In recent years, polyolefin elastomers play an increasingly important role in industry.
The late transition metal complex catalysts, especially α-diimine Ni(II) and α-diimine
Pd(II) complex catalysts, are popular “chain-walking” catalysts. They can prepare polyolefin
with various structures, ranging from linear configuration to highly branched configuration.
Combining the “chain-walking” characteristic with different polymerization strategies, polyolefins
with good elasticity can be obtained. Among them, olefin copolymer is a common
way to produce polyolefin elastomers. For instance, strictly defined diblock or triblock copolymers
with excellent elastic properties were synthesized by adding ethylene and α-olefin
in sequence. As well as the incorporation of polar monomers may lead to some unexpected
improvement. Chain shuttling polymerization can generate multiblock copolymers in one pot
due to the interaction of the catalysts with chain shuttling agent. Furthermore, when regarding ethylene as the sole
feedstock, owing to the “oscillation” of the ligands of the asymmetric catalysts, polymers with stereo-block structures
can be generated. Generally, the elasticity of these polyolefins mainly comes from the alternately crystallineamorphous
block structures, which is closely related to the characteristic of the catalytic system. To improve performance
of the catalysts and develop excellent polyolefin elastomers, research on the catalytic mechanism is of great
significance. Electron spin resonance (ESR), as a precise method to detect unpaired electron, can be applied to study
transition metal active center. Therefore, the progress on the exploration of the valence and the proposed configuration
of catalyst active center in the catalytic process by ESR is also reviewed.