Mayo initiation mechanism

The kinetic model of styrene auto-initiation proposed by Hui and Hameilec [27] was used as a starting point for this work. The Mayo initiation mechanism was assumed (Figure 7.2) but the acid reaction was of course omitted. After invoking the quasi-steady-state assumption (QSSA) to approximate the reactive dimer concentration, Hui and Hameilec used different simplifying assumptions to derive initiation rate equations that are second and third order in monomer concentration. 

The initiation mechanism for styrene has been established [Graham et al., 1979 Kaufman, 1979 Kothe and Fischer, 2001 Mayo, 1968 Olaj et al., 1976, 1977a,b]. It involves the formation of a Diels-Alder dimer (XII) of styrene followed by transfer of a hydrogen atom from the dimer to a styrene molecule (Eq. 3-63). Whether formation of the... 

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... 

The Mayo mechanism involves a thermal Diels-AIder reaction between two molecules of S to generate the adduct 95 which donates a hydrogen atom to another molecule of S to give the initiating radicals 96 and 97. The driving force for the molecule assisted homolysis is provided by formation of an aromatic ring. The Diels-AIder intermediate 95 has never been isolated. However, related compounds have been synthesized and shown to initiate S polymerization."110... 

Despite the body of evidence in favor of the Mayo mechanism, the formation of diphenylcyclobutanes (90, 91) must still be accounted for. It is possible that they arise via the 1,4-diradical 94 and it is also conceivable that this diradical is an intermediate in the formation of the Diels-Alder adduct 95 (Scheme 3.64) and could provide a second (minor) source of initiation. Direct initiation by diradicals is suggested in the thermal polymerization of 2,3,4,5,6-pentafluorostyrene where transfer of a fluorine atom from Diels-Alder dimer to monomer seems highly unlikely (high C-F bond strength) and for derivatives which cannot form a Diels-Alder adduct. 

The polymerization of St with 56 as the initiator is considered to proceed via a reaction mechanism in Eq. (56), being identical to the models in Eqs. (18) and (20). The structure of both chain ends of the resulting polymer was confirmed by NMR using the deuterated St as the monomer. The polymerization with BPO and TEMPO without isolation of the adduct would also proceed via a similar path. In the absence of BPO, it has been reported that the radicals produced by spontaneous initiation according to the Mayo mechanism react with TEMPO to yield the adducts, and then they initiate polymerization [206]. 

Mayo, F. R. Chain transfer in the polymerization of styrene. VIII. Chain transfer with bromobenzenc and mechanism of thermal initiation. J. Am. Chem. Soc. 75, 6133 (1953). 

Spontaneous homopolymerization of a single monomer must occur by a free radical mechanism. The most studied example is styrene [120-121]. Flory originally proposed a 1,4-diphenylbutanediyl as the initiating species [122], More recent work has supported a later proposal by Mayo involving molecule-assisted... 

In 1933, M. S. Kharasch and F. W. Mayo found that some additions of HBr (but not HC1 or HI) to alkenes gave products that were opposite to those expected from Markovnikov s rule. These anti-Markovnikov reactions were most likely when the reagents or solvents came from old supplies that had accumulated peroxides from exposure to the air. Peroxides give rise to free radicals that initiate the addition, causing it to occur by a radical mechanism. The oxygen-oxygen bond in peroxides is rather weak, so it can break to give two alkoxy radicals. 

A distinctive characteristic of styrene polymerization is its thermal selfinitiation at high temperatures (without the presence of a chemical initiator). The mechanism of styrene thermal initiation was first described by Mayo [12]. The kinetics of thermal initiation were described by Weickert and Thiele [13] as a second-order reaction, while Hui and Hamielec [14], Husain and Hamielec... 

Mayo [16] proposed an alternative mechanism that is currently widely supported. Figure 7.7 shows a schematic of the Mayo mechanism. A Diels-Alder reaction between two styrene molecules produces an intermediate dimer (DH), also referred to as Mayo dimer . DH is highly reactive and has never been isolated. To complete the auto-initiation, DH reacts with a third styrene molecule via molecular assisted homolysis [17] to form a phenyltetraline radical (D ) and a phenethyl radical (SH ). A second reaction involving DH is to undergo chain transfer with a growing radical chain to produce a dead polymer chain (PS-H) and a new growing radical. The chain transfer constant (A ct) of DH has been estimated at 10, which is the highest Kcl ever reported for a molecule that contains no heteroatoms [18,19]. 

