Insertions of ethylene

The full ab-initio molecular dynamics simulation revealed the insertion of ethylene into the Zr-C bond, leading to propyl formation. The dynamics simulations showed that this first step in ethylene polymerisation is extremely fast. Figure 2 shows the distance between the carbon atoms in ethylene and between an ethylene carbon and the methyl carbon, from which it follows that the insertion time is only about 170 fs. This observation suggests the absence of any significant barrier of activation at this stage of the polymerisation process, and for this catalyst. The absence or very small value of a barrier for insertion of ethylene into a bis-cyclopentadienyl titanocene or zirconocene has also been confirmed by static quantum simulations reported independently... 

Random insertion of ethylene as comonomer and, in some cases, butene as termonomer, enhances clarity and depresses the polymer melting point and stiffness. Propylene—butene copolymers are also available (47). Consequendy, these polymers are used in apphcations where clarity is essential and as a sealant layer in polypropylene films. The impact resistance of these polymers is sligbdy superior to propylene homopolymers, especially at refrigeration temperatures, but still vastiy inferior to that of heterophasic copolymers. Properties of these polymers are shown in Table 4. 

Figure 9.21. The Cossee-Arlman mechanism of chain growth in ethylene polymerization involves the insertion of ethylene in the...

Dimethyltitanium complex 25, bearing an ethylene and methyl ligands, catalyzed the dimerization of ethylene via a metallacyclopentane intermediate 26 (Eq. 1) [30]. During the dimerization, no insertion of ethylene into the Ti-Me bond was observed due to the perpendicular orientation between methyl and ethylene ligands. This inertness could be attributed to the low oxidation state of 25, i.e. Ti(II). 

The most famous mechanism, namely Cossets mechanism, in which the alkene inserts itself directly into the metal-carbon bond (Eq. 5), has been proposed, based on the kinetic study [134-136], This mechanism involves the intermediacy of ethylene coordinated to a metal-alkyl center and the following insertion of ethylene into the metal-carbon bond via a four-centered transition state. The olefin coordination to such a catalytically active metal center in this intermediate must be weak so that the olefin can readily insert itself into the M-C bond without forming any meta-stable intermediate. Similar alkyl-olefin complexes such as Cp2NbR( /2-ethylene) have been easily isolated and found not to be the active catalyst precursor of polymerization [31-33, 137]. In support of this, theoretical calculations recently showed the presence of a weakly ethylene-coordinated intermediate (vide infra) [12,13]. The stereochemistry of ethylene insertion was definitely shown to be cis by the evidence that the polymerization of cis- and trans-dideutero-ethylene afforded stereoselectively deuterated polyethylenes [138]. 

An example of a complex that reacts slowly with ethylene in a manner consistent with insertion of ethylene into the metal alkyl bond is Co(ti5-C5H5)(PPh3)Me2 Evitt, E.R. Bergman,... 

A ruthenium dihydrogen complex G or a ruthenacycle D, which was proposed as a potential intermediate, catalyzed the insertion of ethylene into sp2-C-H bonds, with TONs reaching 19 after 48 h of reaction and under very mild conditions (room temperature as opposed to the usual 135 °C) (Equation (96)).91,91a91c... 

Insertion of ethylene into the Ni-C bond in 3a leads to the alkyl complex 4a via the transition state TS[3a-4a] with a barrier [13a] of 17.5 kcal/mol relative to 3a It is worth to note that in TS[3a-4a both ethylene and the a-carbon of the growing (propyl) chain are situated in the N-Ni-N plane. For the corresponding palladium complex the insertion barrier [13c] is somewhat higher at 19.9 kcal/mol. 

The hydrosilylation of ethylene by the early-late transition-metal heterodinuclear complexes [CpTa( t-CH2)2lr(CO)2] has been studied mainly in a bid to recognize the mechanism of reaction, which occurs via a predominant alkene/Ir—H insertion pathway over a minor insertion of ethylene into the Ir—Si bonds [21]. 

