Heterogeneous polymerization dispersed media

Recently, compositions made up of OEAs and various linear polymers have gained importance. In the initial stage, this system is similar to that made up of a plastified polymer [55—62]. Later on the system is transformed because of unsaturated zones found in the oligomer and the polymer. Ideally, the polymerization process is developed in a heterogeneous system made up of the dispersed oligomer and the polymeric dispersion medium. 

Supercritical carbon dioxide has been used as a dispersing medium for the manufacture and processing of polymeric materials. The process allows for the synthesis of high molar mass acrylic polymers in the form of micrometer-sized particles with a narrow size distribution. This procedure represents an environmentally responsible alternative to aqueous and organic dispersing media for heterogeneous dispersion polymerizations (Fox, 1994). 

Compared to water-another green solvent which is frequently adopted as a dispersing medium in industrial heterogeneous polymerizations-scC02 eliminates the need for energy-intensive drying processes, as the polymer product can... 

Suspension Polymerization - Refers to a heterogeneous polymerization regime in which the product of the reaction is a solid forming a suspension in the liquid medium of reaction. Little or no surfactant is added to the reaction medium. Characteristics of the process include high agitation rate and poor particle size control. An advantage of this reaction is high purity of the polymer product as compared to that of the dispersion method. 

If possible from the point of view of efficiency, polymerization of the reactants without any use of a dispersing medium is desired. This method is called Bulk Polymerization. However, in order to absorb the heat released in the reaction, other methods like suspension or emulsion polymerization are frequently used. In this case, the water absorbs heat and provides good control of temperature. Another (less popular) method is the solution (homogeneous or heterogeneous) polymerization, which is mostly utilized in ionic initiation. We compare advantages and disadvantages of each method. (Table 2-4 describes the most useful polymerization methods for some commercial polymers). 

Dispersion pol3nnerizatlon, as used in this paper, differs from the other forms of heterogeneous polymerization described above in that the monomer is soluble in the polymerization medium and the polymer is not. 

Heterophase polymerization systems can be defined as two-phase systems in which the resulting polymer and/or starting monomer are in the form of a fine dispersion in an immiscible liquid medium defined as the polymerization medium , continuous phase , or outer phase . Even if oil-in-water (o/w) systems are greatly preferred on an industrial scale, water-in-oil (w/o) systems may also be envisaged for specific purposes. Heterogeneous polymerization processes can be classified as suspension, dispersion, precipitation, emulsion, or miniemulsion techniques according to interdependent criteria which are the initial state of the polymerization mixture, the kinetics of polymerization, the mechanism of particle formation and the size and shape of the final polymer particles (Fig. 4.2) [18]. 

Th reactions in polymerization are highly exothermic and highly viscous, often carried out in a heterogeneous dispersion medium. 

Heterogeneous polymeric compositions include most commercially important materials. All these systems are two-phase systems in which heterogeneity is inherent in the very principle of production of these materials. Pol3mieric blends and alloys are noted for a two-phase structure (see Chapter 6) in which both phases are continuous, and for this reason it is impossible to determine which phase is the dispersion medium and which the dispersed phase (of course, this depends on the ratio of components). In contrast to blends, pol3miers, filled with polymeric fillers (including fibers), comprise systems which are character-... 

Polymer latexes are heterogeneous systems which consist of two phases, namely, a dispersion medium and a disperse phase. For example, conventional emulsion polymerization systems comprise a continuous aqueous phase and a dispersed oil (polymer) phase. Inverse emulsion systems comprise the continuous oil phase and the dispersed aqueous phase. The dispersion medium is known as the continuous phase or the external phase. As shown, it is aqueous in nature or organic in the case of inverse systems. The disperse phase of a latex is known as the discrete phase, the internal phase and the dispersed monomer or polymer. The polymer latex comprises... 

Another heterogeneous process of great industrial importance is emulsion polymerization. The reaction components differ from those used in suspension polymerization only in that the initiator must not be soluble in monomer but soluble only in the aqueous dispersion medium. Nevertheless, this difference has far-reaching consequences for the mechanism and kinetics of polymerization, and also for the form of the reaction product which is a colloidally-stable dispersion of particulate polymer in water known as a latex. The polymer particles generally have diameters in the range 0.05 -l/ m, i.e. considerably smaller than for suspension polymerization. 

Lu et al. [86] also studied the effect of initiator concentration on the dispersion polymerization of styrene in ethanol medium by using ACPA as the initiator. They observed that there was a period at the extended monomer conversion in which the polymerization rate was independent of the initiator concentration, although it was dependent on the initiator concentration at the initial stage of polymerization. We also had a similar observation, which was obtained by changing the AIBN concentration in the dispersion polymerization of styrene conducted in isopropanol-water medium. Lu et al. [86] proposed that the polymerization rate beyond 50% conversion could be explained by the usual heterogenous polymer kinetics described by the following equation ... 

Heterogeneous Copolymerization. When copolymer is prepared in a homogeneous solution, kineiic expressions can be used to predict copolymer composition Bulk and dispersion polymerization are somewhat different since the reaction medium is heterogeneous and polymeri/aiion occurs simultaneously in separate loci. In bulk polymerization, for example, the monomer swollen polymer particles support polymerization within the particle core us well as on the particle surface, lit aqueous dispersion or emulsion polymeri/aiion the monomer is actually dispersed in two or three distinct phases a continuous aqueous phase, a monomer droplet phase, and a phase consisting of polymer particles swollen at Ihe surface with monomer. This affect the ultimate polymer composition because llie monomers are partitioned such that the monomer mixture in the aqueous phase is richer in the more water-soluble monomers than the two organic phases. 

For purposes of clarity, it is perhaps instructive to describe the distinction between a solution and dispersion. The term solution refers to a complete solvent shell around an atom, molecule, or a single polymeric chain on the molecular scale. It refers to a single phase and a homogeneous solution. The term dispersion means that particles larger than Inm are surrounded by and embedded in a medium. Thus, dispersions are not homogeneous, but consist of at least two phases. This heterogeneous system contains a pure solvent phase with no dissolved molecules and the pure dispersed phase. Both phases approach each other at the interfaces. 

Dispersion Polymerization - This technique is a heterogenous regime where a significant amount of surfactant is added to the polymerization medium. Characteristics of the process include small uniform polymer particles which may be unstable and coagulate if they are not stabilized. 

Emulsion pol)m erization is a complex process in which the radical addition polymerization proceeds in a heterogeneous system. This process involves emulsification of the relatively hydrophobic monomer in water by an oil-in-water emulsifier, followed by the initiation reaction with either a water-soluble or an oil-soluble free radical initiator. At the end of the pol)nnerization, a milky fluid called "latex", "synthetic latex" or "pol)rmer dispersion" is obtained. Latex is defined as "colloidal dispersion of polymer particles in an aqueous medium". The pol)nner may be organic or inorganic. In general, latexes contain 40-60 % pol)nner solids and comprise a large population of polymer particles dispersed in the continuous aqueous phase (about lO particles per mL of latex). The particles are within the size range 10 nm to 1000 run in a diameter and are generally spherical. A typical of particle is composed of 1-10000 macromolecules, and each macromolecule contains about lOO-lO " monomer units [10-16]. 

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