Self-initiation

Self-initiated reaetions, e.g. pymvie aeid on storage ean beeome oxidized by air (or airborne yeasts) to form suffieient gaseous earbon dioxide to overpressurize the eontainer ... 

R1 - Regulatory Requirement for the Waste R2 - Reduction of Troafment/Disposal Costs R3 - Other Process Cost Reduction R4 - Self-Initiated Review... 

This section describes polymerizations of monomer(s) where the initiating radicals are formed from the monomer(s) by a purely thermal reaction (/.e. no other reagents are involved). The adjectives, thermal, self-initialed and spontaneous, are used interchangeably to describe these polymerizations which have been reported for many monomers and monomer combinations. While homopolymerizations of this class typically require above ambient temperatures, copolymerizations involving certain electron-acceptor-electron-donor monomer pairs can occur at or below ambient temperature. 

Various mechanisms have been proposed to explain the initiation processes. The self-initiated copolymerizations of the monomer pairs S-MMA and S-AN proceed at substantially faster rates than pure S polymerization. For S-AN333 and S-MAHJJ the mechanism of initiation was proposed to be analogous to that of S homopolymerization (Scheme 3.62) but with acrylonitrile acting as the dicnophile in the formation of the Diels-Alder adduct (Scheme 3.66). 

The data described above proved that isomerization of alkyl and peroxyl radicals plays a very important role in polymer oxidation. They influence the composition of products of polymer oxidation including the structure of hydroperoxy groups. The competition between reactions of alkyl radical isomerization and addition of dioxygen appeared to be very important for the self-initiation and, hence, autoxidation of PP (see later). 

Photoinduced copolymerization of donor-acceptor monomer pairs (Scheme 1) can be either self initiated by excitation of the charge transfer complex (charge transfer initiation) or by polymerization of the charge transfer complex/monomer equilibrium... 

The latest addition to the theories of cationic polymerisation is the self-initiation theory of Kennedy [12]. According to this theory, initiation can take place in certain systems by a reaction between the metal halide and the olefin in which a hydride ion is abstracted by the metal halide from an allylic position in the monomer ... 

Zwitterion Complex Figure 4 Plausible self-initiation mechanism of acrylic monomers with PIDAA. 

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. 

There is evidence that Lewis acids initiate a slow polymerization in some (but not most) systems by a self-ionization process in addition to the coinitiation process [Balogh et al., 1994 Grattan and Plesch, 1980 Masure et al., 1978, 1980], Two mechanisms are possible for self-initiation. One involves bimolecular ionization... 

Hot stars have an intense radiation field, which by absorption due to UV metal lines leads to an outward accelerating force, which is undoubtedly present. Lucy and Solomon (1970) and Abbott (1979) proved convincingly that this radiation force is sufficient to initialize and to maintain stellar winds. The domain of self-initializing winds predicted by the theory coincides almost perfectly with the occurence of mass-loss in the upper left part of the HR-diagram (see Abbott,... 

The tetrahydro-y-pyrone serves as a self-initiator through the light absorption of the ketone group in the molecule. The possibility that the reactive radical is formed through the collapse of the excited tetrahydro-y-pyrone molecule cannot be excluded, although excited ketone molecules do not tend to collapse in such a manner (69). 

R1 Regulatory Requirement for the Waste R2 Reduction of Treatment/Disposal Costs R3 Other Process Cost Reduction R4 Self-Initiated Review R5 Other (e.g., discontinuation of product, occupational safety). 

Vinyl acrylate (VA) is an intriguing compound capable of self-initiation under UV irradiation. Photopolymerization of VA proceeds faster when PI is added, with other conditions being the same. The question is which double bond is attacked by r of PI (Scheme 12.9). 

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