Structural characterization of α-terminal group of natural rubber. 2. Decomposition of branch-points by phospholipase and chemical treatments

The treatment of deproteinized natural rubber (DPNR) latex with phospholipases A₂, B, C, and D decreased significantly the long-chain fatty acid ester contents in DPNR and also the molecular weight and Higgins' k′ constant, except for phospholipase D treatment. This indicates the presence of phospholipid molecules in NR, which combine rubber molecules together. Transesterification of DPNR resulted in the decomposition of the functional group at the terminal chain-end (α-terminal), including phospholipids and formed linear rubber molecules. The addition of small amounts of ethanol into the DPNR solution reduced the molecular weight and shifted the molecular weight distribution (MWD) comparable to that of transesterified DPNR (TE-DPNR). The addition of diammonium hydrogen phosphate into DPNR-latex in order to remove Mg²⁺ ions yielded a slight decrease in molecular weight and a slight shift in MWD. The phospholipids are expected to link with mono- and diphosphate groups at the α-terminal by hydrogen bonding and/or ionic linkages. The decrease in the molecular weight and Huggins' k′ constant of DPNR demonstrates the formation of linear molecules after decomposition of branch-points by this treatment, showing that phospholipids participate in the branching formation of NR. The branch-points formed at the α-terminus are postulated to originate predominantly by the association of phospholipids via micelle formation of long-chain fatty acid esters and hydrogen bonding between polar headgroups of phospholipids.