Application Of The Polyhedral Oligomeric Silsesquioxane(POSS) In Ziegler-Natta Catalyst

Ultra high molecular weight polyethylene?UHMWPE?is a kind of linear polyethylene with molecular weight greater than 1×106g·mol-1.It has unique properties,such as wear resistance,fatigue resistance,high strength,chemical corroSiOn resistance,etc.However,UHMWPE has more molecular chain entanglement,which leads to the increase of melt viscosity and molding processing difficulty.Secondly,the existence of a large number of chain entanglement structures will destroy the crystal structure of the polymer and produce a large number of crystal defects,which will greatly reduce the tensile strength and modulus of UHMWPE fibers and other adverse effects.These limits the synchronous improvement of UHMWPE processing and service performance.After decades of development,polyolefin industry has successively developed Ziegler-Natta catalyst,metallocene catalyst and post-transition metal catalyst,but Ziegler-Natta catalyst still dominates.POSS with porous and cage structure is used as the carrier of Z-N catalyst.POSS compounds have outstanding advantages such as high reactivity,temperature resistance,nano-size effect and structural designability,and are commonly used in modified polyolefin catalysts.However,POSS-modified Juyi Hall catalysts are mainly focused on the functionalization of chemical mechanisms on the surface of the carrier and the regulation of the spacing between active centers.The main purpose is to improve the activity of the catalyst and the stereoregularity of propylene polymerization.No research has been found on the regulation of the entanglement structure of nascent state polyethylene chains.Based on the idea of"the space size effect of POSS nano-aggregates and the excellent mass transfer effect of macroporous carriers",this thesis designs and synthesizes efficient Z-N catalysts.Low entanglement UHMWPE was prepared with high activity at the first re-industrial polymerization temperature?60??.The copolymerization performance of POSS functionalized Ziegler-Natta catalyst was studied in detail.In this thesis,the following three aspects of research work have been carried out:?1?Preparation of cluster POSS/MgCl₂/TiCl₄ catalyst and research on catalytic ethylene polymerization;?2?Research on POSS/MgCl2 nano-aggregates as active center blockers;?3?Study on copolymerization of ethylene/1-hexene catalyzed by POSS modified Ziegler-Natta catalyst.The specific work is as follows:Firstly,according to the cage structure and self-aggregation characteristics of POSS,POSS/MgCl₂ is used as a bridge,POSS is aggregated into nuclear capability,and TiCl₄ is induced to nucleate by Cl-Mg-Cl double chlorine bridge to prepare cluster POSS/MgCl₂/TiCl₄ catalyst.Through STEM,BET,XPS,ICP and scanning electron microscopy,the effects of bichloride bridging of MgCl2/TiCl4 on the structure and morphology of cluster-type catalysts,the promotion of different reducible alkyl aluminum on the activity of cluster-type POSS/MgCl₂/TiCl₄,and the morphology characterization of polymer products were investigated.The results show that POSS/MgCl2 can be used as the active center of bridge payload TiCl4.When the cocatalyst is n-trihexylaluminum?THEA?,more active centers can be activated to improve the polymerization rate,and UHMWPE with low entanglement is synthesized.Thirdly,based on the idea of"space barrier function of POSS/MgCl₂ nano-aggregates",POSS is utilized to capture MgCl₂ molecules through coordination function to form POSS/MgCl₂ nano-aggregates adhered to the surface of macroporous SiO2,which are used as horizontal barriers between active centers and polyethylene primary segments to synthesize POSS-modified Z-N catalyst with barrier function.SEM,XRD,XPS,NMR,BET and molecular simulation?DFT?were used to investigate the microstructure of POSS/MgCl₂ nano-aggregates and the effect of re-primary segment de-entanglement.Realize the efficient synthesis of low entanglement UHMWPE at industrial polymerization temperature?60??.Finally,the copolymerization of ethylene/1-olefin catalyzed by POSS modified Ziegler-Natta catalyst was investigated.We used POSS synthesized in the previous chapter to modify Z-N catalyst with macroporous structure Cat-POSS-20%and Cat-POSS-0%to conduct comparative study on copolymerization behavior.The copolymerization performance of POSS modified macroporous catalyst was analyzed by DSC,GPC,infrared characterization,high temperature nuclear magnetic resonance and rheology.The results show that the insertion rate of monomer 1-hexene in the copolymerization product increases with the increase of monomer concentration.This is because increasing monomer concentration can activate more active centers.What is important is that the macroporous SiO₂ carrier provides an unrestricted space environment,which is conducive to improving the diffuSiOn rate of reactants and the polymerization activity of the catalyst.However,the melting point,crystallinity and relative molecular weight of the polymerization products all decrease with the increase of comonomer concentration.