Qingtao Niu,Aihua He,Chongdian Si,Guodong Liu,Mingxuan Xiao,Guifan Chen,Manman Jin
Four TiCl 4 /MgCl 2 Ziegler-Natta catalysts (Cat-1∼Cat-4) with titanium contents in the range of 0.038–0.578 wt% were synthesized by the chemical rout and used for isoprene polymerizations. Four catalysts had similar possessed rod-like morphologies and their surface areas gradually increased as Ti contents increased. Cat-1 with the lowest Ti content (0.038 wt%) had the highest catalytic activity (28039.6 g∙P/(g∙Ti∙h)). To our knowledge, this was the highest activity achieved for isoprene polymerization using the heterogeneous TiCl 4 /MgCl 2 Ziegler-Natta catalyst. With the increasing Ti content, the catalyst activity gradually decreased in the order of Cat-1 < Cat-2 < Cat-3 < Cat-4, whereas the trans -1,4 stereoselectivity increased. Cat-4 with the highest Ti content (0.578 wt %) gave highest trans -1,4 units (97.7 mol%). The polymers were mainly composed of fraction C, D and G. C and D had high trans -1,4 structure while G fraction had low trans -1,4 structure. The plausible stereospecific transformation models of MgCl 2 -supported Ziegler-Natta catalyst with different Ti contents were discussed.
Series of isoprene polymerizations with different polymerization time were performed by TiCl 4 /MgCl 2 type Ziegler-Natta catalysts. Polymerization kinetics showed catalytic activity had two-stage polymerization feature and the polymer chain microstructure gradually changed from the trans -1,4 and cis -1,4 mixed structure with low molecular weight ( M w ) as well as broad molecular weight distribution ( M w / M n ) at the early stage, to the final high trans -1,4 structure with high M w and narrow M w / M n . The high trans -1,4 units can enhance the tensile strength and hardness of the synthetic polyisoprenes, which are widely used for golf ball cover, medicinal material and shape memory material. Polymers were mainly isolated into Fraction C, D and G by the stepwise isothermal crystallization fractionation method. Fraction C and D had high trans -1,4 content, high trans -1,4-[II] dyad sequence concentration, high trans - trans unit linkage content and high crystallinity, which were generated by high trans -1,4 stereoselective active centers. While G fraction had low trans -1,4 content, low trans -1,4-[II] dyad sequence concentration, low trans - trans unit linkage content and low crystallinity, which were formed by the active center with low trans -1,4 stereoselectivity. The stereoselective transformation of active centers as well as trans -1,4 stereospecific polymerization mechanism of isoprene with TiCl 4 /MgCl 2 -AlEt 3 catalyst system was discussed. Graphical
Zhenliang Zhu,Bei Li,Xing Liu,Peixin Zhang,Shixia Chen,Qiang Deng,Zheling Zeng,Jun Wang,Shuguang Deng
Adsorptive separation of xenon (Xe) from krypton (Kr) is challenging owing to their low concentrations and similar physical properties. Herein, we report two metal–organic frameworks (MOFs) assembled from the same aliphatic ligand ( trans -1, 4-cyclohexanedicarboxylic acid, H 2 CDC), but showing distinct pore shapes. The rhombus pores on Al-CDC show a narrower pore aperture of 4.5 Å × 6.0 Å, contracting to the pseudo-square pores (5.8 Å × 6.8 Å) on Cu-CDC. As a result, Al-CDC exhibits a high Xe adsorption capacity (2.45 mmol g −1 ) and Xe/Kr selectivity (10.7) at 298 K and 1.0 bar. The confined pore space and abundant saturated C-H groups provide stronger van der Waals interactions toward Xe, which is disclosed by the Grand Canonical Monte Carlo (GCMC) and Density Functional Theory (DFT) calculations. Dynamic breakthrough experiments further confirm their industrial application potential and stable reusability.
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