Suspension polymerisation produced

Suspension polymerisation produces latex particles in the size range from 1 to 1000 pm. This process comprises monomer + initiator + solvent (usually water) + surfactant. The monomer and the initiator are both insoluble in the solvent (water), e.g. styrene and benzoyl peroxide hence the monomer is dispersed as droplets (as in emulsion polymerisation), but the initiator is present in these droplets (and not in the aqueous phase). The role of the surfactant is purely to stabilise these droplets. 

The final polymerised product is formed in particles much smaller (50-500 nm) than produced with suspension polymerisation. Emulsion polymerisation can lead to rapid production of high molecular weight polymers but the unavoidable occlusion of large quantities of soap adversely affects the electrical insulation properties and the clarity of the polymer. 

As a result the suspension polymerisation of methyl methacrylate was developed to produce commercial material such as Diakon made by ICI. Such a polymerisation can be carried out rapidly, usually in less than an hour, because there is no serious exotherm problem. 

Hydroxy propyl cellulose, like methyl cellulose, is soluble in cold water but not in hot, precipitating above 38°C. It was introduced by Hercules in 1968 (Klucel) for such uses as adhesive thickeners, binders, cosmetics and as protective colloids for suspension polymerisation. The Dow company market the related hydroxypropylmethyl cellulose (Methocel) and also produce in small quantities a hydroxyethylmethyl cellulose. 

Wulff and collaborators, for instance, reported the preparation of TSA imprinted beads for the hydrolysis of carbonate and carbamate [61, 62], exploiting the amidine (33) functional monomer previously developed by the same group and successfully applied to the bulk format [63]. The polymers were prepared using a suspension polymerisation that produced beads with sizes in the range 8-375 pm, depending on the polymerisation conditions. The pseudo-first order reaction rate of the imprinted beads (Tyrrp/ soin) was enhanced by a factor of 293 for the carbonate hydrolysis and 160 for the carbamate, when compared with the background. 

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 effects of some polymerisation conditions on PVC particles produced by suspension polymerisation were studied. The different stages of vinyl chloride suspension polymerisation were investigated by using an on-line sample withdrawal technique during reaction. The effect of change of stabiliser on particle size, porosity, and morphology is discussed. 10 refs. 

Fig. 12.3. Electron micrographs of imprinted beads produced by suspension polymerisation of typical EDMA/MAA imprinting mixtures in a liquid fluorocarbon. The porogenic solvents were (a) toluene, (b) chloroform and (c) acetonitrile.

Using a similar philosophy to the perfluorocarbon liquid approach, Piletsky carried out suspension polymerisations in silicone oil. This liquid is also immiscible with some organic liquids, although the range of immiscible combinations is much smaller than for perfluorocarbons. Beads were successfully produced containing imprints of ATP and poly-A in an EDMA/A,A-diethyl-2-aminoethylmethacrylate system with DMF as solvent and an excess of silicone oil as dispersant [21]. 

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]. 

In practice the type of support used most widely is a lightly crosslinked poly(styrene-divinylbenzene) system usually in the form of spherical beads —50-500 pm in diameter produced by suspension polymerisation [51]. Typically the level of the crosslinking comonomer divinylbenzene used is only —0.5-2.0 vol%. This is crucial in terms of structural analysis by NMR, since such lightly crosslinked systems can swell considerably in suitable solvents (up to —15 fold), such that the local environment around a functional group attached to the polymer network can approach closely to that in isotropic solution (see later). 

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. 

The best-known elastomer is natural rubber, poly-isoprene (Scheme 6.10). Isoprene (Scheme 6.10a) is a liquid at room temperature, which polymerises readily to give the elastomer polyisoprene (Scheme 6.10b). The polymerisation produces two main geometrical isomers (see Section S2.1.) Natural rubber is the aU-c form of polyisoprene (Scheme 6.10c), in which the methyl (—CH3) groups and hydrogen (H) atoms are on the same side of the carbon-carbon double bond. Rubber latex, a milky liquid, is a suspension of rubber in water. It is found in many plants (e.g. in dandelions) as well as mbber... 

The suspension polymerisation approach is often used in the production of polyvinyl chloride dispersions for use in plastic pipes. Polystyrene and styrenic copolymers are also produced by suspension polymerisation. The polystyrene beads are often prepared by suspension polymerisation for use as packing material in gel permeation chromatography (GPC) columns. Ion exchange resin beads are also commonly produced by suspension polymerisation. Because suspension beads are relatively large, it is easier to separate the polymer by coagulation than in the case of emulsion polymerisation. 

Condensation methods, i.e. nudeation and growth. Here, one starts with molecular units that are condensed to form nuclei that grow further to produce the particles. An example of such a process is the formation of particles by a predpita-tion technique, e.g. production of colloidal Agl particles by reaction of AgNOa with KI. Many suspensions are produced by addition of a solution of the chemical in a suitable solvent, which is then added to another miscible solvent in which the drug is insoluble. A third example of condensation methods is the production of polymer particles from their monomers by a suitable polymerisation technique. 

Direct electron microscopic evidence for basic particles in a polymerising system is scarce, but they have been identified in equilibrium with primary particle nuclei (41). However Behrens (42) reported that the smallest particles resolvable by electron microscopy in a grain produced by suspension polymerisation were lOnm diameter. This view has been supported by Barclay (43) and more recently by Soni et al (44) who concluded that each basic particle was composed of a crystalline core surrounded by less ordered material in a likely fringed micelle structure. 

The former type is produced by solution polymerisation, the second by using solution, bulk and suspension polymerisation techniques. 

These polymers, which are produced using suspension polymerisation techniques, are supplied in granular form and then are simply dissolved in aliphatic hydrocarbon solvent using high speed mixers. 

The synthesis of the high molecular weight polymer from chlorotrifluoroethylene [79-38-9] has been carried out in bulk (2 >—21 solution (28—30), suspension (31—36), and emulsion (37—41) polymerisation systems using free-radical initiators, uv, and gamma radiation. Emulsion and suspension polymers are more thermally stable than bulk-produced polymers. Polymerisations can be carried out in glass or stainless steel agitated reactors under conditions (pressure 0.34—1.03 MPa (50—150 psi) and temperature 21—53°C) that require no unique equipment. 

Acrylate esters can be polymerised in a variety of ways. Among these is ionic polymerisation, which although possible (6—9), has not found industrial apphcation, and practically all commercial acryUc elastomers are produced by free-radical polymerisation. Of the four methods available, ie, bulk, solution, suspension, and emulsion polymerisation, only aqueous suspension and emulsion polymerisation are used to produce the ACMs present in the market. Bulk polymerisation of acrylate monomers is hasardous because it does not allow efficient heat exchange, requited by the extremely exothermic reaction. 

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