Monomers polymer particle

As the micelles grow by absorption of more monomer and formation of polymer, they become relatively large particles that absorb soap from micelles that have not been inoculated or stung by oligoradicals. Thus, in stage II, when about 20% of the monomer has been converted to polymer, the micelles disappear and are replaced by large, but fewer, monomer-polymer particles. 

Fate of Radicals Escaping from Polymer-Monomer Particles. The fate of a radical escaping from a polymer-monomer particle has been a hot topics and discussed quantitatively in the latest decade. An oligomer radical once entering a monomer-polymer particle seems to find it hard to escape from the particle because... 

A period of constant particle number and constant monomer concentration in the monomer-polymer particles, with constant reaction rate. 

Vinylidene chloride and chloroprene (Figures 7 and 8) under the given conditions produce curves which more or less resemble the styrene curve. Vinylidene chloride especially shows a long period of a rather constant reaction rate. By the theory of Harkins and Smith-Ewart this would be interpreted as a period of constant particle number and of constant monomer concentration at the reaction site—i.e., the monomer-polymer particles. The first assumption seems justified (15). The second assumption of constant monomer concentration at the reaction site can be true only in a modified sense because poly (vinylidene chloride) is insoluble in its monomer, and the monomer-polymer particles in this system therefore have a completely different structure as compared with the monomer-polymer particles in the styrene system. 

Most monomers polymerizing by the radical mechanism are almost insoluble in water. Intensive stirring of a mixture of such a monomer with water produces an emulsion which remains stable, however, only in the presence of a surface active compound (tenside), e. g. soap. By the addition of a water-soluble initiator to this emulsion, the monomer polymerizes at a rate several times higher than would be observed by any other radical method with an initiator of equal efficiency. At the same time, a higher polymer with a narrower molecular mass distribution is formed. At the initial stages of the reaction, the monomer is present as three types of particle in tenside-stabilized monomer droplets of diameter 10-3 to 10 4cm (about 1012 such droplets are present in 1 cm3 of emulsion of average concentration) in solubilized micelles about 10 nm in size and concentration 1018 cm 3 and in the growing, emulsifier-stabilized monomer—polymer particles 50-100 nm in size. This situation is illustrated schematically in Fig. 14(a). 

Fig. 14. Monomer placement in particles during emulsion polymerization, a, b, c, Various stages of polymerization (see text) d monomer particle stabilized by emulsifier diam. = 104 nm e monomer-polymer particle f, monomer solubilized in a micelle q, emulsifier. See p. 180 of ref. 126.

The proceeding monomer—polymer transformation is accompanied by an increasing need for stabilization of the monomer—polymer particles. The necessary emulsifier is drawn from the micelles so that, at a certain moment, the micelles disppear [at a conversion x = 0.1-0.2 see Fig. 14(b). 

Finally, the monomer supply from the droplets is exhausted so that, in the final stage of emulsion polymerization, only the monomer—polymer particles are present in the system in an amount of about 1016cm-3 [see Fig. 14(c)]. 

When all generated radicals enter the micelles forming monomer— polymer particles, then k = 0.53. When the radicals simultaneously diffuse into micelles and into monomer particles at rates proportional to the surface of each of these formations, then k = 0.37 (the experimentally found value of k in styrene polymerization is 0.40). 

The idea of particle inhomogeneity was supported experimentally by Williams [149], However, his representation of growth is more complicated. In phase II, the monomer concentration in the particle decreases with conversion, while the rate remains constant. The particle has a core with a relatively high polymer content surrounded by a monomer-rich layer (see Fig. 16). Polymerization occurs at the polymer—monomer interface. Under these conditions, monomer concentration at the interface remains constant, even when its amount in the particle decreases. Napper presented the idea of an exactly opposite composition of the monomer—polymer particle [150], The core should be enriched in monomer and surrounded by a layer with a higher polymer content. Van den Hul and van der Hoff found most growing ends of macromolecules at the particle surface [151], which supports Napper s model. 

These examples illustrate the great variety of current ideas. A generally accepted theory of emulsion polymerization, based on a non-uniform composition of the monomer—polymer particles, is presently being developed [152-155]. 

The presence of the emulsifier, and thus of the micelles, is not a necessary condition for emulsion polymerization. Styrene, the acrylates, vinyl acetate, and vinyl chloride can be polymerized in aqueous medium without an emulsifier [126]. The mechanism of the formation of monomer—polymer particles is in agreement with the ideas of Fitch and Tsai [139] which were refered to above. 

The polymerization of vinyl chloride evidently proceeds in the monomer phase and in monomer-polymer particles. Crosato-Arnoldi et al. [17] and... 

The fourth factor determining polymerization rate is the monomer concentration in the particles. For some monomers the ratio of monomer to polymer in the particles is about constant during part of the polymerization. Smith (57) suggested that this results from a balance between the eflFect on the monomer activity of the dissolved polymer and the eflFect of interfacial tension of the very small particles. This equilibrium was put in a quantitative form by Morton, Kaizerman, and Altier (44), who derived the following equation by combining an expression for the interfacial free energy of the particle with the Flory-Huggins equation for the activity of the solvent (monomer) in the monomer-polymer particle. 

The first part of the log function up to the maximum may be interpreted as follows. At the beginning, the reaction proceeds according to the conception of Harkins. Dispersed PMMA takes up 3 to 4 times its monomer (by weight) contrary to PMMA, dispersed polystyrene dissolves only 1 to 1.5 times its own weight of monomer (Table I). In the case of the emulsion polymerization of MM A, the pure monomer phase therefore disappears at about 30% conversion. From here on, the Trommsdorff (38) or gel effect acts to increase the over-all reaction rate until the maximum is reached. During this period, the bulk of the monomer in monomer-polymer particles is consumed. 

The following sharp drop of t Br niay probably be explained by the assumption that because of high concentration of polymers of high molecular weight in monomer-polymer particles, diffusion of not only polymer molecules but also monomer molecules is retarded. Therefore further polymerization takes place more slowly, despite the high mean radical concentration (gel effect) 1... 

Considering a polymerizing emulsion system at its distribution balance, the three phases must show the same monomer activity the monomer-polymer particles, the micellar phase, and the water phase. Both monomer-polymer particles and the organic part of the micelles are lipophilic, and, therefore, compete for monomer. It does not seem plausible to assume that equal monomer activities in these two phases belong to monomer concentrations which differ by several orders of magnitude. Therefore it is likely that new particles are formed also after the disappearance of the pure monomer phase, provided there is a micellar phase, and enough monomer in the monomer-polymer particles as well. 

Micelle converts into monomer -polymer particle... 

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