An annual production of approximately 46 million metric tons exemplifies the industrial importance of polyolefins . Combinatorial programs for catalysis and materials can be utilized for both discovery and optimization purposes. The discovery process involves, for example, the identification of catalyst systems for novel copolymerizations, whereas in the optimization process the influence of new ligands, metals, activators, alkylators, solvents, and temperature is tested.
Recent reports have shown that certain late transition metal diimine-based catalyst systems exhibit olefin polymerization activities similar to those reported for commercially employed early-metal single-site metallocene-based systems [131— 133]. These new systems have sparked considerable interest in the polyolefins industry because of their high activity, ease of synthesis and handling, and tolerance toward functionalized olefins such as methacrylate and vinyl acetate. Over the past few years, an increasing effort has been expended, in both academic and industrial research laboratories, towards the discovery of new olefin polymerization catalysts differing dramatically from the forefront group 4 metallocenes and half-sandwich titanium amide catalysts . Most commercial-scale polyolefin processes employ high-surface area supports for immobilizing olefin polymerization catalysts, and only few reports have appeared examining the use of polystyrene as a catalyst support .
The researchers Powers, Murphy and colleagues at Symyx Technologies have developed a parallel synthesis and screening protocol for a polymer-bound 96-member library of 1,2-diimine-transition metal complexes (Scheme 32.30) [136, 137].
The key intermediate, a resin-bound diketone, was converted in a titanium-mediated condensation with 48 commercially available anilines with varying steric and electronic substituents to furnish a 48-member 1,2-diimine library. In these catalytic systems both substituent topography and electronic perturbation have been reported to play a dramatic role in catalyst activity, molecular weight, and yield of the polymer . Splitting of the ligand library followed by conversion into the corresponding 48 Ni(II) or Pd(II) complexes with (DME)NiBr2 or (COD)PdMeCl, respectively, afforded a polymer-bound 96-member library of 1,2-diimine-transition metal complexes. In order to compare performance of the resin-bound catalysts with that of the corresponding catalysts in solution, a corresponding 1,2-diimine library based on a related diketone framework was synthesized . To efficiently complex the solution-phase 1,2-diimine library with Ni(II) and Pd(II), metal-delivery agents (MDAs), a novel class of polymeric reagents, were used as metal ion sources . After activation of the Ni(II) or Pd(II) catalyst precursors (1,2-diimine complexes) with MAO (methylalumoxane) or sodium tetrakis-(3,5-bistrifluoromethyl) phenyl borate (Na[(3,5-(F3C)2C6H3)4B]), respectively, a custom high-pressure par-
Ni" or Pd"-Sources
Polyethylene Granula with Imbedded Catalyst Bead Scheme 32.30. Synthesis of Brookhart-type polymer-bound Ni(II) or Pd(II) 1,2-diimine complexes and their use in the polymerization of ethylene. M = Ni(II), X = Y = Br; M = Pd(II), X = Me, Y = Cl; R, alkyl, aryl,
Ni" or Pd"-Sources
^^ = Polystyrene Resin
^^ = Polystyrene Resin heteroaryl, halogen, functional groups; tag, chemical code; MAO, methylalumoxane; ArF, 3,5-(F3C)2C6H3; dme, dimethoxyethane; cod, 1,5-cyclooctadiene.
Polyethylene Granula with Imbedded Catalyst Bead Scheme 32.30. Synthesis of Brookhart-type polymer-bound Ni(II) or Pd(II) 1,2-diimine complexes and their use in the polymerization of ethylene. M = Ni(II), X = Y = Br; M = Pd(II), X = Me, Y = Cl; R, alkyl, aryl, allel polymerization reactor with a modular series of 48 reaction chambers was used to screen for ethylene polymerization. The device was equipped with individual ethylene pressure controls, and reactants were loaded using a three-axis liquid-handling robot (Fig. 32.14) .
Compared with the corresponding free complexes screened under identical conditions in solution, catalyst performance consistently proved to be decreased for the on-bead Ni(II) catalysts and increased for the solid support-bound Pd(II) complexes. For example, assaying the isolated discrete polyethylene granules by rapid high-temperature gel permeation chromatography, molecular weights (MWs) up to 59,000 g mol-1 were found for resin-bound Ni catalysts and up to 213,000 g mol-1 for the corresponding diimine-Ni catalysts in solution. Assuming that catalyst performance is proportional to the "growth" of the polystyrene support beads (2-10 times from the initial diameter of 70 mm), visual inspection of the beads allowed a distinction between Ni(II)- and Pd(II)-derived catalysts in mixed assays (Fig. 32.15). These results were confirmed by a chemical encoding/deconvolution strategy with cleavable tertiary amine tags, followed by HPLC analysis.
The present work demonstrates the feasibility of applying a multitude of combinatorial techniques, including solid-phase synthesis, on-bead screening, and the encoding/deconvolution of pooled libraries of catalysts, for the discovery and optimization of new olefin polymerization catalysts. Moreover, the technology appears to be suitable for catalytic processes other than ethylene polymerization. In this
respect, Symyx Technologies has developed general methodologies for the combinatorial synthesis, high-throughput screening, and characterization of libraries of supported and unsupported organometallic compounds and catalysts [139a, 140]. For the discovery of novel polymerization catalysts, libraries of ligands in combination with various metals are screened in the presence of different monomers. In order to optimize the yield and selectivity of a given organometallic complex in a polymerization reaction, a variety of factors are tested in a high-throughput fashion, including the form of the ancillary ligand precursor, the choice of the metal precursor, the reaction conditions (e.g. solvent, temperature, time), and the stability of the desired product.
