Determination of Sequence Distribution of Styrene Units in Cured SBR

The sequence distribution of styrene units in cured SBR was analyzed by high-resolution GPC measurement on the ozonolysis products obtained by ozonization of finely powdered samples suspended in methylene chloride followed by reductive degradation with lithium aluminum hydride. The conversion of the ozonolysis products, which were soluble in the organic layer, approximated that of an uncured sample when 100% of the calculated amount of the ozone required to react with the double bonds in the butadiene units was used. The conversion decreased slightly as the amount of ozone was increased to 220%. The fraction of monad, diad, and triad styrene sequences which were flanked by 1,4-butadiene units, and styrene long sequences—observed at the GPC exclusion limit of 3×103—leveled off at about 100 to 150% of the required ozone. In the GPC fraction corresponding to long styrene sequences, the residual butadiene units, as measured by 1H-NMR, decreased from 77 to 47% as the ozone increased from 3.7 to 80% of the required amount. Ozonization was believed to proceed rapidly on the surface of the swollen rubber particles in the initial stage, and then it progressed gradually into the inner part upon introducing excess ozone. Cured SBR samples, with and without carbon filler, showed sequence distributions of the styrene units which were similar to the uncured sample when 130% of ozone was used.