Anionic polymerization of (Z)-1,3-pentadiene using Ba/Al/Li trimetallic initiators to synthesize crystalline high trans-1,4-polypentadiene

( Z )-1,3-pentadiene (Zp) exhibits a rigid planar molecular structure and is known for the typical 1,5-sigmatropic hydrogen shift, while it is commonly considered an inert monomer during polymerization. Herein, by utilizing the steric effect of the terminal methyl in Zp, the high trans -1,4-polypentadiene (HTPD) with highly ordered microstructure and well-defined sequence are successfully synthesized using barium dithymol/ tert- butyllithium/ tri -ethylaluminum trimetallic initiators based on anionic polymerization. The significant dependence of polymerization rate and trans -1,4-selectivity on ratio of Ba/Al/Li is demonstrated. The modification in microstructure ( trans -1,4 content ranging from 75 % to 92 %) and stereo-regularity (isotactic triad mm % > 65 %) are confirmed by nuclear magnetic resonance (NMR). Compared with the poly(( Z )-1,3-pentadiene) (PZp) prepared by n -BuLi containing σ -ligand modifiers, the strategy using a large steric initiator to achieve regular insertion of Zp into polymer chains is proved to be effective. Kinetics studies demonstrated that Ba–Al–Li system is characterized by retarded anionic polymerization (RAP), yet it shows a quasi-living process and enabling the preparation of stereometric block polymers such as polyisoprene- b -HTPD (PI- b -HTPD). Thermal performance analysis indicates that the obtained highly ordered polymers show superior thermoplastic properties ( T m  = 77.4 °C) with crystallinity as an elastomer. Furthermore, reasonable interpretations for the structure formation, sequence, and rate reduction effected by the Ba/Al/Li trimetallic initiator are proposed.