Alkenes, addition reactions polymerization

The same high reactivity of radicals that makes possible the alkene polymerization we saw in the previous section also makes it difficult to carry out controlled radical reactions on complex molecules. As a result, there are severe limitations on the usefulness of radical addition reactions in the laboratory. Tn contrast to an electrophilic addition, where reaction occurs once and the reactive cation intermediate is rapidly quenched in the presence of a nucleophile, the reactive intermediate in a radical reaction is not usually quenched, so it reacts again and again in a largely uncontrollable wav. 

Synthetic polymers can be classified as either chain-growth polymen or step-growth polymers. Chain-growth polymers are prepared by chain-reaction polymerization of vinyl monomers in the presence of a radical, an anion, or a cation initiator. Radical polymerization is sometimes used, but alkenes such as 2-methylpropene that have electron-donating substituents on the double bond polymerize easily by a cationic route through carbocation intermediates. Similarly, monomers such as methyl -cyanoacrylate that have electron-withdrawing substituents on the double bond polymerize by an anionic, conjugate addition pathway. 

The term carbometallation was most probably coined only about a quarter of a century ago.1 However, the history of those reactions that can be classified as carbometallation reactions is much older. If one includes not only the Ziegler-Natta-type organometallic alkene polymerization reactions2 but also various types of organometallic conjugate addition reactions,3 carbometallation collectively is easily more than a century old. In its broadest definition, carbometallation may be defined as a process of addition of a carbon-metal bond to a carbon-carbon multiple bond. As such, it may represent either a starting material-product relationship irrespective of mechanistic details or an actual mechanistic microstep of carbon-metal bond addition to a carbon-carbon metal multiple bond irrespective of the structure of the product eventually formed. 

Ethene or ethylene is the most important organic chemical used in commercial applications. Annual production of ethylene in the United States was over twenty-five million tons in the year 2000. Propylene is also used in large quantities with an annual production of over thirteen million tons. Alkenes such as ethylene and propylene have the ability to undergo addition polymerization. In this process, multiple addition reactions take place and many molecules link together to form a polymer. A polymer is a long chain of repeating units called monomers. For example, the addition of two ethylene molecules can be represented as... 

Additions. Homolytic bimolecular addition reactions of carbon-centered radicals to unsaturated groups have been studied in detail because these are the reactions of synthesis and polymerization. Within this group, radical additions to substituted alkenes are by far the best understood. An excellent compilation of rate constants for carbon radical additions to alkenes is recommended for many specihc kinetic values. ... 

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. 

Other additions, such as addition of alkyl halides and carbonyl compounds, are discussed in Chapter 5, whereas Chapter 7 covers addition reactions involving carbon monoxide (hydroformylation, carboxylations). Hydrogen addition is discussed in Chapter 11. The nucleophilic addition of organometallics to multiple bonds is of great significance in the anionic polymerization of alkenes and dienes and is treated in Chapter 13. 

Rhodium(II) acetate catalyzes C—H insertion, olefin addition, heteroatom-H insertion, and ylide formation of a-diazocarbonyls via a rhodium carbenoid species (144—147). Intramolecular cyclopentane formation via C—H insertion occurs with retention of stereochemistry (143). Chiral rhodium (TT) carboxamides catalyze enantioselective cyclopropanation and intramolecular C—N insertions of CC-diazoketones (148). Other reactions catalyzed by rhodium complexes include double-bond migration (140), hydrogenation of aromatic aldehydes and ketones to hydrocarbons (150), homologation of esters (151), carbonylation of formaldehyde (152) and amines (140), reductive carbonylation of dimethyl ether or methyl acetate to 1,1-diacetoxy ethane (153), decarbonylation of aldehydes (140), water gas shift reaction (69,154), C—C skeletal rearrangements (132,140), oxidation of olefins to ketones (155) and aldehydes (156), and oxidation of substituted anthracenes to anthraquinones (157). Rhodium-catalyzed hydrosilation of olefins, alkynes, carbonyls, alcohols, and imines is facile and may also be accomplished enantioselectively (140). Rhodium complexes are moderately active alkene and alkyne polymerization catalysts (140). In some cases polymer-supported versions of homogeneous rhodium catalysts have improved activity, compared to their homogenous counterparts. This is the case for the conversion of alkenes direcdy to alcohols under oxo conditions by rhodium—amine polymer catalysts... 

