Both bifunctional initiators, the new low cost bBCB-diCl [4,9-dichloro,2,4,7,9-tetramethyl-tricyclo[6.2.0.0(36)]deca-1(8),2,6-triene] and the universally used hindered HDCCl [1-(tert-butyl)-3,5-bis(2-chloropropan-2-yl)benzene] induce the living bidirectional block copolymerization of isobutylene (IB) followed by styrene (St), and produce PSt-b-PIB-b-PSt (SIBS) triblocks. We discovered that the molecular weights of triblocks kept significantly increasing long after St conversion reached completion during syntheses. Results were explained by the formation of blends consisting of the expected linear SIBS plus hyperbranched SIBS, HB(SIBS)(n). The structure of high molecular weight (>10(6) g/mol) HB(SIBS)(n) was characterized by various techniques, and key properties of SIBS/HB(SIBS)(n) blends were investigated. The mechanism of HB(SIBS)(n) formation and the synthesis of SIBS/HB(SIBS)(n) blends was elucidated. The properties of SIBS/HB(SIBS)(n) blends are superior to those of SIBS. Thus, whereas SIBS exhibits approximate to 25 MPa tensile strength and approximate to 450% elongation, SIBS/HB(SIBS)(n) blends exhibit 25-27 MPa tensile strength and >400% elongation; deformation under constant load of SIBS is approximate to 12%, whereas that of SIBS/HB(SIBS)(n) is <1%; permanent set of SIBS is 1.3% whereas that of SIBS/HB(SIBS)(n) is <0.5%. SIBS/HB(SIBS)(n) blends also exhibit higher yield, yield strength, and toughness than SIBS. The microstructure/property relationship of HB(SIBS)(n) is discussed and the reasons for enhanced properties of SIBS/HB(SIBS)(n) blends are analyzed. (c) 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 705-713