In the presence of a nonoxidizing radical initiator, azo-tert-butane, and a high concentration of LiCB11(CH3)(12), isobutylene undergoes thermal or light-induced radical polymerization to b-PIB, a highly branched polymer of modest molecular weight (mostly a few thousand and up to similar to 25 000 g/mol based on GPC with polystyrene standards). The structure of b-PIB was elucidated by NMR spectroscopy of a low-molecular-weight fraction. The polymer is branched on every carbon atom of the main chain; one chain end carries an isobutenyl group, and the other carries a tert-butyl group originating in the initiator. The branches are short segments of l-PIB (linear polyisobutylene), on the average composed of five IB units. A mechanism of formation if this dendrimer-like structure is proposed.