Polymerization self-initiated

Fig. 3-9 Inhibition and retardation in the thermal, self-initiated polymerization of styrene at 100°C. Plot 1, no inhibitor plot 2, 0.1% benzoquinone plot 3, 0.5% nitrobenzene plot 4, 0.2% nitrosobenzene. After Schulz [1947] (by permission of Verlag Chemie GmbH and Wiley-VCH, Weinheim).

The effect of temperature on the rate and degree of polymerization is of prime importance in determining the manner of performing a polymerization. Increasing the reaction temperature usually increases the polymerization rate and decreases the polymer molecular weight. Figure 3-13 shows this effect for the thermal, self-initiated polymerization of styrene. However, the quantitative effect of temperature is complex since Rp and X depend on a combination of three rate constants—kd, kp, and kt. Each of the rate constants for initiation, propagation, and termination can be expressed by an Arrhenius-type relationship... 

Fig. 3-13 Dependence of the polymerization rate (O) and polymer molecular weight ( ) on the temperature for the thermal self-initiated polymerization of styrene. After Roche and Price [1952] (by permission of Dow Chemical Co., Midland, MI).

For a purely photochemical polymerization, the initiation step is temperature-independent (Ed = 0) since the energy for initiator decomposition is supplied by light quanta. The overall activation for photochemical polymerization is then only about 20 kJ mol-1. This low value of Er indicates the Rp for photochemical polymerizations will be relatively insensitive to temperature compared to other polymerizations. The effect of temperature on photochemical polymerizations is complicated, however, since most photochemical initiators can also decompose thermally. At higher temperatures the initiators may undergo appreciable thermal decomposition in addition to the photochemical decomposition. In such cases, one must take into account both the thermal and photochemical initiations. The initiation and overall activation energies for a purely thermal self-initiated polymerization are approximately the same as for initiation by the thermal decomposition of an initiator. For the thermal, self-initiated polymerization of styrene the activation energy for initiation is 121 kJ mol-1 and Er is 86 kJ mol-1 [Barr et al., 1978 Hui and Hamielec, 1972]. However, purely thermal polymerizations proceed at very slow rates because of the low probability of the initiation process due to the very low values f 1 (l4 IO6) of the frequency factor. 

Ej has a value of about —60 kJ mol-1 for thermal initiator decomposition, and Xn decreases rapidly with increasing temperature. Ej is about the same for a purely thermal, self-initiated polymerization (Fig. 3-16). For a pure photochemical polymerization Ej is positive by approximately 20 kJ mol-1, since Ed is zero and X increases moderately with temperature. For a redox polymerization, Ej is close to zero, since Ed is 40-60 kJ mol-1, and there is almost no effect of temperature on polymer molecular weight. For all other cases, Xn decreases with temperature. 

Boron trichloride and tribromide successfully polymerize styrenes and isobutene. These Lewis acids are typically used in combination with water or alkyl chlorides, acetates, ethers, and alcohols [105,153]. In contrast to earlier reports, BC13 can self-initiate polymerization of styrene and isobutene [137] by haloboration, and subsequent activation of the resulting alkyl chlorides by excess Lewis acid. Direct initiation was confirmed by the formation of lower molecular weight polymers than pre-... 

Polymerizations which are second order in Lewis acid have also been observed, including self-initiated polymerizations with AlBrj [181], and isobutene polymerizations initiated by alkyl esters and halides activated by TiCU [175,182],... 

Problem 6.14 There is evidence [30] that thermal self-initiated polymerization of styrene may be of about five-halves order. Show that this is in agreement with the established initiation mechanism involving a Diels-Alder dimer formation [Eqs. (6.95) and (6.96)]. 

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 ... 

Self-Initiated Polymerization. Free radical polymerizations can also be initiated by the monomer itself or peroxy compoimds that are formed via exposure of the reaction mixture to molecular oxygen. The processes (mainly) take place at high temperatures. However, imder very pure conditions in exhaustively purified reaction vessels, most monomers do not tend to polymerize spontaneously by increasing the temperature. Styrene is one of the rare monomers that—also in its purest state—does exhibit initiation processes without additionally added initiator. The self-initiation of styrene has been studied in great detail with respect to its kinetics and mechanism (56-58). The underlying reaction is a self-Diels-Alder cycloaddition of two styrene moieties as shown in equation 14 ... 

The Self-Initiated Polymerization of Styrene and the Chemistry of Methylenecyclohexadiene (Isotoluene)... 

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. 

A prominent example for the latter is the self-initiation of styrene, which proceeds via Diels-Alder reaction of two monomers, as depicted in Scheme 1.7. Such self-initiated polymerization processes are typically limited to elevated temperatures, and can often be prevented under very pure conditions. Few monomers are capable of self-initiation even under very pure conditions, one of which is styrene, reacting via a self Diels-Alder cycloaddition mechanism. The self-initiated bulk polymerization of styrene has a substantial activation energy a 50% monomer conversion needs 400 days at 29 °C, but only 4h at 127 °C. However, the produced polystyrene is very pure due to the absence of initiators and other additives. 

Except in the rare case of self-initiated polymerization, transfer to initiator is also unavoidable, but as long as initiator concentration is kept low, its impact on overall reaction kinetics is small. For some common initiators, transfer constants Q are given in Table 1.3. 

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