Ethylene And Propylene Polymerization Catalyzed By Ziegler-Natta Catalysts Supported On Doped MgCl₂

Two series of doped supports of MgF₂/MgCl₂ and AlCl₃/MgCl₂ were prepared by means of ball milling or co-precipitation in this paper, and the catalysts of TiCl₄ anchored on the doped supports were also prepared. We have investigated (1) polymerization of ethylene, propene as well as copolymerization of ethylene and 1-hexene, (2) distribution of active center of the doped catalysts, (3) structure of copolymer of ethylene and 1-hexene.The XRD spectrum of the catalysts supported on MgCl₂ doped with MgF₂ showed that the peaks of MgF₂ appeared and the intensity of these peaks enhanced with increase of dosage of MgF₂, it can be deduced that MgF₂ exists in MgCl₂ as a microcrystalline. Some new peaks, neither belonged to MgF₂, nor to MgCl₂, appeared in the XRD curves showed that doping MgF₂can change the crystal structure of MgCl₂ to some extent.The activity of the catalysts supported on MgCl₂ doped with MgF₂ is much higher than that of the undoped catalyst supported on MgCl₂ in ethylene polymerization, and the catalyst system exhibits higher activity when TEA is used as cocatalyst instead of TBA. But the catalyst supported on pure MgF₂ can not catalyzed ethylene polymerization because MgF₂ is a noneffective support for TiCl₄ and little Ti can be anchored on the MgF₂ support. Doping MgF₂ in the catalysts showed little effect on the polydispersity of Polyethylene. By means of the deconvolution of the MWD curves by multiple Flory functions, we found that the reason was that introducing MgF₂ into the MgCl₂ support did not change the number of active center types existed in the catalysts.The activity of the catalysts supported on MgCl₂ doped with MgF₂ is also much higher than that of the undoped catalyst supported on MgCl₂ in propylene polymerization, and the catalyst supported on pure MgF₂ also exhibited no activity of propylene polymerization. Doping much more MgF₂ (≥17.0 wt.%) in the catalyst increased the number of active center types in the absence of internal donor (ID), and the increased active center type was probably atactic active center, whereas the number of active center types did not varied by doping MgF₂ in the presence of ID.The supports of MgCl₂/AlCl₃-EtOH were prepared by means of co-precipitation, and the catalysts were formed by TiCl₄ anchored on the AlCl₃ doped support. WAXD analysis of the AlCl₃ doped catalysts showed that no peak of AlCl₃ appeared in the curves, it mean that the catalysts contained free of AlCl₃ microcrystalline and AlCl₃ was dispersed in MgCl₂ at molecular level, and the solid solution of AlCl₃/MgCl₂ was formed.The AlCl₃-doped catalyst exhibited slightly higher activity in ethylene polymerization than catalyst supported on pure MgCl₂. Although the catalyst supported on pure AlCl₃ presented much lower activity than the MgCl₂ supported catalysts, its activity was as high as 1320 g PE/(gTi.h), this mean that AlCl₃ may also act as active support for TiCl₄, though not so efficient as MgCl₂. The MW and MWD of PE were influenced by doping AlCl₃, the polydispersity index of PE increased strongly with increasing AlCl₃ content in the catalysts. Accordingly, the MWD of PE can be adjusted over a wide range by varying AlCl₃ content in the catalyst. The deconvolution of the MWD curves by multiple Flory functions showed that the broadening of MWD of PE is due to increase of types of active centers by introducing AlCl₃ into the support.Doping AlCl₃ can also slightly improve the catalyst's activity in propylene polymerization, but the catalyst supported on pure AlCl₃ can not catalyze propylene polymerization. The isotacticity of polypropylene decreased markedly with increasing of AlCl₃ content in catalysts, whereas the MW and MWD of PP influenced slightly by doping AlCl₃ in catalysts.Copolymerization of ethylene and 1-hexene catalyzed by MgCl₂/TiCl₄ and MgCl₂/AlCl₃/ TiCl₄ showed that the two catalysts presented obvious "comonomer effect"; activity of the AlCl₃-doped catalyst was obviously higher than that of undoped catalyst; content of comonomer in copolymers increased by doping AlCl₃, TBA used as cocatalyst and in absence of ID; TBA catalyst system was apt to produce random copolymer while TEA system to produce block copolymer, and the sequence length of ethylene in copolymers obtained by TBA system was shorter than TEA system; and doping AlCl₃ also shorten the sequence length of ethylene.The creative points in this paper:1. Two series of doped catalysts of TiCl₄/MgF₂/MgCl₂ and TiCl₄/AlCl₃/MgCl₂ were prepared by means of ball milling or co-precipitation. The structure of catalysts was analyzed by XRD, and we found that MgF₂ microcrystalline existed in the MgF₂-doped catalyst; doping MgF₂ can change the crystal structure of MgCl₂ and markedly improve activity of the catalysts in ethylene and propylene polymerization. In AlCl₃-doped catalysts, AlCl₃ was dispersed in MgCl₂crystalline at molecular level, and the solid solution of AlCl₃/MgCl₂ was formed.2. The catalyst supported on MgCl₂ doped with MgF₂ exhibited quite high activity in ethylene and propylene polymerization, but the effect of doping MgF₂ on active center distribution of the catalyst was very little.3. The MgCl₂/TiCl₄ catalysts with varied weight of doped MgF₂ were prepared. It is shown that doping AlCl₃ has little effect on the activity of ethylene and propylene polymerization but broaden the active center distribution of the catalyst. The isotacticity index of PP obtained from the AlCl₃ doped catalysts is lower than 30%. TiCl₄ can be anchored on pure AlCl₃ and the obtained catalyst exhibits a little activity in ethylene polymerization, and the MWD curve of PE catalyzed by the catalyst supported on pure AlCl₃ presents multiple peaks and the polydispersity index is very broad. The multi-peak MWD curves can be deconvoluted with multiple Flory functions, it indicated that the method of the deconvolution is reasonable.4. The catalyst of MgCl₂/AlCl₃/TiCl₄ presents very strong copolymerization ability in ethylene and 1-hexene copolymerization. The sequence distribution of copolymer is random and the variation of composition distribution is not obvious.