Synthesis and Performance of Vinyl Butadiene-Pentadiene Copolymers with Tunable Melting Points Based on Cobalt Catalytic Systems: Toward Low-Melting, High-Strength, and High-Toughness Syndiotactic 1,2-Polybutadiene

In this study, a series of vinyl-rich butadiene–isoprene copolymers was synthesized using a commercially available cobalt naphthenate/triisobutylaluminum/carbon disulfide/water catalytic system. Incorporation of isoprene lowered the melting point and enabled a continuous transition from semicrystalline plastics to amorphous elastomers, improving the processability of high-vinyl 1,2-polybutadiene. The copolymer sequence structure was elucidated in detail by 1D and 2D NMR spectroscopy, and their microstructure, morphology, and mechanical properties were systematically characterized. The results showed that higher 1,2-polybutadiene content increased the modulus, low isoprene contents enhanced toughness and strength (tensile strength up to 13.5 MPa), and high isoprene contents imparted rubber-like behavior with favorable dynamic viscoelasticity.