Supported Polyethylene Catalysts

While the control resins were deep red in color due to the presence of soluble porphyrin complexes, the methacrylate resins obtained after removal of the polyethylene-supported catalysts varied from light yellow to nearly water-white (APHA < 25). UV-Vis spectrophotometric analysis of the yellow resins indicated an absorption signal for the cobalt porphyrin complex Soret band (wavelength of cobalt(ll) porphyrin species appears at -415 nm free porphyrin ligand is formd at -423 tun). Resin samples that visttally appear as water-white show little or no porphyrin species present in the spectrum. Measured catalyst activity and PDl of the polyethylene-supported porphyrin complexes are in the expected range for soluble porphyrin CCT catalysts (PDl = M /Mn - 1.2- 2.0)." The screening resrrlts clearly... 

Polyethylene-supported catalysts that are initially insoluble but that become soluble on heating and are separated as insoluble materials on cooling are also used as catalysts in polymerization reactions. Infact, this was the first way a polyethylene-bound catalyst was used (Eq. 11) [34]. However, soluble polymers used in this manner appear to have several deficiencies. First, separation of the products from the catalyst may notalways be as simple as was the case with catalysts like 11 or 12 and low molecular weight products. For example, while a hot solution of a polyethylene-bound neodymium salt was successfully used in the stereoselective polymerization of butadiene to form high molecular weight (Z)-poly( 1,4-butadiene), the product mixture after cooling was a thick, viscous sus-... 

These siUca-supported catalysts demonstrate the close connections between catalysis in solutions and catalysis on surfaces, but they are not industrial catalysts. However, siUca is used as a support for chromium complexes, formed either from chromocene or chromium salts, that are industrial catalysts for polymerization of a-olefins (64,65). Supported chromium complex catalysts are used on an enormous scale in the manufacture of linear polyethylene in the Unipol and Phillips processes (see Olefin polymers). The exact stmctures of the surface species are still not known, but it is evident that there is a close analogy linking soluble and supported metal complex catalysts for olefin polymerization. 

Polymerization. Supported catalysts are used extensively in olefin polymerization, primarily to manufacture polyethylene and polypropylene. Because propylene can polymerize in a stereoregular manner to produce an isotactic, or crystalline, polymer as well as an atactic, or amorphous, polymer and ethylene caimot, there are large differences in the catalysts used to manufacture polyethylene and polypropylene (see Olefin polymers). 

Large yields of polymer seem to be obtained only when polymerization proceeds on the outer catalyst surface, because the transport of high molecular polyethylene from catalyst pores is impossible (112). The working part of the specific surface of the catalyst can be expected to increase with diminishing strength of links between catalyst particles (112). Therefore, to obtain a highly active catalyst a support with large pore volume should be used (e.g. silica with pore volume >1.5 cm8/g). 

While cobaloximes are the most active CCT catalysts known, they are sensitive to air and moisture, so the more robust porphyrins were considered a better choice for a recoverable catalyst. To better understand how to design an optimal thermomorphic catalyst we initiated an investigation to learn how polyethylene length, number, and the covalent linker influence the catalyst activity and the final color of the methaciylate resin. A series of polyethylene-supported CCT catalysts were thus prepared for study (Scheme 36.2). 

Scheme 36.2. Polyethylene-supported cobalt porphyrin catalysts.

Scheme 36.3. Preparation of polyethylene-supported cobalt phthalocyanine catalyst.

We have demonstrated a new class of effective, recoverable thermormorphic CCT catalysts capable of producing colorless methacrylate oligomers with narrow polydispersity and low molecular weight. For controlled radical polymerization of simple alkyl methacrylates, the use of multiple polyethylene tails of moderate molecular weight (700 Da) gave the best balance of color control and catalyst activity. Porphyrin-derived thermomorphic catalysts met the criteria of easy separation from product resin and low catalyst loss per batch, but were too expensive for commercial implementation. However, the polyethylene-supported cobalt phthalocyanine complex is more economically viable due to its greater ease of synthesis. 

