Diolefin mechanism

A concerted pairwise exchange of alkylidene moieties via a quasicyclobutane transition state, also known as the diolefin mechanism, was first suggested to interpret the transformation 3... 

The reactive species that iaitiate free-radical oxidatioa are preseat ia trace amouats. Exteasive studies (11) of the autoxidatioa mechanism have clearly estabUshed that the most reactive materials are thiols and disulfides, heterocycHc nitrogen compounds, diolefins, furans, and certain aromatic-olefin compounds. Because free-radical formation is accelerated by metal ions of copper, cobalt, and even iron (12), the presence of metals further compHcates the control of oxidation. It is difficult to avoid some metals, particularly iron, ia fuel systems. 

The mechanism of the polymerization contains ionic intermediate steps. The free H+ goes to a carbenium ion and, as shown in route B, rearranges to form tetrapropylene. It is highly likely that this actual tetrapropylene exists only in very small concentrations. The product variety is explained by the rearrangement of the carbenium ion to dodecene isomers according to route C. In addition, short-chain olefins formed by fragmentation (route D). Polymerization proceeds at almost 100% to mono olefins. Aromatics, paraffins, and diolefins exist only in trace amounts. The propylene tetramer is best characterized by its distillation range. 

In this chapter the topochemical [2+2] photoreactions of diolefin crystals are reviewed from the viewpoints of organic photochemistry, analysis of reaction mechanism, and crystallography as well as in terms of synthetic polymer chemistry and polymer physics. 

The polymerization of olefins and di-olefins is one of the most important targets in polymer science. This review article describes recent progress in this field and deals with organo-transition metal complexes as polymerization catalysts. Recent developments in organometallic chemistry have prompted us to find a precise description of the mechanism of propagation, chain transfer, and termination steps in the homogeneously metal-assisted polymerization of olefins and diolefins. Thus, this development provides an idea for designing any catalyst systems that are of interest in industry. 

Grosjean, D., and S. K. Friedlander. Kinetics and Mechanism of Aerosol Formation from Cyclic Olefins and Diolefins. Paper No. 75 Presented at the Pacific Conference on Chemistry and Spectroscopy, North Hollywood, California, October 28-30, 1975. (American Chemical Society—11th Western Regional Meeting and Society for Applied Spectroscopy—14th Pacific Meeting)... 

The driving force for this reaction is the formation of the thermodynamically more stable aromatic compound. The mechanism (E) involves doublebond isomerization, as was discussed previously, imtil an endocyclic diolefin is obtained. 

Pines and Kolobielski (18) have shown that phenylcyclohexene, although it is not a cyclic diolefin, will also undergo reactions similar to those that cyclic diolefins undergo when treated with base catalysts. When heated to 200-220 with a sodium-benzyl-sodium catalyst, it underwent a hydrogen transfer reaction resulting in the formation of biphenyl and of phenyl-cyclohexane molecular hydrogen was not produced. The mechanism of this reaction may be pictured as an elimination of sodium hydride from one molecule with the hydride ion being accepted by another molecule (A"-E"). 

The cyclic diolefin formed can then dehydrogenate as was discussed previously for this type of compound, and the hydrogen eliminated may be transferred to -methylstyrene, as was previously discussed for phenyl-cyclohexene, resulting in the formation of cumene. The diaryldiolefin shown in this mechanism was synthesized and successfully cyclized to p-terphenyl in the presence of sodium 55). 

Given the vastness of the subject matter I have limited myself to dealing with the structural (or static) aspects of macromolecular stereochemistry. An adequate treatment of the stereochemistry of polymerization, with specific regard to the polymerization of olefins and conjugated diolefins, would have occupied so much space and called for such a variety of additional information as to make this article excessively long and complex. I trust that others will successfully dedicate themselves to this task. However, the connection between polymer structure and polymerization mechanism is so important that the fundamentals of dyruunic macromolecular stereochemistry cannot be completely ignored in this chapter. 

Intramolecular bond formations include (net) [2 + 2] cycloadditions for example, diolefin 52, containing two double bonds in close proximity, forms the cage structure 53. This intramolecular bond formation is a notable reversal of the more general cycloreversion of cyclobutane type olefin dimers (e.g., 15 + to 16 +). The cycloaddition occurs only in polar solvents and has a quantum yield greater than unity. In analogy to several cycloreversions these results were interpreted in terms of a free radical cation chain mechanism. 

This reaction is based on a stoichiometric reaction of multifunctional olefins (enes) with thiols. The addition reaction can be initiated thermally, pho-tochemically, and by electron beam and radical or ionic mechanism. Thiyl radicals can be generated by the reaction of an excited carbonyl compound (usually in its triplet state) with a thiol or via radicals, such as benzoyl radicals from a type I photoinitiator, reacting with the thiol. The thiyl radicals add to olefins, and this is the basis of the polymerization process. The addition of a dithiol to a diolefin yields linear polymer, higher-functionality thiols and alkenes form cross-linked systems. 

