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.