Palladium polymers

An interesting application of organic polymers is their use as catalyst carriers, by which special properties can sometimes be transferred to the catalyst. The hydrogenation of several unsaturated compounds on palladium/polymer catalysts was measured, and a comparative study showed that the activity of the catalyst decreased in the following sequence as a function of the nature of the polymer 122> ... 

Tyruenkova, O. A. Hydrogenation of Unsaturated Compounds on Palladium/Polymer Catalysts. Russian J. Phys. Chem. (English Transl.) 43, 1167 (1969). 

The spectra of both polymers can be readily separated into methine and methylene regions those resonances appearing upheld of 40 ppm in the C NMR spectrum are due to methylene carbons and those downheld are due to methine carbons. Of special interest are the cross-peaks due to the bridging methylene group C7 (between 36 and 39 ppm, circled in the spectra). As expected, the carbons in this region correlate with the two inequivalent protons II , and Hy, attached to the bridge carbon, C7. However, in the nickel-based polymer, there are two distinct types of bridging carbons, while in the palladium polymer there is only one... 

Second, the use of a supporting sub-layer such as titanium and chromium with high adhesion may improve the stability of the palladium sensing layer. Different materials have proved effective for this purpose including nickel (Butler 1994), VOx (Smith et al. 2002), (Liu et al. 2002), CaF (Fedtke et al. 2004), and MgF (Chtanov and Gal 2001). For example, Fedtke et al. 2004 found that structures with CaFj buffer layers and palladium cap showed a better time stability and behavior up to 6% H. Palladium-polymer nano-composite thin films allowed the accommodation of the palladium lattice expansion during hydrogen exposure up to 10% Liu et al. (2002) have shown that PdA jO,... 

A second difference is the poorer, more limited solubility of the palladium compounds compared to the analogous platinum polyamines. Most of the platinum polyamines thus far synthesized are soluble in a number of dipolar aprotic solvents with some even soluble in chloroform. Only the palladium polyamines derived from dissymmetrical diamines exhibit solubility in any attempted solvent and here solubility is limited to only dipolar aprotic solvents to an extent of 3% and less. Solubilization in dipolar aprotic liquids is dependent on a number of factors - the major one being the ability of the dipolar aprotic liquid molecules to polarize the solute molecules. The smaller size and corresponding poorer polarizability of the palladium atom is probably responsible for this trend. A further, but related factor, is the possible greater tendency for the palladium polymers to form crystalline regions. 

Allylic acetoxy groups can be substituted by amines in the presence of Pd(0) catalysts. At substituted cyclohexene derivatives the diastereoselectivity depends largely on the structure of the palladium catalyst. Polymer-bound palladium often leads to amination at the same face as the aoetoxy leaving group with regioselective attack at the sterically less hindered site of the intermediate ri -allyl complex (B.M. Trost, 1978). 

The allyl bromides formed by method (A) contain 25% of the undesired (Z)-isomer. The selectivity of the palladium-catalyzed amination can be steered by the application of polymer-bound systems (see section 2.6.3 B. M. Trost, 1978),... 

Vinyl acetate (ethenyl acetate) is produced in the vapor-phase reaction at 180—200°C of acetylene and acetic acid over a cadmium, 2inc, or mercury acetate catalyst. However, the palladium-cataly2ed reaction of ethylene and acetic acid has displaced most of the commercial acetylene-based units (see Acetylene-DERIVED chemicals Vinyl polymers). Current production is dependent on the use of low cost by-product acetylene from ethylene plants or from low cost hydrocarbon feeds. 

Ceramic, Metal, and Liquid Membranes. The discussion so far implies that membrane materials are organic polymers and, in fact, the vast majority of membranes used commercially are polymer based. However, interest in membranes formed from less conventional materials has increased. Ceramic membranes, a special class of microporous membranes, are being used in ultrafHtration and microfiltration appHcations, for which solvent resistance and thermal stabHity are required. Dense metal membranes, particularly palladium membranes, are being considered for the separation of hydrogen from gas mixtures, and supported or emulsified Hquid films are being developed for coupled and facHitated transport processes. 

It has been discovered that styrene forms a linear alternating copolymer with carbon monoxide using palladium II—phenanthroline complexes. The polymers are syndiotactic and have a crystalline melting point - 280° C (59). Shell Oil Company is commercializing carbon monoxide a-olefin plastics based on this technology (60). 

