Strategy for isoprene-styrene multi-block copolymers obtained by stereospecific copolymerization through TiCl4/MgCl2 catalyst

Abstract

Multi-block stereoregular copolymer of styrene and isoprene was realized by stereospecific copolymerization with supported titanium Ziegler-Natta catalyst [TiCl4/MgCl2-Al(i-Bu)3] successfully. Effects of initial monomer ratio (Ip/St) on catalyst efficiency (CE) and composition and sequence structure of the as-obtained copolymers were studied. Apparent reactivity ratios, which represent the apparent relative reaction rate of Ip and St, were calculated based on the Kelen-Tüdös method. The activity of isoprene (raIp = 11.12) was much higher than that of styrene (raSt = 0.14) under this catalyst system. The copolymers were fractionated by the n-hexane extraction. The nascent copolymers and the fractions were characterized by FT-IR NMR, DSC and GPC. The soluble fraction of n-hexane extraction were multi-block copolymers with long trans-polyisoprene (trans-PI) segments and single/short isotactic polystyrene(iPS) segments. The insoluble ones were copolymers with long iPS segmets and short trans-PI segments. DSC analysis showed two glass transition temperatures close to each other by adjusting the amount of St in copolymers. The whole copolymers were composed of crystallizable copolymers with different segment length of trans-PI blocks and iPS blocks. This work offers a novel way for stereospecific copolymerization of alpha-olefin and conjugated diene by using traditional supported titanium Ziegler-Natta catalyst.