlebrush Polymer-Conjugated Melittin Exhibits Enhanced Antitumor Activity and Better Safety Profile

Despite potency against a variety of cancers in preclinical systems, melittin (MEL), a major peptide in bee venom, exhibits non-specific toxicity, severe hemolytic activity, and poor pharmacological properties. Therefore, its advancement in the clinical translation system has been limited to early-stage trials. Herein, we report a biohybrid involving a bottlebrush-architectured poly(ethylene glycol) (PEG) and MEL. Termed pacMEL, the conjugate consists of a high-density PEG arrangement, which provides MEL with steric inhibition against protein access, while the high molecular weight of pacMEL substantially enhances plasma pharmacokinetics with a ∼10-fold increase in the area under the curve (AUC∞) compared to free MEL. pacMEL also significantly reduces hepatic damage and unwanted innate immune response and all but eliminated hemolytic activities of MEL. Importantly, pacMEL passively accumulates at subcutaneously inoculated tumor sites and exhibits stronger tumor-suppressive activity than molecular MEL. Collectively, pacMEL makes MEL a safer and more appealing drug candidate.

Recent Advances on Diels-Alder-Driven Preparation of Bio-Based Aromatics

The preparation of high value-added chemicals from renewable resources is a crucial approach towards a sustainable economy. One prominent alternative to the production of petroleum-based chemicals from fossil resources is through the sequential Diels-Alder/aromatization reactions of biomass-derived furan platforms. This Concept is focused on the recent boom in bio-based furan DA strategies for aromatization of bio-based platform chemicals, particularly that of furfurals, ranging from indirect use and activation strategies to recent examples of direct DA reaction of these electron-withdrawing biomass-derived furans.

Ring-Opening Metathesis Polymerization of endo- and exo-Norbornene Lactones

New ester-functionalized bicyclic aliphatic polymers are synthesized through the ring-opening metathesis polymerization (ROMP) of endo- and exo-norbornene lactones (endo-NBL and exo-NBL) and their oxa-norbornene analogue (exo-oxa-NBL) followed by hydrogenation. The polymerizability between endo- and exo-NBLs, and the thermal properties between the six types of polymers before and after hydrogenation are compared and discussed. The ROMP of all three monomers proceeded in a living fashion under optimized conditions, which is confirmed by chain extension experiments. Endo-NBL shows a much lower homo- and copolymerizability than exo-NBL probably owing to six-membered chelation to the Ru center and steric hindrance in the ruthenacyclobutane intermediate. Stereo-block and stereo-gradient copolymers of poly(endo-NBL) and poly(exo-NBL) are also synthesized. The hydrogenation catalyzed by RuHCl(CO)(PPh₃ )₃ in the mixed solvents of o-xylene and N,N-dimethyl acetamide (DMAc) results in more than 95% conversion. The obtained hydrogenated polymers, H-poly(endo-NBL) and H-poly(exo-NBL), are amorphous, soluble in chlorinated aliphatic solvents, and thermally stable until 400 °C without a weight loss. Their glass transition temperatures are 163 and 131 °C, respectively; the values are appropriate in terms of both thermal stability and processing deformation for the application of transparent resin materials.

Substituted Ortho-Benzynes: Properties of the Triple Bond

Ortho-benzyne has been well studied by both experiment and theory. Its substituted variants, however, have been less carefully examined. Benchmark data are computed for unsubstituted ortho-benzyne using several density functional theory functionals and basis sets, up to cc-pVQZ. Optimized geometries for the substituted ortho-benzyne as well as harmonic vibrational frequencies and singlet-triplet splittings are computed using the benchmarked functionals. A proximal (syn)OH substitution causes a mean θ₁ distortion of +8.1 ± 1.4° from ortho-benzyne. Substituting in the proximal position with F shifts the singlet-triplet splitting by +4.5 ± 0.4 kcal mol^-1 from ortho-benzyne. Natural bond orbital analysis, including natural Coulomb electrostatics, elucidates the presence of three influences from the selected substituents: hyperconjugative, resonance, and electrostatic effects.

Safety Assessment of Benzyne Generation from a Silyl Triflate Precursor

Silyl triflate precursors to benzyne and related intermediates have emerged as valuable synthetic building blocks. However, data addressing the safety of employing these silyl triflate precursors are lacking. We report the calorimetric analysis of a typical Kobayashi procedure for forming and trapping benzyne using a silyl triflate precursor. Our findings suggest that, unlike benzenediazonium carboxylate precursors to benzyne, silyl triflates may be employed under mild conditions without severe concern for runaway reaction.

Arynes and Their Precursors from Arylboronic Acids via Catalytic C-H Silylation

A new, operationally simple approach is presented to access arynes and their fluoride-activated precursors based on Ru-catalyzed C-H silylation of arylboronates. Chromatographic purification may be deferred until after aryne capture, rendering the arylboronates de facto precursors. Access to various new arynes and their derivatives is demonstrated, including, for the first time, those based on a 2,3-carbazolyne and 2,3-fluorenyne core, which pave the way for novel derivatizations of motifs relevant to materials chemistry.

Regioselective arene homologation through rhenium-catalyzed deoxygenative aromatization of 7-oxabicyclo[2.2.1]hepta-2,5-dienes

A facile approach to PAHs with an arm-chair edge via regioselective bromination, Diels–Alder reaction, and rhenium-catalyzed deoxygenative aromatization is described.

Rapid access to substituted 2-naphthyne intermediates via the benzannulation of halogenated silylalkynes

A ZnCl₂-catalyzed, regioselective benzannulation of halogenated silylacetylenes provides access to 2-naphthyne precursors.

Aryne intermediates are versatile and important reactive intermediates for natural product and polymer synthesis. 2-Naphthynes are relatively unexplored because few methods provide precursors to these intermediates, especially for those bearing additional substituents. Here we report a general synthetic strategy to access 2-naphthyne precursors through an Asao-Yamamoto benzannulation of ortho-(phenylethynyl)benzaldehydes with halo-silylalkynes. This transformation provides 2-halo-3-silylnaphthalenes with complete regioselectivity. These naphthalene products undergo desilylation/dehalogenation in the presence of F^– to generate the corresponding 2-naphthyne intermediate, as evidenced by furan trapping experiments. When these 2-naphthynes are generated in the presence of a copper catalyst, ortho-naphthalene oligomers, trinaphthalene, or binaphthalene products are formed selectively by varying the catalyst loading and reaction temperature. The efficiency, mild conditions, and versatility of the naphthalene products and naphthyne intermediates will provide efficient access to many new functional aromatic systems.