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Propagation addition-abstraction polymerization

Cases of addition-abstraction" polymerization have also been reported where propagation occurs by a mechanism involving sequential addition and intramolecular 1,5-hydrogen atom transfer steps (Section 4.4.3.4). [Pg.208]

If we consider as an example the addition of HC1 to ethylene, we find that whereas the propagation step for polymerization will be exothermic by about 30 kcal mole-1,146 abstraction of H from HC1 by the R—CH2- radical will be endothermic by 5 kcal mole-1. Activation energies for typical polymerization propagation steps are in the range of 6-10 kcal mole-1,147 and that for abstraction from HC1 will have to be greater than the 5 kcal mole-1 endothermicity. These data are at least indicative that radical addition of HC1 will not be favorable experimentally, it is indeed rare, but can be made to occur with excess HC1.148 With HBr the situation is different. Now the hydrogen abstraction is exothermic by about 10 kcal mole-1 and occurs to the exclusion of telomeriza-tion.149 Hydrogen iodide does not add successfully to olefins because now the initial addition of the iodine atom to the double bond is endothermic. [Pg.507]

Polymerizations of vinyl ketones such as methyl vinyl ketone are also complicated by nucleophilic attack of the initiator and propagating carbanion at the carbonyl group although few details have been established [Dotcheva and Tsvetanov, 1985 Hrdlovic et al., 1979 Nasrallah and Baylouzian, 1977]. Nucleophilic attack in these polymers results in addition, while that at the ester carbonyl of acrylates and methacrylates yields substitution. The major side reaction is an intramolecular aldol-type condensation. Abstraction of an a-hydrogen from a methyl group of the polymer by either initiator or propagating carbanion yields an a-carbanion that attacks the carbonyl group of the adjacent repeat unit. [Pg.420]

The initiation mechanism for cationic polymerization of cyclic ethers, vinyl amines, and alkoxy styrenes has been investigated by A. Ledwith. He used stable cations, like tropylium or triphenylmethyl cations with stable anions, like SbCl6, and distinguished between three initiation reactions cation additions, hydride abstraction, and electron transfer. One of the typical examples of cationic polymerization, in which the propagating species is the oxonium ion, is the polymerization of tetra-hydrofuran. P. and M. P. Dreyfuss studied this polymerization with the triethyloxonium salts of various counterions and established an order of... [Pg.11]

Initiation with Tropylium Ion. Tropylium hexachlorantimonate reacts with vinyl alkyl ethers in a manner very similar to the reactions of triphenylmethyl salts. Again, rapid initiation is followed by propagation without apparent termination. Termination can be demonstrated to be absent from experiments in which fresh samples of monomer are added to completed polymerizations, whereupon the measured reaction rates parallel those previously recorded (Table II). Molecular weights of the polymers from isobutyl vinyl ether are very similar to those obtained with triphenylmethyl salts as initiators and again give clear evidence for excessive monomer transfer. Gas chromatographic analysis of the reaction mixtures showed that cycloheptatriene (product of hydride abstraction) was not present which indicates clearly that initiation must arise via addition of the tropylium ion to the vinyl ether—i.e.,... [Pg.339]

Chain Propagation. In the chain propagation step, an olefin molecule reacts with a tertiary butyl carbonium ion as postulated by Whitmore (1934). This addition reaction produces a larger carbonium ion which then either undergoes isomerization or abstracts a hydride from an isobutane molecule. (Under some circumstances, the larger carbonium ion may add a second molecule of olefin this reaction will be discussed under "Polymerization".) Hydride abstraction regenerates a chain-carrying, tertiary butyl carbonium ion and also forms a molecule of isoparaffin. Reactions follow ... [Pg.30]