In 1996, Brookhart and co-workers developed a remarkable class of Pd complexes with sterically encumbered diimine ligands (Scheme 4, S4-1, S4-2, S4-4, and S4-5). These examples are capable of mediating the co-polymerization of ethylene with methyl acrylate (MA) to furnish highly branched PE with ester groups on the polymer chain ends by a chain-walking mechanism (Scheme 10). " This represents the first example of transition metal-catalyzed ethylene/MA co-polymerization via an insertion mechanism. The mechanism for co-polymerization is by 2,1-insertion of MA and subsequent chelate-ring expansion, followed by the insertion of ethylene units. The discovery of these diimine Pd catalysts has stimulated a resurgence of activity in the area of late transition metal-based molecular catalysis. Recently, the random incorporation of MA into linear PE by Pd-catalyzed insertion polymeriza-... 

Insertion of ethylene into the Pt—H bond of Pmr-PtHCl(PEt3)2 was reported by Chatt and Shaw,73,149 who obtained a 25% yield of fra s-Pt(Et)Cl(PEt3)2 after 18 hours when the reaction... 

When DCN is substituted for HCN, propionitrile is formed in which deuterium is found in both methyl and methylene groups, indicating that the insertion of ethylene into the nickel-hydrogen bond is reversible and occurs rapidly compared to the irreversible coupling of Et and CN to give propionitrile. A singlet at z 7.97 is assigned to coordinated ethylene. 

Ti—13Cp 40.1 ppm), and only weak signals of 13C-labeled alkyl groups at the aluminium were observed (see Fig. 6). These results indicate that an insertion of ethylene takes place into a titanium-carbon bond of a titanium-aluminum complex and no alkyl exchange between the bonds of titanium-alkyl and aluminum-alkyl occurs. 

Benzils (94) react with typical Michael acceptors in the presence of a catalytic amount of CN to give 1,4-diketones (95), which arise by insertion of ethylene group between the carbonyls of the benzil units (Scheme 21).82... 

The intermediacy of 2 was confirmed by the formation of l,2,5,6-tetrakis(2,6-diisopro-pylphenyl)-l,2,5,6-tetrasilatricyclo[3.1.0.02 6]hexane (6) (Scheme 2)33 in the reductive coupling reaction of 2,6-triisopropylphenyltrichlorosilane (5) by the Mg/MgBr2 reagent (which was generated in situ by the reaction of Mg and BrC CTHBr). The tricyclic 6 is presumed to be derived from the insertion of ethylene (formed in situ, see Scheme 2) into the reactive Si—Si bond of 2. None of 6 was formed after the complete removal of ethylene from the reaction system. 

The catalytic cycle of the Ni-catalysed dimerization of ethylene to give 1-butene (65) is explained by the insertion of ethylene to the nickel hydride 62 twice to form the ethyl complex 63 and the butyl complex 64. The elimination of /1-hydrogen gives 1-butene (65), and regenerates the Ni—H species 62. The reaction is chemoselective. Curiously, no further insertion of ethylene to 64 occurs. 

Several linear cooligomers of butadiene are prepared with alkenes and alkynes. Commercially important 1,4-hexadiene (103) is prepared by the reaction of ethylene and butadiene catalysed by Ni [40], Fe [41] and Rh [42], The experiment carried out using deuterated ethylene (100) supports the mechanism that the insertion of butadiene to M—H forms the 7i-allyl complex 99. Insertion of ethylene (100) to 99 gives 101, and its -elimination affords the cooligomer 102, tetradeuterated at C-1,1,2,6 of 103. 

In systems completely free of chloride, complex formation between the titanium or zirconium compound and the trialkylaluminum was not detected by either NMR or UV spectroscopy 191), or by cryoscopy (189). Such systems were thus assumed to be inactive for polymerization. However, NMR-spectroscopic studies in the presence of ethylene revealed a weak but lasting insertion of ethylene, with formation of polyethylene. 

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