The end-to-end cyclization of telechelic polyisobutylenes (PIB's) toward cyclic polyisobutylenes is reported, using either ring-closing metathesis (RCM) or the azide/alkyne-"click"-reaction. The first approach uses bisallyl-telchelic PIB's (M-n = 1650, 3680, 9770 g mol(-1)) and Grubbs 1st-, 2nd-, and 3rd-generation catalyst leading to cyclic PIB's in 60-80% yield, with narrow polydispersities (M-w/M-n = 1.25). Azide/alkyne-"click"-reactions of bisalkyne-telechelic PIB's (M = 3840 and 9820 g mol(-1)) with excess of 1,11-diazido-undecane leads to the formation of mixtures of linear/cyclic PIB's under formation of oligomeric cycles. Subsequent reaction of the residual azide- moieties in the linear PIB's with excess of alkyne-telechelic PEO enables the chromatographic removal of the resulting linear PEO-PIB-block copolymers by column chromatography. Thus pure cyclic PIB's can be obtained using this double-"click"-method, devoid of linear contaminants. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 671-680, 2010