Various linear and three-arm star end-reactive prepolymers carrying one, two or three terminal functions have been prepared by carbocationic techniques. The higher molecular weight varieties of these materials are valuable intermediates for the synthesis of block copolymers, whereas the low molecular weight telechelics (liquid oligomers) are useful starting materials for extension by end-linking. This presentation outlines the synthesis of some new polyisobutylene-based linear and three-arm star, symmetric and asymmetric end-reactive prepolymers such as, α-phenyl-ω-tertchloro-PIB, αω-diaryl-PIB's, and their derivatives carrying the -C6H/NO2, -C6H4NH2, etc. end groups. Telechelic PIB's with -C6H4NH2end groups react with diepoxy compounds (EPON 826) to give rise to hydrolytically and thermally stable flexible networks. An efficient semicontinuous synthesis procedure is outlined that leads to well-defined linear and three-arm star tert.-chloro-telechelic PIB's with close to theoretical end group functionalities and molecular weight distributions. Three-arm star hydroxyl-telechelic PIB's in conjunction with OCN-C6H4,-CH2-C6H4-NCO give new model polyurethane networks exhibiting outstanding hydrolytic and thermal properties.