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

Abstract

A series of star-branched polyisobutylenes with varying arm molecular weights was synthesized using the 2-chloro-2,4,4-trimethylpentane/TiCl 4 /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 (N w and dilute solution viscosity behavior. As the molecular weight of the arm (M w,arm ) was increased, dramatically longer star-forming reaction times were needed to produce fully developed star polymers. It was calculated that N w varied from 50 to 5 as the M w,arm was increased from 13,000 to 54,000 g/mol. The radius of gyration, R g , of the star polymers was observed to increase as M w,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 [γ] 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 M w,arm , but decrease with increasing M w,star . The branching coefficient, g', calculated for the stars at 25°C, increased as N w decreased and agreed well with literature values for other star polymer systems.