Styrene resonance stabilization

This trans-selectivity ultimately results from the fact that trans-alkenes are more stable than their cis-isomers. This energy difference is especially pronounced for the alkenes in Figure 4.3 because they are styrene derivatives. Styrenes with one alkyl group in the trans-position on the alkenyl C=C double bond enjoy the approximately 3 kcal/mol styrene resonance stabilization. This is lost in cis-styrenes because in that case the phenyl ring is rotated out of the plane of the alkenic C=C double bond to avoid the cis-alkyl substituent. However, the transselectivity documented in Figure 4.3 is not a consequence of thermodynamic control. This could occur only for a reversible elimination or if the alkenes could interconvert under the reaction conditions in some other way. Under the conditions of Figure 4.3, alkenes are almost always produced irreversibly and without the possibility of a subsequent cis/frans-isomeriza-tion. Therefore, the observed trans-selectivity is the result of kinetic control. 

All /1-eliminations from the benzyl derivative in Figure 4.5 exhibit a certain stereoselectivity, in this case -stcreoselectivity. This is true regardless of whether the elimination is syn- or awft -selective or neither. The reason for the preferred formation of the /. -product is again product-development control. This comes about because there is a significant energy difference between the isomeric elimination products due to the presence (E isomers) or absence (Z isomers) of styrene resonance stabilization. 

We might be hard pressed to estimate the individual resonance stabilization energies in Eqs. (7.23) and (7.24), but the qualitative apphcation of these ideas is not difficult. Consider once again the styrene-vinyl acetate system ... 

Define styrene to be monomer 1 and vinyl acetate to be monomer 2. The difference in resonance stabilization energy ep. - > 1, since... 

The Q-e Scheme. The magnitude of and T2 can frequentiy be correlated with stmctural effects, such as polar and resonance factors. For example, in the free-radical polymerization of vinyl acetate with styrene, both styrene and vinyl acetate radicals preferentially add styrene because of the formation of the resonance stabilized polystyrene radical. 

The side-chain alkylation reaction of aromatic hydrocarbons has also been studied using unsaturated aromatic olefins, especially styrene. Pines and Wunderlich 43) found that phenylethylated aromatics resulted from the reaction of styrenes with arylalkanes at 80-125° in the presence of sodium with a promoter. The mechanism of reaction is similar to that suggested for monoolefins, but addition does not take place to yield a primary carbanion a resonance stabilized benzylic carbanion is formed [Reaction (23a, b)j. 

The resonance stability of the macroradical is an important factor in polymer propagation. Thus, for free radical polymerization, a conjugated monomer such as styrene is at least 30 times as apt to form a resonance-stabilized macroradical as VAc, resulting in a copolymer rich in styrene-derived units when these two are copolymerized. 

The higher stability of primary anion 37 as compared to secondary anion 38 explains the predominant formation of branched isomers. The high reactivity of conjugated dienes and styrenes compared with that of monoolefins is accounted for by the formation of new resonance-stabilized anions (39 and 40). Base-catalyzed alkylation with conjugated dienes may be accompanied by telomerization. The reason for this is that the addition of a second molecule of diene to the 39 monoadduct anion competes with transmetallation, especially at lower... 

A large number of accurate rate constants are known for addition of simple alkyl radicals to alkenes.33-33 Table 2 summarizes some substituent effects in the addition of the cyclohexyl radical to a series of monosubstituted alkenes.36 The resonance stabilization of the adduct radical is relatively unimportant (because of the early transition state) and the rate constants for additions roughly parallel the LUMO energy of the alkene. Styrene is selected as a convenient reference because it is experimentally difficult to conduct additions of nucleophilic radicals to alkenes that are much poorer acceptors than styrene. Thus, high yield additions of alkyl radicals to acceptors, such as vinyl chloride and vinyl acetate, are difficult to accomplish and it is not possible to add alkyl radicals to simple alkyl-substituted alkenes. Alkynes are slightly poorer acceptors than similarly activated alkenes but are still useful.37... 

