Molecular weight distribution emulsion polymerization

The application of these comprehensive models to the prediction of the emulsion polymer molecular weight distribution requires a fundamental understanding of the very conaplex reaction mechanisms and knowledge of various kinetic parameters (e.g., the rate coefficients for the absorption of free radicals by the latex particles, the desorption of radicals out of the particles, and the bimolecular termination reaction). However, these mathematical models in combination with the polymer molecular weight distribution data may serve as a useful tool for estimating the values of the kinetic parameters involved in emulsion polymerization. 

Successful NMP in emulsion requires use of conditions where there is no discrete monomer droplet phase and a mechanism to remove any excess nitroxide formed in the particle phase as a consequence of the persistent radical effect. Szkurhan and Georges"18 precipitated an acetone solution of a low molecular weight TEMPO-tcrminated PS into an aqueous solution of PVA to form emulsion particles. These were swollen with monomer and polymerized at 135 °C to yield very low dispersity PS and a stable latex. Nicolas et at.219 performed emulsion NMP of BA at 90 °C making use of the water-soluble alkoxyamine 110 or the corresponding sodium salt both of which are based on the open-chain nitroxide 89. They obtained PBA with narrow molecular weight distribution as a stable latex at a relatively high solids level (26%). A low dispersity PBA-WocA-PS was also prepared,... 

Much has been written on RAFT polymerization under emulsion and miniemulsion conditions. Most work has focused on S polymerization,409-520 521 although polymerizations of BA,461 522 methacrylates382-409 and VAc471-472 have also been reported. The first communication on RAFT polymerization briefly mentioned the successful semi-batch emulsion polymerization of BMA with cumyl dithiobenzoate (175) to provide a polymer with a narrow molecular weight distribution.382 Additional examples and discussion of some of the important factors for successful use of RAFT polymerization in emulsion and miniemulsion were provided in a subsequent paper.409 Much research has shown that the success in RAFT emulsion polymerization depends strongly on the choice of RAFT agent and polymerization conditions.214-409-520027... 

Models for emulsion polymerization reactors vary greatly in their complexity. The level of sophistication needed depends upon the intended use of the model. One could distinguish between two levels of complexity. The first type of model simply involves reactor material and energy balances, and is used to predict the temperature, pressure and monomer concentrations in the reactor. Second level models cannot only predict the above quantities but also polymer properties such as particle size, molecular weight distribution (MWD) and branching frequency. In latex reactor systems, the level one balances are strongly coupled with the particle population balances, thereby making approximate level one models of limited value (1). 

Lichti, G., R. G. Gilbert, and D. H. Napper, J. Polym. Sci. Polym. Chem. Ed., 18,1297 (1980) Theoretical Predictions of the Particle Size and Molecular Weight Distributions in Emulsion Polymerization, Chap. 3 in Emulsion Polymerization, I. Piirma, ed., Academic Press, New York, 1982. 

Figure 15. Molecular weight distribution of anionically polymerized styrene-butadiene random copolymer and emulsion polymerized SBR.

Tagata and Homma47 analyzed in the aforementioned manner the compositional heterogeneity of two typical commercial SBR samples, E-SBR and S-SBR, which had 23.5 and 20.0 average styrene wt.%, and were produced by an emulsion polymerization and by a solution polymerization with an organometallic catalyst, respectively. The result was that the former gave a distinctly narrower compositional distribution than the latter. GPC experiments on these samples were also carried out, which indicated that the above situation was just the opposite for the molecular-weight distributions. 

Free-radical initiation of emulsion copolymers produces a random polymerization m which the trans/ds ratio cannot be controlled. The nature of ESBR free-radical polymerization results in the polymer being heterogeneous, with a broad molecular weight distribution and random copolymer composition. The microstructurc is not amenable to manipulation, although the temperature of the polymerization affects the ratio of trans to cis somewhat... 

Inisurfs, Transurfs and Surfmers may be used to reduce/avoid the use of conventional surfactants in emulsion polymerization. However, when Inisurfs and Transurfs are used, the stability of the system cannot be adjusted without affecting either the polymerization rate (Inisurfs) or the molecular weight distribution (Transurfs). Furthermore, the efficiency rate of Inisurfs is low due to the cage effect. It is therefore not obvious yet that these classes will become commercially significant. 

In the following section on reactor and process types, the preponderance of work on mass/solution polymerization is pointed out with a brief review of the more limited work on suspension and emulsion polymerization. In another broad view of the field (the next section) the distinction between polymerization rate and product distribution is discussed, particularly the preoccupation with molecular weight distribution. 

Tn emulsion polymerization and in some suspension polymerizations, free radicals are generated in a continuous phase and diffuse into a dis-persed-phase particle or droplet where polymerization takes place (5). The molecular weight distributions or, equivalently, the polymer size distributions of these systems depend on the relative rates of radical arrival and termination. Frequently in emulsion polymerization the radicals are terminated so quickly that each particle in the dispersed phase... 

Fig. 15. Molecular weight distribution of polymer in concentrated emulsion polymerization and in bulk polymerization (SDS 0.3 g, water 3 ml, styrene 40 ml, 40 °C, 30 h)...

Figure 18, which plots conversion vs monomer volume fraction, exhibits a maximum at about during polymerization, some cells of the gel coalesce and form a bulk phase in which the conversion is smaller. Visual observations indeed indicated a separated thin layer at the upper part of the tube after polymerization. Since no appreciable separated liquid phase was observed before polymerization, it is likely that during polymerization some cells did coalesce. Molecular weight distribution curves have been determined for various values of (j>. The GPC curves (see Fig. 19) have a tail which is consistent with the molecular weight distribution of the polymer prepared by bulk polymerization. Therefore it is likely that this tail is due to the polymerization in bulk. The greater amount of bulk phase formed for values of < ) greater than 0.9 is probably due to the decreased stability of the concentrated emulsion in such cases. 

Figure 22 presents the GPC curves of polystyrenes obtained in concentrated emulsions at various temperatures. The molecular weight distribution broadens because of a greater amount of low molecular weight polymers generated in the bulk as the polymerization temperature increases. The greater the temperature, the greater is the coalescence and hence the amount of bulk phase formed. 

Figure 26 compares the conversion as a function of time in concentrated emulsion and bulk polymerization and shows that polymerization proceeds much faster in a concentrated emulsion. The concentrated emulsion has an internal phase ratio of 0.93 and a molar ratio of MAA/styrene of 0.036. The molecular weight distributions of the polymers generated by both processes are presented in Fig. 27, which shows that concentrated emulsion polymerization leads to molecular weights an order of magnitude higher. Since the copolymer composition changes with conversion, the GPC curves were recorded at the same conversion. 

Fig. 27. Molecular weight distribution of the polymers produced in (a) concentrated emulsion (polymerization time = 9 h) (b) bulk polymerization (polymerization time = 41.5 h) (styrene 37 ml, methacrylic acid 1 ml, AIBN 0.3 g, SDS 0.4 g, water 6 ml)...

Keywords Emulsion polymerization Kinetics Particle nucleation Particle growth Molecular weight distribution Nonlinear polymers... 

Tobita H (2003) Bimodal molecular weight distribution formed in emulsion polymerization with long-chain branching. Polym React Eng 11 855 BarabasiA-L, Albert R (1999) Science 286 509... 

The emulsion polymerization process has several distinct advantages of providing a polymer of exceptionally high molecular weight, and narrow molecular weight distribution, while permitting efficient control over the exothermic polymerization reaction because the aqueous phase absorbs the heat of reaction. 

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