Effect of alkylaluminium on the regio- and stereoselectivity in copolymerization of isoprene and butadiene using TiCl4/MgCl2 type Ziegler-Natta catalyst

Alkylaluminums as one of the most important components of the heterogeneous MgCl2-supported (TiCl4/MgCl2) type Ziegler-Natta catalysts, play significant roles in yielding active species with varied regio- and stereoselectivity. Studying the contribution of alkylaluminums on the conjugated diene isoprene (Ip) and butadiene (Bd) copolymerizations with TiCl4/MgCl2 - alkylaluminum catalysts system is little reported. In this work, effects of various alkylaluminums including triethylaluminium (TEA), triisobutylaluminium (TiBA), diisobutylaluminium hydride (DiBAH) and diethylaluminum chloride (DEAC) on the copolymerization behaviors of Ip and Bd using the TiCl4/MgCl2 - alkylaluminum catalysts system, such as catalytic activity, active center numbers, butadiene incorporation, trans-1,4 stereoselectivity, molecular weight M-w as well as molecular weight distribution M-w/M-n of copolymers were first investigated. TEA with smaller size ethyl groups showed higher copolymerization activity, higher reducing ability and then yielding more active center numbers ([C*]/[Ti]) when compared with TiBA, DiBAH and DEAC. Microstructure of copolymers gradually transformed from the cis-1,4/trans-1,4 mixed structure in the initial polymerization period to the finally high trans-1,4 structure, indicating a transformation of the active center nature during the early polymerization stage, which can be accelerated effectively through the addition of DEAC into the TiCl4/MgCl2-TEA catalyst system. TEA with smaller size ethyl group showed higher transformation rate from the mixed structures of trans-1,4 and cis-1,4 to highly trans-1,4 structure. The active center numbers of the copolymerization systems increased gradually with increasing time until reaching a stable level. The M-w/M-n of copolymers showed interesting evolution with increasing polymerization time from broad distribution with low to medium molecular weight fractions in the initial polymerization stage to relatively narrow distribution with high molecular weight fractions. The contribution of alkylaluminium compounds on the structure of active species which determining the regio- and stereoselectivity of Ip and Bd copolymerization using heterogeneous TiCl4/MgCl2 type catalysts were discussed in details