Radical chain polymerization thermodynamics

The starting point for the Ifee-radical polymerization is to choose a monomer (M) that will react with a free radical R to add to the radical and also create another radical centre R M- of high reactivity that may add another monomer molecule. The source of the free radical to initiate the process may be the monomer itself or an added initiator (1) chosen to produce two free radicals, R -, per molecule cleanly and efficiently at a temperature suitable for the addition of monomer to occur. In the case of self-initiation, heat or radiation must be supplied in order to fragment the monomer or otherwise create the radical to initiate the chain polymerization. Table 1.5 gives examples of typical vinyl monomers that undergo free-radical polymerization, and Table 1.6 shows some initiators. Provided that certain thermodynamic... 

Now that we have looked at the kinetics of polymerizations, let s briefly examine the thermodynamics. Table 13.3 provides some quantitative data. Recall that the pathway (mechanism) does not affect the thermodynamics. Instead, the relative energies of the monomers and polymers set the thermodynamics. As we noted in discussing autoacceleration, the propagation step of a radical chain reaction is typically quite exothermic. This is also seen in the thermodynamics of many addition polymerizations (see Table 13.3), because in each step a tt bond is converted to a a bond (which is stronger). Note, however, that the entropies of polymerizations are quite negative. This is due to the fact that the free translation of individual monomers is lost for each additional propagation step. 

ESI mass spectrometry ive mass spectrometry ESR spectroscopy set EPR spectroscopy ethyl acetate, chain transfer to 295 ethyl acrylate (EA) polymerizalion, transfer constants, to macromonomers 307 ethyl methacrylate (EMA) polymerization combination v.v disproportionation 255, 262 kinetic parameters 219 tacticity, solvent effects 428 thermodynamics 215 ethyl radicals... 

Polymerization reactions can proceed by various mechanisms, as mentioned earlier, and can be catalyzed by initiators of different kinds. For chain growth (addition) polymerization of single compounds, initiation of chains may occur via radical, cationic, anionic, or so-called coordinative-acting initiators, but some monomers will not polymerize by more than one mechanism. Both thermodynamic and kinetic factors can be important, depending on the structure of the monomer and its electronic and steric situation. The initial step generates... 

The analysis of the reaction serum (the continuous phase without polymer particles) at the end of polymerization led to the conclusion that the molecular weight of the soluble oligomers of styrene and PEO macromonomer varied from 200 to 1100 g mol-1. This indicates that the critical degree of polymerization for precipitation of oligomers in this medium is more than ten styrene units and only one macromonomer unit per copolymer chain. Several reasons for the low molecular weight of the soluble copolymers were proposed, such as the thermodynamic repulsion (or compatibility) between the PEO chain of the macromonomer and the polystyrene macroradical, the occurrence of enhanced termination caused by high radical concentration, and, to a lower extent, a transfer reaction to ethanol [75]. 

Rhombic sulfur is a brittle, crystalline solid at room temperature. Heating to 113 °C causes it to melt to a reddish-yellow liquid of relatively low viscosity. Above approximately 160 °C, the viscosity increases dramatically because of the free-radical polymerization of the cyclic molecules into long, linear chains.6,8 14 30 47-51 At this point, a degree of polymerization of approximately 105 is obtained. If the temperature is increased to above approximately 175 °C, depolymerization occurs, as evidenced by a decreasing viscosity. A similar type of depolymerization occurs with the polysiloxanes discussed in Chapter 4. In thermodynamic terms, the negative -TAS term overcomes the positive AH term for chain depolymerization. (The temperature at which the two terms are just equal to one another is called the ceiling temperature for the polymerization.)... 

Since larger free radicals are more stable than those with small molecules, the fragmentation in the middle of the polymeric chain is favored thermodynamically compared to the formation of small molecules. However, kinetic factors also may play a role in determining the abundance of a specific compound. The formation of small radicals from the end of a polymeric chain can be kinetically favored, and, as a result, formation of small radicals in the initiation step is more common than expected based on the thermodynamic criteria. Taking as an example polystyrene, an end chain p-scissions to the aromatic ring can be written as follows ... 

The chain scission also can start truly randomly and not only at the weaker bond. For polymers containing linear backbones, in addition to p-scissions, it is possible to have a-scissions, methyl scissions or even hydrogen scissions. The scission of a C-H bond is thermodynamically unfavorable at low temperatures and is not too common at temperatures where the other scission can take place. The a-scission is more frequent. It refers to the breaking of a o bond to an sp2 carbon. For polystyrene for example, the a-scission leads to the formation of a phenyl radical and a polymeric radical (and it is not a chain scission). 

It must be noted that the process of seeded emulsion polymerization does not lead to an equilibrium structure. Hence, the sharp interface between PS and PMMA observed in the above core-shell particles cannot be explained by thermodynamic arguments. A possible mechanism maybe sought in the adsorption of oligo(methylmethacrylate) radicals from the water phase onto the PS-seed particles [45]. The temperature of the seeded emulsion polymerization (80 °C [45]) is well below the glass transition temperature of polystyrene and the adsorbed chains bear a sulfate endgroup. The adsorbed oligomers will therefore remain at the surface of the core particles and in consequence there is no extended interface between PS and PMMA in these. particles. 

It should be noted that, whereas the preceding discussion has been cast in terms of free-radical polymerizations, the thermodynamic argument is independent of the nature of the active species. Consequently, the analysis is equally valid for ionic polymerizations. A further point to note is that for the concept to apply, an active species capable of propagation and depropagation must be present. Thus, inactive polymer can be stable above the ceiling temperamre for that monomer, but the polymer will degrade rapidly by a depolymerization reaction if main chain scission is stimulated above T.. 

Some information on (he expected course of events in a miniemulsion polymerization of VC may be obtained from results of suspension and bulk polymerization of VC, which has been subject to extensive studies by a number of authors. Ugelstad [93] proposed a simple model for explaining the steady increase in rate with conversion, assuming a thermodynamic equilibrium distribution of radicals below the cntical chain length for precipitation. A very good correlation... 

When a polymeric chain having a polaron is subjected to further oxidation, an electron is removed from either the polaron or the rest of the chain. In the former case, a polaron radical is removed and two new positive charges resnlt, which are coupled through lattice distortion. In the latter case, two polarons are formed. However, the formation of a bipolaron causes a further decrease in ionisation compared to two polarons, indicating that bipolaron formation is thermodynamically more favourable. It has been reported through quantum chemical calculations that bipolaron energies are lower than those of polarons by 0.4 eV [13]. 

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