Olefins chain termination

Chain termination probabilities are readily separated into the individual contributions from each type of chain termination [Eq. (4)]. The net termination to olefins becomes the difference between jSo, the probability of forming an a-olefin, and j8r, the probability that it will readsorb before leaving the catalyst bed. On both Co and Ru catalysts, the net olefin chain termination probability (j8 - j8r) decreases rapidly with increasing carbon number (Figs. 9a and 10a). This termination probability becomes zero for hydrocarbon chains larger than about C25 because olefins disappear from the reactor effluent, even at very short bed residence times. Bed residence times shorter than 2 s (and CO conversions less than about 10%) do not affect chain termination probabilities even for short hydrocarbon chains, because fast convective transport removes a-olefins from the catalyst bed before readsorption occurs. Yet, readsorption within pellets occurs even at short bed residence times, because intrapellet residence times do not depend on space velocity consequently, chain termination to olefins and the olefin content in products still decrease strongly with increasing hydrocarbon size. 

Fro. 13. Site density effects on selectivity and chain termination probability Co/TiOa (catalyst A 11.7 wt% Co, 1.5% Co dispersion, 79 h site-time yield, site density I.O /ig-atom surface Co m catalyst B 12.1 wt% Co, 5.8% Co dispersion, 82 h site-time yield, site density 3.3 /cg-atom surface Co m" 473 K, 2000 kPa, H2/CO = 2.1, <10% CO conversion). (a) C5+ selectivity (b) olefin chain termination probability (c) paraffin chain termination probability. 

Carbon number distribution plots also become linear when olefins readsorb very rapidly (large /3r) or when severe intrapellet transport restrictions (large ) prevent their removal from catalysts pellets before they convert to paraffins during chain termination (Fig. 24, jSr = 100). In this case, chain termination to olefins is totally reversed by fast readsorption, even for light olefins. Chain termination occurs only by hydrogen addition to form paraffins, a step that is not affected by secondary reactions and for which intrinsic kinetics depend only on the nature of the catalytic surface. The product distribution again obeys Flory kinetics, but the constant chain termination probability is given by )8h, instead of po + pH- Clearly, bed and pellet residence times above those required to convert all olefins cannot affect the extent of readsorption or the net chain termination rates and lead to Flory distributions that become independent of bed residence time. 

Straight-chain terminal olefins are more reactive than straight-chain internal olefins, which are more reactive than branched-chain olefins (133). 

Thermal decomposition of LiR eliminates a /6-hydrogen atom to give an olefin and LiH, a process of industrial importance for long-chain terminal alkenes. Alkenes can also be produced by treatment of ethers, the organometallic reacting here as a very strong base (proton acceptor) ... 

If the nucleophilicity of the anion is decreased, then an increase of its stability proceeds the excessive olefine can compete with the anion as a donor for the carbenium ion, and therefore the formation of chain molecules can be induced. The increase of stability named above is made possible by specific interactions with the solvent as well as complex formations with a suitable acceptor 112). Especially suitable acceptors are Lewis acids. These acids have a double function during cationic polymerizations in an environment which is not entirely water-free. They react with the remaining water to build a complex acid, which due to its increased acidity can form the important first monomer cation by protonation of the monomer. The Lewis acids stabilize the strong nucleophilic anion OH by forming the complex anion (MtXn(OH))- so that the chain propagation dominates rather than the chain termination. 

In their early studies, Schwartz and co-workers [5, 80] reported the zirconocene hydrido chloride [Cp2Zr(H)Cl] (1) as a reagent capable of reacting under mild conditions with a variey of non-functionalized alkenes to form isolable alkylzirconi-um(lV) complexes Cp2Zr(R)Cl in which the zirconium is attached to the least-hindered terminal primary carbon, irrespective of the original location of the double bond in the olefin chain. As an example, at room temperature in benzene, 1-octene, cis-4-octene and trows-4-octene all yield the n-octylzirconocene derivative (Scheme 8-6) [80]. 

In contrast to the failure to metathesize terminal olefins, internal olefins such as cis-2-pentene can be metathesized to the extent of 50 turnovers. The chain terminating reaction in this case is rearrangement of intermediate ethylidene and propylidene complexes (equation 4). Both rearrangement of intermediate trisub-... 

The reaction rates of various types of olefins follow much the same pattern with both cobalt- and rhodium-catalyzed systems. Wender and co-workers (47) classified the nonfunctional substrates as straight-chain terminal, internal, branched terminal, branched internal, and cyclic olefins. The results they obtained are given in Table III. 

Some significant observations can be made from these results. Straight-chain terminal olefins are the most reactive. Little if any difference exists between 2- and 3-internal, linear olefins. Branching is important only if present at one or more of the olefinic carbon atoms reaction becomes more difficult as branching increases. Cyclic olefins react in an irregular fashion, but all are less reactive than terminal, linear olefins. 

