Initiation thermal

Very carefully purified styrene polymerizes slowly [1 ]. Initiation is evidently due to the monomer itself. In 1936, Flory had already advanced the hypothesis on the formation of a diradical from two monomer molecules [2] 

Doubt has been cast on the initiating ability of this type of diradical. In would tend rather to cyclization the probability of propagation is negligible [3], However, it appears again in more recent theories. 

1 In certain cases, depending on the initiation mechanism, this number could be doubled. 

The idea of the formation of Diels-Alder intermediates from two styrene molecules was first postulated by Mayo [5] and has recently been revived by several authors. Sato et. al. [6] had described path 1 [see reaction (2)] before Barr [4] they did not consider polymerization on diradicals. Kauffmann [7] supported the postulation of the formation of the 4n-2n Diels-Alder adduct (5-phenylbicyclo- [4,4,0]-deca-1,7,9-triene) spectrophotometrically. Kinetic 

Methyl methacrylate may also give rise to initiating radicals [13]. A collision of two monomer molecules leads to the formation of linear dimers, trimers, two cyclobutane derivatives and a diradical [14] 

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. 

Aspects of thermal initiation have been reviewed by Moad et al., w Pryor and Laswell, 10 Kurbatov/ and Hall.312 It is often difficult to establish whether initiation is actually a process involving only the monomer. Trace impurities in the monomers or the reaction vessel may prove to be the actual initiators. Purely thermal homopolymerizations to high molecular weight polymers have only been demonstrated unequivocally for S and its derivatives and MMA. For these and other systems, the identity of the initiating radicals and the mechanisms by which they are formed remain subjects of controversy. 

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Using typical activation energies out of Tables 6.2-6.4, estimate the percent change in the rate of polymerization with a 1°C change in temperature at 50°C for thermally initiated and photinitiated polymerization. 

Finally we recognize that a 1°C temperature variation can be approximated as dT and that (dRp/Rp) X 100 gives the approximate percent change in the rate of polymerization. Taking average values of E from the appropriate tables, we obtain E j = 145, E = 16.8, and Ep = 24.9 kJ mol . For thermally initiated polymerization... 

Photoinitiation is not as important as thermal initiation in the overall picture of free-radical chain-growth polymerization. The foregoing discussion reveals, however, that the contrast between the two modes of initiation does provide insight into and confirmation of various aspects of addition polymerization. The most important application of photoinitiated polymerization is in providing a third experimental relationship among the kinetic parameters of the chain mechanism. We shall consider this in the next section. 

Water-soluble peroxide salts, such as ammonium or sodium persulfate, are the usual initiators. The initiating species is the sulfate radical anion generated from either the thermal or redox cleavage of the persulfate anion. The thermal dissociation of the persulfate anion, which is a first-order process at constant temperature (106), can be greatly accelerated by the addition of certain reducing agents or small amounts of polyvalent metal salts, or both (87). By using redox initiator systems, rapid polymerizations are possible at much lower temperatures (25—60°C) than are practical with a thermally initiated system (75—90°C). 

When initiator is first added the reaction medium remains clear while particles 10 to 20 nm in diameter are formed. As the reaction proceeds the particle size increases, giving the reaction medium a white milky appearance. When a thermal initiator, such as AIBN or benzoyl peroxide, is used the reaction is autocatalytic. This contrasts sharply with normal homogeneous polymerizations in which the rate of polymerization decreases monotonicaHy with time. Studies show that three propagation reactions occur simultaneously to account for the anomalous auto acceleration (17). These are chain growth in the continuous monomer phase chain growth of radicals that have precipitated from solution onto the particle surface and chain growth of radicals within the polymer particles (13,18). 

However, because of the high temperature nature of this class of peroxides (10-h half-life temperatures of 133—172°C) and their extreme sensitivities to radical-induced decompositions and transition-metal activation, hydroperoxides have very limited utiUty as thermal initiators. The oxygen—hydrogen bond in hydroperoxides is weak (368-377 kJ/mol (88.0-90.1 kcal/mol) BDE) andis susceptible to attack by higher energy radicals ... 

Another interesting feature of these azirine reactions is that many of the thermally initiated reactions can be effected at room temperature in the presence of a suitable transition metal catalyst. Some typical examples are displayed in Scheme 95. 

Overpressure of Store drum at proper temperature material in drum, Keep drum away from heat source due to external heat input or self reaction is complete before drumming heating. Allow adequate freeboard for material Provide adequate sprinkler protection Thermally initiated venting (e.g., melt-out bungs) CCPS G-3 CCPS G-15 CCPS G-22 CCPS G-29... 

This thermal initiation generates two free radicals by breaking a covalent bond. The aldehyde radical is long-lived and does not markedly influence the subsequent mechanism. The methane radical is highly reactive and generates most reactions. 

Anhydrous HX are versatile and vigorous reagents for the halogenation of metals, non-metals, hydrides, oxides and many other classes of compound, though reactions that are thermodynamically permissible do not always occur in the absence of catalysts, thermal initiation or photolytic encouragement, because... 

