Vinyl mechanism

Reactions of (ii)-l-decenyl(phenyl)iodonium salt (6a) with halide ions have been examined under various conditions. The products are those of substitution and elimination, usually (Z)-l-halodec-l-ene (6b) and dec-l-yne (6c), as well as iodobenzene (6d), but F gives exclusively elimination. In kinetic studies of secondary kinetic isotope effects, leaving-group substituent effects, and pressure effects on the rate, the results are compatible with the in-plane vinylic mechanism for substitution with inversion. The reactions of four ( )-jS-alkylvinyl(phenyl)iodonium salts with CP in MeCN and other solvents at 25 °C have been examined. Substitution with inversion is usually in competition with elimination to form the alk-l-yne. 

In conclusion, the experimental material is in accordance with the O-vinylation mechanism. It is probable, however, that depending upon the reaction conditions and the structure of reactants, some alternative routes shown in Scheme 84 are realized in particular cases. At the same time, it should not be ignored that any scheme, even those that seem to be most reasonable, cannot be considered as adequate unless they explain why the pyrrolization of ketoximes with acetylene succeeds only in the presence of specific superbase systems (strong base/DMSO). 

No monochlorophenylated ethylenes were reported. The monochlorinated product would be expected from both mechanisms since Pd—Cl bonds would be easily substituted rather than C—H in the vinylic mechanism, whereas in the phenylpalladation mechanism Pd—Cl elimination would be much more facile than Pd—H elimination. 

Vinyl pyridine is nucleophilic and so starts the polymerization of vinyl pyridinium salts. In aqueous media polymerization, high-molar-mass poly-(vinyl pyridinium salts), II, are formed at high monomer concentrations by the normal vinyl mechanism, but polyadduct formation to oligomers. III, occurs at low monomer concentration ... 

A highly efficient protocol for C2 selective alkenylation of thiazoles and benzothiazoles provides easy access to 2-alkenyl-thiazole derivatives 95 (140L1798). Interestingly, when 1-octene is used under standard conditions, the branched product 97 is obtained in 58% yield. A plausible Pd-catalyzed vinylation mechanism has been proposed. Thiazoles 93 undergoes C2 selective electrophilic C-H substitution of palladium acetate to generate the palladium(II) intermediate 98. Next, the resulting... 

Lithiation at C2 can also be the starting point for 2-arylatioii or vinylation. The lithiated indoles can be converted to stannanes or zinc reagents which can undergo Pd-catalysed coupling with aryl, vinyl, benzyl and allyl halides or sulfonates. The mechanism of the coupling reaction involves formation of a disubstituted palladium intermediate by a combination of ligand exchange and oxidative addition. Phosphine catalysts and salts are often important reaction components. 

The bactericidal and enzymatic action of dyes, particularly of vinyl derivatives of 3,4,5-substituted thiazolium, for example, 45 (Scheme 70) (139), have been systematically studied to know more about the basic mechanisms involved (140). 

Both compounds react by an S l mechanism and their relative rates reflect their acti vation energies for carbocation formation Because the allylic chloride is more reactive we reason that it ionizes more rapidly because it forms a more stable carbocation Struc turally the two carbocations differ m that the allylic carbocation has a vinyl substituent on Its positively charged carbon m place of one of the methyl groups of tert butyl cation... 

A key step in the reaction mechanism appears to be nucleophilic attack on the alkyl halide by the negatively charged copper atom but the details of the mechanism are not well understood Indeed there is probably more than one mechanism by which cuprates react with organic halogen compounds Vinyl halides and aryl halides are known to be very unreactive toward nucleophilic attack yet react with lithium dialkylcuprates... 

The time-temperature superpositioning principle was applied f to the maximum in dielectric loss factors measured on poly(vinyl acetate). Data collected at different temperatures were shifted to match at Tg = 28 C. The shift factors for the frequency (in hertz) at the maximum were found to obey the WLF equation in the following form log co + 6.9 = [ 19.6(T -28)]/[42 (T - 28)]. Estimate the fractional free volume at Tg and a. for the free volume from these data. Recalling from Chap. 3 that the loss factor for the mechanical properties occurs at cor = 1, estimate the relaxation time for poly(vinyl acetate) at 40 and 28.5 C. 

