Styrene monomers, Diels-Alder dimerization

Not only the case of vinyl chloride but also styrene shows that the observed chain transfer to monomer is not the simple reaction described by Eq. 3-112. Considerable evidence [Olaj et al., 1977a,b] indicates that the experimentally observed Cm may be due in large part to the Diels-Alder dimer XII transferring a hydrogen (probably the same hydrogen transferred in the thermal initiation process) to monomer. 

Styrene (and derivatives) also possesses the rare monomer quality that the neat material, without initiator, may be spontaneously polymerized by simply heating to 80-100°C for 24-48 hr. It is thought that this occurs via the initial Diels-Alder dimerization of styrene to the two diasteomers A and B [14]. The two diastereomers appear to have an extremely labile hydrogen, which is both doubly allylic and tertiary. However, only dimer A has the correct stereochemistry (an axial phenyl), which enables the excess styrene to abstract a hydrogen atom from it, producing two radical species (Eq. 23.5). 

The higher than second-order rate observed for thermal conversion of monomer indicates that Eq. (6.96) is the slow step. Representing the concentration of Diels-Alder dimer (V) by [D] and that of styrene by [M],... 

Exit must be of a small radical, and this must be a species formed by chain transfer, either to chain-transfer agent or to monomer. Now, for a system such as that shown in Figure 5.2, there is no added chain-transfer agent, so the exiting species must have arisen from chain transfer to monomer. There are two points about exit which often seem strange. First, styrene contains no labile hydrogens, so transfer to monomer would appear impossible. Second, even if this were to occur, the resulting species would be so insoluble that it would never exit. In fact, the transfer process may well be [30] to a Diels-Alder dimer of styrene ... 

A few monomers, like styrene and methyl methaciylate, will, after careful purification and presumably free from all impurities, polymerize at elevated temperatures. It is supposed that some ring-substituted styrenes act similarly. The rates of such thermal self-initiated polymerizations are slower than those carried out with the aid of initiators. Styrene, for instance, polymerizes only at a rate of 0.1 % per hour at 60 C, and only 14% at 127 C. The rate of thermal polymerization of methyl methacrylate is only about 1% of the rate for styrene.Several mechanisms of initiation were proposed earlier. The subject was reviewed critically. More recently, the initiation mechanism for styrene polymerization was shown by ultraviolet spectroscopy to consist of an initial formation of a Diels-Alder dimer. The dimer is believed to subsequently transfer a hydrogen to a styrene molecule and form a free radical ... 

A comprehensive kinetic mechanism is proposed to describe the combined chemical and thermal free-radical polymerization of styrene. Thus, besides the commonly employed reactions (e.g., chemical initiation, propagation and termination), thermal initiation and chain transfer to monomer and to Diels-Alder adduct reactions are included. In particular, the so-called AH thermal initiation mechanism of Mayo comprises a reversible Diels-Alder dimerization of styrene to form l-phenyl-1,2,3,9-tetrahydronaphtalene (AH), the formation of a styryl (m) and a 1-phenyltetralyl radical... 

Figure L (A) This chart outlines the Diels-Alder dimerization of 2 styrene monomers (or similar vinyl aromatics) to form AH and the subsequent possible reactions of AH to give oligomers or to produce free radicals. The [2 + 2] cycloaddition of 2 monomer units to form the 1,4-airadical Mt also is shown. (B) The reactions of 1,4-diradical that convert it to oligomers such as dicyclobutanes (DCB) or to monoradicals. ( See Refs. 5,6, and 7.)...

However, the probability for the reaction progression greatly depends on the monomer conversion. Because the viscosity of the dispersed phase, in the first stage, is fairly low and the quantity of styrene is sufficiently high, the decomposition process (Figure 9.4) occurs only up to the benzoyloxy radical, which can directly start the kinetic chain. The purely thermal start of chains with reactive dimers of styrene, as a result of Diels-Alder reaction, can be ignored at fairly low temperatures of suspension polymerization, in contrast to the conditions for the bulk styrene process [4-7]. 

One monomer that is known to self-initiate, even at high purity is styrene [8-10]. As shown in Scheme 1.2, styrene undergoes a Diels-Alder reaction to give a styrene dimer. This dimer can then react with another styrene monomer to give a styrene radical or RJ. Significantly, the activation energy for the self-initiation is rather large. 

For styrene, the conversion of monomer per hour rises from —0.1% at 60°C to about 14% at 140°C. Thus, the effect has to be encountered, especially for polymerizations at higher temperatures. Furthermore, when a styrene-based monomer is to be purified by distillation, the addition of inhibitors and distillation at reduced pressure is advisable in order to avoid the distillate from becoming viscous. Another difficulty occurring during distillation is the formation of polymer in the column, which can also be prevented by distilling in vacuo. The initiation of a styrene-based monomer is assumed to involve a (4-1-2) cycloaddition of the Diels-Alder type with a subsequent hydrogen transfer from the dimer to another monomer molecule ... 

The dimer of hexafluorothioacetone (217) reacts, as shown in Scheme 8, with mercuric fluoride-potassium fluoride to give the mercuric mercaptide of perfluoropropane-2-thiol in DMF, potassium flucnide promotes the reaction of (217) with alcohols, thiols, thiophenols, and olefins to give products derived from the thioacetone monomer. Reaction with styrene gave the bis-Diels-Alder adducts (218) and (219) thietan (220) is obtained with cyclohexene. Amino-alcohols, o-aminophenol, o-aminothiophenol, and o-phenylenediamine yield 2,2-bis(trifluoromethyl)-l,3-het ocyclic derivatives, e.g. (221). 1,3-Dithietans did not react with Fe2(CO) and were discarded as possible intermediates in the ortho-metallation reactions of thiobenzophenones. The hydrolysis reactions of a 1,3-cyclodisiladithietan and its reaction with o-hydroxymercuribenzoic acid have been investigated. ... 

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