Natural aging mechanism of buried polyethylene pipelines during long-term service

Currently,accelerated aging tests are widely used to study the aging process of polyethylene pipelines.However,this approach can only simulate one or several main influencing factors in the natural environment,which are often quite different from the actual environment of the buried pipelines.In this study,five types of PE80 buried pipelines in service for 9e18 years were taken as the research object,while new PE80 pipelines were taken as the reference group.The aging process and mechanism of polyethylene buried pipelines were studied through mechanical and chemical property tests and microstructural analysis.The results showed that the pipeline exhibited cross-linking as the main aging mechanism after being in service for 0e18 years.The aging degree and law of the inner and outer surface of the pipeline were compared,and the observed mechanism of both surfaces was explained.After 18 years in service,the elongation at the break of the pipe decreased by 16.2%,and the toughness of the matrix in the main collapse area of the tensile sample was the fundamental reason responsible for changes in the mechanical properties.Finally,after 18 years in service,the oxidation induction time of the pipeline was 25.7 min,which was 28.5% higher than the national standard value.There were no potential safety hazards during continuous long-term service.The results of this paper provide reference data and theoretical guidance for the aging process study of buried polyethylene pipelines.

Construction of polycyclic spirooxindoles through[3+2]annulations of Morita–Baylis–Hillman carbonates and 3-nitro-7-azaindoles

A mild and efficient dearomatic [3＋2] annulation reaction of 3-nitro-7-azaindoles and Morita Baylis Hillman carbonates from isatins was developed catalyzed by DMAP, affording the corresponding polycyclic spirooxindoles containing fused azaindoline architectures and vicinal quaternary centers in excellent yields（up to 96%） with high regio- and diastereoselectivity（dr 〉 19：1）. Moderate enantioselectivity（79% ee） was obtained by employing a chiral DMAP-type Lewis base catalyst.

Phosphine-Catalyzed Interrupted Morita-Baylis-Hillman Reaction and Switchable Domino Reactions of a-Substituted Activated Olefins with Formaldehyde and Mechanism Elucidation

Summary of main observation and conclusion 3-Olefinic oxindoles can undergo interrupted Morita-Baylis-Hillman reaction with formaldehyde. Apart from the previous reported dephosphoration to access reductive aldol-type products, here we uncovered that completely different pathways were followed by tuning the substitutions of 3-olefinic oxin doles. In com bin ation with formalin, an unexpected domino phosphorus-ylide formation, aldol-type addition, hemeacetal formation and O-substitution process was observed to produce 1,3-dioxolane derivatives. Moreover, a switchable sequence to produce formate derivatives via a key hydride transfer process was furnished by simply replacing formalin with paraformaldehyde. Density functional theory calculations were conducted to well elucidate the catalytic reaction mechanism.

Solvent-Regulated Diastereodivergent [3+2] Annulations of CF_(3)-Containing Morita-Baylis-Hillman Carbonates with Cyclic Sulfonimines

A solvent-regulated diastereodivergent[3+2]annulation reaction between Morita-Baylis-Hillman carbonates from aryl trifluoromethyl ketones and cyclic sulfonimines is developed under the catalysis of a modified cinchona alkaloid,furnishing diverse polycyclic products bearing vicinal quaternary stereogenic centers with outstanding enantioselectivity and moderate to high diastereoselectivity.

New Amines and Activation Modes in Asymmetric Aminocatalysis

Asymmetric aminocatalysis has become one of the most powerful strategies for the transformations of carbonyl substances over the past two decades.Here,we describe the research from our laboratory that significantly expands the horizon of aminocatalysis.It includes the development and application of cinchona-based primary amines,fruitful reactions based on HOMO-raising strategy,and the disclosure of amine/thiol double activation catalysis.

Palladium-catalysed diastereodivergent inverse- electron-demand oxa-Diels–Alder reactions of in situ formed cyclopentadienones via ligand- control

Here we report Pd(0)-catalysed asymmetric inverse-electron-demand oxa-Diels–Alder reactions between the carbonates of 4-hydroxy-2-cyclopentenones andα-cyano chalcones by in situ generating η^(2)-Pd(0)-cyclopentadienone complexes as HOMO-raised dienophiles,and diastereodivergent synthesis could be realised by employing a bifunctional monophosphine ligand from a chiral aminoalcohol or a proline-derived bisphosphine ligand.