Polyisoprene, catalytic hydrogenation

Staudinger published an important study of the catalytic hydrogenation of polyisoprene (rubber) in 1922. If the high experimental molecular weight of this substance arose from micelles composed of numerous smaller molecules, then hydrogenation should destroy the micellar structure and produce much lower molecular weight substances. In fact, the... 

The mechanism for this catalyst for the hydrogenation of ds-l,4-polyisoprene (CPIP) is slightly different [69]. As there was no clear evidence that coordination of hydrogen occurs prior to the coordination of C=C to the RuHCl(CO)(PCy3)2, there may be two possible pathways for the hydrogenation of CPIP in the presence of Ru(CH=CH(Ph))Cl(CO)(PCy3)2, namely an unsaturated path and a hydride path. The catalytic mechanism for these two pathways is represented in Scheme 19.6. 

The hydrogenation of unsaturated polymers like polyisoprene is based on the mobility of a soluble catalyst in the reaction medium. In the hydrogenation of such unsaturated polymers the soluble catalyst brings its active site to the C=C bonds in the polymer chain. In contrast, a heterogeneous catalyst requires that the polymer chain unfold to gain access to a catalytically active site on the surface of a metal particle. 

Unlike regular block copolymer micelles which are well permeable for reagents, triblock nanospheres with hydroxylated polyisoprene coronas, cross-linked poly(2-cinnamoyloxyethyl methacrylate) shells, and poly(acrylic acid) cores, filled with Pd nanoparticles, showed slower hydrogenation of alkenes than Pd blacks due to the need for the reactant(s) to diffuse into and the products to diffuse out of the encapsulating nanospheres [13]. On the other hand, microspheres formed by diblock poly(t-butyl acrylate)-hlocfe-poly(2-cinnamoyloxyethyl methacrylate) and filled with Pd nanoparticles demonstrated good permeability and higher catalytic activity in the hydrogenation of methyl methacrylate than the commercial Pd black catalyst [14]. 

Representative diene-based polymers include natural rubber (NR), polyisoprene (PIP), PBD, styrene—butadiene rubber (SBR), and acrylonitrile-butadiene rubber (NBR), which together compose a key class of polymers widely used in the rubber industry. These unsaturated polyolefins are ideal polymers for chemical modifications owing to the availability of parent materials with a diverse range of molecular weights and suitable catalytic transformations of the double bonds in the polymer chain. The chemical modifications of diene-based polymers can be catalytic or noncatalytic. The C=C bonds of diene-based polymers can be transformed to saturated C—C and C—H bonds (hydrogenation), carbonyls (hydrofbrmylation and hydrocarboxylation), epoxides (epoxidation), C—Si bonds (hydrosilylation), C—Ar bonds (hydroarylation), C—B bonds (hydroboration), and C—halogen bonds (hydrohalogenation). ... 

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