Olefins unstable, polymerization

Sterically hindered Zr and Ti chelated phenoxide complexes represent a new class of homogeneous olefin oligomerization/polymerization catalysts when combined with cocatalysts such as MAO and FAB (eq 36).Spectroscopic investigations of the reaction between the Zr dibenzyl complex with FAB in toluene reveals the formation of the corresponding cationic complex associated with a benzylborate anion via Ph coordination (64 eq 36). Similar findings were obtained from bis(o-arylphenoxide)M(CH2Ph)2 complexes (M = Zr, Ti),2° while the corresponding dimethyl complexes yield unstable species after FAB activation. The products mediate the polymerization of ethylene and propylene. 

The discussed reactions of carbene and carbyne complexes show that they have essential significance as catalysts or unstable transient intermediate compounds in such catalytic processes as metathesis of olefins and other unsaturated compounds, Fischer-Tropsch synthesis, syntheses of cyclopropanes from diazoalkanes and olefins, and polymerization of olefins and alkynes as well as in organic synthesis. Except for alkynes [reaction (5.132) ] some compounds containing double bonds react with carbon monoxide and carbene ligands to form bonds with those groups. Examples of such compounds are enamines, ynamines, and Schiff bases. The JV-vinylpyrrolidone (enamine), methoxyphenylcarbene, and excess of CO (higher pressure) react to furnish enaminoketone. 

Primary radicals are unstable, lowest members such as dimet peroxide are shock sens and dangerous expls sensitivity lessens with increasing mw polymeric peroxides (copolymers of olefins and 02) explode on heating... 

Nitrile oxides are usually prepared via halogenation and dehydrohalogenation of aldoximes [11] or via dehydration of primary nitro alkanes (Scheme 1) [12]. However, it is important to note that nitrile oxides are relatively unstable and are prone to dimerization or polymerization, especially upon heating. 1,3-Dipolar cycioaddition of a nitrile oxide with a suitable olefin generates an isoxazoline ring which is a versatile synthetic intermediate in that it provides easy access to y-amino alcohols, )5-hydroxy ketones, -hydroxy nitriles, unsaturated oximes, and a host of other multifunctional molecules (Scheme 1) [5a]. Particularly for the formation of )5-hydroxy ketones, nitrile oxide-olefin cycioaddition serve as an alternative to the Aldol reaction. 

As a typical case, olefin-metal complexation is described first. Alkene complexes of d° transition metals or ions have no d-electron available for the 7i-back donation, and thus their metal-alkene bonding is too weak for them to be isolated and characterized. One exception is CpfYCH2CH2C(CH3)2CH=CH2 (1), in which an intramolecular bonding interaction between a terminal olefinic moiety and a metal center is observed. However, this complex is thermally unstable above — 50 °C [11]. The MO calculation proves the presence of the weak metal-alkene bonding during the propagation step of the olefin polymerization [12,13]. 

The decomposition of aliphatic peroxides produces oxygen radicals too unstable for paramagnetic measurement. These radicals initiate the polymerization of olefins and give the complex mixtures of decomposition products associated with radical mechanisms. On the other hand, aliphatic peroxides are also capable of polar decomposition reactions, a subject to be taken up in Chapter VIII. The characteristic reactions of the less stable oxygen free radicals are /3-cleavage to form... 

Olefins are hydrocarbon compounds with at least two carbon atoms and having a double bond. Their unstable nature and tendency to polymerize makes them one of the very important building blocks for the chemical and petrochemical industry (Gary and Handwerk, 1994). Although olefins are produced by fluid catalytic cracking in refineries, the main production source is through steam cracking of liquefied petroleum gas (LPG), naphtha or gas oils. 

The interaction of norbornene with iron, chromium, molybdenum, and tungsten atoms affords highly unstable and uncharacterized systems of polymeric materials (102, 134). The use of other strained olefins, such as /ran.v-cyclooctene, in metal atom syntheses has yet to be explored. 

