Suspension polymerisation process

Another possible way of overcoming the limitations posed by the presence of water in the suspension polymerisation process is to substitute the continuous water phase with alternative solvents that could still act as dispersing medium for the monomer mixture but better preserve noncovalent interactions in the template-monomer assembly. For example, liquid fluorocarbons are chemically inert and do not affect interactions which are used in noncovalent imprinting. Use of such solvents for the preparation of MIP microbeads has been demonstrated already in 1996 by Mayes and Mosbach [16,17]. A range of MIPs were prepared using Boc-l-phenylalanin as the template, MAA as the functional monomer and different kinds and amounts of crosslinkers and porogenic solvents. The resulting MIP microbeads... 

The suspension polymerisation process (after EVC International NV with permission)... 

A number of commercial vinyl chloride copolymers are predominantly made up of VCM units with comonomer units randomly distributed in minor proportions. The suspension polymerisation process is normally used. 

Even in microporous resins, one can expect some heterogeneities due to the different reactivities of the monomers and crosslinking mixtures involved in the suspension polymerisation process [22, 26]. However, it has been demonstrated experimentally that the distribution of reactive sites is homogenous throughout the whole bead [27-30]. Based on geometrical considerations, one should also be aware of the fact that in microbeads with a diameter of 100 pm, 50% of the active sites are within the first 10 pm of the outer shell [27]. 

Duguet, E. et al. (2000) PMMA encapsulation of alumina particles through aqueous suspension polymerisation processes. Macromolecular Symposia, 151,365-370. 

Suspension polymerisation of styrene is widely practised commercially. In this process the monomer is suspended in droplets 5 -Min. in diameter in a fluid, usually water. The heat transfer distances for the dissipation of the exotherm are thus reduced to values in the range s-fisin. Removal of heat from the low-viscosity fluid medium presents little problem. The reaction is initiated by monomer-soluble initiators such as benzoyl peroxide. 

Bead Processes. These processes have generally replaced the above techniques. The styrene is polymerised by bead (suspension) polymerisation techniques. The blowing agent, typically 6% of low boiling petroleum ether fraction such as n-pentane, may be incorporated before polymerisation or used to impregnate the bead under heat and pressure in a post-polymerisation operation. 

For both types of polymerisation mechanisms, different polymerisation processes can be used ranging from simple bulk and solution polymerisation processes to more sophisticated ones such as suspension, emulsion, interfacial, plasma,... polymerisation processes. 

This is superficially similar to suspension polymerisation. But in this process a monomer dispersed in water, in presence of a surface active agent is polymerised to give a stable polymer latex. 

Bulk and suspension polymerisation are the most commonly used techniques. In bulk polymerisation styrene is heated to 80°C for about 2 days to get a viscous solution of polymer in styrene. The solution is then fed to a tower wherein polymerisation is completed at 100°C, 150C° and 180C° stagewise. In suspension process, styrene is suspended in dimineralised water in presence of suspending agent and initiator like benzoyl peroxide and heated to 20°C. The product is washed with acid, water and dried. 

The influence of dicarboxylic acid ester plasticisers on the thermal degradation of PVC significantly depends on the physical state of the PVC-plasticiser system. If PVC retains the structure formed in the stage of suspension polymerisation, the additive produces inhibition of the process of thermal dehydrochlorination. In the case of true diluted PVC solutions in ester plasticisers, the polymer exhibits accelerated degradation, in accordance with a high value of the solvent basicity. 7 refs. 

The polymeric adsorbents are usually prepared by variations of two-phase suspension processes. These refer to systems where microdroplets of monomers and solvent are converted into solid beads upon polymerisation. In the case where the monomers are not water soluble, as in the case of styrene-based polymers and many methacrylate-based polymers, the monomers, a solvent and a droplet stabiliser are suspended as droplets by stirring in water and then polymerised (o/w suspension polymerisation). The particle size and dispersity can be influenced by the stirring speed and the type of stabiliser. So far, only a few examples of the preparation of imprinted polymers in bead format have been described [4-8] and these are thoroughly reviewed in Chapter 12. In non-covalent imprinting, the main limitation to the use of these techniques is that the imprinting method often requires the use of polar partly water soluble monomers or templates in combination with less polar water insoluble components. Use of the o/w suspension method... 

An ideal solution to the problem of finding a universal method for making monodisperse imprinted beads would be a combination of the inertness of fluori-nated liquids with the quality of beads produced by the two-step swelling process. Unfortunately, to date it has not proved possible to make a microemulsion of the organic phase in a liquid fluorocarbon. It might be possible to get some control of particle size, however, by swelling a seed latex with the smallest droplet-size suspension which can be made. Work is in progress to evaluate this possibility. Other methods are available to achieve better particle size control in suspension polymerisation and these should also be evaluated [54]. 

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... 

Polystyrene can be polymerised by foam processes, mass polymerisation, solution polymerisation, suspension polymerisation and emulsion polymerisation. Each contributes to the final characteristics of the plastics and the likely residues. A limit for styrene monomer may be imposed. Styrene can be readily analysed down to a 0.01% level. Polystyrene exhibits low shrinkage, hence is an excellent moulding material. 

The process used to produce more than 80% of commercial PVC polymers is suspension polymerisation. An aqueous suspension of vinyl chloride monomer is agitated vigorously in a pressurised vessel together with colloids (detergents) to hold monomers in suspension, and buffers to control pH. The resulting PVC particles are roughly spherical and range from 50-250 qm in diameter. 

Microporous beads are weakly crosslinked resins obtained by suspension polymerisation of styrene and divinylbenzene in the absence of any porogen agent. This process leads to the formation of a homogeneous network evidenced by a glassy and transparent appearance. The most commonly used supports for solid-phase organic synthesis and catalysis are styrene-divinylbenzene copolymers crosslinked with only 1-2% DVB. Many of their derivatives are commercially available [20]. 

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