POSS-induced High-performance Ziegler-Natta Catalysts And Investigation Of In Situ Infrared Spectroscopy

Featured by excellent mechanical properties,ultra-high molecular weight polyethylene(UHMWPE)has shown excellent potential in the fields of military industry,national defense,and medical equipment et,al.Currently,traditional spherical MgCl2 supported Ziegler-Natta(ZN)catalysts are responsible for most commercial UHMWPEs.However,the active sites distribute randomly and arrange closely in this catalytic system,which inevitably causes a large number of chain entanglements during the ethylene polymerization.This results in the extremely high melt viscosity of the UHMWPE,thereby increasing the difficulty of processing and reducing the mechanical properties to 1/3 of the theoretical value.It is well acepted that the production of high-performance polyethylene resins highly depend on the development of high-performance catalysts.Accordingly,the main reason for this phenomenon is the lack of advanced catalyst that can control the molecular chain structure and the configuration of polyolefins.But more fundamentally,it is because that this system remains mystifying from a mechanistic standpoint,e.g.,the insufficient understanding of the active species structure at the molecular level,the ambiguous location of donors and debatable relationship between different components.This leads to a lack of theoretical guidance in the design of catalyst molecules.Hence,the rational synthesis of advanced polyolefin materials requires not only catalysts with controllable structure and excellent performance,but also a deeper understanding of the mechanism.Following the concept of catalyst molecular design,the chemical reaction between the polysilsesquioxane(POSS)molecule with hydroxyl substituents and the RMgX Grignard reagent were performed in this work to develop advanced MgCl2-supported ZN catalysts.By taking advantage of the self-nucleation behavior,steric hindrance,and electron-donating effect of POSS molecules,we exploited MgCl2/THF/TiCl4 based Ziegler-Natta catalysts with both well-defined structure and excellent catalytic performance.We systematically investigated ?)the effects of POSS molecular structure,solvent,Grignard reagent,and synthesis temperature.on the catalyst yield and morphology;?)the catalytic behavior of the synthsized POSS-induced catalysts under different polymerization reaction conditions;?)the effects of the crystal structure,morphology,structure and distribution of the active sites of the catalysts on the entanglement degree and mechanical properties of the UHMWPE product.More importantly,an diffuse reflectance infrared spectroscopy device was desiged for evaluating all the components of ZN catalysts at the molecular level,which enables online monitoring of the infrared signal of the catalyst under controllable atmosphere,temperature and pressure.Combing this customized instrument,the POSS-induced ZN catalyst,high-sensitivity surface analysis techniques,and theoretical calculations,we traces the chemical properties of all catalytical components during the TiCl4 loading,the triethylaluminum(TEA)activation,and the ethylene polymerization.In addition,a systematic investigation of this catalyst using in situ spectroscopic approaches,with THF and CO acting as molecular probes,offered us a good opportunity to trace how THF participates in each step of active site formation.Detailed results are as follows:i)The preparation and ethylene polymerization investigation of trihydroxy POSS-induced spherical porous MgCl2 supported ZN catalyst:it was found that nano-sized POSS crystals can be generated by the reaction between POSS molecule with 3 silanol groups and BuMgCl in n-heptane solvent.These POSS crystals can serve as the crystal nucleus to induce the crystallization and aggregation of MgCl2,resulting in the MgCl2 support with good fluidity and well-defined structure in one step,The growth of MgCl2 crystals is very sensitive to the reaction temperature of POSS and BuMgCl.At low temperature(0 and 30?),the catalyst displayed network structure cross-linked by the MgCl2 nanofibers.While at higher temperature(60?),the catalyst showed porous core-shell microsphere with in situ formed POSS inner cores and rhombic MgCl2 outer layers,exhibiting good sphericity and high specific surface area(203.7 m2·g-1).The ethylene polymerization results showed that morphology and structure of the catalyst has a direct impact on its polymerization performance and the properties of the polyethylene product.The catalysts with three-dimensional network structure yielded UHWMPE characterized by porousness and low bulk density.While the porous microsphere catalyst exhibits ethylene