Polymers as catalyst supports

M.C. Lefebvre, Z. Qi, and P.G. Pickup, Electronically conducting proton exchange polymers as catalyst supports for proton exchange membrane fuel cells, J. Electrochem. Soc., 146, 2054-2058 999). 

Poly(styrene—divinylbenzene) copolymers can be used as catalyst supports. Attachment of catalytic groups to the polymer supports can be achieved by... 

Modification of polymers is a topic in polymer science, because new highly valued or improved applications often require sophisticated chemical structures along the polymer chains. One of such timely domains of interest comprises the development of modified polymers as catalysts for chemical processes. Of course, we do not have in mind catalysts, wherein polymers function as inert supports for the active centers and nomore. In fact, our aim is to develop polymeric catalysts, which combine advantages of the other type of catalysts, viz. 

Carbon is unique among chemical elements since it exists in different forms and microtextures transforming it into a very attractive material that is widely used in a broad range of electrochemical applications. Carbon exists in various allotropic forms due to its valency, with the most well-known being carbon black, diamond, fullerenes, graphene and carbon nanotubes. This review is divided into four sections. In the first two sections the structure, electronic and electrochemical properties of carbon are presented along with their applications. The last two sections deal with the use of carbon in polymer electrolyte fuel cells (PEFCs) as catalyst support and oxygen reduction reaction (ORR) electrocatalyst. 

More recently, the scope of using hyperbranched polymers as soluble supports in catalysis has been extended by the synthesis of amphiphilic star polymers bearing a hyperbranched core and amphiphilic diblock graft arms. This approach is based on previous work, where the authors reported the synthesis of a hyperbranched macroinitiator and its successful application in a cationic grafting-from reaction of 2-methyl-2-oxazoline to obtain water-soluble, amphiphilic star polymers [73]. Based on this approach, Nuyken et al. prepared catalyticaUy active star polymers where the transition metal catalysts are located at the core-shell interface. The synthesis is outlined in Scheme 6.10. 

These are long chain molecules consisting of multiples of repeat units (monomers). These are linked by covalent bonds in a three-dimensional network which is characteristic of a polymer. The magnitude of the length of a polymeric molecule can extend up to several hundred nanometres. The dimensions of individual polymer molecules and their arrangement define the structure of polymers and their properties. Many catalytic processes are aimed at producing polymers as we describe in the following chapters. (Polymers can also be used as catalyst supports.)... 

The structural perfection of a dendritic support is not required for every application in catalysis, and hyperbranched polymers provide interesting and cheap alternatives as catalyst supports [37]. These hyperbranched polymers are obtained from a simple one-pot synthesis, yielding globular polymeric structures with broad weight distributions compared to their dendritic analogues. 

Ceramic foams can be inserted into microstructure devices made from metals and polymers to enhance the surface area, act as catalyst supports or even work as heaters. Details of these processes can be found in Refs [29-42]. 

In reactions with polymer-bound catalysts, a mass-transfer limitation often results in slowing down the rate of the reaction. To avoid this disadvantage, homogenous organic-soluble polymers have been utilized as catalyst supports. Oxazaborolidine 5, supported on linear polystyrene, was used as a soluble immobilized catalyst for the hydroboration of aromatic ketones in THF to afford chiral alcohols with an ee of up to 99% [40]. The catalyst was separated from the products with a nanofiltration membrane and then was used repeatedly. The total turnover number of the catalyst reached as high as 560. An intramolecularly cross-linked polymer molecule (microgel) was also applicable as a soluble support [41]. 

Polymers, especially poly(styrene-divinylbenzene), have been applied often as catalyst supports, providing the means for using well-defined catalytic groups in a phase separate from that holding the reactants and thereby minimizing the difficulties of product purification and corrosion associated with homogeneous catalysis. Polymers offer several advantages as catalyst supports (1) they are easily functionalized, especially when... 

Metal Species Supported on Organic Polymers as Catalysts for the Epoxidation of Alkenes Ulrich Arnold ... 

Despite the advantage of their easy separation, the use of conventional insoluble polymer-supported catalysts often suffered from a reduced catalytic activity and stereoselectivity, due either to diffusion problems or to a change of the preferred conformations within the chiral pocket created by the ligand around the metal center. In order to circumvent these problems, a new class of crosslinked macromolecule-namely dendronized polymers-has been developed and employed as catalyst supports. In general, two types of such solid-supported dendrimer have been reported (i) with the dendrimer as a hnker of the polymer support and (ii) with dendrons attached to the polymer support [12, 113]. 

Organic polymers offer several advantages as catalyst supports ... 

The subject of soluble polymers as supports in catalysis has been discussed in a number of recent reviews [ 1-5]. These other reviews each focused on a particular polymer or groups of polymers or on some aspect of catalysis (e.g., organic catalysis or asymmetric synthesis) [2, 3, 6-8]. Soluble polymers use as supports in synthesis has also been reviewed, but this topic is not covered below because in synthesis the polymer is used in a stoichiometric amount and is generally not recyclable [5,9-11]. This review takes a general approach, focusing on soluble polymers used as catalyst supports. It discusses these supports within a context of the separation strategies that could be or were used to separate or recover the soluble polymer-bound catalyst from the products. This review emphasizes examples from the last few years where soluble polymers are used but includes, for completeness, earlier examples if a particular... 

Dendrimers are very attractive macromolecules with potential applications as catalyst supports. They are attractive both because they are discrete molecular species versus a mixture of molecules with varying degrees of polymer-... 

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