Selective ethylene trimerization into 1-hexene, which is an important comonomer for production of copolymers with high performance, was first commercialized in 2003 using a chromium tris(2ethylhexanoate) (Cr(EH) 3 )-based catalyst. One milestone during the development of this catalyst is the addition of 1,2-dimethoxyethane (DME) to Cr(EH) 3 /partially hydrolyzed tri-isobutyl aluminum (PIBAO) catalyst, resulting in a transformation from ethylene polymerization into trimerization. However, the mechanism of this transformation is unknown. In this work, the role of DME in the switching mechanism from ethylene polymerization to trimerization was investigated by a density functional theory (DFT) method based on the 10 most plausible molecular models for the active species of Cr(EH) 3 /PIBAO catalyst with or without DME coordination. For neutral models (Cr(I)OR; Cr(I)OR/DME; Cr(I)R; Cr(I)R/DME; R refers to isobutyl ligand), those without DME coordination were found to be dominantly ethylene trimerization sites. After DME coordination, both the weak electron-donating effect and steric effect of DME increased the total energy barriers of metallocycle growth, leading to ethylene trimerization with dimerization as a side reaction. So neutral models mainly presented ethylene trimerization with or without DME coordination and were not consistent with experimental evidence of transformation. For cationic models (Cr(I) þ ; Cr(I) þ /DME; Cr(II) þ OR; Cr(II) þ OR/DME; Cr(II) þ R; Cr(II) þ R/DME), metallocycle growth leading to ethylene polymerization occurred on those models without DME. After DME coordination, the strong electron-donating effect of DME was found to facilitate metallocycle expansion, while the dominant role of the steric effect of DME made nine-membered ring formation unfavorable, and thus 1-hexene was liberated from the seven-membered ring. So all the cationic models presented a transformation from ethylene polymerization to trimerization after DME coordination and might be the most plausible active sites for the Cr(EH) 3 /PIBAO catalyst system. The results provided a much deeper understanding of the highly selective trimerization mechanism and for further development of new catalysts with high performance as well.
A pH-sensitive and mechanically strong graphene oxide/sodium alginate/polyacrylamide nanocomposite semi-IPN hydrogel was designed and prepared. The composite semi-IPN hydrogel showed superior mechanical strength and pH-dependent swelling behavior.
The high density polyethylene (HDPE) nanocomposites were prepared by melt mixing HDPE with multi-wall carbon nanotubes (MWCNTs). In this work, the morphological, nucleation, crystallization and mechanical properties of the HDPE nanocomposites were studied by scanning electron microscopy, different scanning calorimetry, small-angle X-ray scattering and tensile testing. It was found that the tensile strength and Young’s modulus is increased by 42.4% and 116.5% at 3.wt% MWCNT loading compared to the pure HDPE. According to SEM results combined with SAXS, well-defined nanohybird shish-kebab (NHSK) entities exist in the composites, and in the shish-kebab structures fibrillous carbon nanotubes (MWCNTs) act as shish while HDPE lamellae act as kebab. The crystallization behavior, probed by DSC, suggests that MWCNTs have strong nucleation ability and shear stress plays an important role in polymer crystallization process. The mechanical properties demonstrated that the formation of the Shish-kebab structures improved the interfacial adhesion and brought obvious mechanical enhancement for the HDPE/MWCNTs nanocomposites.
Size-dependent elastic properties of thin films: surface anisotropy and surface bonding SCIENCE CHINA Technological Sciences 57, 680 (2014); Clay surface modification and its coagulation of red tide organisms Chinese Science Bulletin 44, 617 (1999);Experimental study about the influence of adhesive stiffness to the bonding strengths of adhesives for ceramic/metal targets
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.