Free radical addition Chain transfer

In a free radical polymerization, chain transfer is an important reaction. Chain transfer to a monomer, solvent, mercaptan, or other growing chain can take place. When a chain transfer reaction to another chain takes place, it creates a radical, which acts as a site for further chain growth and grafting (see Chapter 2 for additional details) ... 

Oxidations initiated by thermally induced electron transfer in an oxygen-CT complex represent the thermal analog of the Frei photo-oxidation and are properly classified as hybrid type IlAOi-type IIaRH oxidations (Fig, 2), Such reactions require either zeolites with high electrostatic fields or substrates with low oxidation potentials. In addition, elevated temperatures are known to promote the thermally initiated electron-transfer step, although the possible intrusion of a classical free-radical initiation chain oxidation at higher temperatures must be considered. 

Essentially, TFE in gaseous state is polymerized via a free radical addition mechanism in aqueous medium with water-soluble free radical initiators, such as peroxy-disulfates, organic peroxides, or reduction-activation systems.15 The additives have to be selected very carefully since they may interfere with the polymerization. They may either inhibit the process or cause chain transfer that leads to inferior products. When producing aqueous dispersions, highly halogenated emulsifiers, such as fully fluorinated acids,16 are used. If the process requires normal emulsifiers, these have to be injected only after the polymerization has started.17 TFE polymerizes readily at moderate temperatures (40 to 80°C) (104 to 176°F) and moderate pressures (0.7 to 2.8 MPa) (102 to 406 psi). The reaction is extremely exothermic (the heat of polymerization is 41 kcal/mol). 

Figure 5.9. Reactions involved in free-radical addition polymerization. Shown are (a) (i)-(iii) generation of free radicals from a variety of initiators, (b) initiation of polymer chain growth through the combination of a free radical and unsaturated monomer, (c) propagation of the polymer chain through the combination of growing radical chains, (d) chain-transfer of free radicals between the primary and neighboring chains, and (e) termination of the polymer growth through either combination (i) or disproportionation (ii) routes.

If the A-B bond is weak and A—B is in sufficiently high proportion with respect to the alkene, then chain transfer will compete effectively with propagation, allowing overall free-radical addition of A—B to the double bond to occur preferentially (Figure 7.57). 

Conversely, when the rate of propagation is faster than chain transfer, products arising from telomerisation and polymerisation are formed in greater concentration. In this section, free-radical addition to fluoroalkenes will be dealt with first, in order to establish... 

The interconversion of norbornaneyl and nortricyclyl radicals is very rapid, with the latter radical being favored. Therefore, the majority of free radical additions to 2,5-norbornadiene (X = Br) yield mainly tricyclic products and limited amounts of the 1,2-adduct. When the addend is an extremely reactive chain transfer agent (X = I), the 1,2-adduct is the predominant product. 

Among several radical techniques, the free-radical addition-fragmentation chain transfer reaction appears to be an unrivaled method for the synthe-... 

Emulsion polymerization is a free-radical-initiated chain polymerization in which a monomer or a mixture of monomers is polymerized in the presence of an aqueous solution of a surfactant to form a product, known as a latex. The latter is defined as a colloidal dispersion of polymer particles in an aqueous medium. The main ingredients for conducting diese polymerizations include, in addition to the monomer and water, surfactants, initiators and chain transfer agents. 

Free-radical Reactions.—Publications have appeared dealing with the bidirectional addition of trifluoroiodomethane and hydrogen bromide across the C=C bond in the olefin CF3 CH CHMe, peroxide-initiated addition of 1,2-dibromotetrafluoroethane to ethylene, propene, and isobutene, the addition of pentafluoroiodoethane to 3,3,4,4-tetrafluorohexa-l,5-diene (see p. 29), peroxide-initiated cyclodimerization of 3,3,4,4-tetrafluoro-4-iodobut-l-ene (see p. 29), and telomers from tribromofluoromethane or tetrabromomethane and bromotrifluoroethylene as high-density fluids for gyroscope flotation. The telomerization of chloromethanes with tetra-fluoroethylene provides a measure of the relative reactivity for both chlorine and hydrogen abstraction by the CF2 CF2 radical. The chain-transfer... 

