Long-chain branching and mechanism controlling molecular weight in Hevea rubber

Abstract

The linear character of transesterified deproteinized natural rubber (DPNR-TE) was confirmed by the analysis of terminal groups with NMR and viscometric analyses. The branch content of DPNR rubber from fresh latex was found to range from 0.3 to 1.3 and 0.7 to 3.2, based on tri-and tetra-functionalities, respectively. The plot between the number of branch-points and molecular weight (MW) can be divided into three fractions: (A) the rubber fractions in MW ranging from 2.4 × 105to 1.9 × 106; (B) between 1.9 × 105and 2.4 × 105; and (C) those of MW less than 1.9 × 105. The fraction (A) showed the number of branch-points per a branched molecule (m) higher than that of tractions (B) and (C). This plot is superimposable with the bimodal molecular-weight distribution (MWD) of Hevea rubber, showing a good coinciding of peak-tops at the high and low MW fractions. It seems likely that there is a close relationship between the number of branch-point and bimodal MWD of natural rubber.