It was found that the addition of compounds containing one nitrogen atom

such as pyridine

does not raise the catalitic activity in the polymerization of butadiene for the system of FeCls-(i-C4H9)3Al

and that the compounds containing two nitrogen atoms can incease the activity in the following order: 1

10-phenanthroline>2

2'-dipyridyl >> ethylenediamine or tetramethyl ethylenediamine. The polymerization of butadiene in the presence of FeCl3-(i-C4H9)3Al-phenanthroline have been studied in more detail. A possible mechanism is proposed.