Polymerization in dispersion

K. E. J. Barrett and H. R. Thomas, Kinetics and Mechanism of Dispersion Polymerization, in Dispersion Polymerization in Organic Media, K. E. J. Barret, Ed., F. Wiley and Sons, London, 1975. 

We begin by describing the current understanding of the kinetics of polymerization of classical unsaturated monomers and macromonomers in the disperse systems. In particular, we note the importance of diffusion-controlled reactions of such monomers at high conversions, the nucleation mechanism of particle formation, and the kinetics and kinetic models for radical polymerization in disperse systems. 

The radical polymerization in disperse systems may be divided into several types according to the nature of continuous phase and the polymerization loci the dispersion, emulsion, miniemulsion, microemulsion, suspension, etc. 

A review article by Qiu et al. [212] and references herein [217-226] covers NMCRP in miniemulsions up to 2001. Cunningham wrote a related review in 2002, also covering controlled radical polymerization in dispersed phase systems [227]. Here, the main results reported in the Qiu review will be summarized, and new developments in the field since then will be reviewed. 

Barton J, Capek I (1994) Radical Polymerization in Disperse Systems, E. Horwood, Chichester and Veda, Bratislava... 

Vinyl acetate is polymerized in dispersion form using various initiators. Exanples of ionic initiators commonly used for free-radical emulsion polymerizations are ammonium, sodium or potassium persulfate. Topical nonionic hydrophobic initiators include 2,2 -azobis(isobutyronitrile) (AIBN) and benzoyl peroxide. Water-soluble nonionic initiators such as tertiary-butyl hydroperoxide are also employed. The initiator 4,4 -azobis(4-cyanovaleric acid) in its acid state is oil soluble, while neutralization causes it to become water soluble providing for further diversity in initiators. 

J. Barton and 1. Capek, Radical Polymerization in Disperse Systems. Ellis Horwood, Chichester, 1994 (Slovak edition VEDA, Bratislava. 1991)... 

Barton, J. and I. Capek (1994). Radical Polymerization in Disperse Systems. New York, Elhs Horwood. 

In the domain of polymer stabilization (polymerization in dispersed media as well as using physico chemical procedures), it becomes of first importance to anchor the surfactant onto the surface of the particles not only to avoid flocculation, but also to limit water pollution. In the case of emulsion polymerization, a good way to overcome these drawbacks is to use polymerizable surfactants (also called "surfmers") [4-6]. 

D. Charmot, P. Corpart, H. Adam, S. Z. Zard, T. Biadatti, G. Bouhadir, Controlled radical polymerization in dispersed media, Macromol. Symp. 2000, ISO, 23-32. 

M. F. Cunningham, Living/controlled radical polymerization in dispersed systems, Prog. Polym. Sci. 2002, 27, 1039-1067. 

Extension Toward Heterogeneous Polymerization in Dispersed Media... 

This section pays attention to the standard modeling tools to simulate the polymer micro stmcture for polymerizations in dispersed media for which mesoscale phenomena are also relevant. In such polymerizations, a surfactant is present so that a dispersed phase can be... 

Besides polymerization in dispersed media as described in Section 10.4, another form of heterogeneous polymerization involves the use of a solid catalyst. The most commonly known examples are Ziegler-Natta and Phillips catalysts for the production of polyethylene products (Tobita and Yanase, 2007). In this section, the most frequently applied modeling approaches for the calculation of the polymer microstructure in such polymerization processes are highlighted, neglecting macroscale effects for simplicity. For a more detailed description, the reader is referred to Asua (2007) and Tobita and Yanase (2007). 

Zetterlund, P.B., Kagawa, Y., Okubo, M., 2008. ControUed/fiving radical polymerization in dispersed systems. Chem. Rev. 108, 3747-3794. 

Fully exfoliated PS/clay nanocomposites were prepared via free radical polymerization in dispersion [58, 59]. Thermally stable organoclay was obtained by modifying MMT with 3-(trimethoxysilyl)propyl methacrylate (MPTMS) (Table 3.4, Figure 3.14). Analyses by XRD and TEM revealed that nanocomposites with low clay loadings exhibited exfoliated structures, whereas intercalated structures were obtained at higher clay loadings. Another silane-type organoclay was prepared by modification of vermiculite with the... 

