Influence of alkylaluminium on the copolymerization of isoprene and butadiene with supported Ziegler-Natta catalyst

The heterogeneous TiCl4/MgCl2 Ziegler-Natta catalysts activated by alkylaluminium (AlR3) yield active centers with complex structures which are sensitive to the structure and concentration of AlR3. Effects of AlR3 on the copolymerizations of isoprene (Ip) and butadiene (Bd) by using TiCl4/MgCl2-AlR3 were studied. The catalytic activity(CA), microstructure and molecular weight of copolymers, and the number of active centers were studied by H-1 NMR, GPC and fluorescence sulfur analyzer. An increase in the Al/Ti molar ratio led to a significant decrease in the molecular weight of the copolymers, while the microstructure of the copolymers changed little. In the copolymerization activated with AlEt3 with increased Al/Ti molar ratio from 50 to 800, the catalytic activity increased firstly and then decreased gradually till kept a certain value upon further increasing the Al/Ti molar ratios (Al/Ti > 600). The guard effects from the growing active polymer chains and the adsorption of AlEt2Cl nearby the Ti species, prevent the further reduction of the Ti species to the lower valence state. On the basis of XPS analysis of elemental compositions and oxidation state on Ti, Ti3+ was deemed as the active species for diene copolymerization by using TiCl4/MgCl2-AlR3 catalyst. The influences of alkylaluminium on the active centers and its catalytic behaviors were proposed. It is anticipated that this study would supply a further understanding on the catalytic behavior and mechanism of the heterogeneous Z-N catalysts for conjugated diene polymerization.