Free radical polymerization thermodynamics

The various kinetic and thermodynamic factors involved in vinyl free radical polymerization have been considered for the case of a batch (or semi-batch) polymerization being carried out to very high conversion. In particular, computations have been done for the final stage of the reaction when monomer concentration is reduced from approximately 5 volume % to 0.5 volume %. 

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.)... 

The emulsion polymerization methodology is one of the most important commercial processes. The simplest system for an emulsion (co)polymerization consists of water-insoluble monomers, surfactants in a concentration above the CMC, and a water-soluble initiator, when all these species are placed in water. Initially, the system is emulsified. This results in the formation of thermodynamically stable micelles or microemulsions built up from monomer (nano)droplets stabilized by surfactants. The system is then agitated, e.g., by heating it. This leads to thermal decomposition of the initiator and free-radical polymerization starts [85]. Here, we will consider a somewhat unusual scenario, when a surfactant behaves as a polymerizing comonomer [25,86]. 

They also tested (10) Case II for the free radical polymerization of styrene (Mi) and methyl methacrylate at 132°C. by the dilution technique. These data are also shown in Table I, where the good agreement between theory and experiment is apparent. The applicability of the theory to different mechanisms of polymerization is a nice verification of the statement that the composition is governed by end-state thermodynamics rather than by mechanism. 

Thermodynamics of free-radical polymerization. The free energy of polymerization, AGp, is given by the first and second laws of thermodynamics for a reversible process as... 

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... 

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.. 

The great number of possible elementary reactions in free radical polymerizations explains why only a fraction of the many thermodynamically polymerizable groups can be converted free radically to un-cross-linked high-molar-mass polymers. Vinyl, vinylidene, and acrylic compounds, as well as some strained saturated rings, belong to this fraction. Allyl compounds only polymerize to branched oligomers, but diallyl and triallyl compounds produce high-molar-mass cross-linked networks. 

Not all petrochemical processes are catalytic—the steam cracking of hydrocarbons to lower olefins is a thermal process at 700 to 800°C or more. However, excluding free-radical polymerization processes, this is a rare example, though severe conditions may still be required in some catalysed processes on thermodynamic grounds or to achieve acceptable rates (several mol h per litre of reaction volume). As we shall see in this and the following chapter, the major impact of catalysis is to provide a remarkably wide range of products from a small number of building blocks. 

Thermodynamics of the Constrains of the Free-Radical Polymerization Reaction... 

Free-radical polymerization reactions are equilibrium reactions. The equilibrium between the monomer and the growing polymer is subject to thermodynamic conditions. At equilibrium, therefore, the change in free energy is zero ... 

Discuss the effect of monomer structure on the thermodynamics of the free-radical polymerization process. 

Figure 11.7 shows the temperature history at a fixed point in the reaction tube as a front passes. The temperature at this point is ambient when the front is far away and rises rapidly as the front approaches. Hence, a polymerization front has a very sharp temperature profile (Pojman et al., 1995b). Figure 11.7 shows five temperature profiles measured during frontal free-radical polymerization of methacrylic acid with various concentrations of BPO initiator. Temperature maxima increase with increasing initiator concentration. For an adiabatic system, the conversion is directly proportional to the difference between the initial temperature of the unreacted medium and the maximum temperature attained by the front. The conversion depends not only on the type of initiator and its concentration but also on the thermodynamic characteristics of the polymer (Pojman et al., 1996b). 

Strictly speaking, any model based on the time-independent thermodynamics cannot be used to adequately predict the concentration of monomer in latex particles during Smith-Ewart Interval II. This is because the free radical polymerization of monomer in the discrete latex particles is governed by the simultaneous kinetic events such as the generation of free radicals in the continuous aqueous phase, the absorption of free radicals by the particles, the propagation of free radicals with monomer molecules in the particles, the bimolecular termination of free radicals in the particles, and the desorption of free radicals out of the particles. The equilibrium (or saturation) concentration of monomer in the growing latex particles may not be achieved if the rate of consumption of monomer in the major reaction loci is much faster than that of diffusion of monomer molecules from the monomer droplets to the reaction loci. Therefore, the equilibrium concentration of monomer in the latex particles represents an upper limit that is ultimately attainable in the course of polymerization. Nevertheless, the general... 

Table 3 lists the reactivity ratios of some comonomer pairs in free-radical polymerization. While the values of Xi and f2 can vary a lot, their products are close to or lower than 1, which is thermodynamically favored with entropic contributions. For a given pair of monomers, the reactivity ratios are very different in the different typies of polymerization mechanisms. Tfrey are also temperature dependent because of different acdr tion energies of the propagation reactions. 

All the above schemes include, in essence, different variants of empirical linear equations in which the rate constants for chain propagation in the free radical polymerization are brought into correlation with thermodynamic (heat, Hammett constant (a), the change in the Gibbs energy in the equilibrium reaction) and kinetic (loga-ridims of the rate constants of the reference reaction and the reaction under study) characteristics of the addition reaction. 

Free Radical Polymerizations in Nanopores Reaction Kinetics and Thermodynamics... 

Nakano T, Matsuda A, Okamoto Y, Okamoto Y. Pronounced effects of temperamre and monomer concentration on isotactic specificity of triphenylmethyl methacrylate polymerization through free radical mechanism. Thermodynamic versus kinetic control of propagation stereochemistry. Polym J. 1996 28 556-558. 

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