Ethylene hindered

In the NMR spectrum of cis-l,2-bis[2-diethylamino-5-nitrothiazol-4-yl] ethylene (17) (1570), the nonequivalence of olefinic protons requires that the rotation of the NO2 group be hindered. 

Dow catalysts have a high capabihty to copolymetize linear a-olefias with ethylene. As a result, when these catalysts are used in solution-type polymerisation reactions, they also copolymerise ethylene with polymer molecules containing vinyl double bonds at their ends. This autocopolymerisation reaction is able to produce LLDPE molecules with long-chain branches that exhibit some beneficial processing properties (1,2,38,39). Distinct from other catalyst systems, Dow catalysts can also copolymerise ethylene with styrene and hindered olefins (40). 

The biodegradation of poly(alkylene glycols) is hindered by their lack of water solubiUty, and only the low oligomers of poly(propylene glycol) are biodegradable with any certainty (179—181), as are those of poly(tetramethylene glycol) (182). A similar xo-oxidation mechanism to that reported for poly(ethylene glycol) has been proposed. 

The polyols used are of three types polyether, polyester, and polybutadiene. The polyether diols range from 400 to about 10,000 g/mol. The most common polyethers are based on ethylene oxide, propylene oxide, and tetrahydrofuran or their copolymers. The ether link provides low temperature flexibility and low viscosity. Ethylene oxide is the most hydrophilic and thus can increase the rate of ingress of water and consequently the cure rate. However, it will crystallize slowly above about 600 g/mol. Propylene oxide is hydrophobic due to hindered access to the ether link, but still provides high permeability to small molecules like water. Tetrahydrofuran is between these two in hydrophobicity, but somewhat more expensive. Propylene oxide based diols are the most common. 

A 17a-hydroxyl group reduces the reactivity of the 20-ketone but direct ketalization with ethylene glycol is not impeded, Ketalization can also be effected in the presence of 17a- and/or 21-hydroxy substituents. Thus the 3,20-biscycloethyleneketal (88) is obtained from (87) in high yield by the direct procedure, or better by distillation under vacuum without a diluent. A bromine atom at C-17 and a 21-acetoxy group even in the absence of a 17a-hydroxyl group strongly hinder ketalization at C-20. ... 

As early as 1904, 1,2-Propanediol Dinitrate was proposed (Ref 3) as an additive to lower the freezing temp of NG, but its practical application on a large scale was hindered by lack of the raw material, propane-1,2-diol. It is only recently that the synthesis of glycol from ethylene led to the development of a method for producing methyl glycol from propylene via cnioro-hydrin. Even so, propylene-1,2-glycol is somewhat more expensive than glycols derived from ethylene (Ref 9)... 

A cyclobutene ROCM sequence was also used in a synthesis of racemic sporochnol (410), a naturally occurring feeding deterrent toward herbivorous fish (Scheme 80) [170]. Exposing cyclobutene 406 (0.01 M in boiling 1,2-dichloroethane) in the presence of ethylene to second-generation catalyst C (8 mol%) led to 1,5-diene 407 in 73% yield, along with 9% of the homodimer derived from 407 by involving the less hindered double bond. Site-selective hy-... 

An alternative mechanism by which additives may protect polymers from photo-oxidation is radical trapping. Additives which operate by this mechanism are strictly light stabilizers rather than antioxidants. The most common materials in this class are the hindered amines, which are the usual additives for the protection of poly (ethylene) and poly (propylene). The action of these stabilisers is outlined in Reactions 8.3-8.5. 

The Dow corporation has recently developed constrained geometry addition polymerization catalysts (CGCT), typically Me2Si(C5Me4)(NBut)MCl2 (M = Ti, Zr, Hf) (141) activated with MAO. The homo-polymerization of a-olefins by CGCT afford atactic or somewhat syndiotactic (polypropylene rr 69%) polymers. The metal center of the catalyst opens the coordination sphere and enables the co-polymerization of ethylene to take place, not only with common monomers such as propylene, butene, hexene, and octene, but also with sterically hindered a-olefins such as styrene and 4-vinylcyclohexene [202]. 

Ketones are formed under conditions of low total pressure and high olefin concentration. Bertrand et al. (41) pictured the formation as outlined in Eqs. (20) and (21). Ketone formation is most pronounced with ethylene (least hindered olefin), and diethyl ketone has been obtained in... 

Yet another possibility is illustrated by the propene (or ethylene) dimerization catalyzed by 7r-l,l,3,3-tetraphenylallylnickel bromide (26) activated with ethylaluminum dichloride the isolation of considerable amounts of 1,1,3,3-tetraphenylpropene (27) from the reaction mixture suggests that a hydrogen atom has been transferred from the substrate olefin to the sterically hindered 1,1,3,3-tetraphenylallyl system under formation of 3 [Eq. (7)] (81). The subsequent formation of the HNiY species from 3 can then take place by insertion of a second propene molecule and /3-hydrogen elimination, as discussed above. 

Clear formation of ketene—zirconocene complexes upon treatment of acylzirconocene chlorides with a hindered amide base indicates that the carbonyl group of the acylzirconocene chloride possesses usual carbonyl polarization (Scheme 5.10). However, these zirconocene—ketene complexes are exceptionally inert due to the formation of strongly bound dimers [13a], Conversion of the dimer to zirconocene—ketene—alkylaluminum complexes by treating with alkylaluminum and reaction with excess acetylene in toluene at 25 °C has been reported to give a cyclic enolate in quantitative yield. Although the ketene—zirconocene—alkylaluminum complex reacts cleanly with acetylene, it does not react with ethylene or substituted acetylenes [13b]. Thus, the complex has met with limited success as a reagent in organic synthesis. 

These results have been interpreted in terms of trans addition of mercuric ion and nucleophile where the attack of the mercuric ion takes place from the more hindered side of the diene molecule. A transition state 197, involving an endo attack of mercuric ion with some stabilization by coordination to the 8,9-ethylenic bond to the mercury atom, has been proposed to support the suggested mechanism. Analogously, and in sharp contrast to the results obtained167 in the mercuration of norbomadiene which reacts with mercury salts via the usual scheme of exo-syn addition, the principal pathway in the mercuration of bicyclo[2.2.2]octa-2,5-diene is the formation of endo-syn products (equation 165). 

Ethylene was one of the first systems subjected to detailed vibrational analysis using HOCM modified to account for lattice anharmonicity. Agreement with experiment is excellent (Fig. 5.5). The differences in the VPIE s of the equivalent isotopomers cis- trans-, and gem-dideuteroethylene (Fig. 5.6) are of considerable interest since they neatly demonstrate the close connection between molecular structure and isotope chemistry. The IE s are mainly a consequence of hindered rotation in the liquid (moments of inertia for cis-, trans-, and gem-C2D2H2 are slightly... 

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