Metallocene And (?-diimine) Nickel Catalysts Catalyzed Olefin Polymerization:Mechanism And Chain Structure Control

Thermoplastic elastomers(TPEs)are polymer materials with both thermoplastic and elastomeric properties.TPEs show advantages in processing performance typical of plastic resins and elastic property typical of rubbery materials.TPEs have more economic and social benefits than traditional rubbers in respect of lower energy consumption and environmental impacts.Thermoplastic polyolefin elastomer(TPO)is an important category of TPE.Among various TPOs,olefin copolymers with block structures have attracted intensive investigations.On the other hand,even though metallocenes and late transition organometallic catalysts have been successfully applied in the production of many high performance polyolefins(including TPE),microscopic mechanism of olefin polymerization with these organometallic catalysts has not been thoroughly understood.The number of active centers([C*]/[MT])and microscopic kinetic behavior in the olefin polymerization,as well as the relationship between catalysis mechanism and chain structure of polyolefins are still pending topics for basic research so far.In this work,we have studied the variation of[C*]/[MT]in different homogeneous polymerization systems by quenching the active centers with 2-thiophenecarbonyl chloride(TPCC).Furthermore,the mechanism of olefin polymerization has been investigated based on[C*]/[MT]and other kinetic parameters.In addition,multiblock olefin copolymers have been synthesized with various homogeneous catalysts,and the detailed relationship between copolymer structure and material property has been studied.The results may offer some theoretical basis and new ideas for producing polyolefin-based thermoplastic elastomers.1.A method of quenching active centers with TPCC has been established to count the active centers in olefin polymerization with homogeneous organometallic catalysts.By systematical study on the effects of quenching time and TPCC amount-,efficient and selective quenching of the active centers was found to be accomplished with adequate quenching time(3?20 min)and TPCC amount(TPCC/Al= 2?8),and each active chain would be connected with one thiophenecarbonyl group.The molar number of sulfur thus equaled to the molar number of active centers.2.Using the quenching method,ethylene polymerization catalyzed by various homogeneous catalysts including metallocenes and a-diimine nickel catalysts were investigated.The values of[C*]/[Ni]in a-diimine nickel catalysts were found to be lower than 50%under different conditions.The values of[C*]/[Zr]in bridged metallocene rac-(ethylenebis(indenyl))zirconium dichloride(Met.l)were highly dependent on cocatalyst.High degree of metallocene activation with different cocatalysts were observed for unbridged metallocene bis(2,4,7-trimethylindenyl)zirconium dichloride(Met.2).For unbridged metallocenes bis(2,4,6-trimethylindenyl)zirconium dichloride(Met.3)and bis(2,4,5,6-tetramethylindenyl)zirconium dichloride(Met.4),[C*]/[Zr]were found to decrease after a short period of polymerization even though relatively high values of[C*]/[Zr]were observed at the initial stage.3.Phenomenon of multiple active centers present in metallocene catalyzed olefin polymerization was studied.Multiple active centers were found to be present in ethylene polymerization catalyzed by bridged metallocene Met.l,including contact ion pairs of metallocenium cation and the counter anions,loosely associated ion pairs and ion pairs with medium distance.Different aluminoxane cocatalysts were found to cause distinguishable activation of different ion pairs,and trimethyl aluminum was found to be crucial in the formation of contact ion pairs.Propylene homopolymerization and ethylene/propylene copolymerization catalyzed by bridged metallocene were found to be predominantly catalyzed by loosely associated ion pairs.The maximum[C*]/[Zr]values in Met.l catalyzed polymerizations were found to decrease in the order of ethylene/propylene copolymerization>ethylene homopolymerization>propylene homopolymerization.4.Multiple active centers were observed in ethylene polymerization with unbridged metallocene Met.2,and methylaluminoxane(MAO)or dried methylaluminoxane(dMAO)could lead to high activation rate.Propylene homopolymerization catalyzed by unbridged metallocene Met.3 was found to be predominantly catalyzed by loosely associated ion pairs.Multiple active centers were present in ethylene/propylene copolymerization catalyzed by unbridged metalloceneMet.2.Propylene was mostly inserted into propagating chain in a random pattern.5.Ethylene/propylene copolymerization with unbridged metallocene under elevated monomer pressure was systematically studied.The moderate polymerization temperature,relatively high ethylene/propylene feed ratio would lead to great improvement in mechanical strength of the copolymer.By optimizing polymerization conditions,ethylene/propylene copolymer with multiblock structures can be obtained,which showed characteristics of TPE.Various unbridged metallocenes were further used in ethylene/propylene copolymerization under optimized conditions,and using dMAO or 2,6-di-tert-butyl-4-methylphenol treated MAO(BHT-MAO)as cocatalyst can effectively improve mechanical properties of the copolymer.6.In the investigation of ethylene/long chain a-olefins(LCOs)catalyzed by thermostable a-diimine nickel catalyst,we obtained copolymer fractions that showed obvious melting temperature(Tm =12?103 ?),low melting enthalpy and relatively high molecular weight.Characterization of this type of copolymer has proved its multiblock chain structure.In the copolymer,highly branched polyethylene(HBPE)segments form the amorphous phase,and the incorporated LCOs units experienced chain straightening during its insertion,which provide crystalline blocks(either as segments in the main chain and the side chains)in the crystaline phases.This type of multiblock copolymer could be a potential TPE material.The main innovations and findings of this work are:1.TPCC was used as effective quencher of active centers in organometallic catalytic systems for the first time,and suitable conditions of the quenching reaction have been successfully established.2.By combining the study of active centers based on TPCC quenching method with deconvolution of polymer molecular weight distribution and polymer chain structure characterization,in-depth investigations on multiple active centers in metallocenes catalyzed olefin polymerization have been made for the first time.Many important findings revealing microscopic mechanism of olefin polymerization have been made.3.We have studied the relationship between copolymer structure and material property in unbridged metallocenes catalyzed ethylene/propylene copolymerization at high pressure for the first time.Multiblock copolymers synthesized in this work have potential application as TPE.4.We have investigated ethylene/LCO copolymerization catalyzed by thermostable ?-diimine nickel catalyst for the first time.The copolymer has multiblock type chain structure with crystalline segments both in the main chain and the side chains.