28. Molecular Weight Distribution Control with Supported Metallocene Catalysts
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Molar mass distribution
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Branched polyethylene has been prepared by using asymmetric metallocene and MAO catalytic system. By changing the conditions of polymerization, such as temperature, catalyst concentration and the ratio of c (Al)/c ( Zr) , the influence on catalytic activity and polymerization kinetic curve has been revealed. The probable mechanism for forming the branched polyethylene is also discussed.
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Copolymerization of ethylene and 1-hexene was carried out with different catalysts (homogeneous Et[Ind]2ZrCl2, supported Et[Ind]2ZrCl2 and in-situ supported Et[Ind]2ZrCl2). The novel in-situ supported metallocene catalyst showed higher activity than the corresponding supported metallocene catalyst. 13C NMR, gel permeation chromatography and crystallization analysis fractionation studies showed that the microstructure of ethylene/1-hexene copolymers depends upon catalyst type. At the same polymerization conditions, the relative reactivity of 1-hexene increases in the following order: supported metallocene ≈ in-situ supported metallocene < homogeneous metallocene. The molecular weights of the produced copolymers with the three different catalysts are similar, but the molecular weight distribution of the copolymer made with the in-situ supported metallocene is broader than that of those made with the other catalysts. The short chain branching distribution (SCBD) of the copolymer produced with the in-situ supported metallocene catalyst is the broadest with a shoulder in the high crystallinity range, while the copolymers produced with the homogeneous and supported metallocene catalysts show unimodal SCBD. This may indicate that there are at least two different active species with the in-situ supported metallocene catalyst in the copolymerization of ethylene and 1-hexene.
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1-Hexene
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Morphology
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Study on the polymerization of ethylene with carbon bridged dinuclear metallocene of Ti-Sm and Zr-Sm
It has been synthesized two new dinuclear metallocene complexes ,which were used for ethylene polymerization with MAO as cocatalyst. Detailed study on the effect of polymerization caused by catalyst concentration, molar ratio of /, temperature and time, showed that the catalytic activity of complex 7 is higher than that of the corresponding mononuclear metallocene of Cp_2TiCl_2, while the catalytic activity of complex 5 is lower than that of the corresponding mononuclear metallocene of Cp_2ZrCl_2; The molecular weight (M_η) of polyethylene produced by above two catalyst systems is somewhat decreased, and the molecular weight distribution (MWD) becomes broader. With prolongation of time, the molecular weight of polyethylene obtained by complex 7 decreases, while the molecular weight of polyethylene obtained by complex 5 increases.
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Low-density polyethylene
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Films made of metallocene catalyzed polyethylene (mPE), low-density polyethylene (LDPE), and their blend were prepared to investigate how LDPE influences the hot tack of film. Experimental results showed hot tack is independent of film thickness. The addition of 30 wt % of LDPE can increase the hot tack of mPE film. The thermograms of differential scanning calorimetry (DSC) suggest the respective partial melting and recrystallization of those smaller size crystals at the bond forming and joint fracture stages play very important roles. The large amount of partial melting and high flow may induce a higher degree of molecular diffusion. Higher residual crystallinity and recrystallization at the hot tack testing process may induce higher resistant to bond fracture. Those two positive influences show that the mPE/LDPE film has the higher hot tack. The evidence from optical (higher optical transmission and lower haze) as well as viscoelastic (higher storage modulus and lower melt viscosity) properties further support this hypothesis. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1769–1773, 1999
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Abstract The molecular weight (MW) and the molecular weight distribution (MWD) of polyethylene (PE) prepared with various metallocene catalysts were investigated. The weight‐average molecular weight ( M w ) of PE prepared with dicyclopentadienyltitanium dichloride/aluminoxane (Cp 2 TiCl 2 /MAO) was about 4,0 · 10 5 , while that of PE obtained with dicyclopentadienylzirconium dichloride/aluminoxane (Cp 2 ZrCl 2 /MAO) is about 1,0 · 10 5 . PE of bimodal MWD was obtained with the mixed catalyst Cp 2 TiCl 2 /Cp 2 ZrCl 2 . Narrow MWD PE was obtained with the mixed catalyst Cp 2 ZrCl 2 /Et(ind) 2 ZrCl 2 , because each catalyst produced an M w of 1,0 · 10 5 . However, the change of temperature during the polymerization with the mixture Cp 2 ZrCl 2 /Et(ind) 2 ZrCl 2 produced PE with a bimodal MWD and a value of the ratio of weight‐ to number‐average molecular weights of M w / M n = 4.
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A series homopolyme of ethylene synthesised by metallocene catalyst was analyzed by the high temperature gelatin permeation chromatography.The results showed that the molecular weight distribution changed not too much when the ratio of Al/Ti ranging between 1?000 and 4?000,while the molecular weight distributed increased from 2.48 to 2.81 when the ratio ranging between 4?000 and 7?000;catalyst D had the lowest activity to be used as asymmetric metallocene catalyst;the curve of molecular weight moved much to the left with the increases of temperature and two active centres appeared when the temperature is 70?℃.
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Olefin polymerization
Coordination polymerization
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