Effects of chain microstructure of butadiene-isoprene copolymers on their glass transition and crystallization

A series of butadiene-isoprene copolymers (BIR) with various compositions were synthesized with a neodymium-based catalyst system. The microstructure, composition and sequence of copolymers were characterized by FTIR and ¹³C-NMR spectroscopy. The crystallization behavior of the BIR copolymers was investigated by DSC analysis. The results demonstrate that the content of cis-1,4 configuration in both butadiene (Bd) and isoprene (Ip) units are around 98% when Bd content in feed (ƒ _Bd) covering the range from 55.7 mol% to 96.0 mol%. The reactivity ratios of Bd and Ip were determined to be 1.40 and 0.48 respectively. The random copolymers of Bd and Ip show only one glass transition temperature (T _g) from −107.4°C to −80.5°C, which is dependent on the composition and fits nicely with Fox equation. The sequence distribution followed the first-order Markov statistical model. It is found that the copolymer chains with higher Bd content contain longer polybutadiene (PBd) segments, and the sequence length of PBd segments (N _Bd) exhibits great influence on the crystallization behavior of the copolymer. The copolymers with N _Bd ≥ 11.8 could crystallize at low temperatures (−71°C to −43°C). The crystallization temperature and enthalpy values decreased gradually with decreasing N _Bd. The copolymers with N _Bd ≤ 7.9 are amorphous even at very low temperatures (0°C to −150°C) due to the short PBd segments.