Study Of Chromium/vanadium Bimodal Polyethylene Catalysts And Pre-reduction Effects Of Alkyl Aluminum At High Pressure

Polyethylene(PE)is one of the most important and universal materials in the world and it can achieve such a great success to replace other kinds of materials on account of its good properties.It is a fact that the domestic PE productivity has increasingly promoted,however the capacity of high-grade bimodal polyethylene is still hardly enough.With the development of domestic society and economy,the missing demands will be continuously expanded.On account of the high requirements on environment,it is a better choice to study advanced one-pot bimodal technology rather than using traditional two-pot bimodal technology.Therefore,what is the most critical is to develop corresponding catalysts.Based on Phillips catalyst,novel chromium/vanadium bimetallic catalyst,which was capable of one-pot synthesis of bimodal polyethylene as high performance pipe material,had been successfully developed.In this work,the polymerization behaviors and products prosperities of this catalyst and its pre-reduction effects of alkyl aluminum will be discussed.The effects of preparation of catalysts,polymerization temperatures,contents of vanadium,concentration of cocatalysts,types of cocatalysts and types of pre-reductants were considered.The results showed that at 0.4MPa,the catalysts made by co-impregnation and consecutive impregnation separately,held similar activities.Therefore,co-impregnation is a better way,considering its lower production cost.Compared with different Cr/V,the catalyst of Cr/V=1:1,got the highest activity in homopolymerization and copolymerization and its products had similar microstructures with catalysts of different Cr/V.So this Cr/V was considered as the best mole ratio and would be used in the following experiments.Then triethyl aluminum(TEA)and triisobutyl aluminium(TIBA)were used as pre-reductors and cocatalysts separately on previous catalyst to study the pre-reduction impact by polymerization behavior of catalysts and the microstructures of polymers.The study showed that the pre-reduced catalysts needed fewer cocatalysts than before.When using the same cocatalyst,the pre-reduced catalysts had a close activity,and the products had a similar molecular weight(MW)and molecular weight distribution(PDI),meaning that the TEA and TIBA had a close pre-reduction effect.The copolymers obtained from the pre-reduced catalysts showed better 1-hexene incorporations than those of before,which is the unique advantage of pre-reduction.