polymerization activity up to 2×106 g PE(mol Ti·h)-1,and the produced UHMWPE displayed good sphericity,low degree of entanglement,and improved mechanical properties.As a result,the POSS-induced catalysts in this work demonstrates good potential for industrial application.ii)Revealing the dynamic behaviors of tetrahydrofuran for tailoring the active species of Ziegler-Natta catalysts:consistent results acquired from the DRIFT spectroscopy tests under independent experimental conditions(heating,titanation,activation by TEA,and CO adsorption)provide strong evidence of the high mobility of THF adsorbed on the MgCl2 surface.Such a dynamic behavior of THF is fundamentally crucial for the construction and distribution of active sites,because it returns part of THF-precovered Mg2+ sites to the unsaturated coordination state.The DFT calculation shows that the generated vacancy facilitates the insertion of TiCl4,where the formed Mg-Cl-Ti promotes the further migration of THF from Mg2+ to the immobilized titanium sites considering the energy optimization.This consequently generates the active site with a more stable structure.Moreover,the TiCl4-THF complexes formed thereafter are even more sensitive to elevated temperature.This indicates that the coordination of THF to TiCl4 is quite weak,which is favorable for the desorption of THF from TiCl4.Thus,the unsaturated coordinated titanium species thereafter formed are available for further activation by an aluminum alkyl activator.TEA has a profound effect on the THF-covered surface.On one hand,it removes most of the THF molecules adsorbed on the MgCl2 surface,leaving only the THF coordinated with the active center.On the other hand,it can reduce the Ti4+ sites to active Ti3+ sites.Moreover,eliminating the interference of solvents allows us to clearly observe a rapid rise in the intensity of the spectral region characteristic of polyethylene.We propose that the absorption peak at 2851 cm-1 can be used as the characteristic signal of polyethylene in the in situ diffuse reflectance infrared test to evaluate the kinetic characteristics of the catalyst online.iii)Aiming at overcome the problem of instability in the crystal structure of trihydroxy POSS,tetrahydroxy POSS molecules with enhanced rigidity were applied in this section to prepare POSS crystals which not only induced the crystallization of MgCl2,but also regulated the composition and distribution of catalyst active sites.It was found that the POSS nanocrystals can be also in situ formed through the chemical reaction between tetrahydroxy POSS molecule and BuMgCl Grignard reagent in THF solvents.Different from the trihydroxy POSS,this POSS crystal still existed after the loading of TiCl4.Therefore,the uniformly dispersed POSS crystals on the ?-MgCl2 surfaces can serve as horizontal spacers between the active sites to reduce the chain overlap in polymerization.Consequently,weakly entangled UHMWPE with a G'(t=0)of 0.19 MPa can be synthesized at high temperature.Moreover,the catalyst also showed significant effects in improving the mechanical properties of UHWMPE products,whose rigidity,especially strength and toughness are comprehensively enhanced.The mechanical properties of nascent UHMWPE showed a gradual upward trend with increasing polymerization temperature.The Young's modulus,tensile strength,elongation at break,and impact strength of UHMWPE prepared at 80? reached 383.5 MPa,34.9 MPa,940.5%and 102.7 kJ/m2,which increased by+13.7%,+11.1%,+39.3%and+24.6%in comparison to the commercial UHMWPE,respectively.iv)Efficient synthesis of low polydispersity UHMWPE by elevating active sites on anchored POSS molecules:it was demonstrated that the chemical reaction between?SiOH of POSS and TEA-treated silica prohibited the occurrence of POSS crystallization and self-aggregation,consequently yielding a molecularly dispersed POSS phase on the surface and pores of the silica.The residual?SiOH of the anchored POSS further provided the grafted sites for the immobilization of MgTi species.This grafted POSS served as vertical support which elevate the immobilized TiCl4 from the silica surface,enhancing to the formation of tetra-coordinated Mg2+sites.A less heterogeneous environment for constructing immobilizing the active sites is thus created,where the UHMWPE with a narrow distribution(i.e.,3.9)is able to be synthesized with a considerable activity.In addition,we found that POSS molecules dispersed at a molecular scale are not effective in separating active centers and in reducing the probability of molecular chain overlap.Accordingly,this indicates that the active center barriers on the catalyst surfaces need to be of appropriate size to reduce the entanglement density of UHMWPE products.