Olefin types were quantitatively determined by PMR and infrared spectroscopic analyses in polyisobutylenes and commercial polybutenes of Mn ranging from 500 to 2000. The catalyst and the polymerization conditions were found to have a great influence on the olefin type produced. In polyisobutylenes produced with BF3, the terminal vinylidene formed initially can diminish under the isomerizing influence of the catalyst. In polyisobutylenes synthesized with AlCl3, the major olefin type is °C(R)=CH(CH3). Its formation is explained by a multistep termination which involves tail‐to‐tail coupling. Internal vinyl‐idenes were also identified in most of the samples. Their formation can be explained by a skeletal rearrangement either in the termination step, or during polymer double‐bond isomerizations. In most polymers, the spectroscopically measured unsaturation was lower than that determined by hydrogenation. The difference is attributed to the presence of tetrasubstituted olefins.

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