Styrene polymerization Mayo mechanism

The identification of both phenylethyl and 1-phenyl-1,2,3,4-lelrahydronaphthalenyl end groups in polymerizations of styrene retarded by FeCl3/DMP provides the most compelling evidence for the Mayo mechanism.316 The 1-phenyl-1.2,3,4-tetrahydronaphthalenvl end group is also seen amongst other products in the TEMPO mediated polymerization of styrene,317318 However, the mechanism of formation of radicals 96 in this case involves reaction of the nitroxide with the Diels-AIder dimer (Scheme 3.63). The mechanism of nitroxide mediated polymerization is discussed further in Section 9.3.6. 

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... 

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. 

The Soviet investigators undertook the task of modeling the process of noniso-thermal styrene polymerization in the same temperature range. Starting from Mayo s kinetic scheme, they reduced the polymerization mechanism to four constants (kb kp, kt, km). The availability of two independent experimental dependencies (W = f (< )) and Pn = q>(q)) makes it possible to determine only two values of constants for each quantity. 

The identification of both phenylethyl and l-phenyl-1,2,3,4-letrahydronaphthalenyl end groups in polymerizations of styrene retarded by FeCl3/DMF provides the most compelling evidence for the Mayo mechanism." The l-phenyl-1,2,3,4-tetrahydronaphthalenyl end group is also seen amongst other... 

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

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. 

A similar mechanism of chain oxidation of olefinic hydrocarbons was observed experimentally by Bolland and Gee [53] in 1946 after a detailed study of the kinetics of the oxidation of nonsaturated compounds. Miller and Mayo [54] studied the oxidation of styrene and found that this reaction is in essence the chain copolymerization of styrene and dioxygen with production of polymeric peroxide. Rust [55] observed dihydroperoxide formation in his study of the oxidation of branched aliphatic hydrocarbons and treated this fact as the result of intramolecular isomerization of peroxyl radicals. 

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). 

The use of organometallic compounds as chain-transfer catalysts in free-radical polymerization has been widely studied. One objective is the production of polymers with terminal vinyl groups and lower molecular weight components compared with polymerization in the absence of chain-transfer catalysts. Gomplexes of cobalt(ii) have been used as effective catalysts, but the instability of the intermediate cobalt hydride does not permit firm establishment of the reaction mechanism. To address this issue, several chromium compounds have been applied as catalysts for the polymerization of methylmethacrylate (MMA) and styrene. The temperature dependence of the rate constant for free-radical polymerization of MMA for catalyzed chain transfer by (GsPh5)Gr(GO)3 has been determined using the Mayo equation. ... 

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 mechanism of the basic feature of a chain reaction may be illustrated by that of a free-radical reaction, particularly the polymerization of styrene, which has been extensively investigated for years by many investigators. Here we describe the mechanism proposed by Mayo and co-workers (1951, 1959). 

A clearly stepwise mechanism for epoxide formation was dononstrated in detail by Mayo and coworkers (18. 19). It was shown that in a chain copolymerization of styrene and molecular oxygen, any polymeric radical terminating in a styrene unit has the possibility of cleaving off an epoxide molecule from the end of the chain ... 

Currently the most popular mechanism to rationalize the self-initiated polymerization of styrene, eqs 11-13, involves the formation of the Diels-Alder adduct, AH, from two styrene molecules, and the MAH reaction of this dimer with a third styrene molecule. This novel and very elegant mechanism was originally suggested by Frank Mayo, and derived from his observation of phenyltetralin and phenylnaphthalene among the oligomers. It is clear that AH is the source of the phenyltetralin-type products. However, while a critical review (7) of the evidence establishes that AH is present during polymerization, there is no conclusive evidence that it is involved in eq 12, the MAH step. 

Candidate for Initiation of Polymerization by 1,4-Diradicals As was described above, my research group has been involved for some time with efforts to establish the Diels-Alder mechanism for initiation in the thermal polymerization of styrene suggested by Mayo. However, we also have argued that some of the initiation in styrene might be due to 1,4-diradicals. (See Figure IB.)... 

---