Elimination reactions polymerization

In ATRP, the initiator (RX) determines the number of growing chains. Ideally, the degree of polymerization is given by eq. 7 and the molecular weight by cq. 8. Note the appearance of the initiator efficiency (/ ) in the numerator of these expressions. In practice, the molecular weight is ofien higher than anticipated because the initiator efficiency is decreased by side reactions. In some cases, these take the form of heterolytic decomposition or elimination reactions. Further redox chemistry of the initially formed radicals is also known. The initiator efficiencies are dependent on the particular catalyst employed. 

The formation of alkenes and alkene-related polymerization products can seriously reduce the yields of desired alkane products from secondary alcohols, which can undergo elimination reactions. For example, reduction of 2-octanol at 0° with boron trifluoride gas in dichloromethane containing 1.2 equivalents of tri-ethylsilane gives only a 58% yield of n-octane after 75 minutes (Eq. II).129 The remainder of the hydrocarbon mass comprises nonvolatile polymeric material.126... 

The parent [3]radialene 1 has been generated from variously functionalized cyclopropane precursors by classical -elimination reactions (Scheme l)2-6. All these reactions have been carried out as gas-phase reactions, and the radialene has been collected at —63 °C or below. At —78 °C, the pure compound is stable for several days, but polymerization occurs when the vapor is exposed to room temperature as well as in carbon tetrachloride at 273 K2, or in contact with oxygen3. 

A number of runaway reaction incidents which have involved the formation of polymeric species differ from usual polymerisation reactions of monomers in that elimination reactions of various types have been involved. Individually indexed examples are ... 

The second important chain termination reaction characteristic of the catalysis of anionic polymerization of epoxy compounds by the TA consists in the abstraction of the hydrogen atom from the p-carbon atom in the tetraalkylammonium cation by the growing alkoxy anion (P-elimination reaction)I58 164). 

This unexpected result may be related to the increase in TOC on fraction G3 and may be further evidence of the polymerization phenomenon discussed earlier. However, this hypothesis must be carefully considered because of our limited knowledge of pyrolysis mechanisms. The possibility of phenol formation during the thermal fragmentation process from elimination reactions followed by cycliza-tion of poly conjugated chains has been suggested by Bracewell (22) and should be investigated. 

The second common method of polymer synthesis involves the stepwise coupling of small molecules which are difunctional by virtue of reactive functional groups. A typical example of step-reaction polymerization would be the synthesis of polyamides by reaction of a diamine with a diacid. In these systems the chain is built up slowly by reaction of any pair of functional groups in the system and it is common for the coupling to involve elimination of a small molecule. Conventionally these polymerizations allow more control over the chain structure but difficulties in reaching very high conversions and problems of reagent purity usually lead to much shorter... 

Depolymerization is a special case of thermal degradation. It can be observed particularly in polymers based on a, a -disubstituted monomers. In these, degradation is a reversal of the synthesis process. It is a chain reaction during which the monomers are regenerated by an unzipping mechanism. This is due to the low polymerization enthalpy of these polymers. For the thermal fission of polymers with secondary and tertiary C-atoms, higher energies are required. In these cases elimination reactions occur. This can be seen very clearly in PVC and PVAC. 

Another type of elimination reaction favoured under plasma conditions is the decarboxylation. Carbocyclic acids easily lose carbon dioxide to form the parent hydrocarbons. In acid anhydrides decarboxylation is followed by a decar-bonylation. Cyclic or bicyclic anhydrides fragment forming unsaturated compounds, a reaction which has been studied with phthalie anhydride 24>. This anhydride decomposes to dehydrobenzene which, in the absence of other compounds, dimerizes, trimerizes or polymerizes. Orientation experiments indicated similar results for aliphatic acid anhydrides. 

The unsaturated hexodialdopyranosides formed are very readily transformed by either alkaline or acid treatment into brown, polymerized products. Acid treatment of 186, the / -elimination product of methyl a-D-g/ co-hexodialdo-l,5-pyranoside, produced the reactive intermediates 187 and 188.391 The 2,3-diacetate of 186 was obtained by Perlin et al. by an oxidation / -elimination reaction.392... 

Optically active vinyl sulfoxide was prepared by a combination of resolution and elimination reaction. Firstly, inclusion complexation of rac-2-chloroethyl m-tolyl- sulfoxide (118) and 14b in benzene gave, after two recrystallizations from benzene, a 1 1 complex of 14b and (+)-118 of 100% ee in 72% yield. Secondly, treatment of the complex with 10% NaOH gave optically pure (+)-m-tolyl vinyl sulfoxide (119) by HC1 elimination as colorless oil. Rapid polymerization of the (+)-119 proceeded by treatment with BuLi or BuMgBr at -78 °C to give optically active polymer (120). Oxidation of 120 with H2O2 gave optically active polysulfone (121).48... 

The involvement of the a-elimination reaction in this cycle has been in question following experiments on cyclopentadienyl cobalt complexes, where evidence for olefin insertion for Ziegler-Natta polymerization catalysis has been obtained by labelling experiments, using C2H4 with a deuterated cobalt complex (70) ... 

The time to tQ is the time for the wood-monomer mass to reach oven or curing temperature at T5. During the period of constant temperature, the induction period, the inhibitor is being removed by reaction with the free radicals. Once the inhibitor is eliminated from the monomer and wood, the temperature rises to a maximum which corresponds to the peak of the exothermic polymerization reaction. Polymerization continues to completion although at a decreased rate and the temperature returns to that of the curing chamber. The time to the peak temperature depends upon the amount of catalyst present, the type of monomer, the type of crosslinker, and the ratio of the mass of monomer to that of the wood. The wood mass acts as a heat sink. Figure 4 illustrates the effect of increased Vazo catalyst on the decrease in time to the peak temperature, and the increase in the peak temperature(10)... 

Organic reactions can be grouped into basic classes of addition, substitution, elimination, and polymerization along with combustion and oxidation reactions. 

Theoretically, it is possible for the process of olefin coordination and insertion to continue as in Ziegler-Natta polymerization (Chapter 52) but with palladium the metal is expelled from the molecule by a p-hydride elimination reaction and the product is an alkene. For the whole process to be catalytic, a palladium(O) complex must be regenerated from the palladium(ll) product of P-hydride elimination. This occurs in the presence of base which removes HX from the palladium(II) species. 

We have already seen in Section 2.2.2 that metal-alkyl compounds are prone to undergo /3-hydride elimination or, in short, /3-elimination reactions (see Fig. 2.5). In fact, hydride abstraction can occur from carbon atoms in other positions also, but elimination from the /8-carbon is more common. As seen earlier, insertion of an alkene into a metal-hydrogen bond and a /8-elimination reaction have a reversible relationship. This is obvious in Reaction 2.8. For certain metal complexes it has been possible to study this reversible equilibrium by NMR spectroscopy. A hydrido-ethylene complex of rhodium, as shown in Fig. 2.8, is an example. In metal-catalyzed alkene polymerization, termination of the polymer chain growth often follows the /8-hydride elimination pathway. This also is schematically shown in Fig. 2.8. 

The reaction of alkenes and other unsaturated substances with transition metal hydrido or alkyl complexes is of prime importance in catalytic reactions such as hydrogenation, hydroformylation, and polymerization (see Chapter 22). It is one of the major methods for synthesizing metal-to-carbon bonds. The reverse reaction, the /3-hydride or /3-alkyl transfer-alkene elimination reaction has already been discussed (Section 21-3). 

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