The syntheses of end-functional polyisobutylenes (PIBs) including hydroxy, amino, carboxy, azide, propargyl, methoxy, and thymine end groups have been accomplished using nucleophilic substitution reactions. S(N)2 reactions on PIB-Allyl-X (X = Cl or Br), the trans-1,4 addition product of the capping reaction of living PIB with 1,3-butadiene, with different nucleophiles successfully yielded the corresponding PIB-Allyl-OH, PIB-Allyl-OMe, PIB-Allyl-NH2, PIB-Allyl-OCH2C CH, PIB-Allyl-N-3, and PIB-Allyl-CH2COOH with quantitative functionalization as determined by H-1 and C-13 NMR, FT-IR, and rnatrix-assisted time-of-flight mass spectroscopy. As expected, the rate of substitution was faster with PIB-Allyl-Br compared to PIB-Allyl-Cl. GPC analysis of the precursor PIB-Allyl-X and the products indicated that the polymer chain is unaffected by the substitution reactions. The synthesis of hydroxy telechelic PIBs was also achieved using X-Allyl-PIB-Allyl-X at reaction conditions similar to that employed for the preparation of PIB-Allyl-OH. The methodology was extended to the synthesis of PIB block copolymers by employing polymeric nucleophiles. PIB-b-poly(ethylene oxide) (PEO) was synthesized by the nucleophilic substitution of PIB-Allyl-Cl with PEO-O-Na+.