Styrene self-initiation

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. 

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-... 

Since the nitroxide and the carbon-centered radical diffuse away from each other, termination by combination or disproportionation of two carbon-centered radicals cannot be excluded. This will lead to the formation of dead polymer chains and an excess of free nitroxide. The build-up of free nitroxide is referred to as the Persistent Radical Effect [207] and slows down the polymerization, since it will favor trapping (radical-radical coupling) over propagation. Besides termination, other side reactions play an important role in nitroxide-mediated CRP. One of the important side reactions is the decomposition of dormant chains [208], yielding polymer chains with an unsaturated end-group and a hydroxyamine, TH (Scheme 3, reaction 6). Another side reaction is thermal self-initiation [209], which is observed in styrene polymerizations at high temperatures. Here two styrene monomers can form a dimer, which, after reaction with another styrene monomer, results in the formation of two radicals (Scheme 3, reaction 7). This additional radical flux can compensate for the loss of radicals due to irreversible termination and allows the poly-... 

Some pure monomers undergo initiation when heated. The subsequent polymerization is free radical in character. Styrene exhibits significant thermal initiation at temperatures of 100°C or more. Methyl methacrylate also self-initiates but at a slower rate. Low-molecular-weight vinyl polymers can often be made simply by heating the monomers, but the molecular weight control is not very close and initiation in some cases at least may be from thermal homolysis of impurities in the reaction mixture. 

All the CRP methods have strengths that can be exploited in particular systems. TEMPO is essentially useful only for the polymerization of styrene-based monomers, whether for the preparation of statistical or block copolymers [38]. The radicals generated through the self-initiation of St help to moderate the rate of polymerization by consuming any excess TEMPO generated by termination reactions, which will not occur with other monomers. Acrylate monomers, for example, are very sensitive to the concentration of free TEMPO and therefore its build-up causes the polymerization to stop. The use of different nitroxides and alkoxyamines like DEPN [73] and TMPAH [71], which provide higher equilibrium constants and allow for faster polymerization rates, has also enabled the homo- and copolymerizations of acrylate monomers, as well as for St at lower temperatures. Block order is important, however, and chain end functionality is reduced when TMPAH functional polymers are chain extended with BA. This may... 

The self-initiation mechanism for styrene polymerization has been established [30]. It involves the formation of a Diels-Alder dimer (V) of styrene followed by transfer of a hydrogen atom from the dimer to a styrene molecule ... 

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)]. 

Another interesting case is the self-initiation of some acrylic and styrenic monomers upon UV irradiation. Since this process is quite slow, a photoinitiator is normally required. Recent findings on the self-initiation of maleic anhydride, [ 10] styrene, [11] and anumber of acrylic monomers [12] have demonstrated that photopolymerization and photografting could possibly be achieved without using photoinitiators or sensitizers. 

Polystyrene was first manufactured commercially (1938) by The Dow Chemical Company. Styrene was non-continuously bulk polymerized, without the aid of a chemical initiator, to high conversion by heating it in metals cans. The cans were opened and the solid PS ground into small pieces. Over the next 35 years, most of the research focused on understanding the mechanism of self-initiated (spontaneous) polymerization of styrene and developing continuous solution polymerization processes. In recent years, solution polymerization research emphasis has focused upon understanding the chemistry of chemical initiators. Today, most PS is produced via continuous solution polymerization with the aid of peroxide initiation. 

The self-initiated (spontaneous) polymerization of styrene has challenged researchers and has received considerable attention over the past fifty years. Two mechanisms explaining spontaneous styrene polymerization have been proposed and supported by considerable drcumstantial evidence. The oldest mechanism, first postulated by Flory [41] (Scheme 1), involves a bond forming reaction l ween two molecules of styrene (S) to form a 1,4-diradical ( D ). However, experiments to test the mechanism showed that there was no signifi-... 

A majority of the literature on ATRP focuses on the synthesis of styrene and its derivatives with copper-based catalysts. One of the most extensively studied systems is the polymerization of styrene conducted with CuBr, nitrogen-based ligands, and alkyl bromides as initiator. Better molecular weight control is obtained at low temperatures, presumably due to a lower contribution of thermal self-initiation during the early stage of the polymerization. For example, the reaction temperature can be lowered to 80-90 °C when efficient catalysts, such as CuBr/PMDETA, are used [19-22, 30]. 

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 ... 

One mechanism is based on an observation that olefins with allylic hydrogens like isoprene, methyl styrene, indene, and cyclopentadiene can be polymerized by superdry, pure Lewis acids alone. This led to a suggestion that the process may involve an allylic self-initiation ... 

Various mechanisms have been proposed to explain the initiation mechanism of self initiated copolymerizations of styrene (S) with electron acceptor monomers such as maleic anhydride (MA), acrylonitrile, vinyliden cyanide or dimethyl l,l-dicianoethane-2-2-dicarboxylate. They... 

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. 

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 ... 

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