The disproportionation activity in the supported species is parallel to the increased activity of ethylene polymerization on supported catalysts. Many of the steps in the reaction may be identical for example, the initial coordination of olefin to the metal center will be common to both systems. Indeed, some of these catalysts are also ethylene polymerization catalysts (see Table IV) although their activities are much less than the corresponding zirconium derivatives. A possible intermediate common to both disproportionation and polymerization could be the hydrocarbyl-olefin species (Structure I). Olefin disproportionation would result if the metal favored /3-hydrogen elimination to give the diolefin intermediate (Structure II) which is thought to be necessary for olefin disproportionation. Thus, the similarity between the mechanism and activation of olefin disproportionation and polymerization is suggested. 

Further investigations are necessary to clarify the mechanism of stereoregulation in the polymerization of olefins and diolefins probably the stereoelective polymerization of vinyl monomers and the asymmetric polymerization of diolefinic compounds will give further interesting contributions to the future progresses in this field. 

The importance of the electrophilic character of the cation in organo-alkali compounds has been discussed by Morton (793,194) for a variety of reactions. Roha (195) reviewed the polymerization of diolefins with emphasis on the electrophilic metal component of the catalyst. In essence, this review willattempt to treat coordination polymerization with a wide variety of organometallic catalysts in a similar manner irrespective of the initiation and propagation mechanisms. The discussion will be restricted to the polymerization of olefins, vinyl monomers and diolefins, although it is evident that coordinated anionic and cationic mechanisms apply equally well to alkyl metal catalyzed polymerizations of polar monomers such as aldehydes and ketones. 

However, the well-known ability of organolithium compounds to form associated species or to form complexes with electron donor compounds (240—242) provides strong support for mechanisms involving cationic attack by the lithium cation on the monomer prior to an anionic addition. With three orbitals available for coordination, a monomeric lithium alkyl should be able to complex both double bonds of a diolefin to provide the orientation for making cis-1,4 polymer and still have an orbital available for forming associated species in hydrocarbon solvents. The lithium orbitals are presumed to be directed tetrahedrally. Looking at the top of a tetrahedron with the fourth lithium oibital above and normal to the plane of the paper, the complex could have structure A below. In the transition state B for the addition step, the structure... 

Catalysts of the Ziegler type have been used widely in the anionic polymerization of 1-olefins, diolefins, and a few polar monomers which can proceed by an anionic mechanism. Polar monomers normally deactivate the system and cannot be copolymerized with olefins. However, it has been found that the living chains from an anionic polymerization can be converted to free radicals in the presence of peroxides to form block polymers with vinyl and acrylic monomers. Vinylpyridines, acrylic esters, acrylonitrile, and styrene are converted to block polymers in good yield. Binary and ternary mixtures of 4-vinylpyridine, acrylonitrile, and styrene, are particularly effective. Peroxides are effective at temperatures well below those normally required for free radical polymerizations. A tentative mechanism for the reaction is given. 

By comparing the photochemical behavior of conjugated diolefinic monomers in the crystalline state and in solution, a crystal matrix effect on four-center type photopolymerization has been revealed. It has been concluded that high molecular weigth linear polymers are produced photochemically from these monomers only by way of a crystal-lattice controlled mechanism. 

From these kinetic features it is concluded that the crystalline state photopolymerization of diolefinic compounds substantially follows a stepwise mechanism. The reaction of DSP is the first example of a stepwise photopolymerization20), not only in the crystalline state but also in other forms of photopolymerization. 

Given the /3-scission hypothesis, an examination of possible dodecenyl radicals indicates no reasonable mechanism of formation of Ci0H20 or CnH22. These products are formed only from the radical addition path. They can be used then to predict the relative importance of this path, XA. Decene is used to do this since it is better separated by GC from the corresponding diolefin than is undecene. XA is given by the ratio of the predicted value for decene from Table IV to the actual experimental yield. (Values for hydrogen atom addition are used here they are fairly close to those for CH3 and C2H5- addition, and available rate data (13, 14) indicate that H- atoms are the most likely radicals to add to dodecene.)... 

The mechanism originally proposed for the vinylic coupling reactions involved decomposition of a diolefin tr-complex 281, 282). However, the information developed since then on reactions of vinylic Pd(II) compounds suggests that these compounds are intermediates in the coupling reaction. The vinyl Pd(II) compounds could either oxidatively couple, since this is known to occur, or the vinyl Pd(II) intermediate could add across the olefinic double bond ... 

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