Study On Mechanism Of Ethylene And 1-hexene Polymerization Catalyzed Supported Ziegler-Natta Catalysts

In the past several decades,supported catalysts of the TiCl₄/ID/MgCl₂ type were widely used in polyolefin industry.However,questions concerning the mechanism of ethylene copolymerization withα-olefins,the effect of internal electron donor(ID) and hydrogen on the structure and nature of active centers have not yet found satisfactory answers.In this paper,some work was carried out in these directions.Using four kinds of TiCl₄/MgCl₂ catalysts[Cat-1(TiCl₄/MgCl₂),Cat-2(TiCl₄/diethyl phthalate(DEP)/MgCl₂),Cat-3(TiCl₄/Anisole/MgCl₂),Cat-4(TiCl₄/(DEP,Anisole)/MgCl₂)] and DQ Catalyst(TiCl₄/dibutyl(o-)phthalate/MgCl₂),we carried out following studies in this paper:(1) the role of internal electron donor(ID) on ethylene homo-polymerization and copolymerization;(2) A new method for measuring the number of active centers was developed,based on use of cinnamoyl chlorides;(3) the effect of IDs on distribution of active centers;(4) the role of H₂ on polymerization and distribution of active centers;(5) the effect of AlR₃,polymerization temperature on polymerization and distribution of active centers.It was attempted to find the new approach to accommodate the ACD(active center distribution) of Ziegler-Natta catalyst and the MWD(molecular weight distribution),CD (composition distribution) of polyolefins products,and provide the useful information of developing new linear low density polyethylene(LLDPE) and polypropylene(PP) products.In the preparation of the supported catalysts,MgCl₂·nEtOH/ID adducts as the precursor of the support were prepared in situ by novel co-precipitation method.Four kinds of MgCl₂·nEtOH/ID adducts were prepared in situ by novel co-precipitation method,and the catalysts of TiCl₄ anchored on the supports were prepared.FT-IR spectra of the catalysts and PA as well as DEP were also recorded.DEP was found to be the actual ID existing in the final catalyst prepared based on MgCl₂·nEtOH/PA.Titanium content of the catalysts containing different internal donors is different:DEP decreases the Ti content and Anisole as internal donor didn't decreases the Ti content.Four kinds of catalysts(Cat-1,Cat-2,Cat-3 and Cat-4) were applied to ethylene polymerization and ethylene-co-1-hexene copolymerization.The addition of IDs increased activity of catalyst and the T_m of products,especially when the catalyst contains DEP as ID. 1-hexene content of the HI(boiling n-heptane insoluble) fraction increased,the difference in 1-hexene content of the HI fraction and the HS fraction decreased,and the blockiness of HI fractions increased.A new method for determining the number of active centers(C_p) in supported Ziegler-Natta catalysts based on quenching by cinnamoyl chlorides was developed.The optimal quenching conditions were found to be:n(Cinnamoyl chloride/Al)=3～5;t_quenching=10 min;T_quenching=20℃～40℃.The change of C_p by adding IDs in the catalyst were studied by this method.The introduction of IDs,especially anisole,increased the number of active center in 1-hexene polymerization.It was found that the introduction of IDs in the catalyst during its preparation can enhance the molecular weight(MW).The Flory non-linear-fitting of products MWD curves showed that the addition of ID didn't change the type of active centers,but change the ACD (active center distribution),increase both the MW and the number of active centerâ… andâ…¡. From distribution of the number of active centers determined by quenching and fractionation, the addition of PA increases the amount and C_p of fraction with high molecular weight.In this paper,the change from homo-polyethylene to ethylene copolymerization with small amount of 1-hexene,from homo-poly(1-hexene) to 1-hexene copolymerization with small amount of ethylene were studied,respectively.Adding small amount of 1-hexene in ethylene polymerization caused marked activation of the low molecular weight components of the polymer.In 1-hexene polymerization system,the activity can also be greatly enhanced by introducing small amount of ethylene.The total number of active centers is markedly increased by adding small amount of ethylene in 1-hexene polymerization,but the average activity of the active centers decreased.The broad composition distribution of the ethylene-1-hexene copolymer can be well understood from the ACD of catalyst and its dependence on the monomer.Adding small amount of ethylene markedly increase the number of active centers that produce polymer with low molecular weight.There is a nearly linear increase of isotacticity with molecular weight of the polymer fraction obtained by precipitationed fraction.However,the addition of small amount of ethylene in 1-hexene polymerization decreased the isotacticity of the fractions with high molecular weight.The distribution of ethylene content in fractions was broad.The content of ethylene in both the low MW and high MW fractions were higher than that of others.So,ethylene tends to be incorporated in both the low MW and high MW active centers in 1-hexene polymerization.Hydrogen(H₂) has been used as a standard chain transfer agent in industrial polyolefin production.Hydrogen played different roles in ethylene and propylene/α-olefin polymerization.In ethylene polymerization system,the addition of small amount of hydrogen markedly decreased catalyst efficiency and the molecular weight of polymers,and increased the polydispersity index(PDI) of polymers.In 1-hexene polymerization system, the addition of small amount of hydrogen caused a marked increased in polymerization rate and a decrease in the molecular weight of polymers.The total number of active centers is markedly increased by adding small amount of H₂ in 1-hexene polymerization.The Flory non-linear-fitting of polyethylenes MWD curves showed that the addition of H₂ decreased both the M_w and the amount of active centerâ… andâ…¡,meanwhile increased both the MW and the amount of active centerⅢ～Ⅴ,which caused decrease in PDI.In 1-hexene polymerization, however,a new type of active center(â…¤) with low molecular weight product emerged after adding H₂,which decreased the MW and broaden the PDI of the products.In 1-hexene polymerization,the change of cocatalyst concentration markedly influences the polymerization behaviors and distribution of active centers.With the concentration of TEA rising,the catalyst activity increased and the molecular weight of products decreased gradually.When the ratio of Al/Ti rising at low TEA concentration,the number of active centers is increased,which mainly increase the active centers producing polymer with low molecular weight.A new model of two group active centers in polymerization was proposed in this paper, in which the interactions between active centers and IDs,comonomer,hydrogen were elaborated.This model can exactly explain the results in this paper.This model afford deep understand of mechanism of olefin polymerization and provide a new approach to improving and modifying the performance of the supported Ziegler-Natta catalyst.