Two structurally closely related three-arm star blocks were synthesized and characterized: tCurn(PIB-b-PNBD), and tCum(PNBD-b-PIB)(3) [where tCum (tricumyl) stands for the phenyl-1,3,5-tris(-2-propyl) fragment and PIB and PNBD are polyisobutylene and polynorbornadiene, respectively]. The syntheses were accomplished in two stages: (1) the preparation of the first (or inner) block fitted with appropriate chlorine termini capable of initiating the polymerization of the second (or outer) block with TiCl, and (2) the mediation of the polymerization of the second block. Therefore, the synthesis of tCum(PIB-b-PNBD)(3) was effected with tCum.(PIB-Cl-t)(3) [where Cl-t, is tert-chlorine and number-average molecular weight (M-n) = 102,000 g/mol] by the use of TiCl4 and 30/70 CH3Cl/CHCl3 solvent mixtures at -35 degreesC. PNBD homopolymer contamination formed by chain transfer was removed by selective precipitation. According to gel permeation chromatography, the M.'s of the star blocks were 107,300-109,200 g/mol. NMR spectroscopy (750 MHz) was used to determine structures and molecular weights. Differential scanning calorimetry (DSC) indicated two glass-transition temperatures (T-g's), one each for the PIB (-65 degreesC) and PNBD (232 degreesC) phases. Thermogravimetric analysis thermograms showed 5% weight losses at 293 degreesC in air and at 352 degreesC in N-2, The synthesis of tCum(PNBD-b-PIB)3 was achieved by the initiation of isobutylene polymerization with tCum(PNBD-Cl-sec)(3) (where Cl-sec is secchlorine and M-n = 2900 g/mol) by the use of TiCl4 in CH3Cl at -60 degreesC. DSC for this star block (M-n = 14,200 g/mol) also showed two T-g's, that is, at -67 and 228 degreesC for the PIB and PNBD segments, respectively. It is of interest that the Cl-sec terminus of PNBD, [GRAPHICS] readily initiated isobutylene polymerization. (C) 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 740-751, 2003.