Molecular weight and arm number of a star‐shaped styrene–butadiene block copolymer synthesized on a pilot‐vessel scale

To improve the rheological properties and processability of industrial rubbers, star‐shaped styrene–butadiene–styrene (SBS) block copolymers were synthesized and characterized in this work. Through the variation of the ratio of divinylbenzene to the diblock anion, a series of SBS samples with three to six arms were prepared. Multi‐angle laser light scattering (MALLS) and size exclusion chromatography (SEC) combined with light scattering (LS) were used to determine the weight‐average molecular weight (M_w), radius of gyration (〈S²〉^1/2), arm number, and chain conformation. The results from MALLS indicated that the M_w values of the star‐shaped SBS copolymers were 9.0, 13.0, 14.9, and 18.1 × 10⁴, which corresponded to three, four, five, and six arms, respectively. There was a lot of M_w and 〈S²〉^1/2 data for the many fractions in the SEC chromatograms of the SBS copolymers in tetrahydrofuran (THF) detected by LS, so the exponent of 〈S²〉^1/2 = KM_w^α was determined to range from 0.59 to 0.30 for the samples having three to six arms. An analysis of the results revealed that the star SBS copolymers existed in a sphere conformation in THF, and their chain density increased with an increase in the arm number. The viscosity of the six‐arm SBS copolymer was reduced significantly, compared with that of the SBS samples having three to five arms, when their M_w values were similar. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1853–1859, 2007