Mass transfer, of monomers

Mass transfer of monomer from the suspended drops through the aqueous phase to the seeded particles continues throughout the polymerization. 

Boocock and Haward [252] have examined the polymerization of ethylene by a- and T-TiCla/AlEtg under conditions such that mass transfer of monomer to the active centres was limiting, with the following observations ... 

Table lOB shows relative polymerization rates for several catalysts prepared under different conditions. At progressively lower concentrations of catalyst (ca. 5 x 10 mole 1" ) and operating so that mass transfer of monomer was not a limiting factor much higher rate coefficients were observed (> 10 1 mole sec at 80°C). Maximum reaction rates were determined for a number of titanium concentrations and propagation rates calculated using the equation on p. 174. In this way fepf was... 

Fig. 14.3 Schematic view of mass transfer of monomer from the poiymer particies, through the aqueous phase, into the COj phase, based on the flim modei.

Miniemulsion copolymerization of a 50 50 styrene/methyl methacrylate monomer mixture, using hexadecane as hydrophobe, was carried out by Rodriguez et al. [78]. The mechanism of mass transfer between miniemulsion droplets and polymer particles in the miniemulsion copolymerization of styrene-methyl methacrylate (AIBN as initiator, hexadecane as hydrophobe) was studied, analyzing the mass transfer of highly water-insoluble compounds from miniemulsion droplets to polymer particles by both molecular diffusion and collisions between droplets and particles [79,80]. 

The effect of mass transfer of vinyl versatate on the mini/macroemulsion polymerization of VAc/VEOVA in batch and semibatch systems was explored. For the batch experiments, the addition of neat VEOVA formed poor dispersions of VEOVA, which resulted in smaller particles, lower polymerization rates and different polymer composition tracks compared to normal mini/macroemulsion polymerization of VAc/VEOVA. The well-dispersed VEOVA seemed to help the monomer-swollen particle to gain more radicals in the nucleation period. 

Therefore, it is easy to understand how some experimental evidences, such as the catalytic activity decay, have been attributed to diffusion limitations w-196,199,200) Some of these limitations, connected to mass transfer of the monomer from the gas to the liquid phase and from the liquid phase to the polymer surface, can be generally eliminated or at least minimized by means of an appropriate choice of operating conditions... 

Water is a chief ingredient in both suspension and emulsion polymerization. As the continuous phase, although inert, it acts to maintain a low viscosity and provides for good heat transfer. In addition, it serves to isolate the polymerization loci. Termed compartmentalization, this is a particular advantage in emulsion polymerization as will be described later in terms of rates of polymerization and molar masses. The water also acts as the medium of transfer of monomer from... 

The catalytic activity of imprinted materials would also be improved if the active sites were located primarily at the solid liquid interface, owing to better mass transfer of reactants and products into and out of these sites. Emulsion polymerization is, as already mentioned, a means by which imprinted polymers can be prepared with a high surface area and accessible sites. This methodology has been adapted to prepare an artificial biocatalyst [67], employing oleoyl imidazole as the functional host monomer, A -a-t-Boc-L-histidine as template and 7V-a-Boc-L-alanine / -nitrophenyl ester as the substrate for hydrolysis (Fig. 20). 

For a binary system, D j is the binary mutual diffusion coefficient Dp. The FRRPP process, however, is essentially a ternary system of the polymer/monomer/ precipitant type (such as polystyrene/styrene/water or poly(methacrylic acid)/ methacrylic acid/water). Let us designate the polymer (poly(methacrylic acid) or polystyrene) as component 1 precipitant (such as water or ether) as component 2 and the monomer (methacrylic acid or polystyrene) as component 3. Therefore, for the FRRPP process, we have four mutual diffusivities in the mixture Dn is approximated as the mutual diffiisivity of the polymer and precipitant D22 is approximated as the mutual diffiisivity of the monomer and the precipitant D 2 is the mutual dif-fusivity for the mass transfer of the polymer due to composition gradient of the monomer and D21 is the mutual diffiisivity for the mass transfer of the monomer... 

In a classic 1978 paper [5,6], L.L. Bohm reported on the experimental parameters needed to establish steady-state polymerization conditions in order to eliminate monomer transport phenomena from the experimental results. As pointed out by Bohm, suspension or slurry polymerization takes place if the polymerization temperature is lower than the polyethylene solubility temperature and, therefore, the semicrystalline polymer precipitates from the suspension medium as the polymerization proceeds. The important physical process is the mass transfer of ethylene, comonomer and hydrogen (chain transfer reagent used to control polymer molecular weight) from the gas phase through the suspension medium and into the growing polymer particle to the active site. In order to obtain correct kinetic results, concentration gradients and temperature gradients within the polymer particle need to be removed from the polymerization process to achieve the necessary steady-state polymerization conditions. 

If a linear mbber is used as a feedstock for the mass process (85), the mbber becomes insoluble in the mixture of monomers and SAN polymer which is formed in the reactors, and discrete mbber particles are formed. This is referred to as phase inversion since the continuous phase shifts from mbber to SAN. Grafting of some of the SAN onto the mbber particles occurs as in the emulsion process. Typically, the mass-produced mbber particles are larger (0.5 to 5 llm) than those of emulsion-based ABS (0.1 to 1 llm) and contain much larger internal occlusions of SAN polymer. The reaction recipe can include polymerization initiators, chain-transfer agents, and other additives. Diluents are sometimes used to reduce the viscosity of the monomer and polymer mixture to faciUtate processing at high conversion. The product from the reactor system is devolatilized to remove the unreacted monomers and is then pelletized. Equipment used for devolatilization includes single- and twin-screw extmders, and flash and thin film evaporators. Unreacted monomers are recovered for recycle to the reactors to improve the process yield. 

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