It might be noted that most (not all) alkenes are polymerizable by the chain mechanism involving free-radical intermediates, whereas the carbonyl group is generally not polymerized by the free-radical mechanism. Carbonyl groups and some carbon-carbon double bonds are polymerized by ionic mechanisms. Monomers display far more specificity where the ionic mechanism is involved than with the free-radical mechanism. For example, acrylamide will polymerize through an anionic intermediate but not a cationic one, A -vinyl pyrrolidones by cationic but not anionic intermediates, and halogenated olefins by neither ionic species. In all of these cases free-radical polymerization is possible. 

Any discussion based on reactivity ratios is kinetic in origin and therefore reflects the mechanism or, more specifically, the transition state of a reaction The transition state for the addition of a vinyl monomer to a growing radical involves the formation of a partial bond between the two species, with a corre sponding reduction of the double-bond character of the vinyl group in the monomer ... 

Dichromated Resists. The first compositions widely used as photoresists combine a photosensitive dichromate salt (usually ammonium dichromate) with a water-soluble polymer of biologic origin such as gelatin, egg albumin (proteins), or gum arabic (a starch). Later, synthetic polymers such as poly(vinyl alcohol) also were used (11,12). Irradiation with uv light (X in the range of 360—380 nm using, for example, a carbon arc lamp) leads to photoinitiated oxidation of the polymer and reduction of dichromate to Ct(III). The photoinduced chemistry renders exposed areas insoluble in aqueous developing solutions. The photochemical mechanism of dichromate sensitization of PVA (summarized in Fig. 3) has been studied in detail (13). 

Nitrile mbber finds broad application in industry because of its excellent resistance to oil and chemicals, its good flexibility at low temperatures, high abrasion and heat resistance (up to 120°C), and good mechanical properties. Nitrile mbber consists of butadiene—acrylonitrile copolymers with an acrylonitrile content ranging from 15 to 45% (see Elastomers, SYNTHETIC, NITRILE RUBBER). In addition to the traditional applications of nitrile mbber for hoses, gaskets, seals, and oil well equipment, new applications have emerged with the development of nitrile mbber blends with poly(vinyl chloride) (PVC). These blends combine the chemical resistance and low temperature flexibility characteristics of nitrile mbber with the stability and ozone resistance of PVC. This has greatly expanded the use of nitrile mbber in outdoor applications for hoses, belts, and cable jackets, where ozone resistance is necessary. 

Alkyl diphenyl phosphate plasticizers can exert flame-retardant action in vinyl plastics by a condensed-phase mechanism, which is probably some sort of phosphoms acid coating on the char. Triaryl phosphates appear to have a vapor-phase action (29). 

Poly(vinyl acetate). The dielectric and mechanical spectra of hybrids produced by mixing a poly(vinyl acetate)—THE solution with TEOS, followed by the addition of HCl have been investigated (45). Mixtures were made which were beheved to be 0, 5, 10, 15, and 20 wt % Si02, respectively. These composites were transparent and Eourier transform infrared spectroscopy (ftir) revealed hydrogen bonding between the siUcate network and carbonyl units of the poly(vinyl acetate) (PVAc). No shift in the T of the composites from that of the pure PVAc was observed. Similarly, the activation... 

Radicals are employed widely in the polymer industry, where their chain-propagating behavior transforms vinyl monomers into polymers and copolymers. The mechanism of addition polymeri2ation involves all three types of reactions discussed above, ie, initiation, propagation by addition to carbon—carbon double bonds, and termination ... 

Unsaturated polyester resins prepared by condensation polymerization constitute the largest industrial use for maleic anhydride. Typically, maleic anhydride is esterified with ethylene glycol [107-21-1] and a vinyl monomer or styrene is added along with an initiator such as a peroxide to produce a three-dimensional macromolecule with rigidity, insolubiUty, and mechanical strength. 

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