Absorbing monomer

Dispersion polymerization involves an initially homogeneous system of monomer, organic solvent, initiator, and particle stabilizer (usually uncharged polymers such as poly(A-vinyl-pyrrolidinone) and hydroxypropyl cellulose). The system becomes heterogeneous on polymerization because the polymer is insoluble in the solvent. Polymer particles are stabilized by adsorption of the particle stabilizer [Yasuda et al., 2001], Polymerization proceeds in the polymer particles as they absorb monomer from the continuous phase. Dispersion polymerization usually yields polymer particles with sizes in between those obtained by emulsion and suspension polymerizations—about 1-10 pm in diameter. For the larger particle sizes, the reaction characteristics are the same as in suspension polymerization. For the smallest particle sizes, suspension polymerization may exhibit the compartmentalized kinetics of emulsion polymerization. 

The rate of reaction, measured by emission of CO2, is proportional to the concentration of absorbed monomer molecules ... 

In a separate investigation by Kent et al. (3) latex compositions containing poly(methyl methacrylate-co-hexyl acrylate) were prepared containing the ultraviolet absorber monomer Norbloc 7966. 

Another reaction path involves the micellization of in situ surfactant. Both the micelles and primary particles can absorb monomer. Particle growth occurs by polymerization fed by entry into these loci of oligomers generated in the water. Figure 8-.3 is a schematic representation of these mechanisms, the details of which are still debatable [9-13]. 

In solvents of high dielectric constant, (e.g. dimethylformamide), formaldehyde polymerized sluggishly and polymers were formed in low yield. In similar solvents, aliphatic aldehydes could not be polymerized. Precipitated aldehyde polymers have the tendency to absorb monomers. The monomer concentration near the propagating site may be much hi er than that in the surrounding solution. This occurs with n-butyraldehyde polymerizations in pentane [5] the polymer precipitated during the polymerization is highly swollen by the monomer. 

Termination of polymerization in micelles finally occurs, since the expelled polymer nuclei absorb monomer and the resultant mixed nucleus adsorbs on to its surface a monolayer of soap. Eventually this process reduces the soap concentration below the critical value for micelle formation, and further initiation practically ceases. This generally occurs at about 13 to 20% yield of polymer, but if as much soap as 6% be used as catalyst, micelles can persist up to the highest yields. 

Figure 1. Olefinic proton resonances of styrene-methacrylic anhydride copolymers containing both uncycttzed methacrylic anhydride units and absorbed monomer (top), and uncyclized methacrylic anhydride units but little absorbed monomer (bottom). This sample was prepared by reacting a styrene-methacrylic acid copolymer with methacrylic anhydride.

Nonylphenyl methacrylate was synthesized as a UV-absorbing monomer to confirm the effect of this type of hydrophobic monomer on the surfactant solution. Results are shown in Figure 5 (the monomer concentration has been adjusted on the basis of the number of double bonds obtained from the NMR results see the Experimental Details section). Although the points are in an almost straight line, the intercept of a least-squares line through all of the points would be about 5000 however, the intercept must be zero. Therefore, there must be an initial linear relationship that includes the origin. At higher nonylphenyl methacrylate concentrations, the MW is lower than extrapolated from the initial line. Thus, this hydrophobic monomer also acts as a cosurfactant. 

Plasma initiation of the chain polymerization is due to formation of a primary free radical R( ), starting the traditional scheme (9-70), and by formation of positive or negative ion radicals, which are also capable of initiating the MMA polymerization. The primary free radical R( ) as well as the charged centers of polymer growth are formed from the absorbed monomers by electron/ion bombardment and UV radiation from plasma. Formation of a positive ion radical from an adsorbed MMA molectrle on the strrface under electron/ion bombardment and UV radiation can be schematically shown as the ionization process... 

Related effects occur when process chemicals are absorbed that may later react, decompose, or solidify within the structure of the polymer. Prolonged retention of the chemicals may lead to their decomposition within the polymer. Although it is unusual, it is possible for absorbed monomers fo polymerize. 

The potentially active centers react with monomer in an initiation step at a rate v, which is proportional to the fraction /mon of the catalyst surface covered by absorbed monomer ... 

The number-average degree of polymerization is determined by the propagation reaction as well as by both the concentration of true active center [C ] and transfer reactions involving both absorbed monomer and absorbed metal alkyl. Thus, at time t... 

The rate of growth of the primary particles due to polymerization of the absorbed monomer in the polymer-rich phase, ly, can be expressed in terms of the overall rates of polymer-rich phase, i pp, using the following equation ... 

Potassium persulfate is dissolved in the aqueous phase and thermally dissociates into free radicals that can initiate polymerization when they diffuse into monomer micelles. As the polymer particle grows by absorbing monomer micelles, surfactant leaves the aqueous phase to cover the increased surface area of the particle. Schork and Ray (1987) demonstrated slow oscillatory behavior in the conversion and in the surface tension of the aqueous phase. (The surface tension increases with decreasing surfactant concentration.)... 

Ultraviolet radiation is the other main stress encountered by polymers in the coatings arena. One hundred percent acrylic polymers are highly resistant to photodegradation because they are transparent to the vast majority of the solar spectrum (59). When uv-absorbing monomers, such as styrene, are incorporated into the polymer backbone, the uv-resistance of the resulting polymer decreases dramatically and a more rapid deterioration in polymer/coating properties is observed. On the other hand, a noncovalently bound uv absorber, such as hydroxybenzophenone [117-99-7], further improves the uv stability of 100% acrylic polymers (59). 

There is an important group of processes where this situation occurs, namely dispersion and emulsion polymerizations. In these processes the reaction starts in the continuous water phase, but after a while another liquid phase begins to form. By carefully selecting the right initiator and emulsifier concentrations, the number of newly formed particles may be controlled. These particles absorb monomer and become the second reaction phase, gradually taking over from the continuous (water) phase. Finally the particles consist mostly of amorphous polymer (see section 13.6). 

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