Polymerization processes free radical influences

Emulsion polymerization is the process of choice for the commercial production of many polymers used for coating and adhesive applications, especially for those products that can be used in latex form. Emulsion polymerization uses free-radical polymerization mechanisms with unsaturated monomers. The heterogeneous nature of the reaction mixture, however, has a significant influence on the chemical and physical reaction mechanisms and on the nature of the final product. 

Emulsion polymerization requires free-radical polymerizable monomers which form the structure of the polymer. The major monomers used in emulsion polymerization include butadiene, styrene, acrylonitrile, acrylate ester and methacrylate ester monomers, vinyl acetate, acrylic acid and methacrylic acid, and vinyl chloride. All these monomers have a different stmcture and, chemical and physical properties which can be considerable influence on the course of emulsion polymerization. The first classification of emulsion polymerization process is done with respect to the nature of monomers studied up to that time. This classification is based on data for the different solubilities of monomers in water and for the different initial rates of polymerization caused by the monomer solubilities in water. According to this classification, monomers are divided into three groups. The first group includes monomers which have good solubility in water such as acrylonitrile (solubility in water 8%). The second group includes monomers having 1-3 % solubility in water (methyl methacrylate and other acrylates). The third group includes monomers practically insoluble in water (butadiene, isoprene, styrene, vinyl chloride, etc.) [12]. 

The most comprehensive simulation of a free radical polymerization process in a CSTR is that of Konopnicki and Kuester (15). For a mechanism which includes transfer to both monomer and solvent as well as termination by combination and disproportionation they examined the influence of non-isothermal operation, viscosity effects as well as induced sinuoidal and square-wave forcing functions on initiator feed and jacket temperature on the MWD of the polymer produced. 

First, in composites with high fiber concentrations, there is little matrix in the system that is not near a fiber surface. Inasmuch as polymerization processes are influenced by the diffusion of free radicals from initiators and from reactive sites, and because free radicals can be deactivated when they are intercepted at solid boundaries, the high interfacial area of a prepolymerized composite represents a radically different environment from a conventional bulk polymerization reactor, where solid boundaries are few and very distant from the regions in which most of the polymerization takes place. The polymer molecular weight distribution and cross-link density produced under such diffusion-controlled conditions will differ appreciably from those in bulk polymerizations. 

Surface free radicals are assumed to be produced by the dissociation of bonds at the polymer surface as well as by the adsorption of radicals from the gas phase. New polymer is formed by the recombination of surface and gas phase free radicals. Within the context of this model, the rate of polymerization should depend upon the rates at which surface and gas phase free radicals are produced. The influence of the discharge frequency on the rates of these processes can now be examined. 

Microstructure of Polybutadienes. Microstructure strongly influences the viscosity of the CTPB prepolymer. The viscosity of CTPB increases with increased vinyl content, but for CTPB prepolymers of the required molecular weight, an upper limit of 35% vinyl groups is satisfactory from the standpoint of propellant processing characteristics. It has also been found that the microstructure changes markedly with the synthesis process. Lithium-initiated polymerization yields prepolymers with slightly higher vinyl content than those produced by free radical initiation. 

Vinyl-type addition polymerization. Many olefins and diolefins polymerize under the influence of heat and light or in the presence of catalysts, such as free radicals, carbomum ions or carbamons. Free radicals are particularly efficient in starting polymerization of such important monomers as styrene, vinylchloride, vinylacetate, methylacrylate or acrylonitrile. The first step of this process—the so-called initiation step—consists in the thermal or photochemical dissociation of the catalyst, and results in the formation of two free radicals-. 

Spherical beads that can be expanded into foam under the influence of heat or steam are produced directly by suspension polymerization in the presence of blowing agent. The term suspension polymerization describes a process in which water-insoluble monomers are dispersed as liquid droplets with suspension stabilizer and vigorous stirring to produce polymer particles as a dispersed solid phase. Initiators used in suspension polymerization are oil-soluble. The polymerization takes place within the monomer droplets. The kinetic mechanism of the suspension process is considered to be a free radical, water-cooled microbulk polymerization [1]. 

Since the separate time scale condition is clearly valid for most of the polymerization process, one may say that each polymer chain is formed inside a particle of unchanging siz wherein all rate coefficients are constant and the distribution of free radicals has its steady-state value, for each volume V. Any residual effect of the PSD on the MWD would reside presumably in the effects of the PSD on the kinetic parameters (e.g., p. c, and to a lesser extent fc). Conversely, the MWD would possibly influence the PSD through its effects on the swelling of the particles by the monomer the effect, if it exists, is likely to ha small. 

Termination reactions cannot be eliminated in radical polymerizations because termination reactions involve the same active radical species as propagation therefore, eliminating the species that participates in termination would also result in no polymerization. Termination between active propagating species in cationic or anionic processes does not occur to the same extent because of electrostatic repulsions. Equation (1) represents the rate of polymerization, Rp, which is first order with respect to the concentration of monomer, M, and radicals, P, while Eq. (2) defines the rate of termination, Rt, which is second order with respect to the concentration of radicals. To grow polymer chains with a degree of polymerization of 1000, the rate of propagation must be at least 1000 times faster than the rate of termination (which under steady state condition is equal to the rate of initiation). This requires a very low concentration of radicals to minimize the influence of termination. However, termination eventually prevails and all the polymer chains produced in a conventional free radical process will be dead chains. Therefore they cannot be used in further reactions unless they contain some functional unit from the initiator or a chain transfer agent. 

The purpose of this review article is to summarize the historical development of the solvent effect on free radical polymerization and to point out possibilities of specific interactions of the propagating radical with solvent. The effect of metal salts on the propagation process will not be described. Emphasis will be laid on the interpretation of experimental results, relating to the influence of aromatic solvents on propagation rate constants, and on the discussion for the molecular interpretation. 

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