Monomers monomer-swollen micelles

Emulsion Polymerization. Emulsion polymerization takes place in a soap micelle where a small amount of monomer dissolves in the micelle. The initiator is water-soluble. Polymerization takes place when the radical enters the monomer-swollen micelle (91,92). Additional monomer is supphed by diffusion through the water phase. Termination takes place in the growing micelle by the usual radical-radical interactions. A theory for tme emulsion polymerization postulates that the rate is proportional to the number of particles [N. N depends on the 0.6 power of the soap concentration [S] and the 0.4 power of initiator concentration [i] the average number of radicals per particle is 0.5 (93). 

Complex formation takes place in an organic solvent or in a water/monomer mixture by reaction of the macroligand with a metal compound (e.g. a Cu(I)-ha-lide). It is supposed that the conditions in the reaction mixture are comparable to those in conventional emulsion polymerization, where monomer droplets stabilized by surfactant molecules coexist with monomer swollen micelles [64]. Reaction sites are presumably the hydrophobic core of the micelles and the monomer droplets as well. Initial results of the micellar-catalyzed ATRP of methyl methacry-... 

The emulsion polymerization system consists of three phases an aqueous phase (containing initiator, emulsifier, and some monomer), emulsified monomer droplets, the monomer-swollen micelles, and monomer-swollen particles. Water is the most important ingredient of the emulsion polymerization system. It is inert and acts as the locus of initiation (the formation of primary and oligomeric radicals) and the medium of transfer of monomer and emulsifier from monomer droplets or the monomer-swollen particle micelles to particles. An aqueous phase maintains a low viscosity and provides an efficient heat transfer. 

In accordance with the Smith-Ewart theory, the nucleation of particles takes place solely in the monomer-swollen micelles which are transformed into polymer particles [16]. This mechanism is applicable for hydrophobic (macro)mon-omers (see Scheme 2). The initiation of emulsion polymerization is a two-step process. It starts in water with the primary free radicals derived from the water-soluble initiator. The second step occurs in the monomer (macromonomer)-swollen micelles by entered oligomeric radicals. 

The presence of a very large number micelles indicates that radicals are captured predominantly by the monomer-swollen micelles. Each entry of a radical to a monomer-swollen micelle leads to a nucleation event and therefore the particle number increases with conversion. The particle growth is supposed to be a result of propagation of monomer and the agglomeration of primary particles. The dead monomer-swollen polymer particles and uninitiated monomer-swollen micelles serve as a reservoir of monomer. Solution or bulk polymerization kinetics seem to govern the microemulsion polymerization process [30,31]. 

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,... 

Therefore, they showed both theoretically and experimentally that the kinetic behavior of the emulsion polymerization of St initiated by AIBN is basically similar to that initiated by KPS, and concluded that this similarity is mainly due to the radicals produced from the water-soluble fraction of the initiator, because the radicals produced pair-wise inside the small volume of a monomer-swoUen latex particle or a monomer-swollen micelle are very hkely to recombine. 

Unlike in conventional emulsion polymerization, no monomer droplets exist in a microemulsion polymerization system, and hence, oil-soluble initiators partition into the monomer-swollen micelles, the resultant polymer particles and the water phase. Therefore, in microemulsion polymerization, the polymerization only proceeds in the monomer-swollen micelles and the resultant polymer particles over the entire course of polymerization. Pairs of radicals produced in volumes as small as monomer-swollen micelles and polymer particles may terminate as soon as they are generated. If so, it is expected that the radicals responsible for the polymerization in the monomer-swollen micelles and the resultant polymer particles would usually be those generated from the fraction of the initiator dissolved in the water phase. In order to examine whether this expectation is correct, oil-in-water (O/W) microemulsion polymerizations of St were carried out using four kinds of oil-soluble azo-type initiators with widely different water-solubilities [209]. It was found that the rates of polymerization with these oil-soluble initiators were almost the same irrespective of their water-solubilities, when the polymerizations were carried out with the same rate of radical production for the whole system for all of the oil-soluble initiators used. Moreoever, the rate of polymerization with any of these oil-soluble initiators was only about 1/3 of that with KPS at the same rate of radical production. Considering that the rate of polymerization was pro-... 

Since the x-value in the relationship of Rp oc Cg depends on the size of monomer-swollen micelles and the latter is, in turn, related to the solubilizing power of the monomer-free micelles and the hydrophobic properties of the monomers, the "micellar size effect" should predict the following ... 

Np = Number of polymer particles per unit volume of aqueous phase Number of starting mixed micelles per unit volume of aqueous phase Percentage of the total micelles to become polymer particles, or, the probability of a monomer-swollen micelle to become a polymer particle, or, the probability of nucleation, and... 

Smith and Ewart (1948) assumed that particles are formed when free radicals diffuse into monomer-swollen micelles. Roe (1968) demonstrated later that the S-E Model, in a mathematical sense, did not depend on the concept of a micelle. Roe referred simply to the stabilizing capability of the... 

I hcse unusual variations of N with emulsifier concentration and temperature suggest that the aqueous monomer droplets may furnish loci for polymerization initiation. In conventional emulsion polymerizations, the monomer droplets usually range in diameter from 1 to 10 microns therefore their total surface area is small relative to that of the micelles, and they serve only as reservoirs in the particle initiation process. However, if these monomer droplets were smaller by one or two orders of magnitude, they would compete effectively with the monomer-swollen micelles for the available radicals and therefore would serve as loci for particle initiation. 

Therefore, it is concluded that, although the monomer-swollen micelles furnish loci for particle initiation, many monomer droplets are small enough to serve also in this capacity and, as a result, particle initiation occurs in both the micellar and droplet ph e. The reverse dependence of N on temperature at low emulsifier concentrations is still unexplained. 

The mean distance of the monomer swollen micelles is of the order of the diameter of the spurs (elementary reaction regions of secondary electrons, produced by the y-radiation). Therefore, possibly, a certain fraction of the radio-lytical radicals does not recombine in the Frank-Rabinovitch cage, but breaks into the micelles or particles and is trapped therein. 

Abstract. Polymerization in direct mini-emulsions is a relatively new polymerization technique which allows the preparation of submicron latex particles within the range 100large population of submicron monomer droplets in water (termed the mini-emulsion) by intensive shear force with the aid of an adequate emulsifier and coemulsifier (or hydrophobe). These stable, homogenized monomer droplets have an extremely large surface area and, therefore, can compete effectively with the monomer-swollen micelles, if present, for the oligomeric radicals gen-... 

Inverse-emulsions can also be formed with monomer droplets very similar in size to the monomer-swollen micelles, due to the large emulsifier levels needed to stabilize these systems. Distinguishing such species may be both arbitrary and rather semantic. 

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