Hyperbranched polyisobutylenes (PIBs) have been synthesized by carbocationic copolymerization of an inimer (initiator-monomer, IM) and isobutylene (IB). The IM used was 4-(2-methoxyisopropyl)styrene (p-methoxycumylstyrene, pMeOCumSt), in conjunction with TiCl4 as a co-initiator in methylcyclohexane (MeCHx)/methyl chloride (MeCl) 60/40 v/v solvent mixture at -80 degreesC, in the presence of 2,6-di-tert-butylpyridine (DtBP) as a proton trap. The effect of reaction conditions on the molecular weights (MW), molecular weight distributions (MWD), and branching frequencies (BR) was investigated. Hyperbranched PIBs with MWs up to M-n approximate to 8 x 10(5) g mol(-1) and MWD as low as 1.2 were obtained within 15-60 min reaction time, where under the same conditions the 2-chloro-2,4,4-trimethylpentane (TMPCl) monofunctional initiator would have yielded MWs in the range of M-n approximate to 10(4) g/mol. BRs were determined by selective destruction of the branching points (links) and taking the ratio of the MWs before and after link destruction. The experimental BR values, ranging from 3 to 57, agreed reasonably well with values calculated using the formula BR = (M-n/Mn-(theo)) - 1, where M-n is the total MW of the hyperbranched PIE, and M-n(theo), would be the MW if the IM would act as initiator only. The process was scaled up to 400 g of PIB/match, sufficient for meaningful physical characterization.