Stereoselective Synthesis and Chain Microstructure Control of Trans-1,4-polyisoprene Enabled Via TiCl4/MgCl2 Ziegler-Natta Catalyst
Series of isoprene polymerizations with different polymerization time were performed by TiCl 4 /MgCl 2 type Ziegler-Natta catalysts. Polymerization kinetics showed catalytic activity had two-stage polymerization feature and the polymer chain microstructure gradually changed from the trans -1,4 and cis -1,4 mixed structure with low molecular weight ( M w ) as well as broad molecular weight distribution ( M w / M n ) at the early stage, to the final high trans -1,4 structure with high M w and narrow M w / M n . The high trans -1,4 units can enhance the tensile strength and hardness of the synthetic polyisoprenes, which are widely used for golf ball cover, medicinal material and shape memory material. Polymers were mainly isolated into Fraction C, D and G by the stepwise isothermal crystallization fractionation method. Fraction C and D had high trans -1,4 content, high trans -1,4-[II] dyad sequence concentration, high trans - trans unit linkage content and high crystallinity, which were generated by high trans -1,4 stereoselective active centers. While G fraction had low trans -1,4 content, low trans -1,4-[II] dyad sequence concentration, low trans - trans unit linkage content and low crystallinity, which were formed by the active center with low trans -1,4 stereoselectivity. The stereoselective transformation of active centers as well as trans -1,4 stereospecific polymerization mechanism of isoprene with TiCl 4 /MgCl 2 -AlEt 3 catalyst system was discussed. Graphical
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