Alkanes olefin polymerization

The siHca-supported Zr(IV) hydrides 1 obtained by hydrogenolysis of the singlesite siHca-supported species [(=SiO)ZrNp3] are well-known catalysts for hydrogen/ deuterium exchange in saturated hydrocarbons [2, 3], olefin polymerization [4] and alkane or polyolefin hydrogenolysis [5, 6]. They also display a key role in stoichio-... 

The most relevant catalytic reactions approached by SOMC are olefin polymerization (and depolymerization), alkane activation (including a new reaction, discovered thanks to SOMC-alkane metathesis), alkene metathesis and epoxidation. All these reactions are discussed in this chapter. 

Since group 4 derived species are of particular interest as catalysts for olefin polymerization and epoxidation reactions, the thermal stability of surface metal-alkyl species, as weU as their reactivity towards water, alcohols and water, deserve some attention. On the other hand, mono(siloxy) metaUiydrocarbyl species can be converted into bis- or tris(siloxy)metal hydrides by reaction with hydrogen [16, 41, 46-48]. Such species are less susceptible to leaching and can be used as pre-catalysts for the hydrogenolysis of C-C bonds, alkane metathesis and, eventually, for epoxidation and other reactions. 

The largest-volume polymers are polyolefins, and the kinetics of olefin polymerization are fairly similar to the ideal addition process just considered. All these olefins form condensation products to form a very long-chain alkane such as... 

Very little skeletal rearrangement occurs via pyrolysis, a fact inherent in the failure of free radicals to readily isomerize by hydrogen atom or alkyl group migration. As a result, little branched alkanes are produced. Aromatization through the dehydrogenation of cyclohexanes and condensation to form polynuclear aromatics can take place. Additionally, olefin polymerization also can occur as a secondary process. 

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). 

Cyclopentadienyl ligands are present on some of the most active catalysts for olefin polymerization, on the first complexes to add alkanes and on the first catalysts to functionalize the terminal position of alkanes regiospecifically. They are ancillary ligands on complexes that undergo all of the classes of reactions described in later chapters of this book, and for these reasons many derivatives of Cp ligands have been prepared. Section 3.6 describes the thermodynamic, electronic, and steric properties of cyclopentadienyl ligands, the synthesis of cyclopentadienyl complexes, and some fundamental reactivity of the Cp ligand. 

Group NIB and f-BIOCk Metallocenes. The need for expensive cocatalysts has always hampered the development of metallocene catalysts for olefin polymerization. It was recognized early on that substitution of the Group IVB metal with a lanthanide or Group IIIB element in the -1-3, state would represent a cocatalyst-free analogue of the Kaminsky system. Active catalysts are indeed obtained from bis-Cp lanthanocenes and yttrium- and scandium-based congeners. The lutetium dimer 29 has an activity of >7 kg/mmol(Lu)/h/atm for ethylene polymerization in cyclohexane (95). (This remarkable compound can also break the C—H bonds of alkanes.)... 

Supported metal oxide catalysts are widely employed in industrial applications alkane dehydrogenation, olefin polymerization, olefin metathesis, selective oxidation/ammoxida-tion/reduction of organic molecules (alkyl aromatics and propylene), and inorganic emissions (N2O, NO , H2S, SO2, and VOC) [1,3,7,11-13]. The initial industrial applications of supported metal oxide catalysts were limited to hydrocarbon dehydrogenation/hydro-genation and olefin polymerization/metathesis reactions. In more recent years, the number of applications of supported metal oxide catalysts for oxidation reactions has grown significantly due to their excellent oxidation characteristics in the manufacture of certain... 

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. 

More recently, the same principle was applied by the same authors to cyclic alkanes for catalytic ring expansion, contraction and metathesis-polymerization (Scheme 13.24) [44]. By using the tandem dehydrogenation-olefin metathesis system shown in Scheme 13.23, it was possible to achieve a metathesis-cyclooligomerization of COA and cyclodecane (CDA). This afforded cycloalkanes with different carbon numbers, predominantly multiples of the substrate carbon number the major products were dimers, with successively smaller proportions of higher cyclo-oligomers and polymers. 

This is a major achievement, mainly due to Basset and his group, in surface organometallic chemistry because it has been thus possible to prepare single site catalysts for various known or new catalytic reactions [53] such as metathesis of olefins [54], polymerization of olefins [55], alkane metathesis [56], coupHng of methane to ethane and hydrogen [57], cleavage of alkanes by methane [58], hydrogenolysis of polyolefins [59] and alkanes [60], direct transformation of ethylene into propylene [61], etc. These topics are considered in detail in subsequent chapters. 

Since Zr-H is able both to (i) activate the C-H bonds of alkanes (via cr-bond metathesis) [15, 48] and to carry out their hydrogenolysis (transfer of a least two carbons via a P-alkyl transfer) and (ii) polymerize olefins (via insertion), the ability of such supported Zr-H was tested in the homologation of propane. 

Dimetalloalkanes are formed in those systems in which separation of alkanes occurs without polymerization of olefins. 

Indeed, let us consider, for example, olefin self-hydrogenation. This is generally a multistage process beginning with disproportionation of a pair of olefin molecules to the corresponding alkane and a polyunsaturated compound (diene or alkyne), which rapidly polymerizes to form high-molecular-weight products P ... 

---