Natural rubber terminal groups

Figure 11.15. Typical chemical groupings in a sulphur-vulcanised natural rubber network, (a) Monosulphide cross-link (b) disulphide cross-link (c) polysulphide cross-link (j = 3-6) (d) parallel vicinal cross-link (n = 1-6) attached to adjacent main-chain atoms and which have the same influence as a single cross-link (e) cross-links attached to common or adjacent carbon atom (f) intra-chain cyclic monosulphide (g) intra-chain cyclic disulphide (h) pendent sulphide group terminated by moiety X derived from accelerator (i) conjugated diene (j) conjugated triene (k) extra-network material (1) carbon-carbon cross-links (probably absent)...

According to this mechanism, natural rubber chains are expected to have one dimethylallyl terminal unit and one isoprenyl pyrophosphate terminal unit the latter may give rise to a hydroxyl group by hydrolysis. From this point of view, acyclic terpenes in the generalized structure (II) may be appropriate models for the structural characterization of natural polyisoprenes by 13C NMR spectroscopy. 

The polysulfides derived from ethylene dichloride were not as elastomeric as natural rubber. It was foimd that increasing the spacing between reactive terminal groups improved the properties. Better products derived from bis(2-chloroethyl) ether were soon developed in the United States by the Thiokol Chemical Corp. 

Natural rubber and synthetic rubbers in general do not have hydroxyl groups in sufficient numbers for them to be used for esterification reactions. Terminal... 

A pioneer group of researchers in UKM has extensively studied utilization of liquid natural rubber (LNR) as a compatibilizer on various NR/polyolefin blends. LNR was produced by photodegradation of NR in toluene and exposure to the ultraviolet for 6 h. The LNR has the same microstructure as NR but with a short polyisoprene chain (of diflerent molecular weight, M )- The Mw of LNR is around 50,000 whereas for NR is 900,000. The formation of new functional groups such as H, C-O and C=0 via oxidation of photochemical sensitization of NR. The presence of the new functional groups is shown in Figure 17.1. The LNR with some active terminals like -OH is... 

PCL (M approximately 1300-6500) with terminal hydroxyl groups was functionalized to provide telechelic PCL with terminal carboxyl groups (XPCL) 31. This polymer was blended with epoxidised natural rubber, epoxidised to either 25% or 50% (ENR25 and ENR50, respectively). The two polymers were compounded on a two-roll mill at room temperature to provide a self-curing system. The blends were heated to 180 °C under a pressure of 1000 N cm , approximately, to cure them. For comparison, samples of the original PCL 32, with hydroxyl endgroups were blended with the epoxidised rubber [171]. 

Based on infrared evidence, ester groups in natural rubber were initially reported to be due to the presence of lactone functional groups. " Subsequently, C-NMR studies on deproteinized natural rubber showed that the ester groups were due to the presence of fatty acids bonded to the rubber molecule, of which 80% was saturated fatty acids and 20% unsaturated fatty acids. The fatty acids were later identified to be the acyl component of phosphoHpids hnked to the a-terminal end of the rubber molecule. The phospholipids were also postulated to be the branching point of the natural rubber molecule. The effects of these groups on natural rubber s properties have been discussed above. 

In another study by Cherian et UPA was modified by blending with functionalized rubbers such as ENR, hydroxyl-terminated natural rubber, and maleated nitrile rubber. The elastomers bearing reactive functional groups showed better compatibility with the resin and improved the toughness and impact resistance of the cured resin substantially, compared to unmodified elastomers. Maleated nitrile rubber (NBR) was far superior to all other... 

Suffice it to say here that the end-groups are entirely determined by the mechanism of biosynthesis of the natural rubber molecule and present views indicate that the terminal groups are likely to be isopropyl at one end and pyrophosphate at the other. 

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