Other Characteristics of Emulsion Polymerization

Polymerizations conducted in nonaqueous media in which the polymer is insoluble also display the characteristics of emulsion polymerization. When either vinyl acetate or methyl methacrylate is polymerized in a poor solvent for the polymer, for example, the rate accelerates as the polymerization progresses. This acceleration, which has been called the gel effect,probably is associated with the precipitation of minute droplets of polymer highly swollen with monomer. These droplets may provide polymerization loci in which a single chain radical may be isolated from all others. A similar heterophase polymerization is observed even in the polymerization of the pure monomer in those cases in which the polymer is insoluble in its own monomer. Vinyl chloride, vinylidene chloride, acrylonitrile, and methacryloni-trile polymerize with precipitation of the polymer in a finely divided dispersion as rapidly as it is formed. The reaction rate increases as these polymer particles are generated. In the case of vinyl chloride ... 

Other aspects of the subject which have received considerable attention in recent years include the preparation of mono-disperse latices by emulsion polymerization, emulsion polymerization in reaction systems to which no surfactant has been added, the special features which attend the emulsion polymerization of polar monomers such as acrylic esters, the preparation of latices of carboxyla-tcd polymers by emulsion polymerization, and the characteristics of emulsion polymerization systems to which new reactants are continuously being added and from which the product is continuously being removed. 

An important characteristics of emulsion polymerization is that, unlike other polymerization processes, the polymerization rate and molecular weight can be increased at the same time. This is due to the compartmentalization of the system that reduces the probability of mutual termination of propagating radicals. The behavior of this compartmentalized system dqiends on the rate of exchange of species between the elements of the system. The main ingredients of an emulsion polymerization system include monomer, dispersant, emulsifier, and initiator. Water is commonly used as the dispergant. A water-insoluble monomer can be dispersed in water by means of an oil-in-water emulsifier and polymerized with a water-soluble initiator. 

S-E cases 1 and 2 correspond to what is known as zero-one systems, in which the radicals grow in isolated compartments, reaching very high molecular weights hence, this characteristic feature of emulsion polymerization is known as compartmentalization. In case 3, this characteristic is relaxed so that radicals in a given particle grow in the presence of other radicals. As more radicals coexist within the particles, the system approaches the behavior of a bulk polymerization (or pseudobulk system). 

Finally, addition polymerization of suitably substituted furans allows incorporation of the furan nucleus into heterocyclic polymers (77MH1102). 2-Vinylfuran apparently exhibits free radical polymerizability comparable with that of styrene, although rates, yields and degrees of polymerization are low under all conditions except for emulsion polymerization. Cationic polymerization is quite facile and leads not only to the poly(vinylfuran) structure (59), as found in free radically produced polymers, but also to structures such as (60) and (61) in which the furan nucleus has become involved. Furfuryl acrylate and methacrylate undergo free radical polymerization in the manner characteristic of other acrylic esters. 

Besides giving latices of narrow particle size distribution, mixed surfactant systems have shown several other interesting characteristics which lighten some aspects concerning the mechanism of particle nucleation in emulsion polymerization process. 

We are currently exploring new routes to the synthesis of ionomers with controlled architecture, i.e. with control over the amount and location of ionic groups in the polymer backbone. One of our main interests is the synthesis of ion containing block copolymers. The applicability of anionic polymerization in the synthesis of block copolymers and other well defined model systems is well documented (22-24) Not as well appreciated, however, is the blocky nature that certain emulsion copolymerizations may provide. Thus, we have utilized both anionic and free radical emulsion polymerization in the preparation of model ionomers of controlled architecture. In this paper, the synthesis and characteristics of sulfonated and carboxylated block ionomers by both free radical emulsion and anionic polymerization followed by hydrolysis will be discussed. 

Typically, the segregated phase has a smaller characteristic length scale than the continuous phase. In a monomer-flooded emulsion polymerization, the aqueous continuous phase will contain monomer drops and polymer particles, although large monomer drops may also contain smaller water droplets or polymer particles (if crosslinked or insoluble). This is the consequence of a thermodynamic principle that acts in the direction of a constant chemical potential for all species, throughout the whole system. In other words, there is a driving force that pushes all of the components of a system to be present in different proportions in all of its phases. This principle has been proven in spontaneous emulsification experiments, where droplet formation is observed on either side of the liquid-liquid interface [7]. Moreover, the chemical potential is size-dependent at the colloidal scale and hence, particles of different size will possess different compositions. 

In nature as well as in technology, polymeric emulsifiers and stabiUzers play a major role in the preparation and stabiUzation of emulsions. Natural materials such as proteins, starches, gums, cellulosics, and their modifications, as well as synthetic materials such as polyvinyl alcohol, polyacryhc add, and polyvinylpyrrolidone, have several characteristics that make them extremely useful in emulsion technology. By the proper choice of chemical composition, such materials can be made to adsorb strongly at the interface between the continuous and dispersed phases. By their presence, they can reduce interfacial tension and/or form a barrier (electrostatic and/or steric) between drops. In addition, their solvation properties serve to increase the effective adsorbed layer thickness, increase interfacial viscosity, and introduce other factors that tend to favor the stabilization of the system. 

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