Most of the vinyl acetate produced in the United States is made by the vapor-phase ethylene process. In this process, a vapor-phase mixture of ethylene, acetic acid, and oxygen is passed at elevated temperature and pressures over a fixed-bed catalyst consisting of supported palladium (85). Less than 70% oxygen, acetic acid, and ethylene conversion is realized per pass. Therefore, these components have to be recovered and returned to the reaction zone. The vinyl acetate yield using this process is typically in the 91—95% range (86). Vinyl acetate can be manufactured also from acetylene, acetaldehyde, and the hquid-phase ethylene process (see Vinyl polymers). 

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]. 

Chemical reduction is used extensively nowadays for the deposition of nickel or copper as the first stage in the electroplating of plastics. The most widely used plastic as a basis for electroplating is acrylonitrile-butadiene-styrene co-polymer (ABS). Immersion of the plastic in a chromic acid-sulphuric acid mixture causes the butadiene particles to be attacked and oxidised, whilst making the material hydrophilic at the same time. The activation process which follows is necessary to enable the subsequent electroless nickel or copper to be deposited, since this will only take place in the presence of certain catalytic metals (especially silver and palladium), which are adsorbed on to the surface of the plastic. The adsorbed metallic film is produced by a prior immersion in a stannous chloride solution, which reduces the palladium or silver ions to the metallic state. The solutions mostly employed are acid palladium chloride or ammoniacal silver nitrate. The etched plastic can also be immersed first in acidified palladium chloride and then in an alkylamine borane, which likewise form metallic palladium catalytic nuclei. Colloidal copper catalysts are of some interest, as they are cheaper and are also claimed to promote better coverage of electroless copper. 

Industry, however, favours electrodeposited palladium-nickel alloy since it is cheaper than palladium, harder and less prone to cracking, fingerprinting and formation of polymer films Its wear resistance is poor, so it is usually given a thin topcoat of hard (sometimes, soft) gold. ... 

Trost and coworkers137 have reported the polymer-supported palladium catalyzed cyclization of 1, l-bis(phenylsulfonyl)epoxyalkene 235 which gives cycloalkanes 236 and 237 in a 2 1 ratio (equation 143). This method has proven useful for the synthesis of macrocyclic compounds under neutral conditions without using high dilution technique. Temperature and concentrations are critical. The best results are achieved if a reaction mixture of 0.1-0.5 m is added to a preheated (at 65 °C) suspension of the catalyst. 

The coupling reaction of aryl-alkenyl halides with alkenes in the presence of a palladium catalyst and a base is known as the Heck coupling (Scheme 9.4).6 Since the early 1980s, this type of coupling reaction has been used for die syndiesis of poly(arylenevinylene) and related polymers by polymerization of AB- or AA/BB-type of monomers (Scheme 9.5).7... 

At about die same time, die application of the Suzuki coupling, the crosscoupling of boronic acids widi aryl-alkenyl halides in die presence of a base and a catalytic amount of palladium catalyst (Scheme 9.12),16 for step-growth polymerization also appeared. Schliiter et al. reported die synthesis of soluble poly(para-phenylene)s by using the Suzuki coupling condition in 1989 (Scheme 9.13).17 Because aryl-alkenyl boronic acids are readily available and moisture stable, the Suzuki coupling became one of die most commonly used mediods for die synthesis of a variety of polymers.18... 

A mixture of l,4-dibromo-2,5-bis(3-sulfonatopropoxy)benzene 61 (0.78 g, 1.39 mmol), 60 (0.23 g, 1.39 mmol), Na2C03 (0.99 g) in doubly distilled water (47 mL), and DMF (20 mL) was heated at 85°C until the solids were completely dissolved. The resulting solution was cannulated to a 200-mL Schlenk flask with tris[(sulfonatophenyl)phosphine]palladium(0) (0.045 g) and the mixture was stined at 85°C for 10 h. The reaction mixture was concentrated to 25 mL by boiling and filtered. The filtrate was added dropwise to cold acetone (250 mL) to precipitate out the polymer. The polymer was collected by filtration, redissolved in a minimum of hot water, and reprecipitated by cooling. After repeating this procedure twice, the polymer was redissolved in distilled water and dialyzed for 72 h in 3500 gmol 1 cutoff membrane. After drying under vacuum, polymer 63 was obtained in 64% (0.42 g). 

The N-substituted aminoacids required could be prepared by microwave-assisted reductive amination of aminoacid methyl esters with aldehydes, and although in the Westman report soluble NaBH(OAc)3 was used to perform this step, other reports have shown how this transformation can be performed in using polymer-supported borohydrides (such as polymer-supported cyanoborohydride) under microwave irradiation [90]. An additional point of diversity could be inserted by use of a palladium-catalyzed reaction if suitably substituted aldehydes had been used. Again, these transformations might eventually be accomplished using supported palladium catalysts under microwave irradiation, as reported by several groups [91-93]. 

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