Also indicated in Figure 7.7 is the possibility of acid-catalyzed aromatization of DH to an unreactive dimer (DA). Under neutral conditions, only traces of DA are found. However, when a small amount of CSA is added to styrene undergoing polymerization by auto-initiation, significant levels of DA are formed along with polystyrene of higher than expected MW. We believe this is strong support for the Mayo mechanism since acid would have little affect on the Flory diradical intermediate. 

Figure 7.7 Auto-initiation of styrene monomer by the Mayo mechanism and acid-catalyzed aromatization of the intermediate reactive dimer...

Although neither of the chain transfer models lead to a suitable tool for production planning, they both exhibited the correct trends for conversion and Mw relative to the addition of acid (Figure 7.13). Since the model modifications are based solely on altering the amount of DH available for initiation and chain transfer, the model results are consistent with the Mayo mechanism. 

To account for this peroxide effect, Kharasch and Mayo proposed that addition can take place by two entirely different mechanisms Markovnikov addition by the ionic mechanism that we have just discussed, and anti-Markovnikov addition by a free-radical mechanism. Peroxides initiate the free-radical reaction in their absence (or if an inhibitor, p. 189, is addedX addition follows the usual ionic path. 

In principle, no added conventional initiator is required for S polymerization within the temperature range 100-130 C, since radicals formed from monomer through thermal initiation by the Mayo mechanism generate alkoxyamine initiators (Section 3.3.6.1). However, this method is seldom used in practice because the alkoxyamine generation step constitutes a very long inhibition period ( 24 hours depending on reaction temperature and nitroxide concentration). 

Scheme 2. Mayo mechanism for spontaneous initiation of styrene polymerization...

Clearly then, a major portion of the primary initiating radicals are derived by the Mayo mechanism. However, the formation of styrene dimers 1,2-diphenylbutane and 1,3-diphenyl-l-butene must be accounted for. Otsu and Sato [61] believe that the formation of primary initiating radicals from styrene... 

Olaj et al. [63] expand the Mayo mechanism focusing on the chemistry of the dimer intermediate. They performed UV spectroscopic measurements (315-365 nm) on polymerizing styrene and presented evidence to support the formation of two stereoisomers of the Mayo dimer (DHa and DHb). They suggest that both isomers are consumed during styrene polymerization. Possible consumption pathways are copolymerization, chain transfer, and formation of initiating radicals by MAH with monomer. They believe that only the axial phenyl isomer DHa is capable of generating initiating radicals by reaction with monomer. 

This mechanism has been extended to the spontaneous copolymerizations of styrene with maleic anhydride and other electron-acceptor monomers (see Scheme 1, Several authors have studied the spontaneous polymerization of st5rene with acrylonitrile focusing on isolated trimers that are produced presumably as a result of the initiation step however, the trimer structures do not suffice to differentiate between the Mayo mechanism and the Flory diradical mechanism. 

There is no doubt that a cycloadduct can be obtained in the self-initiation of styrene-maleic anhydride, although this is not necessarily a concerted reaction. A semi empirical calculus comparing the energy difference between reactants (styrene and maleic anhydride) and their cycloaduct and between two styrene molecules and their cycloadduct was carried out with the PM3 semi- empirical method provided by the Hyperchem program. Molecular geometries were calculated initially by molecular mechanics and afterwards by the PM3 calculation, at 0.01 convergence limits. The results were AE = 3. l Kcal/mol for the S-MA cycloadduct and AE = 122.8 Real/ mol for the Mayo styrene cycloadduct. However, the formation of the biradical of styrene requires 11.7 Kcal/mol and that of S-MA requires only 5.5 kcal/mol. The resonance stabilization of the biradical would easily produce the cycloadduct. 

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