Trans -1,4- stereospecific copolymerization of isoprene and butadiene catalyzed by TiCl 4 /MgCl 2 type Ziegler-Natta catalyst II. Copolymerization Kinetics and Mechanism

Stereospecific copolymerization mechanism of isoprene (Ip) and butadiene (Bd) with heterogeneous TiCl4/MgCl2-Al(i-Bu)3 Ziegler-Natta catalyst was investigated for the first time in this paper. Copolymerization kinetics of isoprene and butadiene revealed that copolymers with high cis-1,4 units were observed in the initial stage, then the trans-1,4/cis-1,4 ratio increased gradually to the finally dominant trans-1,4-sequecnc with increasing polymerization time. The Al/Ti ratio influenced the catalytic efficiency, trans-1,4-sequecnc percentage and the butadiene incorporation (FBd). The copolymers obtained at different conversion and Al/Ti ratio were isolated to a series of fractions by stepwise isothermal crystallization fractionation from 80 °C to −20 °C. The trans-1,4-unit percentage, trans-trans and trans-cis placement, butadiene-isoprene (BI or IB) dyad sequence concentration, FBd, molecular weight and molecular weight distribution of the main fractions were analyzed by 1H NMR,13C NMR, TGA, GPC, DSC and WAXD characterization, which provided powerful evidences for the presence and evolution of active centers with different catalytic stereospecificity. Therefore, the possible active centers and trans-1,4 stereospecific copolymerization mechanism of isoprene and butadiene using heterogeneous Ziegler-Natta catalyst system was proposed accordingly.