Monomer transport

No monomer transport through the solid catalyst support is considered. The diffusivity Z)PV in Eq. (73) is only a formal notation because the flux between the void phase V and the polymer P (or P ) micro-elements is calculated from the assumption of no accumulation and sorption equilibrium at the phase interface. 

When the active centre is surrounded by a layer of solid polymer, further propagation will be controlled by the rate of monomer diffusion through the polymer layer. Usually it will be retarded. With a porous polymer layer surrounding the active centres, monomer transport will be easier. These effects must be considered when highly crystalline polymers are formed, especially when the chains grow from a non-transferring monomer as, for example, with coordination polymerizations [56],... 

The yield of the catalyst, 0, was measured at various ethylene concentrations (see Fig. 10). According to the results, initiation is rapid and the catalytic system maintains full capacity for a long time, for at least 1 h. In this interval, the polymeric particles increase their size 5-10 fold. Thus the monomer supply into the pores of the particles by diffusion cannot be hindered. In the subsequent phase, activity already decreases. Either the conditions for monomer transport to the centres by diffusion are deteriorating, and/or the centres are slowly decaying. The polymerization rate, i>pol, can be determined from the slopes of the curves in Fig. 10. The determined values of the initial rates are directly proportional to monomer concentration (except for the lowest values of [M]), as shown in Fig. 11. 

At lower stirring speeds, on the other hand, stirring controls the rate of monomer transport from the monomer droplets to the polymer particles, thereby controlling the rate of polymerization. The rate-determining step is usually the monomer-transport step from the monomer droplets to the aqueous phase, because the monomer-transport step from the aqueous phase to the polymer particles is much faster than the former step due to the much greater total surface area of the polymer particles compared to that of the monomer droplets. 

One of the most unique properties of miniemulsion polymerization is the lack of monomer transport. Recall from Fig. 1 that with macroemulsion polymerization, the monomer must diffuse from the monomer droplets, across the aqueous phase, and into the growing polymer particles. In contrast, in an ideal miniemulsion (nucleation of 100% of the droplets), there is no monomer transport, since the monomer is polymerized within the nucleated droplets. This lack of monomer transport leads to some of the most interesting properties of miniemulsions. For most monomers, macroemulsion polymerization is considered to be reaction, rather than diffusion limited. However, for extremely water insoluble monomers, this might not be the case. In this instance, polymerization in a miniemulsion might be substantially faster than polymerization in an equivalent macroemulsion. For copolymerization in a macroemulsion, where one of the comonomers is highly water insoluble, the comonomer composition at the locus of polymerization might be quite different from the overall comonomer composition, resulting in copolymer compositions other than those predicted by the reactivity ratios. 

Kitzmiller et al. [148] have found the rate of copolymerization for VAc and vinyl 2-ethylhexanoate to be much slower in macroemulsion than in miniemulsion. They attribute this to monomer transport effects for the less water-soluble monomer. 

While the rate of monomer transport in macro emulsions may or may not limit the rate of polymerization, it is quite possible that unequal rates of diffusions for comonomers may make the comonomer composition at the locus different (richer in the more water-soluble monomer) from the overall composition. 

The Mayo Lewis equation, using reactivity ratios computed from Eq. 18, will give very different results from the homogenous Mayo Lewis equation for mini-or macroemulsion polymerization when one of the comonomers is substantially water-soluble. Guillot [151] observed this behavior experimentally for the common comonomer pairs of styrene/acrylonitrile and butyl acrylate/vinyl acetate. Both acrylonitrile and vinyl acetate are relatively water-soluble (8.5 and 2.5%wt, respectively) whereas styrene and butyl acrylate are relatively water-insoluble (0.1 and 0.14%wt, respectively). However, in spite of the fact that styrene and butyl acrylate are relatively water-insoluble, monomer transport across the aqueous phase is normally fast enough to maintain equilibrium swelling in the growing polymer particle, and so we can use the monomer partition coefficient. 

