The chain structure of 1

2-polybutadiene prepared with molybdenum catalyst system has been studied by IR

13C-NMR

X-ray and thermal analysis

and the method to regulate the chain structure has been investigated. It was found that some of the polar additives

e.g.

allyl halides are able to regulate not only the content of 1

2-unit in the polymer

but also its configuration and sequence distribution. Attempt has been made to estimate the sequence length of stero-isomers of 1

2-unit by statistical theory.