Trans-1,4-stereospecific copolymerization of isoprene and butadiene catalyzed by TiCl4/MgCl2 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 (F B d). The copolymers obtained at different conversion and Al/Ti ratio were isolated to a series of fractions by stepwise isothermal crystallization fractionation from 80 degrees C to -20 degrees 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 H-1 NMR, C-13 NMR, TGA, GPC, DSC and WARD 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. (C) 2018 Elsevier Ltd. All rights reserved.