Effect of external electron donor on stereoselective Copolymerization of isoprene and butadiene with MgCl2-Supported Ziegler-Natta catalyst

Abstract

External electron donors (ED) play significant roles in regulating stereoselectivity and catalytic activity of MgCl2-supported (TiCl4/MgCl2) type Ziegler-Natta catalysts. Investigating the contribution of ED on copolymerization of isoprene (Ip) and butadiene (Bd) with TiCl4/MgCl2 Ziegler-Natta catalysts is little reported. Herein, we reported the effects of three alkoxysilanes including dimethyldimethoxysilane (DMDMS), diphenyldimethoxysilane (DPDMS), phenyltrimethoxysilane (PTMS) added as ED on trans-1,4 stereoselective copolymerization of Ip and Bd using TiCl4/MgCl2-triisobutylaluminium (Al(i-Bu)3) catalyst system. Copolymers generated by TiCl4/MgCl2 catalyst in the absence of ED featured the microstructure transformation from cis-1,4 and trans-1,4 mixed microstructure on initial stage to trans-1,4 dominated microstructure and the molecular weight (Mw) evolution from low Mw to high Mw, indicating the transformation from low trans-1,4 stereoselective active center C*-1 to high trans-1,4 stereoselective active center C*-2. Appropriate introduction of alkoxysilane donors improved the polymerization activity, trans-1,4 unit content and molecular weight of copolymer in the order of PTMS > DPDMS > DMDMS, while excess alkoxysilane donors caused the decrease in that of polymerization behaviors. Increasing polymerization temperature also promoted the enhance in catalytic activity and trans-1,4 stereoseletivity of TiCl4/MgCl2 catalyst in the presence of ED. The equilibrium between deactivation and activation effect strongly depended on alkoxysilane structure, concentration and temperature. Plausible mechanisms for stereoselective transformation of active centers in Ip and Bd copolymerization using MgCl2-supported type Ziegler-Natta catalyst were proposed in terms of special roles of ED.