Because of the hypernucleofugality of A3-iodanyl groups, alkyl-A3-iodanes are generally unstable and can exist only as short-lived species. For instance, oxidation of iodomethane with dimethyldioxirane in acetone at -78°C produces polymeric iodosylmethane as a pale yellow precipitate (See Sect. 3.4.2) but it decomposes to hypoiodous acid and methanol even at -40°C, probably via nucleophilic substitution by water [Eq. (21)] [46]. Hypoiodous acid is trapped by olefins to give iodohydrines. 

Most of these compounds show a limited solubility in non-polar solvents. In addition, the respective alkyl derivatives are rather unstable in solution and decompose easily [297]. The peculiarity about CpNd complexes is their ability to polymerize various alkenes such as a-olefines, styrene, c -dienes as well as polar acrylates [298,299]. 

This parallelism is reflected in the proposed mechanism for the ionization of methane which shows that (a) the second step of the scheme invoives attack of an ethyl cation on methane, but the reaction cannot stop there, and goes on to (b), the third step, which involves attack of a secondary-isopropyl cation on methane. The primary and secondary alkyl cations are very strongly acidic species and are unstable under the reaction conditions. The condensation reaction essentially terminates with the much more weakly acidic tertiary-butyl ion. Alkane polycondensation and olefin polymerization side reactions producing stable, less acidic, tertiary ions obscured the simple alkylation reactions of the primary and secondary alkyl cations. Implicit in this mechanism, however, is that it is possible to react an acidic energetic primary cation (such as the ethyl cation) with molecules as weakly basic as methane and thus, the door was opened to new chemistry through activation of the heretofore passive, weakly basic, "paraffins" (20-24). 

The diesel fuel has 10% a-olefin content (i.e. terminal unsaturation or double bonds) which make the fuel unstable and prone to polymerization (i.e. sludge formation). There is no proven track record for producing transportation-grade diesel from the Smuda Process (Poland plant produces crude oil from plastics which is subsequently sent to a refinery). 

Dialkyl peroxides (1), R-O-O-R (R and R are = or primary, secondary, tertiary alkyl, cycloalkyl, aralkyl and heterocyclic radicals Homolytic decompn when heated or irradiated with prodn of free radicals for org synthesis difficult to hydrolyze and reduce rearrangement crosslinking and polymerization polymeric peroxides are thick liqs or amorph wh powds used as polymerization catalysts Primary radicals are unstable, lowest members such as dimet peroxide are shock sens and dangerous expls sensitivity lessens with increasing mw polymeric peroxides (copolymers of olefins and Oj) explode on heating... 

Strong bases (pKa > 11) also convert alkyl cobaloximes and alkyl cobalamins into -complexes such as 73. This is usually followed by further decomposition to olefins and alkanes. The stability of complexes such as 73 depends very much upon X and the nature of the axial ligand in the cobalt chelate.98-218-227 230 Strong nucleophiles such as RS or CN can cause decomposition of LCo—R as well.98-231 Under the normal conditions of radical polymerization, Markovnikov organocobaloxime should form whenever the hydride, LCoH, appears in the polymerization mixture. If 1,2-vinylidene monomers are being polymerized, then thermally unstable tert-alkyl-cobaloximes are obtained. These species are expected to undergo homolytic Co—C cleavage to yield tertiary radicals. 

Polymerization also combines two light olefins to produce a high-octane gasoline component. The process employs a fixed catalyst bed at low temperatures and pressure. The process is relatively inexpensive, but the product is less desirable than alkylates or ethers. The polymerization products consist primarily of olefins that are unstable in gasoline (gum forming). 

Acetylene-copper complexes are polymeric in nature, and little is known concerning acetylene-silver complexes. Some unstable solid complexes are known, and in solution they are less stable than olefin complexes 115). 

For a long time, cationic polymerization has been considered to be very difficult to control to obtain polymers of narrow molecular-weight distribution. Usually, molecular weights are unpredictable and M /M is far from one. An active species at the propagating polymer end is a carbocation or an onium ion, which reacts with olefin monomers extremely rapidly. The active polymer end is also highly unstable and readily participates in chain-transfer reactions by loss of p-protons leading to uncontrollable molecular-weight distributions. 

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