Polymerization continued locus

The monomer droplets and the micelles swollen with monomer compete for the free radicals generated in the aqueous phase, but since there are many more micelles than droplets in the system most of the free radicals enter micelles. Polymerization is initiated within individual micelles. The monomer consumed during the resulting polymerization is replenished by diffusion of new monomer molecules from the aqueous phase, which in turn, is kept saturated with monomer from the droplets of monomer. Polymerization continues within a given micelle until a second free radical enters the micelle, in which case termination quickly occurs because of the small volume of the reaction locus. The micelle then remains inactive until a third free radical enters, and so on. As reaction proceeds the micelles become larger and are disrupted to form particles of polymer swollen with monomer which are stabilized by soap molecules... 

The argument on the main polymerization locus continued for several years, until 1980, and closed without finding a clear conclusion. 

In emulsion polymerization the compartmentalization of reaction loci and the location of monomer in polymer particles favor the growth and slow down termination events. The contribution of solution polymerization in the continuous phase is strongly restricted due to the location of monomer in the monomer droplets and/or polymer particles. This gives rise to greatly different characteristics of polymer formation in latex particles from those in bulk or solution polymerization. In emulsion polymerization, where polymer and monomer are mutually soluble, the polymerization locus is the whole particle. If the monomer and polymer are partly mutually soluble, the particle/water interfacial region is the polymerization locus. 

Emulsion Polymerization in a CSTR. Emulsion polymerization is usually carried out isothermally in batch or continuous stirred tank reactors. Temperature control is much easier than for bulk or solution polymerization because the small (. 5 Jim) polymer particles, which are the locus of reaction, are suspended in a continuous aqueous medium as shown in Figure 5. This complex, multiphase reactor also shows multiple steady states under isothermal conditions. Gerrens and coworkers at BASF seem to be the first to report these phenomena both computationally and experimentally. Figure 6 (taken from ref. (253)) plots the autocatalytic behavior of the reaction rate for styrene polymerization vs. monomer conversion in the reactor. The intersection... 

The polymerization process of two monomers with different polarities was carried out in direct or inverse miniemulsions using the monomer systems AAm/MMA and acrylamide/styrene (AAm/Sty). The monomer, which is insoluble in the continuous phase, is miniemulsified in the continuous phase water or cyclohexane in order to form stable and small droplets with a low amount of surfactant. The monomer with the opposite hydrophilicity dissolves in the continuous phase (and not in the droplets). Starting from those two dispersion situations, the locus of initiation (in one of the two phases or at the interface) was found to have a great influence on the reaction products and on the quality of the obtained copolymers, which can act as hydrogels. 

Once a radical enters a reaction locus, it is presumed to initiate a chain polymerization reaction which then continues at a constant rate until the activity of the radical is lost. The processes whereby the activity of the propagating radicals is lost from the reaction loci can be classified into two broad types ... 

The influence of the emulsifier (SHS) concentration on Np is more pronounced in the conventional emulsion polymerization system (Rp°c[SHS]y, y= 0.68) than in mini-emulsion polymerization (y=0.25). This result is caused by the different particle formation mechanism. While homogeneous nucleation is predominant in the conventional emulsion polymerization, monomer droplets become the main locus of particle nucleation in mini-emulsion polymerization. In the latter polymerization system, most of the emulsifier molecules are adsorbed on the monomer droplet surface and, consequently, a dense droplet surface structure forms. The probability of absorption of oligomeric radicals generated in the continuous phase by the emulsifier-saturated surface of minidroplets is low as is also the particle formation rate. 

The fact that these polymers do not dissolve in flieir own monomers leads to complex phase separation at veiy low conversions during the polymerization process. The phase separation is even more complex than in a conventional emulsion polymerization since, not only is there phase separation between the newly formed polymer and the continuous wat phase, but also betweoi the polymer and monomer present. Due to this phase s aradon the locus of polymerization is not the interior of the polymer particles, simply because there is no monomer in the interiors. Free radicals formed in the water phase toid to precipitate or adsorb at the surface of existing particles r idly after their fonnation. Thus the major part of the propagation process will take place at the surface of the polymer latex particles [5,6]. Evidence for this is that the rate of polymerization is proportional to the total surface area of the latex particles this bend has been determined by varying both the latex particle concentration and size and observing the polymerization rate [5]. 

The variety of structures encountered in microemulsions offers great versatility for choosing the locus of polymerization. Besides polymerization in globular microemulsions, several studies have dealt with polymerization of monomers in the other phases of microemulsions. One of the main goals underlying these studies was to use the microstructure of microemulsions as a template to produce solid polymers with similar characteristics. For example, incorporation of large amount of hydrophobic monomers in the continuous phase of W/O microemulsions should yield solid polymers with a Swiss cheese-like structure capable of encapsulating the disperse phase (water). This would allow the inclusion of materials (metallic colloidal particles as catalysts, photochromic compounds, etc.) in the disperse phase that would otherwise be insoluble in the polymer. 

The kinetics of emulsion polymerization differ completely from that of bulk or suspension. The soap (detergent) is used not only as a stabilizer but mainly as locus of the polymerization—so-called soap micelles. These consist of an array of 20-100 molecules of soap creating a micellar stmcture of 25-50 A in length (radius). The initiator is water soluble and the free radicals and the monomer molecules diffuse into the hydrophobic interior of the micelles while water is attracted to the hydrophilic exterior zone. Thus, the micelles serve as the core of growing polymer particles. At a later stage the micellar structure disappears, and the process continues in the polymer particles swollen by monomers, leaving the soap as a protective layer on the particle surface. The concentration of soap dictates the molecular weight and rate of production. 

The polymerization proceeds in two reaction loci, the continuous phase and the polymer particles. At low conversion, the first locus is dominant and low-molecular-weight chains are predominantly produced. Since no gel effect is operative in supercritical phase, no acceleration is verified in the first part of the conversion versus time curve and the first mode of the MWD is quite narrow, corresponding to a polydispersity of about 1.5. On the other hand, the relevance of the polymerization in the particles is increases rapidly and becomes dominant by the first hour. This behavior explains the increase in polymerization rate and the high molecular weight of the second MWD mode (much lower ter-... 

Polymerization occurs in both the continuous phase and the particles. Because of the compartmentalization of the radicals among the polymer particles, the main polymerization locus changes from the continuous phase at the beginning of the process to the dispersed phase after the nucleation period. The polymerization in the particles is mainly initiated by radicals entering from the continuous phase with some contribution from the initiator located in the particles [132]. 

Simultaneous with the formation of ester groups, free hydroxyl groups are formed in the reaction above, without the release of a small molecule such as water, which is common to most step polymerizations. Given favorable reaction conditions, these hydroxyl groups constitute a new locus for continued reactions. In fact, some care must be exercised for highly unsaturated triglycerides, because too large a functionality may be introduced. 

---