| With the development of the polyolefin industry worldwide,the production capacity of metallocene catalysts and corresponding high-valued polyolefin products will increase rapidly.The study on the influence of metallocene(and post-metallocene)catalysts' structures and reaction conditions on the structures and properties of the resulting polyolefins can further understand the mechanism of olefin polymerization,and promote the development and application of new polyolefin catalysts.It is of great signifcance to ultimately guide the controlled preparation and production of new polyolefin products.In this thesis,ethylene and 1-hexene coordination(co)polymerization are the main topic.A series of branched polyolefins have been prepared by varing polymerization conditions and catalysts.The structures and properties of the polyolefins have been characterized to analyze catalytic systems'characteristics.(1)The comparative study of ethylene/1-hexene copolymerzaiton catalyzed by bridged metallocene catalysts rac-ethylenebis(Ind)ZrCl2(Et(Ind)2ZrCl2)and rac-ethylenebis(4,5,6,7-4H-1-Ind)ZrCl2(Et(IndH4)2ZrCl2)has been done with Cp2ZrCl2 as a reference catalyst and methylaluminoxane(MAO)as cocatalyst.The properties and microstructures of the polyolefins have been characterized by gel permeation chromatography(GPC),differential scanning calorimeter(DSC),'H nuclear magnetic resonance(NMR)and '3C NMR.It is found that,the effect of the reaction temperature on the copolymerization activity is different for each catalytic system.The addition of I-hexene significantly reduces the resulting polyolefin's molecular weight for each catalytic system.An approximately bimodal distribution can be found for the polyolefin prepared by Et(Ind)2ZrCl2 at high temperature.The molecular weight of the polyolefin prepared by Et(Ind)2ZrCl2 under comparable conditions is higher than that of the polyolefin prepared by Et(IndH4)2ZrCl2.Increasing the 1-hexene concentration or lowering the reaction temperature causes the polyolefin's melting temperature(Tm)to decrease.Most DSC curves of the copolymers prepared from Cp2ZrCl2 and Et(IndH4)2ZrCl2 contain multiple melting peaks.While,most DSC curves of the copolymers prepared by Et(Ind)2ZrCl2 have a single melting peak without obvious glass transition phenomenon.However,a copolymer with low glass transition temperature(Tg)can be obtained by Et(Ind)2ZrCl2 at the 1-hexene concentration of 0.32 M.As the reaction temperature or the 1-hexene concentration increases,the resulting polyolefin's branching density trends to increase for each system.Combined with the thermal properties of the polyolefin,the sequence distribution and the reactivity ratio calculated from 13C NMR spectra,it is known that Et(Ind)2ZrCl2 is more sensitive to the reaction conditions than Et(IndH4)2ZrCl2.(2)Using cobaltocene as a reductant,the differences in the performance of olefin polymerization before and after reduction of an a-diimine Ni(?)complex have been studied.The influence of Al/Ni,reaction time,1-hexene concentration,Co/Ni,addition method of cobaltocene and reaction temperture on the activity of the catalytic system containing cobaltocene has also been studied.The properties and microstructures of the polyolefins have been characterized by GPC,DSC,1H NMR and 13C NMR.The states of a-diimine Ni(?)complex before and after reduction and in ethylene polymerization have been characterized by electron spin resonance(ESR).It is found that,the reduced complex shows lower catalytic activity,yielding polyolefin with higher Tm and Tg.Polyolefin's branching density is decreased(?20%).Polyolefin shows good elasticity at room temperature.However,the resulting polyolefin's molecular weight is not markedly changed.The Al/Ni,reaction time and Co/Ni have limited effect on the properties of the polyolefin prepared by the reduced complex.But the catalytic behavior of the Ni(?)complex activated by MAO then mixed with cobaltocene is similar to that of the original catalytic system.At 0?,the reduced catalytic system does not show the“living" feature in ethylene polymerization,yielding higher molecular-weight polyethylene with high Tm,which mainly contains methyl branches.The results of ESR characterization shows that the Ni(?)complex can be reduced by cobaltocene to form Ni(I)complex and a small amount of species beraing radical.Continued addition of MAO to the mixture can produce a large amount of species beraing radical.(3)Using an a-diimine Ni(?)complex to investigate its behavior in ethylene polymerization,1-hexene polymerization and ethylene/1-hexene copolymerization at 0?.Poly(l-hexene)-b-poly(ethylene-co-l-hexene),polyethylene-b-poly(l-hexene)and polyethylene-b-poly(l-hexene)-b-poly(ethylene-co-l-hexene)have been successfully prepared via sequential addition of monomers.The properties and microstructures of the polyolefins have been characterized by GPC,DSC,1H NMR,13C NMR and uniaxial tensile tests.It is found that,ethylene polymerization is close to living/controlled polymerization,yielding semicrystalline polyethylene having a dispersion(D)of 1.3-1.4 and mainly containing methyl and long branches.1-Hexene polymerization at the initial 1-hexene concentration of 0.64 M shows living/controlled feature,yielding amorphous poly(l-hexene)(D<1.2)containing methylene backbone and different types of branches.By controlling the stirring speed,copolymerization of ethylene/l-hexene is close to living/controlled polymerization,yielding polyethylene-like polyolefin(1.5<D<1.6)containing different types of branches.All of the block copolymers have a Tg,of which polyethylene-b-poly(l-hexene)and polyethylene-b-poly(1-hexene)-b-poly(ethylene-co-l-hexene)copolymer have a Tm close to that of polyethylene.The mechanical properties of polyethylene-b-poly(1-hexene)are poor.The polyethylene-b-poly(1-hexene)-b-poly(ethylene-co-l-hexene)shows multi-yielding behavior in the tensile test,but the elongation at break is up to 2829%and the elastic recovery ratio is 82.5%. |
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