Polymer selective catalytic reaction

Figure 8.17 Schematic illustration for selective catalytic reactions provided by the use of protective polymers.

Metal complexes, especially involving transition metals, are known for their role as catalysts in a broad variety of chemical processes including isomerization, oxidization, hydrogenation, and polymerization. Such catalytic reactions play an important role not only in many industrial processes, such as petroleum and polymer industries, but also in many biological systems, e.g., a variety of selective oxidation catalysts with heme (1) and nonheme (2) iron centers. The transition metals in these systems usually constitute a fundamental part of the catalyst, due to their... 

Transition metal-mediated C-C bond formation through reaction of C02 with acetylenes and dienes can serve as a useful method for the construction of various carbon skeletons, such as linear and cyclic carboxylic acids, and esters and lactams. Enantioselective incorporation of C02 can also be achieved, especially when combined with sterically controlled formation of cyclic carbo- or heterocyclic skeletons. In perspective of the future in this area, development of more efficient and more selective catalytic systems for incorporation or transformation of C02 into useful fine chemicals and polymer materials will continue to be an important and attractive research target. 

Examples of catalytic reactions and processes relevant to hydrocarbon chemistry are numerous. The technologies of the oil refinery with extremely low (<0.1) E factors are excellent examples demonstrating the possibilities that can be achieved by the development of selective catalytic methods, particularly by the use of various solid acids (see detailed discussions in Chapter 2). Further examples of commercially highly successful processes are the oxidation catalyst TS-1 developed by Enichem researchers160 161 (see Sections 9.1.1, 9.2.1, and 9.4.1), the homogeneous aqueous-phase Rh-catalyzed hydroformylation (see Sections 7.1.3 and 7.4.1), and single-site metallocene polymerization catalysts, which allow the preparation of tailored polymers with new properties (see Sections 13.3.2).162-164... 

As an example of low-temperature catalytic reactions, hydrogenation of unsaturated hydrocarbons is the most important industrial application. Chemical industrial needs are mainly for unsaturated hydrocarbons, which have reactivities that enable polymer or petrochemical product development. All the processes developed for the production of olefins, diolefins, and aromatics give a mixture of unsaturated hydrocarbons, which are not valuable as such further hydrogenations are necessary to obtain usable products for refining and chemical industry. Sulfur is generally considered to be a poison of hydrogenation catalysts. But in the case of hydrodehydrogenation reactions, this compound can also be used as a modifier of selectivity or even, in some cases, as an activator. 

In the presence of catalysts, heterogeneous catalytic cracking occms on the surface interface of the melted polymer and solid catalysts. The main steps of reactions are as follows diffusion on the surface of catalyst, adsorption on the catalyst, chemical reaction, desorption from the catalyst, diffusion to the liquid phase. The reaction rate of catalytic reactions is always determined by the slowest elementary reaction. The dominant rate controller elementary reactions are the linking of the polymer to the active site of catalyst. But the selectivity of catalysts on raw materials and products might be important. The selectivity is affected by molecular size and shape of raw materials, intermediates and products [36]. 

D 2— —>7—>3—>5 4 Pd(PPh3)4 -polymer (heterogeneous) stable catalytic system but less selective coupling reaction and more reaction steps... 

In Parton et al., a new type of heterogeneous catalyst was proposed consisting of a solid catalyst (iron phthalocyanine zeolite Y) dispersed in a dense PDMS (polydimethylsiloxane) polymer matrix.[l] The system resulted in strongly increased catalytic activities in the oxidation of cyclohexane.[2] Other systems, such as Mn(bipy)2-Y (mangtuiese bipyridine zeolite Y) were also proven to benefit from such incorporation.[3,4] The results presented here using Ti-MCM-41 confirm this for the epoxidation of olefins, an important route for the production of fine chemicals.[5] The influence of the polymer on the reaction activity and selectivity is shown by using different oxidants and solvent conditions in the epoxidation of 1-octene. It will enable the deduction of the advantages and limitations of the reported membrane occluded catalyst system. 

TTie extension of tandem catalysis to polymer chemistry is, however, not trivial. In order to reach high molecular weight polymers, each reaction has to proceed with almost perfect selectivity and conversion. Obviously, combining different catalytic reactions limits the choice of suitable reactions since they must also be compatible with each other. We recently introduced Iterative Tandem Catalysis (ITC), a novel polymerisation method in which chain growth during polymerisation is effectuated by two or more intrinsically different catalytic processes that are both compatible and complementary. If the catalysts and monomers are carefully selected, ITC is able to produce chiral polymers from racemic monomers, as was shown by us for the ITC of 6-MeCL and the DKR polymerisation of sec-diols and diesters. ... 

Molybdenum imido alkylidene complexes also have been employed for a variety of other catalytic reactions of interest to the organic or polymer chemist, among them selective cross-couplings of olefins, [99] polymerization of terminal alkynes, [100-102] step-growth polymerization of dienes, [103,104] and cyclopolymerization of 1,6-heptadiynes. [105-107]... 

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