Propagation reactions alkenes

As a simple computational model for the catalysis of alkene polymerization, let us consider some aspects of the general chain-propagation reaction... 

On the basis of this simple orbital picture, we can also consider the effect of alternative alkene pendant groups R in the catalytic propagation reaction (4.105). In the case of propylene (R = CH3), for example, one can envision two distinct isomers of the alkyl-alkene complex, with either the primary or the secondary alkene carbon atom as the proximal Cp. This leads to the alternative primary and... 

This reaction may account in part for the oligomers obtained in the polymerization of pro-pene, 1-butene, and other 1-alkenes where the propagation reaction is not highly favorable (due to the low stability of the propagating carbocation). Unreactive 1-alkenes and 2-alkenes have been used to control polymer molecular weight in cationic polymerization of reactive monomers, presumably by hydride transfer to the unreactive monomer. The importance of hydride ion transfer from monomer is not established for the more reactive monomers. For example, hydride transfer by monomer is less likely a mode of chain termination compared to proton transfer to monomer for isobutylene polymerization since the tertiary carbocation formed by proton transfer is more stable than the allyl carbocation formed by hydride transfer. Similar considerations apply to the polymerizations of other reactive monomers. Hydride transfer is not a possibility for those monomers without easily transferable hydrogens, such as A-vinylcarbazole, styrene, vinyl ethers, and coumarone. 

Reaction 8 may, therefore, be the major chain-propagating reaction of H02 between 250° and 400°C. The radicals produced will, of course, undergo the same fates as those produced in Reaction 4, regenerating (eventually) alkyl radicals. The main difference between the alkene-H02 addition route and the alkylperoxy radical isomerization route is that in the former case the hydroperoxyalkyl radicals formed are necessarily a-radicals—i.e., radicals in which the unpaired electron is borne by a carbon atom adjacent to that bearing the hydroperoxy group, such as... 

The understanding of the reaction mechanism is directly related to the role of the catalyst, i.e., the transition metal. It is universally accepted that olefin metathesis proceeds via the so-called metal carbene chain mechanism, first proposed by Herisson and Chauvin in 1971 [25]. The propagation reaction involves a transition metal carbene as the active species with a vacant coordination site at the transition metal. The olefin coordinates at this vacant site and subsequently a metalla-cyclobutane intermediate is formed. The metallacycle is unstable and cleaves in the opposite fashion to afford a new metal carbene complex and a new olefin. If this process is repeated often enough, eventually an equilibrium mixture of alkenes will be obtained. 

The resulting mechanism includes 16 isomerization, eight /i-decomposition and nine dehydrogenation reactions, which are the primary propagation reactions of five different K-decyl radicals. As already discussed elsewhere, it is very easy to extend this generation to heavier species (Ranzi et al., 2005). Of course, the complexity, or rather the number of elementary reactions, and the number of intermediate radicals and molecules in particular, rapidly increase with the number of C-atoms. The need to introduce the primary reactions of the primary products, such as alkenes, is also taken into account. 

Figure A2 shows the number of components involved in the primary propagation reactions of normal alkenes. These data clearly indicate that this number rapidly becomes very large, increasing the carbon number of the initial alkene. This fact justifies the need to turn to the component lumping when dealing with detailed and mechanistic models describing the hydrocarbons pyrolysis of such heavy species.

Fig. A2. Components number in the primary propagation reactions of normal-alkenes.

The second important type of propagation reaction is addition to multiple bonds addition to C=C is particularly important. In reaction (6.34), R can be an atom or a group centred on carbon or any element which forms a bond stronger than the n bond which is broken in the reaction (about 250 kJ mol-1). If the alkene is unsymmetrical, addition can in principle take place at either end of the double bond. Addition normally takes place at the end of the double bond which will generate the more stable free radical. Thus for addition of a halogen atom to propene, attack at the CH2 position will give the secondary radical 47 (reaction 6.35) rather than attack at the central carbon atom which would give the less stable primary radical 48 (reaction 6.36). 

Radicals react with an alkene to form a carbon racial (see 156). When the C=C unit of an alkene reacts with a radical X to form a radical intermediate (X-C-C ), this radical can react with another molecule of the alkene to form another radical (X-C-C-C-C ). In this chain propagation reaction, another reaction with more alkene leads to X-C-C-C-C-C-C. If this process continues, the final product will be a large molecule known as a polymer, -(C-C) -, where n is a large number, such as 500 or 2,000, that represents the number of times that unit is repeated. 

