The Hydrogenation of Polymers

These reactions were carried out using a heterogenous catalyst. 

The ease of hydrogenation and the resultant degree of saturation achieved reflect the microstructure of the polymer. Hydrogenation of unsaturated elastomers usually proceeds in a blocky way. This is due to the different reactivities of the various double bonds. In general, double bonds of 1,2 structure are four times more reactive than double bonds of 1,4 structure, cw-1,4 units are more reactive than tmns-, 4 units. Chamberline et al. (1981) have shown that hydrogenation of 1,2 units is statistically random, whereas the hydrogenation of 1,4 units is not. 

The complete hydrogenation of poly(l,4-butadiene), cis or tmns structure, forms a polyethylene with a low melting point of about 115 C. It is believed that this linear polyethylene is a low-density material. Unpublished data obtained 

Polybutadienes made by anionic catalysts in the presence of polar modifiers contain a mixed microstructure of cis-1,4, tmns-1,4, and 1,2 units. Hydrogenation of these polymers leads to interesting products. As mentioned previously, hydrogenation favors the 1,2 units over the 1,4 units by a 3- (or 4)-to-l ratio. Because of this mismatch in reactivity, hydrogenation of a polybutadiene containing 40-50% 1,2 units produces a polymer containing a polyethylene portion with a Tin of 85-95°C and a rubbery portion with a Tg of -62°C. 

Block copolymers can also be hydrogenated to produce unique products. Hydrogenated triblock copolymers of poly(styrene-co-butadiene-co-styrene) (SBS) are commercially available from the Shell Company under the trade name Kraton G. The middle block is usually a mixed microstructure of poly( 1,2-butadiene) and poly( 1,4-butadiene) units. The resulting product is a hydrogenated unsaturated polymer, which exhibits greater thermal and oxidative properties than the parent SBS triblock. 

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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. 

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. 

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. ... 

The next simplest group of linear polymers is the vinylidcnc group. Now two of the hydrogens of ethylene are replaced by radicals. Polymethylmethacrylate (alias PMMA,... 

Ionic liquids have already been demonstrated to be effective membrane materials for gas separation when supported within a porous polymer support. However, supported ionic liquid membranes offer another versatile approach by which to perform two-phase catalysis. This technology combines some of the advantages of the ionic liquid as a catalyst solvent with the ruggedness of the ionic liquid-polymer gels. Transition metal complexes based on palladium or rhodium have been incorporated into gas-permeable polymer gels composed of [BMIM][PFg] and poly(vinyli-dene fluoride)-hexafluoropropylene copolymer and have been used to investigate the hydrogenation of propene [21]. 

ADMET is quite possibly the most flexible transition-metal-catalyzed polymerization route known to date. With the introduction of new, functionality-tolerant robust catalysts, the primary limitation of this chemistry involves the synthesis and cost of the diene monomer that is used. ADMET gives the chemist a powerful tool for the synthesis of polymers not easily accessible via other means, and in this chapter, we designate the key elements of ADMET. We detail the synthetic techniques required to perform this reaction and discuss the wide range of properties observed from the variety of polymers that can be synthesized. For example, branched and functionalized polymers produced by this route provide excellent models (after quantitative hydrogenation) for the study of many large-volume commercial copolymers, and the synthesis of reactive carbosilane polymers provides a flexible route to solvent-resistant elastomers with variable properties. Telechelic oligomers can also be made which offer an excellent means for polymer modification or incorporation into block copolymers. All of these examples illustrate the versatility of ADMET. 

These four types of forces are responsible for the adaptive behavior of smart gels. The different forces come into play when the network of polymer chains composing a gel is disturbed, (a) Charged ionic regions can attract or repel each other, (b) Nonpolar hydrophobic regions exclude water, (c) Hydrogen bonds may form from one chain to another, (d) Dipole-dipole interactions can attract or repel chains. 

In recent years further concepts have been developed for the construction of polymer-based diodes, requiring either two conjugated polymers (PA and poly(A-methyl-pyrrole) 2 > or poly(A-methylpyrrole in a p-type silicon wafer solid-state field-effect transistor By modifying the transistor switching, these electronic devices can also be employed as pH-sensitive chemical sensors or as hydrogen or oxygen sensors 221) in aqueous solutions. Recently a PPy alcohol sensor has also been reported 222). 

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