Extremely water insoluble comonomers are only selectively used in emulsion polymerization because of concerns about monomer transport limitations. 

Typically, copolymer composition can be manually adjusted by slowly feeding the more reactive monomer in throughout the reaction but this may not be helpful when trying to overcome monomer transport limitations. Therefore, Reimers and Schork [ 102] performed identical copolymerization experiments in miniemulsions, where monomer transport is less significant, in order to determine what effect this would have on the evolution of the copolymer composition. Data on the MMA/VS (and other) copolymerizations indicate that the Schuller equation (and not the Samer adaptation) fits the copolymer composition data. This points to the effect of extremely low monomer water solubility on copolymer composition in macroemulsion polymerization, and the relative insensitivity of miniemulsion polymerization to this effect. 

Delgado et al. pubhshed a series of papers [56,58,91,153-157] on the miniemulsion copolymerization of vinyl acetate and butyl acrylate. A very comprehensive mathematical model of the polymerization system was developed. Equihbrium swelling was accoimted for, since the model did not presume complete droplet nucleation, and so monomer transport from unnucleated mini-... 

An investigation of the copolymer composition demonstrated the important effect of monomer transport on the copolymerization. The droplets in the macroemulsion act as monomer reservoirs. In this system, the effect of monomer transport will be predominant when an extremely water-insoluble comonomer, such as DOM, is used. In contrast with the macroemulsion system, the miniemulsion system tends to follow the integrated Mayo Lewis equation more closely, indicating less influence from mass transfer. 

The difference in copolymer composition between miniemulsion and macroemulsion copolymerization in a batch reactor was not observed in a CSTR. In this case, the copolymer composition for the extremely water insoluble comonomer in a miniemulsion recipe decreases from a batch reactor to a CSTR. This difference can be attributed to the fact that monomer transport is... 

Extremely hydrophobic monomers do not polymerize well via macroemulsion polymerization due to their very low rates of monomer transport across the aqueous phase. Obviously, these monomers can be polymerized much more effectively in a miniemulsion system. One example of this is provided by Landfester et al. [320]. In this paper,fluoroalkyl acrylates are polymerized in a miniemulsion with low levels of a protonated surfactant. When fluorinated monomers were copolymerized with standard hydrophobic and hydrophilic monomers, either core-shell structures or statistical copolymers were formed. 

Rodripuez VS (1988), Interparticle Monomer Transport in Miniemulsion Copolymerization, Ph.D. Dissertation, Lehigh University, Bethlehem,... 

Padmanaban et al. have described a new class of polymers containing a photosensitive disilane group in the main chain. Krongauz and Yohannan have developed an interesting non-destructive method for monitoring the kinetics of monomer transport during photopolymer formation. 

C p maintains its saturation value during intervals I and n provided that the interfacial area of monomer droplets is high enough to allow the monomer transport to the growing particles where it replaces the monomer consumed swelling the polymer formed [146]. However, on the basis of Monte-Carlo simulations, Tauer and Hernandez [147] have claimed that latex particles in emulsion polymerization never experience either a period of saturation with monomer or a constant monomer concentration during interval II, as frequently assumed. 

The mechanism by which monomer is transported between miniemulsion droplets and from these to polymer particles was the focus of a study using styrene and methyl methacrylate as comonomers [7]. The SLS/HD surGactant/cosurfactant system was used in this work. Several approaches were applied in an attempt to determine the diffusive mass-transfer coefficients of the monomeis as well as the contribution of monomer droplef polymer particle collision to the monomer transport process. Mass transfer was shown not to be a limitation in miniemulsion polymerization (i.e. equilibrium was satisfied under normal conditions). However, the thermodynamic model developed previously [6] predicted the continued existence of monomer droplets until the end of a miniemulsion polymerization provided that not all droplets were nucleated. Experimentally though, nucleation has been shown to end well before complete conversion which implies another route for monomer droplet disappearance, namely, collision. 

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