Polymerization system, seeded

The monomer conversion in this seeded polymerization system is independent of the degree of segregation as long as an exponential residence time distribution function is maintained. 

The theory also has relevance to the so-called seeded " emulsion polymerization reactioas- In these reactions, polymerization is initial in the presence of a seed latex under conditions such that new particles are unlikely to form. The loci for the compartmentalized free-radical polymerization that occurs are therefore provided principally by the particles of the initial seed latex. Such reactions are of interest for the preparation of latices whose particles have, for instance, a core-shell" structure. They are also of great interest for investigating the fondamentals of compartmentalized free-radical polymerization processes. In this latter connection it is important to note that, in principle, measurements of conversion as a function of time during nonsteady-state polymerizations in seeded systems offer the possibility of access to certain fundamental properties of reaction systems not otherwise available. As in the case of free-radical polymerization reactions that occur in homogeneous media, investigation of the reaction during the nonsteady state can provide information of a fundamental nature not available through measurements made on the same reaction system in the steady state. 

Besides of this classification including the above three processes, another classification is also made according to the way in which the reaction loci are formed. By this way, two types of reaction system are defined "ab initio emulsion polymerization" and "seeded emulsion polymerization" [29]. 

The seeded emulsion polymerization process can be applied for VAc/BuA comonomer system. Seeded reactions allow for the production of mono-dispersee latexes. Key factors in producing narrow PSD latexes are concentration of seed latex and ionic strength. The increasing of seed latex concentration (55% by weight) causes to the largest mono-dispersee particles [93]. 

Several studies followed another philosophy to solve mechanical problems in the use of PCL copolymer instead of changing the fabrication technique. Rat bone marrow cells were seeded and cultured on a porous polymeric system composed of cornstarch blended with PCL. Results revealed that... 

Compartmentalized Free-radical Polymerization.—Considerable interest has been shown in recent years in the solution of the differential difference equations which are obtained when the theory of Smith and Ewart is applied to reaction systems which contain a fixed number of reaction loci, but in which a steady state for the various locus populations has yet to be established. An example of such a reaction system would be a seeded emulsion polymerization system within whose external phase new radicals suddenly begin to be generated, and which does not contain sufficient surfactant to permit the nucleation of new particles. The theory which has been developed is concerned with the question of the nature of the approach to the steady-state distribution of locus populations, and with what might be learned from accurate measurements made during the approach to the steady state. 

In summary, formation of particle nuclei from emulsified monomer droplets is almost certain to occur in any emulsion polymerization system in which these droplets are present. As mentioned earlier, however, monomer droplets containing polymer will primarily serve as reservoirs to provide monomer to the much more numerous and smaller latex particles formed by other particle nucleation mechanisms. Polymerization in monomer droplets can be eliminated or at least minimized by using seed polymer particles and slowly adding monomer (neat or as an emulsion) to supply the growing seed particles (i.e., seeded semibatch emulsion polymerization under the monomer-starved condition). 

To resolve this instability problem, adopting a feed stream of seed latex particles [62] or installing a continuous tubular reactor, which generates seed particles, upstream of the continuous stirred tank reactor [53] have been proved quite effective (Figure 7.4b). For the latter approach, small latex particles form as a seed latex before the reacting stream enters the continuous stirred tank reactor when the monomer conversion at the exit of the tubular reactor is maintained at an adequate level. As a result, the continuous emulsion polymerization system can be operated at a stable steady state. The work of Nomura and Harada [54] also suggests that a tube-stirred tank reactor series... 

Continuous reactors with seed latex particles in the feed stream could be an interesting polymerization system for morphological studies. The broad residence time distribution of the polymerizing latex particles associated with such a reactor configuration results in a broad particle size distribution of the effluent product. By changing the particle size distribution (monodisperse or polydisperse) of seed latex particles and operation conditions (mean residence time, monomer addition method, etc.) simultaneously, one can essentially obtain a variety of morphological structures of latex particles. 

Other than fuel, the largest volume appHcation for hexane is in extraction of oil from seeds, eg, soybeans, cottonseed, safflower seed, peanuts, rapeseed, etc. Hexane has been found ideal for these appHcations because of its high solvency for oil, low boiling point, and low cost. Its narrow boiling range minimises losses, and its low benzene content minimises toxicity. These same properties also make hexane a desirable solvent and reaction medium in the manufacture of polyolefins, synthetic mbbers, and some pharmaceuticals. The solvent serves as catalyst carrier and, in some systems, assists in molecular weight regulation by precipitation of the polymer as it reaches a certain molecular size. However, most solution polymerization processes are fairly old it is likely that those processes will be replaced by more efficient nonsolvent processes in time. 

A modified latex composition contains a phosphorus surface group. Such a latex is formed by emulsion polymerization of unsaturated synthetic monomers in the presence of a phosponate or a phosphate which is intimately bound to the surface of the latex. Thus, a modified latex containing 46% solids was prepared by emulsion polymerization of butadiene, styrene, acrylic acid-styrene seed latex, and a phosphonate comonomer in H20 in the presence of phosphated alkylphenol ethoxylate at 90°C. The modified latex is useful as a coating for substrates and as a binder in aqueous systems containing inorganic fillers employed in paper coatings, carpet backings, and wallboards [119]. 

Seeded dispersion polymerization was extensively investigated for radical systems [17]. Much less is known about seeded dispersion polymerizations with propagation on ionic and/or pseudoionic active centers. Awan et al. reported seeded ionic polymerization of styrene, which at certain conditions produced particles with narrow diameter size dispersity [18,19]. We presented the first data on the seeded ring-opening polymerization with constant number of microspheres. 

Hayashi et al., 1989], involving the addition of monomer and initiator to a previously prepared emulsion of polymer particles, is especially useful for this purpose since it allows the variation of certain reaction parameters while holding N constant. Thus, h in seeded styrene polymerization drops from 0.5 to 0.2 when the initiator concentration decreases from 10-2 to 1CT5 M. At sufficiently low Ru the rate of radical absorption is not sufficiently high to counterbalance the rate of desorption. One also observes that above a particular initiation rate ([I] = lO-2 M in this case), the system maintains case 2 behavior with h constant at 0.5 and Rp independent of Ri. A change in Ri simply results in an increased rate of alternation of activity and inactivity in each polymer particle. Similar experiments show that h drops below 0.5 for styrene when the particle size becomes sufficiently small. The extent of radical desorption increases with decreasing particle size since the travel distance for radical diffusion from a particle decreases. 

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