The observed methane generation points to a plausible I —> III or II - III transformation, but it does not distinguish which of the structures (II or III) is the metathesis-active carbene. This matter is mechanistically significant with regard to the chain termination process. Type III may terminate by a bimolecular dimerization sequence as in Eq. (11), or it may convert to a 7r-olefin complex via an uncommon 1,2-hydride shift ... 

Brookhart and coworkers [1] have recently developed Ni(II) and Pd(II) bis-imine based catalysts of the type (ArN=C(R)-C(R)=NAr)M-CH3+ (la of Figure 1) that are promising alternatives to both Ziegler-Natta systems and metallocene catalysts for olefin polymerization. Traditionally, such late metal catalysts are found to produce dimers or extremely low molecular weight oligomers due to the favorability of the P-elimination chain termination process [2],... 

In this proposed process, p-hydride elimination first yields a putative hydride olefin rc-complex. Rotation of the -coordinated olefin moiety about its co-ordination axis, followed by reinsertion produces a secondary carbon unit and therefore a branching point. Consecutive repetitions of this process allows the metal center to migrate down the polymer chain, thus producing longer chain branches. Chain termination occurs via monomer assisted p-hydrogen elimination, either in a fully concerted fashion as illustrated in Figure 2b or in a multistep associative mechanism as implicated by Johnson1 et al. 

Figure 4. Structures resulting from ethylene insertion and chain termination due to the generic catalyst (HN=C(H)-C(H)=NH)PdC3H7+. Ethylene complex (3a) insertion transition state (TS[ 3a-4a]) termination transition state (TS[3a-5a) new olefin product from termination process (5a).

Propene- and butene-oligomers are complex mixtures. A typical isomer distribution is shown in Fig. 24. According to the thermodynamical stability the double bonds are distributed along the chain, terminal double bonds are present only in traces. To get predominant terminal products, a catalyst must provide extremely fast terminal hydroformylation activity for the traces of terminal olefins, a high isomerization activity to supply the terminal double bonds as fast as they are consumed, and low hydroformylation activity for internal double bonds. 

The overall energy barrier for chain growing is lower than that for chain termination, thus, if a lower molecular weight product is desired, the copolymerisation should be carried out at higher temperature. In the copolymerisation process, the insertion of ethene is the slow step [13-15], thus upon increasing the pressure of the olefin, as well as the total pressure, it is reasonable to expect an increase in the molecular weight. The nature of the... 

A number of heterogeneous systems have been developed for oxidation reactions using H2O2 as oxygen source . In 1981, Taramasso, Notari and collaborators at Enichem opened new perspectives in this field with the discovery of the Ti-silicalite (TS-1) ° , a new synthetic zeolite of the ZSM family. In the TS-1 zeolite, titanium atoms are located in vicariant positions in the place of Si atoms in the crystalline framework . The remarkable reactivity of TS-1 is likely ascribable to the site-isolation of tetrahedral Ti(IV) in a hydrophobic environment. TS-1 has proved to be an efficient catalyst for the epoxidation of unfunctionalized short-chain olefins, especially terminal ones (equation 28). In addition, polyunsaturated compounds are mainly converted into the mono epoxides (equation 29). 

There are two reasonable paths for chain termination. Under the conditions used to liberate the olefin in the Aufbau process (high temperature, low ethene pressure), the main termination reaction is j -elimination [2]. 

At lower temperatures (or in solution) and at high monomer concentration, a second chain termination process that could occur is direct j -hydrogen transfer to a second molecule of monomer. This kind of chain transfer step is now generally accepted for many transition-metal-catalyzed polymerizations, where direct /1-elimination would be too much uphill to explain the observed molecular weights, for olefin oligomerization at aluminium, a similar situation applies. Since insertion and j -hydrogen transfer have an identical concentration dependence, their ratio does not depend much on the reaction conditions (except temperature) and hence limits the molecular weight attainable in the Aufbau reaction. 

Since the migration reaction is always toward the end of a chain, terminal olefins can be produced from internal ones, so the migration is often opposite to that with the other methods. Alternatively, the rearranged borane can be converted directly to the olefin by heating with an alkene of molecular weight higher than that of the product (7-15). Photochemical isomerization can also lead to the thermodynamically less stable isomer.72... 

Straight-chain terminal olefins react most readily in hydroformylation. Internal alkenes exhibit decreased reactivity, whereas branched olefins are the least reactive.6,11,43 Rhodium modified with phosphite ligands, however, was shown to... 

The coordination polymerization of ethylene and a-olefins with Ziegler-Natta catalysts involves, in general, many elementary reactions, such as initiation (formation of active centers), chain propagation, chain transfers and chain terminations. The length of growing polyolefin chains is limited by the chain-terminating processes, as schematically represented (for ethylene) by 21,49 51)... 

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