A series of butadiene-isoprene copolymers(BIR) with various compositions were synthesized with a neodymiumbased catalyst system.The microstructure,composition and sequence of copolymers were characterized by FTIR and ~(13)C-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(f_(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℃to -80.5℃,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℃to-43℃).The crystallization temperature and enthalpy values decreased gradually with decreasing N_(Nd).The copolymers with N_(Bd)≤7.9 are amorphous even at very low temperatures(0℃to-150℃) due to the short PBd segments. A series of butadiene-isoprene copolymers (BIR) with various compositions were synthesized with a neodymiumbased catalyst system. The microstructure, composition and sequence of the copolymers were characterized by FTIR and ~ (13) C-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 (f_ (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 longer polybutadiene (PBd) segments, and the sequence length of PBD segments (N_ (Bd)) show great influence on the crystallization behavior of the copolymers. These copolymers with N_ (Bd) ≥11.8 could crystallize at low temperatures (-71 ℃ to-43 ℃) gradually with decreasing N_ (Nd). The copolymers with N_ (Bd) ≤7.9 are amorphous even at very low temperatures (0 ° C to -1500 ° C) due to the short PBd segments.