Molecular versus Particle Chains

In the silicic acid system, the formation of gel has usually been ascribed to the condensation of Si(OH) into siloxane chains, then branching and cross-linking to form a three-dimensional molecular network. Measurement of gel time has often been used as an indication of rate of polymerization (90, 110-113). 

Such a siloxane gel network might be obtained under conditions where depolymerization is least likely to occur so that the condensation is irreversible and siloxane bonds can not be hydrolyzed once they are formed. Thus SiCOH), made by dissolving an anhydrous orthosilicate in anhydrous methanol-hydrochloric acid, or by hydrolysis of methyl silicate with a theoretical amount of water in methanol, will slowly form a gel. Owing to the insolubility of silica in the system (which is another 

On the basis of studies of the thermal aging of silica gels, Shapiro and Kolthoff (117) agreed with Elkins, Shull, and Roess (118) that the structure of siEca gel could best be visualized as being jnade up of discrete particles. The latter authors estimated, from small angle X-ray scattering, that the average size of the primary particles, assumed to be spherical, in freshly prepared silica gel was around 30-60 A. 

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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-... 

The emulsion polymerization of vinyl acetate (to homopolymers and copolymers) is industrially most important for the production of latex paints, adhesives, paper coatings, and textile finishes. It has been known that the emulsion polymerization kinetics of vinyl acetate differs from those of styrene or other less water-soluble monomers largely due to the greater water solubility of vinyl acetate (2.85% at 60°C versus 0.054% for styrene). For example, the emulsion polymerization of vinyl acetate does not follow the well-known Smith-Ewart kinetics and the polymerization exhibits a constant reaction rate even after the separate monomer phase disappears. The following observations have been reported for vinyl acetate emulsion polymerization [78] (a) The polymerization rate is approximately zero order with respect to monomer concentration at least from 20% to 85% Conversion (b) the polymerization rate depends on the particle concentration to about 0.2 power (c) the polymerization rate depends on the emulsifier concentration with a maximum of 0.25 power (d) the molecular weights are independent of all variables and mainly depend on the chain transfer to the monomer (e) in unseeded polymerization, the number of polymer particles is roughly independent of conversion after 30% conversion. 

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