Revealing the Dynamic Behaviors of Tetrahydrofuran for Tailoring the Active Species of Ziegler–Natta Catalysts

Abstract

A full elucidation of chemisorbed donor behavior is of fundamental importance for tailoring the active species but has not yet been achieved for Ziegler–Natta catalysts. Herein, by exploiting the self-nucleation of polyhedral oligomeric silsesquioxane (POSS) molecules, we present porous microspheres assembled from rhombic MgCl2 nanocrystals with layered laterals. The as-formed POSS–MgCl2 aggregation, after immobilization of TiCl4, shows considerable catalytic activity toward ethylene polymerization, synthesizing ultrahigh molecular weight polyethylene with enhanced processability and mechanical properties as well as reduced entanglement. By tracing catalyst synthesis and titanium activation with in situ spectroscopy, surface approaches, and density functional theory calculations, we acquired fundamental insight into the active role of THF on the MgCl2 surface. We experimentally demonstrate (i) the full coverage of THF on the lateral surfaces of MgCl2 crystals; (ii) the partial desorption and migration of THF on the surfaces owing to its high mobility, resulting in the regeneration of unsaturated Mg2+ sites and the subsequent formation of TiCl4–THF complexes during titanation; and (iii) the active participation of THF in the final active sites (i.e., after activation). The whole data set clearly demonstrates that the dynamic adsorption of donors directly affects the structure and performance of the active species, offering a perspective on the interactions between donors and other components.

Preparation of Weakly Entangled and Fine-sized Ultra-High-Molecular Weight Polyethylene by a MgCl2-Based Ziegler–Natta Catalyst

“…In the spectrum range of 3000–2800 cm –1 in Figure c, all the new absorption peaks at 2981, 2958, 2897, and 2879 cm –1 are attributed to the C–H stretching vibration of THF molecules, while in Figure d, the new absorption peaks at 1041 and 1024 cm –1 are due to the C–O stretching vibration peaks of THF adsorbed on five-coordination and four-coordination Mg 2+ , respectively. The above-mentioned results explain that THF molecules widely exist on the catalyst surfaces induced by the 4-OH POSS molecules, which is consistent with the phenomenon observed for catalysts prepared with 3-OH POSS in the previous studies . It is worth noting that the intensities of these two peaks are relatively weak, which may be affected by the bending vibration signal peaks in the C–H plane of the benzene ring of 4-OH POSS located at 1028 cm –1 .…”

“…The BE of Mg 2p increased slightly from 51.5 to 51.6 eV after TiCl 4 loading. It is due to the partial desorption of the adsorbed THF, leading to the decrement of the electronegativity of Mg 2+ . Besides, the full width at half-maxima (fwhm) increased from 1.7 to 2.0 eV (see Figure a), due to the desorption of THF and the more complicated chemical environment of Mg 2+ on the catalyst surface with the loading of TiCl 4 .…”

“…It is due to the partial desorption of the adsorbed THF, leading to the decrement of the electronegativity of Mg 2+ . 36 Besides, the full width at halfmaxima (fwhm) increased from 1.7 to 2.0 eV (see Figure 6a), due to the desorption of THF and the more complicated chemical environment of Mg 2+ on the catalyst surface with the loading of TiCl 15,38 In our previous work, it was found that the active sites formed by the coordination with TiCl 4 and POSS nanoaggregates also demonstrate extremely low catalytic activity for ethylene polymerization. 8…”

“…The above-mentioned results explain that THF molecules widely exist on the catalyst surfaces induced by the 4-OH POSS molecules, which is consistent with the phenomenon observed for catalysts prepared with 3-OH POSS in the previous studies. 36 It is worth noting that the intensities of these two peaks are relatively weak, which may be affected by the bending vibration signal peaks in the �C−H plane of the benzene ring of 4-OH POSS located at 1028 cm −1 . However, to a certain extent, it also reflects the relatively low concentration of THF in the 4POSS-Sup support, which is due to the reduced amount of Mg 2+ on the support surface for the adsorption of THF.…”

Synthesis of high-performance polyethylene

“…(2) 通过强配位作用化学吸附在不饱和配位Mg 2+ 位点上。三乙基铝的加入会导致部分表面TiCl x 物质聚 集形成Ti团簇。本课题组 [64,65] 在众多聚乙烯的生产工艺中，流化床乙烯聚合工艺占到40%的产能，属于主流工艺 [66] 。该工艺流 程短，操作简便，能够生产全密度的聚乙烯产品 [67] 。但也正是由于其流程短，使得其调节手段少， 流化床工艺的产品结构单一、工艺竞争力弱，生产的产品中通用料占到95%以上，专用料缺口非常突 出(图7a) [68] 。此外，为获得高的单体浓度，流化床反应器通常在露点附近进行操作，进而获得更高 的催化剂活性。由于流化床反应器较差的传热能力，颗粒温度过高往往引发结块的发生，影响装置的 稳定运行 [69~71] 。北欧双星推出的环管-流化床聚乙烯工艺(图7b) ，依次在环管反应器中生产低分量线 性聚乙烯，在流化床反应器中生产高分子量高支化聚乙烯，实现多种聚乙烯在单颗粒内的共混 [72] 结。本课题组 [92,93] 从活性链缠结形成机制出发，开发了具备活性链解缠结功能的催化剂和聚合工艺。 在催化剂方面，将惰性纳米微球植入催化剂表面，增大活性链间距离，实现活性链的解缠结；在聚合 工艺方面，将直径为数十至数百微米的氮气微气泡引入淤浆聚合体系中，微气泡与颗粒间的平均接触 时间(微气泡从接触颗粒至离开该颗粒的时间)的数量级为10 -3 s，该接触时间与聚合过程中链段的…”