Study On Tailoring Comb-branched Polyolefinic Thermoplastic Elastomers Using A Tandem Solution Copolymerization System

Thermoplastic polyolefin elastomers(TPE-O)demonstrate good mechanical and electrical insulation properties as well as resistance to chemicals and weathering.As a result,they are used extensively in the automotive,construction,medical and electronics fields.However,high performance TPE-O products,including ethylene/a-olefin copolymer(POE)and olefin block copolymer(OBC),have not been industrialized in China.Chinese domestic manufacturers do not currently possess the relevant production technology and are dependent on imported raw materials.Therefore,carrying out research on the development of polyolefin elastomers is important for the promotion of polyolefin industrial production technology in China.For the chain-shuttle polymerization mechanism of OBC,precise control over chain length,number and molecular weight distribution of hard and soft segments is not possible,nor can we inhibit the formation of ethylene homopolymers and random copolymers.However,the structural characteristic of OBC proves that good performance polyolefin elastomers can be achieved through the design and regulationof chain structure.Based on this,a comb-branched polyolefin thermoplastio elastomer(CPOE)having crystalline polyethylene(PE)as side chains and amorphous ethylene/a-olefin random copolymers as primary chains were developed.In this work,PE macromonomer catalysts suitable for tandem catalysis were screened to study polymerizations under a variety of conditions.A series of different CPOEs were synthesized,which helped identify relationships between polymerization conditions and chain structure.Mechanical properties of generated CPOEs were characterized and compared.Also,studied were the influence of PE macromonomers on the properties of polyolefin elastomers blends and properties of CPOEs after melt grafting to consume the residual PE macromonomers.Finally,thermoplastic polyolefin elastomers with dual long chain branching structure(DPOEs)were constructed by using diene monomers to participate in the copolymerization process,and the effects of polymerization conditions on the properties of DPOE were explored.The major achievements of this thesis include:(1)The selected phenoxycycloalkylimine catalyst(PTFI)can produce linear polyethylene having vinyl-end contents of>90%and molecular weight of several thousand at 90?.The addition of 1-octene at high concentration had little effect on the molecular weight,melting point and vinyl-end content of PE macromonomers.The molecular weight of PE macromonomers can be effectively adjusted with polymerization pressure.(2)CPOEs having crystalline PE as side chains and amorphous ethylene/1-octene random copolymers as primary chains were developed using a tandem catalyst solution polymerization system consisting of PTFI and a constrained geometry catalyst(CGC-Ti).The synthesized CPOEs possessed high melting points(>120?)and low glass transition temperatures(<-60 ?),which provided for a wider usage temperature compared with OBC.The final product was a complex mixture consisting of unreacted macromonomers,ethylene/1-octene random copolymers,and targeted CPOEs.Phase separation existed in hard and soft segments of all CPOE samples under solid state.The severity of phase separation was influenced by the residual PE macromonomer content and its co-crystallization effect with comb copolymers.Chain structure of CPOE can be regulated by polymerization conditions,including PTFI amounts,pre-polymerization time,1-octene feeding concentration and PE macromonomer chain length.(3)The relationship between chain structure and material performance for CPOE was studied with the finding that CPOE samples showed obvious shear thinning.With the higher molecular weight of PE macromonomers incorporated into the copolymer,the more obvious the shear thinning and the higher the melt elasticity.Increasing incorporation content and chain length of PE macromonomer and decreasing 1-octene content in primary chain resulted in the gradual improvement of tensile strength but lead to poor elastic recovery performance.High amounts of residual PE macromonomers found to be harmful for tensile strength and elastic recovery.The mechanical properties of CPOE were determined by the content and composition of the hard/soft segments,and degree of phase separation.(4)The effect of PE macromonomers on TPE-O/PE macromonomers blends was studied.It was found that PE macromonomers can greatly improve the processing properties while maintaining or even improving the mechanical properties of POE and OBC.CPOEs exhibited better mechanical performance,greater incorporation of PE macromonomers and fewer residual macromonomers.Reducing the residual PE macromonomers by melt grafting modification can improve the tensile strength and elastic recovery properties of CPOE.(5)DPOEs having dual long chain branching structure were synthesized by introducing 1,5-diene into the ethylene/1-octene/PE macromonomers copolymerization step.The tensile strength and elastic recovery performance of DPOE were greatly improved while maintaining the advantages of CPOE such as high melting temperatures,low glass transition temperatures and high elongations at break.