Hydrogenation of polymers

Polymer Synthesis and Characterization. This topic has been extensively discussed in preceeding papers.(2,23,24) However, we will briefly outline the preparative route. The block copolymers were synthesized via the sequential addition method. "Living" anionic polymerization of butadiene, followed by isoprene and more butadiene, was conducted using sec-butyl lithium as the initiator in hydrocarbon solvents under high vacuum. Under these conditions, the mode of addition of butadiene is predominantly 1,4, with between 5-8 mole percent of 1,2 structure.(18) Exhaustive hydrogenation of polymers were carried out in the presence of p-toluenesulfonylhydrazide (19,25) in refluxing xylene. The relative block composition of the polymers were determined via NMR. 

Another palladium complex, namely, a six-membered cyclopalladate complex of 2-benzoyl pyridine, has also been used for the hydrogenation of polymers [77, 78]. Possible catalytic mechanisms for the hydrogenation of natural rubber [76] and NBR [77] catalyzed by these two complexes were proposed, but unfortunately the authors did not provide sufficient evidence to support their proposed mechanisms. 

Although Ziegler-type catalysts have been widely investigated for the homogeneous hydrogenation of polymers, their catalytic mechanism remains unknown. One possible reason for this may be the complexity of the coordination catalysis and the instability of the catalysts. Metallocene catalysts are highly sensitive to impurities, and consequently it is very difficult to obtain reproducible experimental data providing reliable kinetic and mechanistic information. 

The hydrogenation in a liquid-liquid system with ionic liquids as the catalyst phase was also applied to the hydrogenation of polymers. The first studies were presented by the group of Rosso et al. [91], who investigated the rhodium-catalyzed hydrogenation of polybutadiene (PBD), nitrile-butadiene rubber (NBR) and styrene-butadiene rubber (SBR) in a [BMIM][BF4]/toluene and a [BMIM][BF4]/tolu-ene/water system. The activity of the catalyst followed the trend PBD>NBR> SBR, which is the same order as the solubility of the polymers in the ionic liquid. The values in percentage total hydrogenation after 4 h reaction time were 94% for PBD and 43% for NBR, and after a reaction time of 3 h was 19% for SBR. 

Hydrogenation of polymers of substituted norbomenes is frequently a valuable aid to the determination of structural detail and is best carried out using diimide (NH=NH) generated in situ by the decomposition of p-toluenesulphonohydrazide in xylene at 120 °C. This procedure works well and selectively even in the presence of groups such as COOMe and PPh2497. 

Lange J P, Schoon L, ViUena A et al. (2004) Monolithic catalysts forthe fixed-bed hydrogenation of polymers. Chem Commun issue 24 2864—2865... 

Hydrogenation of polymers results in better thermal and oxidative stability, and several attempts were made to employ homogeneous catalysis for this purpose. An ABS copolymer was reduced in the form of an aqueous emulsion with neutral and cationic Rh(I) complexes. The slightly water-soluble cationic complex was more efficiently built into the micelles of the anionic detergents used for polymer solubilization and this resulted in better conversions, e.g., 70% in contrast to 20% with [RhCl(PPh3)3] [76],... 

Hydrogenation of polymers results in improved thermal and oxidative stability. In the hydrogenation of polybutadiene using the [RhCl(4)2]2 and [RhCl(5)2]2 catalysts, the pendant (terminal) vinyl units were hydrogenated preferentially over the internal double bonds [Eq. (8)] [16]. 

The hydrogenation of polymers such as NBR (acrylonitrile-butadiene rubber), SBR (styrene-butadiene rubber), and PBD (polybutadiene) has been also performed by Ru(ll) compounds associated with phosphine ligands immobilized in classical imidazolium ILs or polyether-modified ammonium salts. ... 

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