Figure 5.13. Reactions involved in cationic addition polymerization. Shown are (a) generation of a carbo-cation intermediate from a Lewis acid initiator, (b) propagation of the polymer chain through the combination of the carbocationic polymer chain and additional monomers, and (c) termination of the polymer growth through either proton abstraction (i) or anionic attachment (ii) routes. Figure 5.13. Reactions involved in cationic addition polymerization. Shown are (a) generation of a carbo-cation intermediate from a Lewis acid initiator, (b) propagation of the polymer chain through the combination of the carbocationic polymer chain and additional monomers, and (c) termination of the polymer growth through either proton abstraction (i) or anionic attachment (ii) routes.
Effect of Tertiary Amines.—Though the influence of ethereal solvents and additives in anionic polymerizations has been widely investigated, it is only relatively recently that attention has been focussed on the effect of tertiary amines. The complex between Bu"Li and tetramethylethylene diamine (TMEDA) is known to be a powerful base readily capable, for example, of abstracting a proton from aromatic hydrocarbons and generating lithiated derivatives. Not surprisingly, therefore, tertiary amines do indeed have significant effects in carboanionic propagations. [Pg.260]

The abstraction takes place mostly on the carbon atom alpha to the double bond, although in alcohols and esters other radicals may also be formed. If produced by chain transfer, they terminate the propagation of a chain if produced by a catalyst radical, they destroy a potential polymerization initiator and reduce the effective catalyst concentration. These reactions compete with the normal addition polymerization [reaction (2)], and the over-all result is a slow rate of polymerization and the formation of low-molecular-weight polymer. [Pg.1025]

The chain lengthening of the polymeric radical is the propagation step. The polymer can be formed as any combination of 1,2-, 1,4-cis or 1,4-trans additions the polymer can be a result of one or all of the three addition processes. The termination step of a polymerization reaction puts a stop to the growing polymer. In free radical polymerization, the termination step rids the growing polymer of its free electron. This generally proceeds by any one of three different methods dimerization, disproportionation and abstraction. Dimerization involves the joining of two growing polymer radicals. It can be shown as ... [Pg.292]

In an FRP, all these reactions are susceptible to occur, but in a concentrated monomer environment, the dominating reactions are the addition (propagation) and termination by disproportionation or coupling. The fragmentation, abstraction, and transmutation reactions are detrimental for the chain formation however, sometimes they can be induced to regulate the degree of polymerization. [Pg.66]

Bases were used as polymerization catalyst since early days of polymerizalion studies, but showed limited usefulness due to the encountered low extent of polymerization. Reaction conditions are adjusted to enhance living anionic polymerization as applied to styrene in liquid ammonia through initiation by K NH2. Addition of the NH2 to olelinic part of the monomer generates the intermediate carbanion which propagates until termination is attained by abstraction of proton by the anion from the ammonia. If this type of termination occurred fast, then a limited polymerization process would be attained as shown in Scheme 3.8. [Pg.57]

DBPOX, a low temperature initiator, decomposes easily at room temperature into t-butoxy radicals and carbon dioxide (Eq. (1)) The t-butoxy radical undergoes both addition to the monomer (Eq. (2)) and hydrogen-abstraction from the N-methyl group (Eq. (3)) Radicals 1 and II attack the monomer and propagate yielding poly(NMAAm) radicals (Eq. (4)). The stable poly(NMAAm) radic was similarly formed in the photo-sensitized polymerization with azo-bis-isobutyronitrile(AIBN) or di-tert-butyl peroxide(DBPO). In the latter system, the concentration of living radical III reached 1 x mol/1... [Pg.45]


See other pages where Propagation addition-abstraction polymerization is mentioned: [Pg.212]    [Pg.212]    [Pg.395]    [Pg.107]    [Pg.49]    [Pg.599]    [Pg.138]    [Pg.22]    [Pg.173]    [Pg.207]    [Pg.253]    [Pg.231]    [Pg.234]    [Pg.354]    [Pg.295]    [Pg.515]    [Pg.150]    [Pg.15]    [Pg.16]    [Pg.58]    [Pg.221]    [Pg.280]    [Pg.136]    [Pg.158]    [Pg.158]    [Pg.206]    [Pg.779]    [Pg.194]    [Pg.130]    [Pg.135]    [Pg.180]    [Pg.779]    [Pg.259]    [Pg.1757]    [Pg.5633]   
See also in sourсe #XX -- [ Pg.208 ]




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Abstraction Addition

Addition polymerization

Addition-abstraction polymerization

Additional polymerization

Additives polymerization

Polymeric additives

Propagation, polymerization

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