Soluble polymers that have been used in hquid-phase methodologies are listed in Fig. 5.1 [3, 7, 8, 34, 35]. Polyethylene glycol and non-cross-linked polystyrene are some of the most often used polymeric carriers for organic synthesis and have found frequent use in the preparation of soluble polymer-supported catalysts and reagents consequently, a brief discussion of these polymers is warranted. 

Me) was R = Me > Bu > Pr-opposite to that observed for the heterogeneous system. In addition, the supported catalysts produced polyethylene with higher molecular weights, narrower molecular weight distributions and higher bulk densities than that produced by the homogeneous systems. 

In a similar manner to the [4+2] cycloaddition, Benagha has shown that a polyethylene glycol supported imidazolidinone leads to similar levels of enantioselec-tivity in the [3+2] cycloaddition of nitrones with a,P-unsaturated aldehydes when compared to a non-supported catalyst, however, significantly lower chemical yields were obtained, which deteriorated upon catalyst recycling [65]. 

Substrate selectivity effects were investigated with polystyrene-supported polyethylene glycol) catalysts 51 (n = 3,61% RS) and 56 (Z = phenyl, n = 30, 8 % RS) under solid/solid/liquid conditions 180). The difference in rates of reaction of 1-bromobutane and 1-bromooctane with solid potassium phenoxide was a factor of about 3 (51 was more active). Measurements of the distribution of both bromides between... 

Systems have been developed that allow the recycling of catalysts. The first case study involved simple adsorption of proline onto silica gel [6], but the system suffered from a loss in enantioselectivity. More recently, promising results have been obtained with fluorous proline derivatives [64] used for aldol reactions the recycling of fluorous catalysts has been demonstrated using fluorous solid-liquid extraction. Solid phase-supported catalysts through covalent bonds [65] and through noncovalent interactions [66] were also used for aldol reactions. Proline and other catalysts can be recycled when ionic liquids or polyethylene glycol (PEG) were used as reaction solvents [67]. 

Metallocene supported catalysts are not limited only to those involving inorganic catalyst carriers. Starch [212] and a-cyclodextrin [213] have also been successfully used as supports for zirconocene-AlMe3 [212] and zircono-cene-fA Me b [213] catalysts capable of producing high molecular weight polyethylene. 

As early as in 1985, supported catalysts were described for the use in a solution process [399]. Bergbreiter et al. used catalyst supports on the basis of divinylbenzene-styrene-copolymers as well as on polyethylene. These authors found that the use of supported catalysts has no influence on the stereospecificity of diene polymerization. 

Energx A process for making LLDPE (linear low-density polyethylene). Developed by Eastman Chemical in the 1990s and used at its plant in Longview, TX. Licensed to Chevron Chemical in 1999 for use at its plant in Baytown, TX. By 2002, licenses had been granted in Europe, North America, and Asia. A variation, Energx-DCX, uses a supported catalyst (Sylopol DCX) made by W.R. Grace. The polyethylene products have the trade name Hifor. 

While the passage from the discovery of the supported catalysts to their industrial use was quite rapid for polyethylene, it was much slower for polypropylene because... 

Gas phase polymerizations, using other supported catalysts, are also employed to make isotactic polypropylene, with productivities of the same order as those reported for polyethylene manufacture. 

Phosphine free catalysts and halogen-free reactions are known for the Heck reaction. Improvements on the palladium catalyst system are constantly being reported, including polymer-supported catalysts." °° The influence of the ligand has been examined." Efforts have been made to produce a homogeneous catalyst for the Heck reaction." The Heck reaction can be done in aq. media," ° in perfluori-nated solvents," in polyethylene glycol," ° in neat tricaprylmethylammonium... 

Catalysts prepared by supporting TiCl on silica or polyethylene powder and activated by organoaluminium compounds before polymerization have an activity which is 10 times as high as that of bulk d-TiCls x 0.3 AICI3 (Table 1). This increase is due to the increase of the number of ACj. As it has been shown the dispersed surface phase of TiCls is an active component of these supported catalysts. 

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