Trans-polyisoprene crosslinking

Fig. 49. Standard 13C NMR spectrum (top) and DEPT spectra of sulfur cured natural rubber (6h at 138 °C). The label T indicates peaks from trans-polyisoprene units, X marks residual peaks from other subspectra and arrows indicate peaks due to crosslink sites (adapted from Ref. 194>)...

Halogenation reactions of unsaturated polymers follow two simultaneous paths, ionic and free radical. Ionic mechanisms give soluble products from chlorination reactions of polybutadiene." The free-radical mechanisms, on the other hand, cause crosslinking, isomerization, and addition products. If the free-radical reactions are suppressed, soluble materials form. Natural rubber can be chlorinated in benzene with addition of as much as 30% by weight of chlorine without cycliza-tion. " Also, chlorination of polyalkenamers, both cis and trans, yields soluble polymers. X-rays show that the products are partly crystalline. The crystalline segments obtained from 1,4-trans-polyisoprene are diisotactic poly( 0 /rw-dichlorobutamer)s while those obtained from the 1,4-cis isomer are diisotactic polyOAfieo-l,2-dichlorobutamer)s. ... 

Some examples of the analysis of the apparent activation energy Ea of blends can be found in the literature. When evolution of the Ea of liquid cis-polyisoprene/ trans-polyisoprene (CPI/TPI) blends was analyzed using the VFTH model [41], the Ea for pure TPI was observed to be higher than that of TPI/CPI blends. Moreover, as the concentration of TPI decreased, then the Ea also decreased this was ascribed to the fact that in glass-rubber relaxation the motion of molecules is governed by the crosslink density. As the decrease in TPI content caused a decrease in crosslink density, this in turn enhanced the motion of chains, and consequently less Ea was required to promote segmental cooperative movements. 

The synthetic polyisoprenes are not purely cis or trans but exhibit branching. If a side chain growing on a diene polymer combines with one growing from another molecule, the result is a crosslink between the two molecules. Extensive crosslinking between polymer... 

Fig. 8. Melting temperatures for crosslinked trans 1, 4-polyisoprene, held at various stretch ratios X (11). Theoretical relation from equation 14 for increase in T with X,...

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