Emulsion polymerization initiators

Redox initiation is commonly employed in aqueous emulsion polymerization. Initiator efficiencies obtained with redox initiation systems in aqueous media are generally low. One of the reasons for this is the susceptibility of the initially formed radicals to undergo further redox chemistry. For example, potential propagating radicals may be oxidized to carbonium ions (Scheme 3.44). The problem is aggravated by the low solubility of the monomers (e.g. M VIA. S) in the aqueous phase. 

Ugelstadt, J., El-Aasser, M.S. and Vanderhoff, J.W. (1973) Emulsion polymerization initiation of polymerization in monomer droplets. Journal of Polymer Science Polymer Chemistry Edition, 11, 503-513. 

Sadurm, N., Solans, C., Azemar, N. and Garci a-Celma, M.J. (2005) Studies on the formation of O/W nano-emulsions, by low-energy emulsification methods, suitable for pharmaceutical aplications. Emulsion polymerization initiation of polymerization in monomer droplets., 26, 438-445. 

On the other hand, a number of qualified investigations on the emulsion polymerization initiated by chemical catalysts and on the experimental verification of the quantitative theory of Smith and Ewart were performed by van der Hoff (14) and Gerrens et ah (4,6). The high experimental standard of these papers in determining the kinetic data is rarely met by papers on the gamma-induced emulsion polymerization. 

Several researchers have carried out experimental and/or theoretical investigations on emulsion polymerizations initiated with oil-soluble initiators and reported that the kinetics of the emulsion polymerizations is basically similar to that initiated with water-soluble initiators [193-202]. Breitenbach et al. [193] carried out the emulsion polymerization of St initiated by BPO at 50 and 60 °C. The authors interpreted the experimental results by assuming a relatively rapid exchange of low molecular weight radicals between the micelle-polymer... 

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 ... 

From a fundamental point of view it is interesting to speculate on tbe differences that could exist between the kinetics of emulsion polymerization initiated by y radiation and those of a conventional chemical initiator with, for example, potassium persulfate. At dose rates jiving a free radical flux comparable to those achieved with chemical initiation any differences... 

VANDERHOFF T AL inverse Emulsion Polymerization Initiated Polymerizations... 

Table 1. Kinetic parameters in the mini-emulsion polymerization initiated by KPS in the presence of SDS/classical coemulsifierM...

Emulsion polymerizations initiated by y-radiation have been carried out [141, 185],... 

Monomer conversion can be adjusted by manipulating the feed rate of initiator or catalyst. If on-line M WD is available, initiator flow rate or reactor temperature can be used to adjust MW [38]. In emulsion polymerization, initiator feed rate can be used to control monomer conversion, while bypassing part of the water and monomer around the first reactor in a train can be used to control PSD [39,40]. Direct control of surfactant feed rate, based on surface tension measurements also can be used. Polymer quality and end-use property control are hampered, as in batch polymerization, by infrequent, off-line measurements. In addition, on-line measurements may be severely delayed due to the constraints of the process flowsheet. For example, even if on-line viscometry (via melt index) is available every 1 to 5 minutes, the viscometer may be situated at the outlet of an extruder downstream of the polymerization reactor. The transportation delay between the reactor where the MW develops, and the viscometer where the MW is measured (or inferred) may be several hours. Thus, even with frequent sampling, the data is old. There are two approaches possible in this case. One is to do open-loop, steady-state control. In this approach, the measurement is compared to the desired output when the system is believed to be at steady state. A manual correction to the process is then made, based on the error. The corrected inputs are maintained until the process reaches a new steady state, at which time the process is repeated. This approach is especially valid if the dominant dynamics of the process are substantially faster than the sampling interval. Another approach is to connect the output to the appropriate process input(s) in a closed-loop scheme. In this case, the loop must be substantially detuned to compensate for the large measurement delay. The addition of a dead time compensator can... 

Initiator i- ni-she- at [LL initiatus, pp of initiare, fr. L, to induct, fr. initium] (ca. 1573) vt. An agent that causes a chemical reaction to commence and that enters into the reaction to become part of the resultant compound. Initiators differ from catalysts in that catalysts do not combine chemically with the reactants. Initiators are used in many polymerization reactions, especially in emulsion polymerizations. Initiators most commonly used in polymerizing monomers and resins having ethenic unsaturation (-C=C-) are the organic peroxides. Odian GC (2004) Principles of polymerization. John Wiley and Sons Inc., New York. 

The reactions in the aqueous phase lead initially to a change in the conductivity and subsequently to the formation of latex particles accompanied by the drop in the transmission (cf. Figure 10). Moreover, the shape of the conductivity curve is qualitatively the same as observed for surfactant-free emulsion polymerizations initiated with potassium peroxodisulfate. The bend of the conductivity curves marks the onset of particle nucleation as conducting species are captured in the diffuse electrical double layer of the particles. These results clearly prove that side reactions of carbon radicals in water lead to conducting species. The zeta-potential of the particles is pH-dependent and negative at pH >4. First hints that such radicals can attack water molecules have been obtained by NMR investigations of polymers made by normal emulsion polymerization (i.e. in the presence of surfactants) initiated with azo-initiators.P Ongoing studies try to clarify the reaction mechanisms. 

Emulsion polymerization initiated by sulfur dioxide and sodium ferripyrophosphate [244]. 

The initiator was also found to make an unusual contribution to the polymerization reaction rate. In dispersion (emulsion) polymerization, initiator concenfra-tion is controlled to control the molecular weight of the polymer. Sometimes the reaction is conducted at a lower... 

Figure 16 Two-step SGI-mediated emulsion polymerization initiated with monofunctional (BlocBuilder salt, 72) or difunctional (DIAMA-Na, 73) water-soluble SGI -based alkoxyamines. Synthesis of the living seed latex (a) followed by its chain extension after one shof or continuous addition of...

Figure 2 Schematic representation of (a) miceiiar nucieation and (b) homogeneous-coaguiative nucieation, for an emulsion polymerization initiated by suifate radicais (i.e., persuifate initiator).

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