The properties of a compound with isolated double bonds, such as 1,4-pentadiene, generally are similar to those of simple alkenes because the double bonds are essentially isolated from one another by the intervening CH2 group. However, with a conjugated alkadiene, such as 1,3-pentadiene, or a cumulated alkadiene, such as 2,3-pentadiene, the properties are sufficiently different from those of simple alkenes (and from each other) to warrant separate discussion. Some aspects of the effects of conjugation already have been mentioned, such as the influence on spectroscopic properties (see Section 9-9B). The emphasis here will be on the effects of conjugation on chemical properties. The reactions of greatest interest are addition reactions, and this chapter will include various types of addition reactions electrophilic, radical, cycloaddition, and polymerization. 

The fundamental process in alkene polymerization is a double-bond addition reaction similar to those discussed in Section 23.10. A species called an initiator, In, first adds to the double bond of an alkene, yielding a reactive intermediate that in turn adds to a second alkene molecule to produce another reactive intermediate, and so on. 

The success of the modified patterns treatment shows that radical reactivities in the gas phase are governed to a major extent by polar forces as given quantitative expression in the Hammett equation. These correlations support the conclusions reached in earlier sections that both the polarity of the alkene and the polar character of the radical are important. They also help to establish a common pattern of behaviour for radicals in gas-phase addition reactions and in liquid-phase polymerization processes. 

Ethylene shows all the chemical properties of alkenes. It undergoes combustion, addition reactions and polymerization reactions. It burns with a bright yellow flame. 

The elementary reactions of carbocationic polymerizations can be separated into three types. Deactivation of carbenium ions with anions and transfer to counteranion are ion-ion reactions, propagation and transfer to monomer are ion-dipole reactions, and ionization is a dipole-dipole reaction [274]. Ion-ion and dipole-dipole reactions with polar transition states experience the strongest solvent effects. Carbocationic propagation is an ion-dipole reaction in which a growing carbenium ion adds electro-philically to an alkene it should be weakly accelerated in less polar solvents because the charge is more dispersed in the transition state than in the ground state [276]. However, a model addition reaction of bis(p-methoxyphenyl)carbenium ions to 2-methyl- 1-pentene is two times faster in nitroethane (e = 28) than in methylene chloride (e = 9) at - 30° C [193]. However, this is a minor effect which corresponds to only ddG = 2 kJ morit may also be influenced by specific solvation, polarizability, etc. [276,277]. 

UV studies suggest that in 1-vinyl-imidazoles and -benzimidazoles there is conjugation with the heteroaromatic ring. Certainly the reactivities of such compounds would indicate that they are not normal alkenes, nor are the alkynyl compounds typical of other alkynes. They are very prone to polymerization processes, and electrophilic addition reactions are often quite difficult to accomplish. When 1-styrylimidazoles are prepared, the trans compound is much more likely to eventuate as the more stable isomer 73CJC3765, 76JCS(PD545). 

Alkenyl- and alkynyl-triazoles have received little attention. By analogy with the behaviour of other azoles they are expected to polymerize but to be less reactive in addition reactions than alkenes or alkynes. Although the most promising polymers derived from triazoles are obtained by different methods (see Section 4.12.5.2.3), some information is available on potentially polymerizable vinyltriazole (63MI4120i). The styryltriazole (134) could be oxidized to 3-methyl-l-phenyl-l,2,4-triazole, i.e. without affecting either the triazole or iV-phenyl ring, but hydroxylation of the alkene chain failed (s4JCS4256). 

On a coulometric scale, however, the n values for reduction of 51 and 52 were found to be less than one in DMF (see Table 11), probably due to polymerization under the dry conditions [7,55], a behavior similar to but less pronounced than that found for reduction of monoactivated alkenes. Addition of alkali metal cations did not change the n value significantly for 51 [71] but led to an increase in n for 52 [7,71]. At the same time, the overall rate of reaction of 52 increased considerably [71], an effect interpreted similarly to that observed for monoactivated alkenes (Sec. II.A.7). 

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