Polymerisation dispersion

This method is usually applied for the preparation of nonaqueous latex dispersions, and hence is referred to as NAD [5]. The method has also been adapted to prepare aqueous latex dispersions by using an alcohol-water mixture. 

Typical preformed graft stabilisers based on poly( 12-hydroxy stearic add) (PHS) are simple to prepare and effective in NAD polymerisation. Commerdal 

AB graft with several B chains — Anchor chain A 

Polymeric surfactant Continuous phase Disperse polymer 

Polystyrene-block-poly(dimethyl siloxane Hexane Polystyrene 

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K. E. J. Barrett, ed.. Dispersion Polymerisations in OrganicMedia,]ohxi Wiley Sons, Inc., New York, 1975. 

The success of filler polymers used in increasing quantities in PVC technology paste can be considered as an extension of this principle. These filler polymers are made by suspension (granular, dispersion) polymerisation and by... 

Polystyrene latices can be prepared by the dispersion polymerisation of styrene in alcohol/water mixtures containing polyelectrolytes. The experimental data obtained lend support to the hypothesis that the polymerisation mechanism which operates is analogous to that which occurs in the non aqueous dispersion polymerisation of methyl methacrylate in solutions of degraded... 

Polyacrylic acid stabilised latices have been prepared by aqueous dispersion polymerisation. The method used is analogous to the non-aqueous dispersion (NAD) polymerisation methods originally used to prepare polymethyl methacrylate particles in aliphatic hydrocarbons (1. In effect the components of a NAD polymerisation have been replaced as follows aliphatic hydrocarbon by aqueous alcohol, and degraded rubber, the stabiliser, by polyacrylic acid (PAA). The effect of various parameters on the particle size and surface charge density of the latices is described together with details of their colloidal stability in the presence of added electrolyte. 

Preparation by Sequential Polymerisation. Two-polymer composite latex particles may be prepared using either emulsion or dispersion polymerisation techniques. A dispersion (latex) of particles of a first polymer may be prepared in the usual manner after complete conversion of monomer to polymer, a different monomer or monomer mixture is added and polymerised to provide the second polymer. 

Sequential polymerisation may be operated by producing particles of the first polymer by either emulsion or dispersion polymerisation and by adding the monomer for the second polymer together with free-radical initiator at a slow and controlled rate such that the rate of addition and the rate of polymerisation are equal. The amount of free monomer remains at a low level throughout the process. This monomer-starved process (5) has been used with an aqueous continuous phase in many studies. Samples may be taken at regular intervals to... 

Barrett, K.E.J. Thomas, H.R. In Dispersion Polymerisation in Organic Media J. Wiley and Sons New York, 1975. 

I ] K. Barrett, Ed., Dispersion Polymerisation in Organic Media. Wiley, New York, 1975. 

Unfortunately, to date, this technique has received little attention from the molecular imprinting community and only one report of a dispersion polymerisation method had appeared until very recently [26]. This is probably better classified as a precipitation polymerisation, since random aggregates were produced rather than beads. No colloidal stabilisers were included in this procedure. The aggregates were made in situ in chromatography columns, which avoided the need to grind and sieve the polymer and pack the columns. Due to the rather polar nature of the solvent mixtures used (cyclohexanol, dodecanol, isopropanol), good imprints were only achieved for compounds which interact strongly with functional monomer... 

Fig. 12.6. Electron micrograph of 17/S-estradiol-imprinted TRIM/MAA latex made by dispersion polymerisation in acetonitrile (from [27])...

Overall, the dispersion polymerisation approach might offer an effective solution to bead production in many situations if the precise balance of solvent composition, imprinting recipe and synthetic conditions can be appropriately matched to produce particles with the desired size, morphology, porosity and binding characteristics. 

Aqueous two-step swelling is a technique for producing monodisperse polymer particles over a size range much greater than that achievable by direct dispersion polymerisation. The properties of the particles, such as porosity, can also be controlled. The concept was first introduced by Ugelstadt and Mork [28] and has since been adopted by many workers. It is used to produce a number of commercial products for chromatography and bioseparations, such as Mono-beads by Pharmacia and Dynabeads by Dynal. 

Another in situ preparation of molecularly imprinted columns employs dispersion polymerisation, whereby agglomerated polymer particles are obtained [16]. The procedure is similar to the rod preparation a mixture of the chemicals for the polymer preparation, such as a template, a functional monomer, a cross-linker, a porogen and an initiator is put in a column and heated to effect polymerisation. This method also requires polar solvents, such as cyclohexanol-dodecanol and isopropanol-water, to obtain aggregated polymer particles of well-defined micro-sises. A crucial difference with the rod preparation lies in the volume of the porogen used larger volumes of porogens are used in dispersion polymerisation. 

The in situ molecular imprinting protocol employing dispersion polymerisation has some advantageous features. The dispersion polymer can be removed from a column and re-packed when a column is damaged after repeated use. Back-pressure of agglomerated polymer particles is less problematic therefore, this in situ method can be applied to a wider range of analytical techniques. Here, two applications of in situ dispersion polymer, solid phase extraction (SPE) and CE are described. 

The first reported preparation of capillary columns containing MIPs utilised a thermally initiated dispersion polymerisation procedure [58], The functional monomer MAA and the cross-linking monomer EDM A were used. Agglomerates of micrometre-sized globular polymer particles were claimed to be prepared in situ in the capillary. Molecular imprinting of L-phenylalanine anilide, pentamidine and benzamidine was undertaken. A pH-dependent retardation of pentamidine over benzamidine in the pentamidine capillary was observed, while the opposite. 

Dispersion polymerisation this is usually applied to the preparation of nonaqueous polymer dispersions, and is commonly referred to as nonaqueous dispersion (NAD) polymerisation. In this case, the monomer, initiator, stabiliser (referred as the protective agent) and the solvent initially form a homogeneous solution, and the polymer particles precipitate when the solubility limit of the polymer is exceeded. The particles continue to grow until the monomer is consumed. 

Descriptions of both emulsion and dispersion polymerisation are given below, with particular reference to the control of their particle size and colloid stabiHty, which is greatly influenced by the emulsifier or dispersant used. Particular emphasis will be placed on the effects of polymeric surfactants that have been recently appHed to the preparation of emulsion polymers. 

Most aqueous emulsion and dispersion polymerisation that have been reported are based on a few commercial products with a broad molecular weight distribution and varying block composition. The results obtained from these studies could not establish the effect that the structural features of the block copolymer would have on their stabilising ability and effectiveness in polymerisation. Fortunately, model block copolymers with well-defined structures can be synthesised, and their roles in emulsion polymerisation have been determined using model polymers and model latexes. 

Table 17.4 Block and graft copolymers for dispersion polymerisation.

Several other examples of block and graft copolymers that are used in dispersion polymerisation are listed in Table 17.4, which also provides details of the continuous phase and disperse polymers that can be used with these polymers. 

Dispersion polymerisation may be considered a heterogeneous process which may include emulsion, suspension, precipitation and dispersion polymerisation. In dispersion and precipitation polymerisation, the initiator must be soluble in the continuous phase, whereas in emulsion and suspension polymerisation the initiator is chosen to be soluble in the disperse phase of the monomer. A comparison of the rates of polymerisation of MMA at 80 C for the three systems was given by Barrett and Thomas [11], as illustrated in Figure 17.10. The rate of dispersion polymerisation is much faster than either precipitation or solution polymerisation. TTie enhancement of the rate in precipitation polymerisation over... 

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