Mullen and coworkers tagged silica- or polymer-supported heterogeneous catalysts for industrial olefin polymerization with fluorescent dyes . Here, direct detection of the different product beads obtained by different catalysts is available through fluorescent dyes that exhibit different emission wavelengths. The approach starts with producing the tagged catalysts by supporting various metallocenes with silica, activating them with MAO, and labeling them with different rylene dyes (Fig. 32.16). The dyes were chosen because of their high chemical stability, their high tendency to physisorb on silica, their high fluorescence yield, and because of the fact that they do not influence the polymerization. Also, a great variety of rylene dyes with different emission wavelengths covering the entire visible spectrum is available. These labeled catalysts are then mixed and introduced in a single polymerization vessel. During the olefin polymerization each catalyst particle forms only one product granulate through a particular growth process and can be considered as a microreactor. To assign the different compounds of the product mixture to the employed catalyst, the polymer products are exposed to UV light, and can be directly identified, manually separated, and characterized because of the different emission wavelengths of the labels. The authors could demonstrate the feasibility of this concept for the polymerization of ethylene as well as for the co-polymerization of ethylene/hexene.
Fig. 32.16. Catalysts and dyes for the synthesis of tagged, supported catalysts.
Fig. 32.16. Catalysts and dyes for the synthesis of tagged, supported catalysts.
Coates and coworkers used a pooled polymerization catalyst strategy to identify catalysts for the synthesis of syndiotactic polypropylene . The basis of this concept is that the formed polymer itself serves as a stereochemical recording of the events of the polymerization catalyst. Assuming that the catalyst species do not react with one another, then a group of complexes for stereoselective polymerization can be screened simultaneously. When the desired polymer product has distinguishing chemical or physical properties, techniques such as solubility, spectroscopy, or chromatography can be used to quickly probe the crude product of a pooled polymerization reaction to see if a noteworthy catalyst is present. To probe this concept, a library of 12 salicylaldiminato ligands was synthesized separately by the condensation of three different salicylaldehydes and four amines (Scheme 32.31). Equimolar amounts of these ligands were combined, deprotonated with
Scheme 32.31. Synthesis of a library of salicylaldiminato titanium complexes.
BuLi, and reacted with 0.5 equiv. of TiCl4 to give a library of 78 possible titanium species. This complex library was then activated with MAO ([Al]/[Ti] = 100] in toluene and the resulting catalyst solution was exposed to propylene (2.7 atm.). Even though 90% of the formed polymer could be washed away in refluxing diethyl ether (atactic polypropylene), the remaining 10% of polymer was found to be syndiotactic polypropylene (13C-NMR microstructural analysis). Deconvolution methods, i.e. the synthesis and testing of sublibraries, were used to identify successfully the most active catalyst.
Hinderling and Chen reported the use of electrospray ionization tandem mass spectrometry (ESI-MS/MS) and gas-phase ion molecule reactions for the rapid screening of Brookhart-type Pd(II) olefin polymerization catalysts . A test library of eight catalysts was prepared by reacting eight diimine ligands with [(cod)Pd(CH3)(Cl)], washing and drying ofthe complex, and activation with AgOTf (Scheme 32.32). An electrospray mass spectrum of the mixture of complexes showed that all eight catalysts are present in similar concentrations. The mixture
Scheme 32.32. Rapid screening of Brookharttype Pd(II) olefin polymerization catalysts by ESI-MS/MS. (i) [(cod)Pd(CH3)(Cl)], then AgSO3CF3; (ii) excess ethylene, then quenching with DMSO; (iii) electrospray under mild
desolvation conditions to give polymeric ions (and loss of DMSO); (iv) reject all ions below a certain mass and subject the remaining highmass ions to collision with Xe to induce b-hydride elimination.
Scheme 32.32. Rapid screening of Brookharttype Pd(II) olefin polymerization catalysts by ESI-MS/MS. (i) [(cod)Pd(CH3)(Cl)], then AgSO3CF3; (ii) excess ethylene, then quenching with DMSO; (iii) electrospray under mild desolvation conditions to give polymeric ions (and loss of DMSO); (iv) reject all ions below a certain mass and subject the remaining highmass ions to collision with Xe to induce b-hydride elimination.
was then dissolved in dichloromethane, saturated with ethylene, reacted for 1 h, and quenched with dimethyl sulfoxide (DMSO). The electrospray mass spectrum of the crude reaction mixture was rather complex and showed multiple, overlapping series of oligomeric and polymeric ions corresponding to each catalyst species with between 0 and @100 ethylene units added. After selection of ions with m/z > 2200, these high-mass ions were collided with xenon to induce b-hydride elimination. From the corresponding daughter ion spectrum the structure of the best catalyst can be derived. Similarly, the daughter ion spectrum with a lower mass cut-off shows the next best catalysts. The advantages of this methodology are sensitivity, speed, direct assay, versatility, and possible automation.
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