Possible mechanisms controlling molecular weight of rubbers in Hevea brasiliensis

The M(r) and M(r) distribution were determined by gel-permeation chromatography and osmometry for four rubbers from one-month- to three-year-old Hevea brasiliensis seedlings. A bimodal distribution was usually observed with M(r) peaks at 2.0 x 10(6) and 1.2 x 10(5), although the high M(r) peak was very small. The number-average M(r) (<(M)over bar (n)>) was one-tenth to one-sixth of that from a mature tree. There was a clear tendency for <(M)over bar (n)> to increase with the age of the tree. Transesterification reduced <(M)over bar (n)> by ca 40%. Part of the rubber from the Hevea seedlings was presumed to consist of molecules having the same ester branch-points as in the case of rubber from mature trees.