In Section 3.5.1, it was mentioned that Br2 and Cl2 form resonance-stabilized benzyl cation intermediates with styrene derivatives and that gem-dialkylated alkenes react with Br2 hut not... 

Copolymerization. The importance of VDC as a monomer results from its ability to copolymerize with other vinyl monomers. Its j2 value equals 0.22 and its e value equals 0.36. It most easily copolymerizes with acrylates, but it also reacts, more slowly, with other monomers, eg, styrene, that form highly resonance-stabilized radicals. Reactivity ratios ( and r2) with various monomers are listed in Table 2. Many other copolymers have been prepared from monomers for which the reactivity ratios are not known. The commercially important copolymers include those with vinyl chloride (VC),... 

In Section 3.5.1 it was mentioned that Br2 and Cl2 form resonance-stabilized benzyl cation intermediates with styrene derivatives and that gem-dialkylated olefins react with Br2 but not with Cl2 via halonium ions. Because C—Cl bonds are shorter than C—Br bonds, chloronium ions presumably have a higher ring strain than bromonium ions. Accordingly, a /3-chlorinated tertiary carbenium ion is more stable than the isomeric chloronium ion, but a /3-brominated tertiary carbenium ion is less stable than the isomeric bromonium ion. 

Copolymerization of styrene with diolefins provides further support that monomer coordinates with the cationic site prior to addition. Korotkov (218) showed that in homopolymerizations styrene is more reactive than butadiene, but in copolymerization the butadiene reacted first at its homopolymerization rate and when it was exhausted the styrene reacted at its homopolymerization rate. This interesting result has been duplicated by Kuntz (245) and analogous results have been obtained by Spirin and coworkers (237) for the styrene-isoprene system. Presumably, the diene complexes more strongly than styrene with the lithium and excludes styrene from the site. That the complex occurs at a cationic site, rather than at the anion or the metal-carbon bond, is indicated by the fact that dienes form more stable complexes than styrene with Lewis acids (246). It should be emphasized that selective monomer coordination is not the only factor influencing reactivities in copolymerizations. Of greatest importance are the relative reactivities of the different polymer anions. The more resonance-stabilized anion is more readily formed and is less reactive for polymerizing the co-monomer. 

Many alkenes undergo free-radical polymerization when they are heated with radical initiators. For example, styrene polymerizes to polystyrene when it is heated to 100 °C with a peroxide initiator. A radical adds to styrene to give a resonance-stabilized radical, which then attacks another molecule of styrene to give an elongated radical. 

Each propagation step adds another molecule of styrene to the radical end of the growing chain. This addition always takes place with the orientation that gives another resonance-stabilized benzylic (next to a benzene ring) radical. 

It appears that within each group, the reactivity of monomers towards the styrene radical increases with both q and e (see Chap. 5, Sect. 5.2). Higher q values correspond to greater resonance stabilization of the newly formed radical growth in e is connected with the interaction of the easily polarizable macroradical with the / carbon of the monomer whose electronegativity is increased. 

Hydrides are reactive nucleophilic agents able to initiate growth slowly. The whole process will have the character of degradative transfer the polymerization of a-methylstyrene will be stopped at a certain conversion. The double bond at the chain end will increase the acidity of the penultimate styrene unit and the corresponding C—H bond will be weakened by resonance stabilization of the anion... 

Among the processes used for the formation of polyolefins, the longest-known but least selective one is free radical polymerization. A free radical species X produced e.g. by thermolysis of benzoyl peroxide or by photolysis of azabisisobutyronitrile (AIBN) - can react with the double bond of a vinyl derivative H2C=CHR to form a new radical of the type XCH2-CHR which can then add another H2C=CHR unit repetition of this process leads to polyolefin formation (Figure 2, top). This process works best for vinyl derivatives with unsaturated side groups, which provide resonance stabilization for an adjacent radical centre, e.g. with vinyl and acrylic esters, vinyl cyanides and vinyl chloride and with styrene and 1,3-dienes. It is extensively used in the emulsion polymerization of vinylic and acrylic derivatives and in the light-induced formation of photoresists for the nanofabrication of semiconductor chips and integrated electronic circuits. 

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