Ethylene polymerization spectra

Fig. 4 Fast time-resolved spectra of ethylene polymerization reaction on CO-reduced Cr/Si02 sample. Initial ethylene pressure was 10 Torr. Last spectrum after 15 s. Reprinted from [77]. Copyright (1994) by Elsevier...

The electron paramagnetic resonance spectrum of transition metal ions has been widely used to interpret the state of these ions in systems of catalytic interest. Major emphasis has been placed on supported chromia because of its catalytic importance in low-pressure ethylene polymerization and other commercial reactions. Earlier work on chromia-alumina catalysts has been reviewed by Poole and Maclver 146). On alumina it appears that the chromium is present in three general forms the S phase, which is isolated Cr3+ on the surface or in the lattice the 0 phase, which is clusters of Cr3+ and the y phase, which is isolated Cr5+ on the surface. The S and 0... 

When the Cr02Cl2 adsorbed as chromate, such as on silica that had been calcined at 400 °C, normal polymerization activity was observed at 100 °C and a concentration of ethylene of 1.0 mol L-1 in isobutane. Indeed, the activity was nearly identical to that of Cr03/silica activated at 400 °C. The kinetics profile of the polymerization reaction was also the same, as shown in Figure 7. The polymer FILMI, MW, and MW breadth were also almost the same, as was the UV-vis reflectance spectrum. In contrast, the chlorochromate catalysts were not active for ethylene polymerization under these conditions. Thus, the monochromate species... 

The Lewis acidity of aluminoxanes can arise from functionality other than three-coordinate Al centers. Barron et al. found that, in the case of aluminoxane clusters such as 5, three-coordinate aluminum is not a prerequiste for ethylene polymerization activity when combined with zirconocene dimethyl. Coordi-natively saturated aluminoxane 5 reacts exothermically with Cp2ZrMe2 at room temperature to produce, according to the H NMR spectrum, an ethylene polymerization-active tightly ion paired species, Cp2-ZrMe ( Bu)6Al6( 3-0)6Me (74). The driving force for... 

Ethylene copolymerizations with a-olefins relax some of the constraints imposed upon a-olefin homopolymerizations, since while a-olefins insert only at primary palladium alkyl bonds, ethylene will insert at secondary palladium—alkyl bonds as long as they are not adjacent to a tertiary carbon atom. Thus, for an a-olefin with n carbon atoms, the spectrum of the resulting copolymer shows a superposition of resonances for a normal ethylene polymerization with a spectrum containing not only n — 2 branches but also n — 3 (and presumably n — 1) branches, etc. Ethylene can be inserted into any carbon of the a-olefin. 

Fig. 22 Monitoring of ethylene polymerization over 3f/TiBA (Al/Cr = 4) by in-situ ESR spectroscopy (from 220 to 350 K) (a) 220 K, (b) 270 K, (c) 290 K, and (d) 350 K. For determination of the g factor spectrum recorded with TEMPO (g = 2.0058), the three lines of TEMPO are marked with asterisks...

Reduction with ethylene at 100 C gives blue catalysts. On alumina the spectrum is dominated by bands at 17,200 24,900 and 38,600 with a shoulder at 12,800 cm, while on silica the spectrum consists of bands at 16,400 33,800 and a weak band at 10,000 cm. In any case, the amount of Cr is higher on silica than on alumina. The better catalytic performances of Cr/SiOj for ethylene polymerization [8] can be explained by the higher amount of Cr on the silica surface. 

Figure 2 shows the profile of the 27-29 ppm spectral region of three polymers which served as models (1J ) for ethylene propylene rubber. The better agreement between the observed spectrum and the five-parameter model strongly suggests the three-parameter model is less realistic as an explanation for the polymerization mechanism. Table VII compares the observed profiles of EPDM rubbers made with a Ziegler catalyst system. The ratio of... 

Finally the ESR spectrum of Nb(7r-allyl)4/alumina was unaffected by the addition of ethylene gas to the ESR sample tube. It is assumed that polyethylene is produced in this process since polymer can be isolated from larger scale reactions under similar conditions. The accepted mechanism for the ethylene growth reaction postulates a steady-state concentration of a a-bonded transition metal-hydrocarbon species which would be expected to modify the ESR spectrum of the supported complex. A possible explanation for the failure to detect a change in the ESR spectrum may be that only a small number of the niobium sites are active for polymerization. Although further experiments are needed to verify this proposition, it is consistent with IR data and radiochemical studies of similar catalyst systems (41, 42, 43). 

The unique nature of the alkyl attached to titanium in a polyethylene catalyst has been indicated by Gray (80). Methyltitanium trichloride has an infrared spectrum which is unique and different from the bridged or unbridged methyl of methylaluminum chlorides. Although methyltitanium trichloride is not an effective catalyst to polymerize ethylene, this unique character is an indication of a difference which is developed further in the effective polyethylene catalysts. 

Further evidence is the x-ray photoelectron spectrum of the catalyst, which is about the same whether the reduction is carried out in CO or in ethylene (30). However, more positive evidence comes from Baker and Carrick (32) who measured the valence on a catalyst exposed to ethylene at 125°C, a typical polymerization temperature. Within a few minutes they obtained 85-96% conversion to Cr(II). Formaldehyde was the by-product. 

TVy blending with any one of a multitude of additives, PVC can be transformed into a broad spectrum of resins ranging from highly plasticized to impact resistant. The use of polymeric plasticizers has attracted a great deal of attention because they provide superior permanence in physical properties over their low molecular weight counterparts. Recently a terpolymer of ethylene, vinyl acetate, and carbon monoxide was reported to be miscible with PVC (1,2). The system is of interest because blends of PVC and ethylene-vinyl acetate copolymers range from incompatible to miscible, depending on the content of vinyl acetate in the copolymer (3,4,5). We have therefore undertaken x-ray,... 

Finally, Hinderling and Chen used ESMS to screen the activity of a small library of eight Brookhart-type palladium(II) complexes in the solution-phase polymerization of ethylene [62]. The crude reaction mixture was quenched with DMSO, diluted, and electrosprayed in order to analyze the growing polymers chains. Upon CID in the gas phase, the polymer chain was fragmented from the catalyst by /J-hydride elimination, thus facilitating the identification of the most active catalysts in an otherwise dauntingly complex mass spectrum of a polymer mixture. Since this analysis can be performed simultaneously for a whole catalyst library, ESMS was hereby proven the method of choice for an assay of multiple, competitive and simultaneously occurring catalytic reactions. 

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