Chain-growth reactions

Alternatively, the intermediate acetaldehyde (qv) for this process was obtained from ethylene by the Wacker process (9). A small amount of -butyl alcohol is produced in the United States by the Ziegler-Natta chain growth reaction from ethylene [74-85-1] (10). 

How does a dilatometer work to provide a means of measuring the rate of chain growth reactions From a practical perspective, what issues are likely to arise during this method of measurement ... 

We make polyethylene resins using two basic types of chain growth reaction free radical polymerization and coordination catalysis. We use free radical polymerization to make low density polyethylene, ethylene-vinyl ester copolymers, and the ethylene-acrylic acid copolymer precursors for ethylene ionomers. We employ coordination catalysts to make high density polyethylene, linear low density polyethylene, and very low density polyethylene. 

Characterization results have been reported for dendrigraft-polylpthylene ox-ide)s of generation Gl, prepared by adding different amounts of ethylene oxide to the initiator core in the side chain growth reaction (Table 9.8). Because of the grafting from method used, the molecular weight of the polyethylene oxide) branches cannot be accurately determined, and hence it is impossible to confirm... 

Initially the polymer molecular weight distribution obeys a Poisson distribution, typical of a chain growth reaction without chain transfer. Since the reactions are reversible, at a later stage, also the equilibration between the polymers becomes important and a broad distribution of molecular weights is obtained. As can be seen from Figure 16.5 the presence of linear alkenes causes chain termination (chain transfer) and thus low molecular weights are produced if the cycloalkenes are not sufficiently pure. 

Depending on the nature of the active center, chain-growth reactions are subdivided into radicalic, ionic (anionic, cationic), or transition-metal mediated (coordinative, insertion) polymerizations. Accordingly, they can be induced by different initiators or catalysts. Whether a monomer polymerizes via any of these chain-growth reactions - radical, ionic, coordinative - depends on its con-... 

Steric Defects in Stereospecific Chain growth Reactions and Analysis of Polymer Stereoregularity... 

Figure 10. Hydrogenation and chain growth reaction of ethylene in gas phase with amorphous FetoNieoPio catalyst at 223°C. Key O, ethane and A, propylene.

Now it s time to learn more about the chemistry of chain-growth reactions. To begin, let s consider the most prevalent type of chain reaction, free radical polymerization. A free radical is defined as a species having one unpaired electron. For example, the methyl free radical would look like this ... 

Tihe theory of free-radical addition polymerization, described in numer-ous publications (2, 3, 4, 17, 21), makes it clear that radical chain-growth reactions of polymers are regulated by statistical laws. Because of their statistical character the products from these reactions must be heterodisperse. The ranges extend from a single unit upward, depending upon kinetic details of the reactions. 

The authors conducted an experiment (now regarded as classical) in Fischer-Tropsch catalysis that supports this initiation mechanism (3,4). Using isotopes, they demonstrated that the carbon chain-growth reaction can occur from Ci species generated by the dissociation of CO. As shown below, this hypothesis implies that the rate of CO dissociation should be fast and should not control the overall Fischer-Tropsch reaction. 

In Section 2.1, we demonstrate the execution of our method and deduce kinetics expressions for the methanation reaction and for the chain-growth reaction. 

We can also investigate how the rate of deacfivafing C—C bond formation compares with the rate of initial C—H bond formation. These rates are a function of the strength of inferacfion befween adsorbed carbon atoms and the metal surface (28). When fhe mefhanation reaction expression is generalized such that it can be used to describe the chain-growth reaction, the resultant expression shows the complex CO pressure dependence of the rate of the chain-growth reaction. 

Because the Fischer-Tropsch chain-growth reaction depends strongly on the coverage of fhe cafalyst surface with "Ci" species, 0c(l)/ it is likely... 

Recent simulations by Marin and coworkers (56,57) seem to confirm Equations (12b) and (12c). A single-event microkinetics (SEMK ) model was used to analyze data characterizing Fischer-Tropsch catalysis on iron. The authors reported an activation energy of only 57 kj/mol for CO dissociation, whereas activation energies for the chain-growth reaction and termination reaction leading to alkane or alkene formation were found to be 45,118, and 97 kJ/mol, respectively. 

A characteristic of the Pichler-Schulz mechanism (presented in Scheme 2) is that the chain-growth reaction does not require high "Ci coverage but is favored by a high CO coverage. In contrast to the Biloen-Sachtler... 

To initiate chain growth, a "Ci" surface species may be required as in the Sachtler-Biloen mechanism, but the rate of formation of such species may be low. This scenario is comparable to conventional polymerization catalysis, in which initiation is usually the rate-limiting step. Assuming the generation of "Ci" species to be rate determining contrasts the Pichler-Schulz reaction scheme from the Sachtler-Biloen scheme, in which the slow step of the reaction is the termination. Because of the structure sensitivity of the CO dissociation reaction, and also because of the expected structure sensitivity of the chain-growth reaction, the Pichler-Schulz mechanism requires unique sites. The rate of CO insertion and consecutive steps should be fast compared with the rate of CO dissociation. Of course, the rate of termination should be low compared with that of chain growth. 

C—O bond in the adsorbed aldehyde has a barrier of only 40 kj/mol (see last step in Figure 4). However, the overall barrier for the chain-growth reaction CO + CH2 + 2H CH3CH - - O is 180 kJ/mol as a result of highly unstable intermediates that are postulated along the reaction coordinate between the steps of CO insertion and C—O cleavage (Figure 4). 

For CO insertion to play an important role in the chain-growth reaction, the subsequent cleavage of the C—O bond has to be fast in... 

Iron is also an important Fischer-Tropsch catalyst, but in the active state, it is present as a carbide (70,71) which is characterized by a unique chemistry that we do not discuss. Computations concerning the Fischer-Tropsch reaction on iron were performed by Bromfield et al. (72) and by Lo and Ziegler (73), who investigated the chain-growth reaction. 

Recombination of CH. fragments is an essential step to initiate the chain-growth reaction according to the Sachtler-Biloen carbide mechanism. In a series of elegant papers, Cheng et al. (31-33) reported on the structure dependence as well as on the metal dependence of this class of reactions. Activation energies for CH. —CH recombination on flat and stepped surfaces of cobalt are listed in Table 4. 

The second important conclusion from this section is that, at least on cobalt, the chain-growth reaction proceeds at the edge sites of stepped surfaces by recombination of carbene-type intermediates. This result is consistent with the chain-growth reaction mechanism proposed by Gaube and Klein (37), which is discussed in Section 4. 

For Equations (21a) and (21b) to apply, internal hydrogen atom transfer in the surface intermediates is an elementary step in the chain-growth reaction. Calculations by Ciobica et al. (90) indicate that the activation energies for C—H bond cleavage or C—H bond formation of adsorbed alkylidene or alkenyl intermediates are less than 50 kj/mol. 

---