The trapped radicals, most of which are presumably polymeric species, have been used to initiate graft copolymerization [127,128]. For this purpose, the irradiated polymer is brought into contact with a monomer that can diffuse into the polymer and thus reach the trapped radical sites. This reaction is assumed to lead almost exclusively to graft copolymer and to very little homopolymer since it can be conducted at low temperature, thus minimizing thermal initiation and chain transfer processes. Moreover, low-molecular weight radicals, which would initiate homopolymerization, are not expected to remain trapped at ordinary temperatures. Accordingly, irradiation at low temperatures increases the grafting yield [129]. 

Macroinitiators are macromolecules having peroxygen and/or azo groups that can thermally initiate a vinyl polymerization to obtain block copolymers in one step. They can be classified as macroperoxyinitiators (MPl), macroazoinitiators (MAI), and macroazo-peroxyiniti-ators. 

An important ramification of the photolability of azo-compounds is that, when using dialkyldiazenes as thermal initiators, care must be taken to ensure that the polymerization mixture is not exposed to excessive light during its preparation. 

Peroxides are used most commonly either as thermal initiators or as a component in a redox system. While peroxides are photochemically labile, they seldom find use as photoinitiators other than in laboratory studies because of their poor light absorption characteristics. They generally have low extinction coefficients and absorb in the same region as monomer. Kinetic parameters for decomposition of some important peroxides are given in Table 3.5,... 

Thermal initiation of styrene has been shown to be third order in monomer. The average rate constants for third order initiation determined by Hui and Hamielec is k = 105 34 e(,j8iaT) (M V).- "0 The rate constant for formation of the Mayo dimer determined in trapping experiments with nitroxidcs (Scheme 3.63) or acid (Scheme 3.64) as kn = 104 4 (M ls 1)j21 is substantially higher than is... 

In eq. 8, the rate of polymerization is shown as being half order in initiator (T). This is only true for initiators that decompose to two radicals both of which begin chains. The form of this term depends on the particular initiator and the initiation mechanism. The equation takes a slightly different form in the case of thermal initiation (S), redox initiation, diradical initiation, etc. Side reactions also cause a departure from ideal behavior. 

Disulfide derivatives and hexasubstituted ethanes2,15 may also be used in this context to make cnd-functional polymers and block copolymers. The use of dilhiuram disulfides as thermal initiators was explored by Clouet, Nair and coworkers.206 Chain ends are formed by primary radical termination and by transfer to the dilhiuram disulfide. The chain ends formed are thermally stable under normal polymerization conditions. The use of similar compounds as photoin iferters, when some living characteristics may be achieved, is described in Section 9.3.2.1.1. 

If there is an external source of free radicals (e.g. from thermal initiation in S polymerization or from an added conventional initiator) eq. 5 may again apply. The rate of polymerization becomes independent of the concentration oflX and, as long as the number of radicals generated remains small with respect to [IX] , a high fraction of living chains and low dispersilies is still possible. The validity of these equations has been confirmed for NMP and with appropriate modification has also been shown to apply in the case of ATRP.3... 

Since the dithiocarbatnyl end groups 8 are thermally stable but pholochemically labile at usual polymerization temperatures, only photo-initiated polymerizations have the potential to show living characteristics. However, various disulfides, for example, 9 and 10, have been used to prepare end-functional polymers37 and block copolymers38 by irreversible chain transfer in non-living thermally-initiated polymerization (Section 7.5.1). 

Catala and coworkers167JuiS made the discovery that the rate of TEMPO-mediated polymerization of S is independent of the concentration of the alkoxyamine. This initially surprising result was soon confirmed by others.23 69 Gretza and Matyjaszewski169 showed that the rate of NMP is controlled by the rate of thermal initiation. With faster decomposing alkoxyamines (those based on the open-chain nitroxides) at lower polymerization temperatures, the rate of thermal initiation is lower such that the rate of polymerization becomes dependent on the alkoxyamine concentration, Irrespective of whether the alkoxyamine initiator is preformed or formed in situ, low dispersities require that the alkoxyamine initiator should have a short lifetime. The rate of initiation should be as fast as or faster than propagation under the polymerization conditions and lifetimes of the alkoxyamine initiators should be as short as or shorter than individual polymeric alkoxyamines. 

NMP is most commonly used for S polymerization. For S polymerizations carried out at temperatures greater than 100 °C, thermal initiation provides some rate enhancement and a mechanism for controlling the excess of nitroxide that is formed as a consequence of radical-radical termination and the persistent radical... 

NMP with acrylates and acrylamides with TEMPO provides only very low conversions. Very low limiting conversions and broad dispersities were reported.2 Better results were obtained with DTBN (83),111 151 imidazoline (61-64)I3S and isoindoline (59) nitroxides.111 However, limiting conversions were still observed. The self-regulation provided in S polymerization by thermal initiation is absent and, as a consequence, polymerization proceeds until inhibited by the buildup of nitroxide. The final product is an alkoxyamine and NMP can be continued... 

In S polymerization, thermal initiation will be a source of extra chains. Additional chain formation processes will cause the molecular weight to be lower than anticipated by cq. 7. Sometimes conventional thermal initiators are added with similar effect (see also eq. 12). A pre-tailing molecular weight distribution may result. 

An issue when making the second (and subsequent) blocks from styrenic monomers is that thermal initiation or an added initiator will provide a homopolymer impurity. 

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