Efficient Preparation Of Olefin Block Polymers And Research On Its Structural Properties

Consumption of polyolefin materials is increasing day by day in our country.Although most of the traditional polyolefin materials can basically be self-sufficient,the most important high-performance polyolefin materials have still relied on imports.Coordination reversible chain transfer polymerization is an efficient polymerization method to achieve polymer structure control through the reversible exchange of polymer chains between the catalytic center and the chain transfer agent.In this way,the efficiency of catalysts could be greatly boosted,and molecular weight and molecular weight distribution,chain composition,chain number and ratio of blocks,could be easily controlled.Therefore,the physical properties of the block copolymers can also be tuned in a wide range.In this paper,a series of polyolefin block polymers were prepared by screening and optimizing the reversible coordination chain transfer polymerization system.Microstructures and properties of the polymers prepared from different monomers via different conditions were studied.This study provided a research basis for the development of novel polyolefin block polymers with well-controlled structure and much improved physical properties.Contents and results are as follows:（1）High-density polyethylene-b-isotactic polypropylene diblock copolymers were prepared with a’one-pot,two-step’’method by combining the coordination chain transfer polymerization of ethylene catalyzed by zirconium bis-salicylaldimine catalyst and propylene catalyzed by pyridylamido hafnium catalyst.The polymerization system synthesized a series of diblock copolymers with varying molecular weights（M_n≈900-1×10⁴）,narrow molecular weight distributions（1.2-1.8）and different block ratios,which are expected to be used as compatibilizers for ethylene-propylene plastics recycling agent.（2）Through the matching experiment of pyridylamido hafnium catalyst and different chain transfer agents,we screened out a reversible coordination chain transfer polymerization system using triisobutylaluminum as chain transfer agent,which has a similar chain transfer constant(k_ct)and chain-growth constant（k_p）.A series of isotactic polypropylene-based di-and triblock copolymers containing both hard i PP and soft ethylene-propylene rubbery block was successfully obtained.Compared with the i PP homopolymer,the toughness of the block copolymers was significantly improved to about 47 times,while their yield and breaking strength of the diblock polymer only decreased by 15%and 8%.Respectively,the triblock polymers showed a higher tensile strength（about 50 MPa）and toughness（elongation at break was up to 1100%）.Compared with the impact strength of the i PP（about 2.0 k J/m²）,the triblock polymer with very low ethylene content（about 1 mol%）showed a significant improvement in impact strength（about 5.5 k J/m²）.（3）Through the screening of chain transfer agents in zirconocene catalyzed polymerization systems with different selectivity of norbornene monomers,we selected diethylzinc as a chain transfer agent and verified the efficiency of coordination chain transfer polymerization of each catalyst system.Through chain shuttling polymerization of two catalyst systems under preferred conditions,we prepared cyclic olefin multiblock copolymers with alternating crystalline high-density polyethylene segments and transparent cyclic olefin copolymer segments.The copolymer has a high melting point,high transparency（>80%）and tensile toughness（strain greater than150%）.We can change the polymerization parameters to tune the microstructure of polymers and efficiently synthesize new transparent plastics with excellent comprehensive physical properties in a more economical manner.