1, 3-Butadiene, resonance effects

The monomer addition scheme, shown at the top, requires an initiator which is capable of removing a hydrogen atom from the allylic position of the butadiene, resonance stabilization of the radical from AIBN does not permit this initiator to effect this reaction while benzoyl peroxide is capable of reaction to remove a hydrogen atom and initiate the reaction. On the other hand the polymeric radical addition scheme requires that homopolymerization of the monomer be initiated and this macroradical then attack the polymer and lead to the formation of the graft copolymer. Huang and Sundberg explain that the reactivity of the monomer... 

We assume that the double bonds in 1,3-butadiene would be the same as in ethylene if they did not interact with one another. Introduction of the known geometry of 1,3-butadiene in the s-trans conformation and the monopole charge of 0.49 e on each carbon yields an interaction energy <5 — 0.48 ev between the two double bonds. Simpson found the empirical value <5 = 1.91 ev from his assumption that only a London interaction was present. Hence it appears that only a small part of the interaction between double bonds in 1,3-butadiene is a London type of second-order electrical effect and the larger part is a conjugation or resonance associated with the structure with a double bond in the central position. 

A lower max response at resonance was noted for poly butadiene-acrylic acid-containing pro-pints compared with polyurethane-containing opaque proplnts. Comparison of the measured response functions with predictions of theoretical models, which were modified to consider radiant-heat flux effects for translucent proplnts rather than pressure perturbations, suggest general agreement between theory and expt. The technique is suggested for study of the effects of proplnt-formulation variations on solid-proplnt combustion dynamics... 

Many substituents stabilize the monomer but have no appreciable effect on polymer stability, since resonance is only possible with the former. The net effect is to decrease the exothermicity of the polymerization. Thus hyperconjugation of alkyl groups with the C=C lowers AH for propylene and 1-butene polymerizations. Conjugation of the C=C with substituents such as the benzene ring (styrene and a-methylstyrene), and alkene double bond (butadiene and isoprene), the carbonyl linkage (acrylic acid, methyl acrylate, methyl methacrylate), and the nitrile group (acrylonitrile) similarly leads to stabilization of the monomer and decreases enthalpies of polymerization. When the substituent is poorly conjugating as in vinyl acetate, the AH is close to the value for ethylene. 

In the reduction of acetylene with molybdothiol and molybdoselenol complex catalysts, the effects of structural variation in ligands, variety of coordination-donor atom, kind of transition-metal ion, and other factors have been surveyed systematically. These factors have profound effects on the catalytic activity. The Mo complexes of cysteamine (or selenocysteamine), its N,N-dimethyl derivative, and its /3-dimethyl derivative give ethylene, ethane, and 1,3-butadiene, respectively, as the major product. The Co (I I) complexes of cysteine and cysteamine show higher catalytic activity than do the corresponding Mo complexes, and the order of the activity in the donor atom, namely S >Se 0 in the Co(II) complexes is consistent with that in the Mo complex systems. On the basis of electron spin resonance (ESR) features of these Mo complex catalysts, a relationship between their ESR characteristics and catalytic activities is discussed. 

The results obtained are reported in Table 8 and show that the methyl affinities (k4/k2) of MB, NB and TDE are comparable with those known for acyclic and cydic olefins, respectively, having approximately the same degree of substitution at the double bond. Conversely, the k4/k2 ratio for (III,a) is 5 times greater than that expected for two separated double bonds with comparable degree of substitution, but 1—2 orders of magnitude sillier than those observed for other dienic systems, e. g. butadiene, cydopoitadiene, etc. This result illustrates the opposite effects due to resonance stabilization and steric hindrance. 

QHs, -CH=CH2 > -CN, -COR > -COOR > -Cl > -OCOR, R > -OR The order of monomer reactivities in the above series corresponds to the order of increased resonance stabilization (by the particular substituent) of the radical formed from the monomer. Substituents containing unsaturation are more effective in stabilizing the radicals because of the loosely held TT-electrons, which are available for resonance stabilization. Resonance stabilization becomes more significant in radical polymerization when the monomers contain conjugated C-C double bonds as in styrene, 1,3-butadiene, and similar molecules ... 

The NPA electron distribution can be related to the VB concept of resonance structures. The orbitals corresponding to localized structures and those representing delocalization can be weighted. For example, Scheme 1.5 shows the relative weighting of the most important resonance structures for 1,3-butadiene, benzene, the benzyl cation, formamide, and the formate anion. These molecules are commonly used examples of the effect of conjugation and resonance on structure and reactivity. 

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