N. Greesh, P. C. Hartmann, and R. D. Sanderson, Preparation of polystyrene/clay nanocomposites by free-radical polymerization in dispersion. Macromolecular Materials and Engineering, 294 (2009), 787-94. 

IR is not well suited to monitor polymerizations in dispersed media because water gives a strong absorption, and hence important bands are overlapped or hidden by that of the water. In addition, transmission through fiber-optics is still relatively poor in the infrared region, which makes IR not as suitable as other techniques for remote monitoring. 

Water is a weak scatterer and hence Raman is well suited to monitor polymerization in dispersed media. 

Common features of the different polymerizations in dispersed media considered in this chapter are that the polymerization mostly proceeds through free radicals and the dispersed phase is stabilized by means of surface-active compounds (surfactants, suspension agents). The way in which the monomer dispersion is stabilized in the continuous medium largely defines the differences among these processes. Figure 4.1 summarizes the different options (with the exception of dispersion polymerization, discussed later). 

In polymerizations in disperse phase (emulsion, suspension, and dispersion polymerizations), the viscosity of the dispersed systems depends on the volume fraction of the dispersed polymer, (j>, particle shape and size, particle size distribution, interparticle interaction, and shear rate [54], For monodisperse particles at low (j>, viscosity is proportional to the volume fraction of the disperse phase as given by the Einstein equation ... 

Zetterlund PB, Kagawa Y, Okubo M. Controlled/living radical polymerization in dispersed systems. Chem Rev 2008 108 3747-3794. 

Dispersion polymerization may be considered a heterogeneous process which may include emulsion, suspension, precipitation and dispersion polymerization. In dispersion emd precipitation polymerization, the initiator must be soluble in the continuous phase, whereas in emulsion and suspension polymerization the initiator is chosen to be soluble in the disperse phase of the monomer. The rate of dispersion polymerization is much faster than precipitation or solution polymerization. The enhancement of the rate in precipitation polymerization over solution polymerization has been attributed to the hindered termination of the growing polymer radicals. 

The second major use of polymerizable surfactants is their application as stabilizer in polymerization in dispersed media, chiefly in emulsion polymerization, but also in dispersion polymerization. 

Most MIP today are prepared via standard free radical polymerization in dispersion. The initiator for the radical polymerization has to be selected with regard to its reliability. There are a few azo-bis compounds like AIBN , which are frequently used in thermo-initiated polymerization, but for UV-initiated processes, ABCHC seems to be more reliable than AIBN. For some special approaches, where molecular imprinting takes place in aqueous media, a water-soluble initiator like ABDV has to be chosen. 

Barton J, Capek I (1993) in Kemp TJ, Kennedy JF (eds) Radical polymerization in disperse systems, Horwood location... 

Cunningham, M.F. (2002) Living/controlled radical polymerizations in dispersed phase systems. Progress in Polymer Science, 27,1039-1067. 

It is worth noting that chemical reactions responsible for polymer formation during the polymerizations in dispersed systems are the same as in the case of the polymerizations in the bulk or in solution. However, for the polymerizations in dispersed systems, the processes responsible for particle formation, transport of initiator and growing polymer into particles, and release of the growing chains into continuous medium are just as important. As a result, in dispersion polymerizations, significant differences in concentrations of monomer and propagating chains in the continuous medium and in the particles are very common. Thus, one may expert that the rates of polymerization in dispersed systems and in solution are different even if monomer and initiator concentrations are similar in both systems. 

In the domain of the cationic ring-opening polymerization in dispersion, until now only one system has been investigated. In 1968, Penczek et al published results of the studies of the cationic copolymerization of 1,3-dioxolane and 1,3,5-trioxane initiated with BF3 and carried out in cyclohexane in the presence or the absence of poly(ethylene oxide). Hie initial concentration of 1,3-dioxolane in these studies was 20 times lower than the initial concentration of 1,3,5-trioxane. The former monomer was used with the purpose of protecting poly (1,3,5-trioxane) from depolymerization. It was found that depolymerization stops when 1,3-dioxolane monomeric unit is the terminal one. 

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