Chain Shuttling Polymerization To Prepare Stereoblock Polypropylene

Coordination chain transfer polymerization and chain shuttling polymerization can improve catalyst utilization efficiency,precisely control the polymer structure,and synthesize novel polyolefin materials,which are of great significance to the development of polyolefin materials.However,the chain shuttling polymerization has very strict requirements on the catalyst system,moreover,only several stereoblock polypropylene have been also obtained by chain shuttling polymerization with two specific catalysts in the presence of trimethyl aluminium(TMA)or diethylzinc(DEZ)as a chain shuttling agent(CSA).The paper first studied the coordination chain transfer polymerization of three different stereoselective catalysts under the optimal polymerization conditions.According to the effects of different chain transfer agents on polymerization activity,chain transfer efficiency,molecular weight and molecular weight distribution,it was found that in the presence of triisobutylaluminum,2,6-ⁱPr Ph NHC₅H₃C₈H₆ⁱPr Ph Hf Me₂,Ph₂CFlu Cp Zr Cl₂ and{Me₂Si(2,5-Me₂-3-(2-Me Ph)-cyclopento[2,3-b]thiophen-6-yl)₂}Zr Cl₂ can catalyze the reversible coordination chain transfer polymerization of propylene,respectively.The optimal polymerization conditions of the two zirconium catalysts were same,and polypropylene with different isotacticities can be obtained.In order to prepare stereoblock polypropylene containing soft segments and hard segments,we used the dual catalyst system consisting of two zirconium catalysts proceeding the chain shuttling polymerization under different chain shuttling agents.With triisobutylaluminum as the chain shuttling agent,the GPC curve of the polymer changed from a bimodal without no CSA to a symmetrically unimodal.The GPC of the polymer was still unimodal with a narrow molecular weight distribution when the conditions such as propylene pressure,the dosage of of triisobutylaluminum,and polymerization reaction time were changed.Finally,the representative polymerization products were extracted and characterized by GPC,NMR,DSC.It was determined that the products containing isotactic,syndiotactic and atactic blocks were synthesized.