Unseeded polymerizations

The emulsion polymerization of vinyl hexanoate has been studied to determine the effect of chain transfer on the polymerization kinetics of a water-insoluble monomer. Both unseeded and seeded runs were made. For unseeded polymerizations, the dependence of particle concentration on soap is much higher than Smith-Ewart predictions, indicating multiple particle formation per radical because of chain transfer. Once the particles have formed, the kinetics are much like those of styrene. The lower water solubility of vinyl hexanoate when compared with styrene apparently negates its increased chain transfer, since the monomer radicals cannot diffuse out of the particles. 

Kinetics of Vinyl Hexanoate. A series of standard polymerizations was conducted using 10 ml of vinyl hexanoate in a total volume of 140 ml with varying amounts of emulsifier and initiator. The effect of varying initiator, particle number, and organic volume fraction in seeded runs, as well as for unseeded polymerizations, were also studied. The results are summarized in Table II. 

Moles monomer polymerized per liter aqueous phase 6 Unseeded polymerizations c Seeded polymerizations 10.3 420... 

Gugliotta, L.M., Leiza, ).R Arot arena, M., Armitage, P.D. and Asua, ).M. (1995) Copolymer composition control in unseeded emulsion polymerization using calorimetric data. Industrial Engineering Chemistry Research, 34, 3899-906. 

In unseeded polymerizations, the number of particles increases and they grow in size by polymerization and imibition of monomer, so that both N and r are functions of time. The value of N is computed by iterative numerical integration of Equation 4, whilst the value of r must be calculated from the rate of polymerization Since the particles are spheres,... 

These results explain the findings of Blackley and Haynes who also showed that the molecular weight of the polymer formed in the presence of ethyl benzene was lower than that in its absence. Calculation from their experimental data shows that their n varied from 0.005 to 0.039 radicals per particle, well into Case 1. Thus, their explanation on the basis of the Trommsdorff "gel" effect cannot be correct since this requires the mutual termination of two macroradicals in a particle, which obtains only under Case 3 kinetics. Similar experiments on the effect of the diluents on "insitu" (unseeded) and seeded emulsion polymerization indicates that n decreases due to desorption of free radicals from the particles (27). 

In order to delve deeper into the similarities and differences between the kinetic behaviors of emulsion polymerization initiated by oil-soluble initiators or water-soluble initiators, Nomura et al. [199-202] carried out extensive investigations into the kinetics and mechanisms of the unseeded and seeded emulsion polymerizations of St at 50 °C using sodium lauryl sulfate (NaLS) as the emulsifier and AIBN as the initiator, and obtained the following conclusions ... 

Fig. 1. Conversion-time curve for typical unseeded emulsion polymerization leactioa.

Thus, the heterogeneous reactions of isoprene can be a source of several components of atmospheric aerosols. The evidence is limited, however, and has to be supported by further experimental work. Other work has shown that the formation of polymeric compounds is also possible in non-acid or unseeded gas-phase experiments. 

Figure 84i Conversion transient for methyl methacrylate (MMA) emulsion polymerization in an unseeded CSTR. [20]...

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. 

Hansen, F. K., and J. Ugelstad, Particle Nucleation in Emulsion Polymerization la. Theory of Homogeneous Nucleation, J. Polym. Sci. Polym. Chem., 16,1953,1978 II. Nucleation in Emulsifier-Free Systems Investigated by Seed Polymerization III. Nucleation in Systems with Anionic Emulsifier Investigated by Seeded and Unseeded Polymerization IV Nucleation in Monomer Droplets, J. Polym. Sci. Polym. Chem., 17, 3033-3046, 3047-3068, 3069, 1979. 

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