Tailoring Ethylene/1-Octene Copolymers In A Solution Copolymerization Process

Ethylene/α-olefin copolymer with high comonomer incorporation is one of the most widely used high performance thermoplastic elastomers. However, the industrial production technology is still limited to quite few companies. The catalyst with high activity for α-olefin copolymerization and the solution copolymerization process at high temperature are almost patented. In the whole polyolefin industry, precise control over the polyolefin chain microstructure is still challenging. Recently, the advent of living olefin polymerization technique provides the possibility of precise control of the polyolefin chain microstructure, so as to tailor-make polyolefin materials.This work targeted on the controlled synthesis of ethylene/1-octene copolymer elastomers. First, a bridged metallocene catalyst was systematically studied for the high temperature ethylene/1-octene solution copolymerization, which was performed in our self-designed high temperature and high pressure continuous olefin solution polymerization system. The result is of great benefit to the industrial production of the polyolefin elastomers in our country. Second, a high performance living coordination copolymerization catalyst was synthesized. Ethylene/1-octene living copolymerization behavior was systematically studied and an effective copolymerization kinetic model was established based on this living catalyst system. It was first demonstrated that the chain microstructure of olefin copolymers could be precisely controlled through the programmed monomer feeding policies assisted by the kinetic model design. The chain microstructures and the mechanical properties of the ethylene/1-octene random copolymers and block copolymers produced by living coordination polymerization were finally investigated in comparison with commercial products. The major achievements of this thesis include:(1) The catalytic activity and the comonomer incorporation ability of the bridged bi-metallocene zirconium catalyst reached the industrial requirements. The apparent kinetic parameters in catalyzing ethylene/1-octene copolymerization at 140℃ were: κ’p11= 483 L·mol-1·S-1, κd=0.0013 S-1,r1=10.03, r2=0.123. The apparent kinetic parameters in catalyzing ethylene/1-hexene copolymerization at 80℃ were:kp11=423 L·mol-1·S-1, kd=0.00098 S-1, r1=6.16, r2=0.114. In the continuous solution copolymerization, the catalytic activity, the copolymer molecular weight and the comonomer incorporation reached the steady state after 6τ, the copolymer molecular weight decreased as the 1-octene feeding ratio increased.(2) The fluorinated MFI catalyst synthesized in this work exhibited excellent ethylene/1-octene living copolymerization behavior at room temperature. The 1-octene incorporation in the copolymer reached more than 30 mol%. In the solution polymerization process at room temperature, the reactivity ratios for this catalyst system were estimated as:r1= 44.56±4.45, r2=0.022±0.0023 (13C-NMR method); r1= 54.95±5.33, r2=0.034±0.016 (Fineman-Ross method). The reactivity ratios were found independent on the reaction temperature between 0 to 45℃.(3) An effective kinetic model was established for the semi-batch living copolymerization of ethylene and 1-octene for the MFI catalyst. The model gave good prediction on the ethylene consumption rate, the comonomer conversion, the copolymer composition and the copolymer molecular weight. In the semibatch living copolymerization, the chain microstructure of olefin copolymers was precisely controlled by the comonomer sequential feeding policy assisted by the kinetic model design. Ethylene/1-octene diblock and step triblock copolymers with narrow distributed molecular weight were synthesized. The block copolymers retained the crystallization ability of the hard block with the melting temperature higher than 110℃, together with the low temperature flexibility of the soft block with the glass transition temperature lower than -50 ℃. Ethylene/1-octene diblock copolymers, including the ultrahigh molecular weight polyethylene with blocky structure, were also produced through the ethylene pressure pulse feeding policy. The melting temperature of these block copolymers was higher than 80 ℃, and the glass transition temperature was lower than-60℃.(4) Structure analysis showed that, the living random copolymers possessed narrower intrachain composition distributions than those prepared from conventional metallocene catalysts. The ethylene/1-octene block copolymers made by the living polymerization had very different intrachain microstructures from the commercial olefin multiblock copolymer (OBC) which was produced by the chain shuttling polymerization technique. The interchain composition distribution of OBC was found broader than living block copolymers. However, the crystal thickness distribution of OBC was narrower. The average crystalline methylene sequence length in the OBC polymer chain was longer than that in the living block copolymers.(5) The living random copolymer elastomers synthesized in this work exhibited better elastic recovery than the commercial sample that made by metallocene catalyst, due to the narrower intrachain composition distribution. With the similar comonomer incorporation, the molecular weight had little influence on the material’s elasticity. For the ethylene/1-octene block copolymers, the crystallinity increased with the hard block content, which finally increased the material’s elastic modulus, the strength and elongation at break. With the similar crystallinity, increasing the molecular weight also enhanced the block copolymer’s strength. For the diblock copolymers, the hard to soft block ratio had a significant effect on the material’s elasticity. Either too low or too high content of the hard block is not good to the block copolymer’s elastic recovery.