Multi-arm star polyisobutylenes: 2. The effect of synthesis conditions on the structure of star PIBs



Multi-arm star polyisobutylenes (*-(PIB)n) have been prepared by the “arm-first” method. This synthesis was accomplished by adding various linking agents (“core builders”) such as p- and m-divinylbenzene (DVB) and p- and m-diisopropenylbenzene (DIB) to living PIB® charges and thus obtaining a crosslinked aromatic core holding together a corona of well-defined arms. The products were characterized in terms of overall arm/core composition, molecular weight, and molecular weight distribution (M̄w/Mn). The effect of reaction conditions (time, [linking agent]/[PIB], arm molecular weight) on the kinetics of the star formation and star structure were investigated. The multi-arm star nature of *-(PIB)ns was proven directly by determining the molecular weight (by light scattering) of the intact products, selectively destroying the aromatic polyDVB (or polyDIB) core (“core-destruction”), and finally determining the molecular weight of the surviving aliphatic PIB arms. The synthetic strategy, overall kinetics, and observations during the preparation of star-PIBs were discussed. Among the critical parameters that determine product structures are the rate of crossover PIB + DVB (or DIB) → PIB-DVB (or PIB-DIB), the concentration of the linking agent DVB (or DIB), and the molecular weight of the PIB arm. Evidence for the formation of higher order stars (“secondary”, etc.) by star-star- coupling has been presented.