Preparation of Low Molecular Weight Liquid Polybutadiene Rubber Featuring High 1,4 Content by Nickel-Based Ziegler–Natta Catalytic System

A ligand-free Ni(acac)2/EASC Ziegler–Natta catalytic system was developed for the efficient synthesis of low molecular weight liquid polybutadiene (LPB) featuring high 1,4 content. The influences of key polymerization parameters, including Al/Ni ratio, polymerization temperature, monomer-to-catalyst ratio ([Bd]/[Ni]), and external donors, were systematically investigated to elucidate structure–reactivity relationships. Increasing the Al/Ni ratio significantly enhances catalytic activity while promoting chain transfer reactions, leading to reduced molecular weights and broader molecular weight distributions, with minimal impact on overall 1,4 selectivity. Polymerization temperature strongly affects both activity and stereoselectivity; elevated temperatures accelerate chain transfer processes and broaden dispersity, while inducing a shift from kinetically favored cis-1,4 insertion toward increased trans-1,4 incorporation. Variation of the [Bd]/[Ni] ratio provides an effective handle for molecular weight regulation, where higher ratios favor chain propagation over chain transfer, affording higher molecular weights but lower monomer conversion. Notably, the system maintains consistently high 1,4 content (>98%) across a wide range of conditions. In contrast, the introduction of external donors markedly affects catalytic behavior depending on their coordination ability. Strongly coordinating O- and S-containing donors partially deactivate the catalyst and significantly shift regioselectivity toward 1,2-vinyl incorporation (up to ~20%), while N- and P-containing donors are well tolerated and can increase molecular weight by suppressing chain transfer pathways, which also results in products with higher 1,2 content.