Organic radicals are also included in the larger class of coordi-natively unsaturated carbon atoms, but are neutral. They too will react with alkenes to generate a covalent connection and new radical and can thus participate in polymerization reactions not unlike anionic polymerization. A typical propagation reaction for the radical polymerization of vinyl chloride is shown in Figure 4 as an example. However, radicals can be more indiscriminant in their reactivity and radical polymerizations are not quite as straightforward as anionic polymerizations. The key difference is that when two anions... 

A radical reaction or radical chain propagation (such as in alkene polymerization) is terminated by either the reaction of two radicals or by disproportionation of the radical into alkane and alkene (Scheme 2.2.6). The latter reaction plays the dominant role in petrochemical cracking processes. Alternatively, a radical reaction can be stopped by adding to the reaction mixture substances that react very easily with radicals by forming very stable radicals themselves so that the propagation reaction is terminated. Examples of such radical scavenger molecules are phenols, quinones, and diphenylamines. 

The carbon-centered radical R, resulting from the initial atom (or group) removal by a silyl radical or by addition of a silyl radical to an unsaturated bond, can be designed to undergo a number of consecutive reactions prior to H-atom transfer. The key step in these consecutive reactions generally involves the intra-or inter-molecular addition of R to a multiple-bonded carbon acceptor. As an example, the propagation steps for the reductive alkylation of alkenes by (TMSfsSiH are shown in Scheme 6. 

The most general method for formation of new carbon-carbon bonds via radical intermediates involves addition of the radical to an alkene. The reaction generates a new radical that can propagate a chain sequence. The preferred alkenes for trapping alkyl... 

Radicals for addition reactions can be generated by halogen atom abstraction by stannyl radicals. The chain mechanism for alkylation of alkyl halides by reaction with a substituted alkene is outlined below. There are three reactions in the propagation cycle of this chain mechanism addition, hydrogen atom abstraction, and halogen atom transfer. 

There are several guidelines that should be followed in order to increase the chemoselectivity of the monoadduct. Firstly, radical concentration must be low in order to suppress radical termination reactions (rate constant of activation [fcal and fca2] < < rate constant of deactivation kd t andfcd2]). Secondly, further activation of the monoadduct should be avoided ( al> >kd2). Lastly, formation of oligomers should be suppressed, indicating that the rate of deactivation (kd 2[Cu"LmX]) should be much larger than the rate of propagation ( [alkene]). Alkyl halides for copper-catalyzed ATRA are typically chosen such that if addition occurs, then the newly... 

Syntheses of isolable organometallic species by carbometallations of alkenes are difficult because many side reactions can occur, namely p-hydride elimination and chain propagation. As a consequence, only a few examples have been reported to date, mainly concerning reactions in which the initial carboalumination product is trapped through fast intra-... 

P. H. Plesch, The Propagation Rate-Constants of the Cationic Polymerisation of Alkenes, Progress in Reaction Kinetics, 1993, 18, 1. 

With isobutene and other 1,1-disubstituted alkenes [72,73] the great stability of the tertiary carbonium ion ensures that propagation is energetically by far the most favourable reaction. 

Isobutene - In contrast to the complicated picture presented by the polymerisations of most other alkenes, the polymerisation of isobutene at low temperatures is a clean reaction with apparently few complications [10, 16, 17, 18]. The propagation step seems to be a simple addition to the monomer of the tertiary carbonium ion at the growing end of the chain. This difference between the behaviour of isobutene and of most other olefins is so striking that isobutene could usefully be regarded as a standard of reference it would thus be possible to enquire into the behaviour of other olefins by comparing them and their polymers with isobutene and polyisobutene. 

Therefore further progress in this area depends on the measurement of equilibrium constants. At this stage I simply cannot say how much of the difference of two powers of 10 between the k+Bpl of the alkenes and the styrenes is to be attributed to an intrinsic difference in reactivity and how much to the existence of the P+ G complexes. The negative temperature coefficient of the rate constant for a-methyl styrene found by Chawla Huang (1975) is a strong indication in favour of my view that the propagation is not a simple bimolecular reaction. 

The present author wanted to determine the propagation rate-constant, kp, for the cationic polymerisation of various alkenes (this term here includes vinyl ethers, VE) under such conditions that the interpretation of the measurements should be as unambiguous as possible. If there are no complications from the complexation of the propagating species with constituents of the reaction mixture, the rate of such polymerisations is given generally by equation (1) ... 

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