Free radical polymerization equilibrium monomer concentration

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... 

Free-radical polymerization is a highly exothermic process and reactor temperature control is an important issue for both polymer quality and operation safety. At the temperatures used in commercial practice, most radical polymerizations are irreversible. However, because it is an exothermic reaction, at sufficiently high temperatures the reaction becomes reversible and complete conversion cannot be achieved. Methyl methacrylate is a major monomer that suffers from this problem, with an equilibrium concentration of 0.139 mol at 110°C [12[. 

Thus the values shown in Table 1.8 are for standard conditions and represent just one of a series of ceiling temperatures for various monomer concentrations above which polymer formation is not favoured. Thus, in a bulk polymerization reaction the ceiling temperature may change with conversion in such a way that complete conversion is not achieved. For example, if methyl methacrylate is polymerized at 110°C the value of [M]c calculated from the above equation is 0.139M and this will be the monomer concentration in equilibrium with the polymer. The polymer, when removed from the monomer, will have the expected ceiling temperature as given in Table 1.8 and will depolymerize only if there is a source of free radicals to initiate the depolymerization (Section 1.4.1)... 

The key feature of the use of a dormant species may be seen in the following general scheme (Scheme 1.32) that involves complexation of the propagating species by means of a stable nitroxide radical (Hawker et al, 2001). The P -0 bond of the alkoxy amine P -0-NR is thermally labile at the polymerization temperature, so this becomes the site for the insertion of monomer. Propagation then occurs at a rate that is much slower than for a simple free-radical addition reaction since the propagating radical concentration (which is governed by the position of the equilibrium with the alkoxy amine... 

CRP (also referred to as living radical polymerization ) is a family of promising techniques for the synthesis of macromolecules with well-defined molecular weight, low polydispersities (often close to unity) and various architectures under mild conditions at 20-120°C, with minimal requirements for purification of monomers and solvents. A common feature of the variants is the existence of an equilibrium between active free radicals and dormant species. The exchange between active radicals and dormant species allows slow but simultaneous growth of all chains while keeping the concentration of radicals low enough to minimize termination. The ideal CRP is achieved if all chains are initiated... 

While the SNR-mediated polymerization process comprises heating a mixture of monomer(s) and P-N adduct (that acts both as an initiator and a controlling agent), or a mixture of monomer(s), free-radical initiator, and SNR (or P-N adduct), the best temperature of polymerization, determined by experiments, is the one that leads to (i) a fast initiation rate as compared to the propagation rate (ii) a fast equilibrium between the active species and the dormant ones (iii) a low concentration of active species in order to minimize the termination and/or transfer reactions and (iv) a negligible thermal polymerization (of styrenic monomers). For example, the rate of formation of thermal radicals in styrene is equal to 1.6x10 mol L s at 100°C, 0.6x10 mol L... 

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