Initiation initiator-derived radicals

In some texts the initiator efficiency (/) is defined simply in terms of the yield of initiator-derived radicals (the fraction of radicals I- that undergo cage escape - Section 3.2.8). This number will always be larger than that obtained by application of eq. 1. 

According to cq. 1, the term/should take into account all side reactions that lead to loss of initiator or initiator-derived radicals. These include cage reaction of the initiator-derived radicals (3.2.8), primary radical termination (3.2.9) and transfer to initiator (3.2.10). The relative importance of these processes depends on monomer concentration, medium viscosity and many other factors. Thus/is not a constant and typically decreases with conversion (see 3.3.1.1.3 and 3.3.2.1.3). 

Thus, the size and the reactivity of the initiator-derived radicals and the medium viscosity (or microviscosity) are important factors in determining the initiator efficiency. Thus, the extent of the cage reaction is likely to increase with... 

The concentration of monomers in the aqueous phase is usually very low. This means that there is a greater chance that the initiator-derived radicals (I ) will undergo side reactions. Processes such as radical-radical reaction involving the initiator-derived and oligomeric species, primary radical termination, and transfer to initiator can be much more significant than in bulk, solution, or suspension polymerization and initiator efficiencies in emulsion polymerization are often very low. Initiation kinetics in emulsion polymerization are defined in terms of the entry coefficient (p) - a pseudo-first order rate coefficient for particle entry. 

Initiator efficiency increases with reaction temperature (Table 3.4). It is also worth noting that apparent zero-conversion initiator efficiencies depend on the method of measurement. Better scavengers trap more radicals. The data in Table 3,4 suggest that monomers (MMA, S) are not as effective at scavenging radicals as the inhibitors used to measure initiator efficiencies. The finding suggests that in polymerization the initiator-derived radicals have a finite probability of... 

The low concentration of initiator residues in polymers formed by radical polymerization means that they can usually only be observed directly in exceptional circumstances or in very low molecular weight polymers (Section 3.5.3). Thus, the study of the reactions of initiator-derived radicals with monomers has seen the development of some novel techniques. Three basic approaches have been employed. These involve ... 

Various methods have been described whereby polymers are formed with an initiator that contains chromophores or other functionality to permit ready detection of initiator-derived end groups by chemical or spectroscopic methods/7 1" 150 A potential disadvantage of this procedure is that the initiator is chemically modified and the specificity shown by the initiator-derived radicals may be different from that of the corresponding unlabeled species. 

Even though the rate of radical-radical reaction is determined by diffusion, this docs not mean there is no selectivity in the termination step. As with small radicals (Section 2.5), self-reaction may occur by combination or disproportionation. In some cases, there are multiple pathways for combination and disproportionation. Combination involves the coupling of two radicals (Scheme 5.1). The resulting polymer chain has a molecular weight equal to the sum of the molecular weights of the reactant species. If all chains are formed from initiator-derived radicals, then the combination product will have two initiator-derived ends. Disproportionation involves the transfer of a P-hydrogen from one propagating radical to the other. This results in the formation of two polymer molecules. Both chains have one initiator-derived end. One chain has an unsaturated end, the other has a saturated end (Scheme 5.1). 

It is also necessary to select the initiator according to the particular monomer(s) and the substrate. Factors to consider in this context, aside from initiator half-lives and decomposition rates, are the partition coefficient ot the initiator between the monomer and polyolefin phases and the reactivity of the monomer vs the polyolefin towards the initiator-derived radicals. 

Scheme 9.45 (Q is an initiator-derived radical or a propagating radical)...

The initiator-derived radical products generate a-tocopheroxyl radicals (2) from a-tocopherol (1). The radicals 2 are further oxidized to ort/io-quinone methide 3 in a formal H-atom abstraction, thereby converting benzoyloxy radicals to benzoic acid and phenyl radicals to benzene. The generated o-QM 3 adds benzoic acid in a [ 1,4] -addition process, whereas it cannot add benzene in such a fashion. This pathway accounts for the observed occurrence of benzoate 11 and simultaneous absence of a 5 a-phenyl derivative and readily explains the observed products without having to involve the hypothetical C-centered radical 10. 

Due to the significant importance of dienes as monomers, absolute as well as relative rate data have been determined for the addition of initiator derived radicals. Photolysis of (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (TMDPO) 5 leads to the formation of... 

TABLE 6. Relative bimolecular addition rates for the reaction of initiator derived radicals obtained by polymer end group analysis15 163... 

Kinetics is used to investigate mechanisms of radical additions to alkenes. Outside the solvent cage, the initiator-derived radicals may undergo the desired bimolecular reaction with the substrate, or side reactions. When the substrate is an alkene, the exothermic intermolecular addition of the reactive radical (R ) to the double bond results in the formation of two new single carbon-carbon bonds in place of the double bond. This reaction represents conversion of an initiator into a propagating radical in radical polymerisations, and is becoming increasingly important in a number of synthetically useful intermolecular small molecule reactions. The addition of R to monosubstituted and 1,1-disubstituted alkenes is nearly always at the unsubstituted carbon atom (tail addition), and thus is normally not affected by the individual steric demand of the alkene substituents. Equation 10.4 is the expression for the rate of addition (R ) of R to an alkene where [M] is the monomeric alkene concentration ... 

One of the most important parameters in the S-E theory is the rate coefficient for radical entry. When a water-soluble initiator such as potassium persulfate (KPS) is used in emulsion polymerization, the initiating free radicals are generated entirely in the aqueous phase. Since the polymerization proceeds exclusively inside the polymer particles, the free radical activity must be transferred from the aqueous phase into the interiors of the polymer particles, which are the major loci of polymerization. Radical entry is defined as the transfer of free radical activity from the aqueous phase into the interiors of the polymer particles, whatever the mechanism is. It is beheved that the radical entry event consists of several chemical and physical steps. In order for an initiator-derived radical to enter a particle, it must first become hydrophobic by the addition of several monomer units in the aqueous phase. The hydrophobic ohgomer radical produced in this way arrives at the surface of a polymer particle by molecular diffusion. It can then diffuse (enter) into the polymer particle, or its radical activity can be transferred into the polymer particle via a propagation reaction at its penetrated active site with monomer in the particle surface layer, while it stays adsorbed on the particle surface. A number of entry models have been proposed (1) the surfactant displacement model (2) the colhsional model (3) the diffusion-controlled model (4) the colloidal entry model, and (5) the propagation-controlled model. The dependence of each entry model on particle diameter is shown in Table 1 [12]. 

Maxwell et al. [ 11 ] proposed a radical entry model for the initiator-derived radicals on the basis of the following scheme and assumptions. The major assumptions made in this model are as follows An aqueous-phase free radical will irreversibly enter a polymer particle only when it adds a critical number z of monomer units. The entrance rate is so rapid that the z-mer radicals can survive the termination reaction with any other free radicals in the aqueous phase, and so the generation of z-mer radicals from (z-l)-mer radicals by the propagation reaction is the rate-controlling step for radical entry. Therefore, based on the generation rate of z-mer radicals from (z-l)-mer radicals by propagation reaction in the aqueous phase, they considered that the radical entry rate per polymer particle, p p=pJNp) is given by... 

When it is assumed that initiator-derived radicals do not exit and only monomeric and CTA radicals produced by chain transfer to a monomer and/or a CTA can desorb (s=l), Eq. 17 can be simplified as... 

A typical polymerization system comprises many components besides the initiators and the monomers. There will be solvents, additives (e.g. transfer agents, inhibitors) as well as a variety of adventitious impurities that may also be reactive towards the initiator-derived radicals. 

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