Polymers, kinetic modeling radical polymerization

In Section 5.2.1.1 we provide an overview of the classical treatment of polymerization kinetics. Some aspects of termination kinetics are not well understood and no wholly satisfactory unified description is in place, Nonetheless, it remains a fact that many features of the kinetics of radical polymerization can be predicted using a very simple model in which radical-radical termination is characterized by a single rale constant. The termination process determines the molecular weight and molecular weight distribution of the polymer. In section... 

Advanced computational models are also developed to understand the formation of polymer microstructure and polymer morphology. Nonuniform compositional distribution in olefin copolymers can affect the chain solubility of highly crystalline polymers. When such compositional nonuniformity is present, hydrodynamic volume distribution measured by size exclusion chromatography does not match the exact copolymer molecular weight distribution. Therefore, it is necessary to calculate the hydrodynamic volume distribution from a copolymer kinetic model and to relate it to the copolymer molecular weight distribution. The finite molecular weight moment techniques that were developed for free radical homo- and co-polymerization processes can be used for such calculations [1,14,15]. 

While vinyl acetate is normally polymerized in batch or continuous stirred tank reactors, continuous reactors offer the possibility of better heat transfer and more uniform quality. Tubular reactors have been used to produce polystyrene by a mass process (1, 2), and to produce emulsion polymers from styrene and styrene-butadiene (3 -6). The use of mixed emulsifiers to produce mono-disperse latexes has been applied to polyvinyl toluene (5). Dunn and Taylor have proposed that nucleation in seeded vinyl acetate emulsion is prevented by entrapment of oligomeric radicals by the seed particles (6j. Because of the solubility of vinyl acetate in water, Smith -Ewart kinetics (case 2) does not seem to apply, but the kinetic models developed by Ugelstad (7J and Friis (8 ) seem to be more appropriate. 

On the other hand, Nomura and Harada [14] proposed a kinetic model for the emulsion polymerization of styrene (St), where they used Eq. 7 to predict the rate of radical entry into both polymer particles and monomer-swollen micelles. In their kinetic model, the ratio of the mass-transfer coefficient for radical entry into a polymer particle kep to that into a micelle kem> K lk,... 

In general, a polymerization process model consists of material balances (component rate equations), energy balances, and additional set of equations to calculate polymer properties (e.g., molecular weight moment equations). The kinetic equations for a typical linear addition polymerization process include initiation or catalytic site activation, chain propagation, chain termination, and chain transfer reactions. The typical reactions that occur in a homogeneous free radical polymerization of vinyl monomers and coordination polymerization of olefins are illustrated in Table 2. 

A number of workers have reported on kinetic models for plasma polymerization. Williams and Hayes (36) first suggested that the reaction occurred exclusively on solid surfaces within the reaction zone. Initially, monomer is adsorbed onto the electrode surface, where a portion is converted to free radical species after bombardment by ions and electrons produced in the plasma. Surface radicals then polymerize with adsorbed monomer to yield the thin film product. Based on this scheme, Denaro, et. al. derived a simple rate expression which showed reasonably good agreement with deposition rate data at various pressures and power levels (16). It is, however, unrealistic to assume that the plasma polymerization reactions occur exclusively on the surface. A more likely mechanism is that both gas phase and surface reactions proceed simultaneously in plasma polymer formation. 

Cavin, L., Rouge, A., Meyer, X, Renken, A. Kinetic modeling of free radical polymerization of styrene initiated by the bifunctional initiator 2,5-dimethyl-2,5-bis(2-ethyl hexanoyl peroxy)hexane. Polymer 41(11), 3925-3935 (2000)... 

Krajnc, M., Poljansek, I., Golob, J. Kinetic modeling of methyl methacrylate free-radical polymerization initiated by tetraphenyl biphosphine. Polymer 42(9), 4153 162 (2001)... 

Hence, the dynamical scaling of DMDAACh radical polymerization allows to describe quantitatively the kinetic curve Q, and can be used for its prediction. In this approach base the key physical principles and models (scaling, universahty classes, irreversible aggregation models, fractal analysis) are placed. The three key process properties, characterized reactive centers concentration (c ), diffusive characteristics of reactive medium (q) and accessibility degree of reactive centers (Df), are used for the kinetic cmves description. It is supposed, that the offered approach will be vahd for the description of radical polymerization process of any polymer. 

Since there are no direct measurements of radical capture available, it is always necessary to extract the kinetics of radical capture from the overall kinetics of emulsion polymerization. This deficient situation means that model-independent radical entry data are not available [36], and that erroneous conclusions might be obtained from this type of experimental result [37]. In addition, the precise determination of the radical capture mechanism can be reliable only if a wide range of values for the polymer volume fraction, from highly diluted ( 0.1%) to concentrated (>10%) dispersions, is considered [38]. 

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