Multi-arm star-branched polyisobutylenes were synthesized by the "arm-first, core-last" method using the 2-chloro-2,4,4-trimethylpentane/pyridine/TiCl4 initiating system and the reactive core-forming comonomers 1,3-diisopropenylbenzene (DIPB) and divinylbenzene (DVB). Star formation was confirmed by RI and UV GPC and static light-scattering analyses. It was determined that DVB was significantly superior to DIPB. Using DVB, star polymers formed more rapidly and contained a much lower amount of residual PIB arms. Increasing the concentration of the reactive comonomer from 1 to 10 times the concentration of chain ends, [CE], increased the efficiency of the star-forming reaction substantially. Modest increases in the fraction of PIB arms incorporated into the star could be obtained by increasing the duration of the star-forming reaction. The timing of addition of the reactive comonomer to the PIB arms seems to be the process parameter most critical to the star development, since early addition at excessively low IB conversion hinders star formation by causing a copolymerization between IB and the core-forming comonomer. Late addition risks loss of a significant fraction of PIB chains due to spontaneous beta-proton expulsion. A fully developed multi-arm star-branched PIB was synthesized by utilizing 10 : 1 [DVB] : [CE], 24 h star-forming reaction time, 14,000 g/mol target arm M(n), and addition of DVB at 99% IB conversion. The resultant star polymer contained only 4% unreacted PIB arms and possessed M(omega) = 345,000 g/mol by light scattering. The weight-average number of arms per star polymer was calculated to be 23.