Micelle, monomer-swollen

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). 

As reaction proceeds the micelles become swollen with monomer and polymer and they eject polymer particles. These particles which are stabilised with soap molecules taken from the micelles become the loci of further polymerisation, absorbing and being swollen by monomer molecules. 

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-... 

As the conversion to polymer proceeds, the micelles become progressively more and more swollen by the polymer-monomer mixture. As with other microemulsions, it eventually becomes problematic as to whether the resulting dispersed particles should be called micelles or swollen polymer particles with adsorbed surfactant. 

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]. 

If one provides the system with preformed nuclei, for example in the form of "seed" latex particles, monomer droplets (in sufficient number), dust, or monomer-swollen soap micelles, the AG will have been provided and the system progresses on a downward slope (dashed curve in Figure 1) from the beginning. This constitutes heterogeneous nucleation. Since it is energetically favored, it will tend to occur whenever such conditions obtain. 

The preparation of a latex by emulsion polymerization comprises two stages (i) particle nucleation (ii) particle growth. For the latex to be monodisperse, the particle nucleation stage must be short relative to the particle growth stage. Despite many investigations, there is disagreement as to the locus of particle nucleation (i) monomer-swollen emulsifier micelles (ii) ad-... 

The description of phase 1 of the SE theory was refined by Gardon [133] and Harada et al. [134] the effect of particles in which polymerization has been terminated by radical entrance is included. Paris et al. [135] and Sautin et al. [136] calculate the balance of micelles and of the growing and dead particles. Pismen and Kuchanov [137] and Sundberg and Eliassen [138] included the effect of particle size distribution in their calculations. According to Fitch and Tsai [134] and Roe [140], the monomer swollen particles are produced by the polymerization of the monomer which is dissolved in water. 

On the other hand, they derived an expression that predicts the number of polymer particles produced, ATp, assuming that (i) a monomer-swollen emulsifier micelle is transformed into a polymer particle by capturing a free radical from the aqueous phase, (ii) the volumetric growth rate per particle p is constant, at least during particle formation, and (iii) free radical activity does not transfer out of a growing particle... 

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,... 

When a free radical in the aqueous phase enters a monomer-swollen emulsifier micelle and polymerization proceeds therein (micellar nucleation). 

A monomer-swollen emulsifier micelle is transformed into a polymer particle by capturing a free radical from the aqueous phase [4,5]. 

The free radicals produced from the fraction of initiator dissolved in the water phase are responsible for particle formation and growth in the emulsion polymerization of St initiated by AIBN. The free radicals produced in pairs in the polymer particles play almost no role in the polymerization inside the polymer particles because pairs of radicals produced within a volume as small as a monomer-swollen 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-... 

Strictly speaking, if the size of the micelle, is to play a role in emulsion polymerization, it should be the size of the monomer-swollen surfactant micelle, and not the monomer free one. However, these two sets of "sizes" should be proportional in the present case... 

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... 

The probability, p(n), of a monomer-swollen surfactant micelle to become a polymer particle is a function of its size. 

---