Bead polymerisation

Solid grades suspension of bead polymerisation process... 

The use of AZDN is accompanied by the release of nitrogen into the reaction mixture. This would not normally present a problem, but where the polymer is produced by a suspension or bead polymerisation process, the presence of nitrogen can lead to voids being formed in the polymer bead. These alter the specific gravity of the particle causing it to float. This leads to practical difficulties, and yield loss when filtering and collecting the final polymer. 

Suspension Polymerization. Suspension polymerisation yields polymer in the form of tiny beads, which ate primarily used as mol ding powders and ion-exchange resins. Most suspension polymers prepared as mol ding powders are poly(methyl methacrylate) copolymers containing up to 20% acrylate for reduced btittieness and improved processibiUty are also common. 

In a suspension polymerisation monomer is suspended in water as 0.1—5-mm droplets, stabilised by protective coUoids or suspending agents. Polymerisation is initiated by a monomer-soluble initiator and takes place within the monomer droplets. The water serves as both the dispersion medium and a heat-transfer agent. Particle sise is controlled primarily by the rate of agitation and the concentration and type of suspending aids. The polymer is obtained as small beads about 0.1—5 mm in diameter, which are isolated by filtration or centrifugation. 

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. 

Polymerisation proceeds to 100 per cent conversion and the product formed is obtained as spherical beads or pearls, because of this it is sometimes known as bead or pearl polymerisation. 

A good imprinting effect also relies on the existence of stable assemblies between the template molecule and the functional monomers in the course of the polymerisation process therefore, any preparation method for beaded MIPs must preserve such template-monomer assemblies. 

As it will be apparent in the following, both these issues quite severely limit the possible application of standard polymerisation techniques to MIPs. On the other hand, they provide guidelines for critically evaluating the application of the various beaded polymer preparation methods to MIP synthesis. 

Suspension polymerisation is arguably the simplest and most widely employed preparation method for polymers in beaded form [6], In this method, polymerisation occurs entirely within monomer droplets that are dispersed inside a monomer-immiscible phase the stability of the droplet dispersion is often increased by addition of suitable stabilisers. 

Apart from strong hydrogen bonds, hydrophobic and n—n stacking interactions, which are unaffected or possibly even strengthened by the presence of water have been also very recently employed for the preparation of polymer beads imprinted with pyrene by suspension polymerisation in water [15], although it must be borne in mind that the inherent weakness of these interaction may limit their exploitation for imprinting purposes. 

Liquid fluorocarbon was used as continuous phase by Perez-Moral and Mayes [19] as well. They proposed a new method for rapid synthesis of MIP beads, in that they prepared 36 polymers imprinted for propranolol and morphine with different amounts of EDMA as a cross-linker and different functional monomers (MAA, acrylic acid, hydroxyethyl methacrylate, 4-vinylpyridine) directly in SPE cartridges. The properties of MIP microspheres prepared by this method were very similar in terms of size, morphology and extent of rebinding to microspheres prepared by conventional suspension polymerisation in perfluorocarbons as well as to bulk polymers prepared in the same solvent. The most notable advantages of this method are no waste production (no transfer of beads during washing steps) and possible direct use for a variety of screening, evaluation and optimisation experiments. 

Silicone oil was also recently employed by the group of Peng as an alternative nonaqueous solvent system for suspention polymerisation [22]. Silicone oil has the advantage of being cheap, inert and rather viscous, which enhances the stability of the monomer droplets in suspension. In this case, composite MIP beads were prepared including magnetic nanoparticles. FesCL nanoparticles (average diameter... 

Fig. 1 Scanning electron micrographs of beads prepared by suspension polymerisation in mineral oil (top) or liquid fluorocarbon (bottom) using the microreactor (left) or the conventional approach (right). Reproduced with permission from [23]...

The availability of MIP microparticles through this synthetic method has also stimulated the development of analytical techniques that make use of them as sensing elements. Apart from competitive radioassays [30] and immunoassays [32], which were already performed with ground bulk polymers, the small, regular size of the beads prepared by dispersion/precipitation polymerisation enables their use in CEC [45, 46], scintillation proximity assays [35], fluorescent polarisation assays [47], and chemiluminescence imaging [48]. 

Finally, very recently Shea et al. successfully employed inverse microemulsion polymerisation for the preparation of MIP beads in the tens of nanometers range using hydrophilic peptides as template molecules. In this case it was the template molecule which was prefunctionahsed with a hydrophobic chain to orient it towards the surface of the growing bead during polymerisation. The rebinding efficiency of the resulting nanoparticles was however found to depend markedly on the nature of the employed template and to be lower than that recorded with beads of similar... 

In this connection, different strategies are in principle possible. The first involves use of porous seeds that can be swollen or (in the case of a rigid porous seed) imbibed with the monomer mixture yielding the MIP. Subsequent polymerisation within the seeds yields a composite in beaded form containing the MIP component. The seed can be subsequently removed prior to MIP use or left in the composite. 

Since the multi-step swelling and polymerisation method is time consuming, in 2005 Chen et al. [97] prepared uniformly sized MIP beads for separation of... 

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