Abstract:
Structural studies showed that natural rubber (NR) contains functional groups
at the initiating- and terminating-ends of rubber chain, i.e., ω- and α-terminals,
bonding with peptides and phospholipids, respectively. Both terminal ends have been
presumed to originate branch-points and soft-gel in NR. The formation of
branch-points by the α-terminal in NR chain was presumed based on the presence of
about two long-chain fatty acid groups and one phosphorus atom accompanied with
rubber chain independent of molecular weight even after deproteinization followed by
acetone extraction of NR. The decomposition of the ester linkages in deproteinized NR
(DPNR) by transesterification or saponification resulted in the formation of linear
rubber molecules. The molecular structure of α-terminal group was elucidated by
selective decomposition of ester linkages with enzymatic reactions using lipase and
phospholipases A2, B, C and D. The origin of branching formation was confirmed by
chemical treatment with ethanol.
The presence of monophosphate, diphosphate and phospholipid linked to
rubber molecule was confirmed by treatment of DPNR with lipase, phosphatase,
phospholipases A2, B, C followed by combination with 1H-, 13C- and 31P-NMR
analyses and molecular weight analysis by GPC and dilute solution viscometry. The
α-terminal of rubber molecule was postulated to be monophosphate and diphosphate
groups linking to phospholipids by Mg2+ linkage and hydrogen bonding. Quantitative
analysis by solid state 31P-NMR indicates the presence of about two to three
phosphorus atoms per α-terminal group in rubber chain. The formation of
branch-points in DPNR was postulated to be derived from micelle formation via
hydrogen bonding or ionic linkages between phospholipids and phosphate groups.
Mg2+, tetramethylthiuram disulfide (TMTD), zinc oxide (ZnO) and diammonium
hydrogenphosphate (DAHP) were the origin of additional branching-points in
long-storage fresh and commercial high-ammonia latices, which form hard gel.
At present, it can be postulated that NR molecule is terminated with
monophosphate and diphosphate groups at α-terminal. Branching formation is
originated from phospholipids associated by hydrogen bonding or ionic linkage.