Structure and origin of long-chain branching and gel in natural rubber

NMR studies have revealed the fundamental structure of natural rubber (NR) composed of the initiating terminal (ω-terminal), two trans-1,4 isoprene units, a long sequence of cis-1,4 isoprene units and the chain-end group (α-terminal). The functional ω- and α-terminal groups have been postulated to form branch-points by association with proteins and phospholipids, respectively. The treatment of NR latex with a proteolytic enzyme decomposes the branch-points containing proteins to form branched molecules, the residual branch-points in which can be decomposed by transesterification with sodium methoxide in toluene solution to form linear molecules. The branch-points at the α-terminal were postulated to be originated by phospholipids which linked to the phosphate or diphosphate terminal group of rubber chain. The selective decomposition of phospholipids in NR by treatment of lipase, phospholipases A₂, B, C and D, and phosphatase provided conclusive evidence on the formation of long-chain branchings composed of phospholipids.