Synthesis and characterization of multiarm star-branched polyisobutylenes: Effect of arm molecular weight

A series of star-branched polyisobutylenes with varying arm molecular weights was synthesized using the 2-chloro-2,4,4-trimethylpentane/TiCl4/pyridine initiating system and divinylbenzene (DVB) as a core-forming comonomer (linking agent). The resulting star-branched polymers were characterized with regard to the weight-average number of arms per star molecule (N̄w) and dilute solution viscosity behavior. As the molecular weight of the arm (M̄w, arm) was increased, dramatically longer star-forming reaction times were needed to produce fully developed star polymers. It was calculated that N̄w varied from 50 to 5 as the M̄w, arm was increased from 13,000 to 54,000 g/mol. The radius of gyration, Rg, of the star polymers was observed to increase as M̄w, arm was increased. The solution properties of the star polymers were evaluated in heptane using dilute solution viscometry. It was determined that the stars had a much higher [η] compared to the respective linear PIB arms, but a much lower [η] compared to a hypothetical linear analog of an equivalent molecular weight. The dependence of [η] on temperature for the stars and linear arms was very small over the temperature range 25 to 75°C, with only a very slight decrease with increasing temperature. [η]star was also determined to increase with increasing M̄w, arm, but decrease with increasing M̄w, star. The branching coefficient, g′, calculated for the stars at 25°C, increased as N̄w decreased and agre ed well with literature values for other star polymer systems.