Transition metal complexes functioning as redox catalysts are perhaps the most important components of an ATRP system. (It is, however, possible that some catalytic systems reported for ATRP may lead not only to formation of free radical polymer chains but also to ionic and/or coordination polymerization.) As mentioned previously, the transition metal center of the catalyst should undergo an electron transfer reaction coupled with halogen abstraction and accompanied by expansion of the coordination sphere. In addition, to induce a controlled polymerization process, the oxidized transition metal should rapidly deactivate the propagating polymer chains to form dormant species (Fig. 11.16). The ideal catalyst for ATRP should be highly selective for atom transfer, should not participate in other reactions, and should deactivate extremely fast with diffusion-controlled rate constants. Finther, it should have easily tunable activation rate constants to meet sped c requirements for ATRP monomers. For example, very active catalysts with equilibrium constants K > 10 for styrenes and acrylates are not suitable for methacrylates. 

Radical Addition. Free-radical addition of toluene-/>-sulphonyl iodide or methanesulphonyl iodide to acetylenes proceeds readily and stereoselectively when the two are mixed in ether and illuminated. The high yields of crystalline products (102) obtained imply that in the mechanism (Scheme 7), chain transfer by the sulphonyl iodide ( 3) is much faster than isomerization of the intermediate vinyl radical (k. The /ra 5-addition was confirmed by zinc-acetic acid reduction to the sulphone, and by Z-ray analysis of one of the adducts. 

In addition to information concerning chain transfer ability, the data in Table 4 indicated a relationship between conversion and the complexing ability of the particular crown ether utilized as phase transfer catalyst. In fact, when percent conversion was plotted vs the log of the binding constants (log K) of the respective crowns for potassium cation, an apparently linear correlation was obtained. On the basis of some simple assumptions and free radical addition polymerization theory, however, it may be deduced that conversion should correlate with K rather than log K (Equation 1). This anomaly was resolved by using inhibitor-free monomer (Table 5), whereupon a good correlation (R=0.948) between the variables was obtained. 

In practice, another type of reaction sometimes occurs in free-radical addition polymerizations. These chain-transfer reactions kill a growing chain radical and can start a new one in its place (as long as the radical transfers to another monomer) ... 

FIGURE 9.4 Instantaneous number- and weight-fraction distributions of chain lengths in free-radical addition polymerization. Equations 9.47 and 9.48 are for termination by disproportionation and/or chain transfer Equations 9.58 and 9.59 are for termination by combination. 

Consider a free-radical addition reaction in which there is negligible conventional termination. Instead, growing chains are killed by a degradative chain-transfer reaction ... 

A batch reactor producing a free-radical addition polymer is sampled at various conversions. Analysis of these samples reveals that the weight-average chain length varies linearly with conversion from 10,000 at X = 0 to 5000 at X = 1. The polymer is known to terminate by combination and there is no significant chain transfer occurring. Obtain an expression for the polydispersity index of the product as a function of conversion. 

A bewildering array of names are used to describe the various controlled/living radial polymerization techniques currently in use. These include stable free radical polymerization (SFRP) [35-38], nitroxide mediated polymerization (NMP) [39], atom transfer radical polymerization (ATRP) [40-42 ] and degenerate transfer processes (DT) which include radical addition-fragmentation transfer (RAFT) [43, 44] and catalyst chain transfer (CCT). These techniques have been used to polymerize many monomers, including styrene (both linear and star polymers) acrylates, dienes, acrylamides, methacrylates, and ethylene oxide. Research activity in this field is currently expanding at a very high rate, as is indicated by the many papers published and patents issued. 

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