Molecular dispersion polymerisation

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... 

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. 

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. 

Micro-emulsion is another variant of emulsion polymerisation. Such emulsions are thermodynamically stable systems including swollen monomer micelles dispersed in a continuous phase. In general, they require fairly large concentrations of surfactants to be produced compared with the other dispersed polymerisation systems. Hence, the interfacial tension of the oil/water is generally close to zero. Polymers with ultra-high molecular weight, i.e. above 10 g/mol, can be obtained, as can copolymers with a very well-defined, homogenous composition. Whereas polymerisation can take 24-48 h in the normal emulsion process, it proceeds at a fast rate in micro-emulsion, as total conversion can be obtained in less than 30 min. Polymer particles of very small size (diameter < 100 nm) and narrow distribution can be obtained by this process. 

The dispersion polymerisation of styrene was studied in a mixed ethanol-water medium using 0.1-2 wt% of omega-methoxy poly(ethylene oxide)40 undecyl-alpha-methacrylate macromonomer as a steric stabiliser. The polymerisation rate reached a maximum at a styrene conversion of around 18% and remained almost constant up to about 55% conversion. The molecular weight of the polymer increased with increasing conversion of styrene up to about 55%. The polymerisation rate and activation energies were studied and discussed. Spherical monodisperse particles of around 250 nm diameter were obtained for the final stable latices.The grafted poly(ethylene oxide) macromonomers were enriched and anchored on the surface of PS latex particles with a top surface composition of 28% macromonomer. 33 refs. SINGAPORE Accession no.665916... 

A mass of polymer will contain a large number of individual molecules which will vary in their molecular size. This will occur in the case, for example, of free-radically polymerised polymers because of the somewhat random occurrence of ehain termination reactions and in the case of condensation polymers because of the random nature of the chain growth. There will thus be a distribution of molecular weights the system is said to be poly disperse. 

Emulsion polymerisation is a special case of heterogeneous addition polymerisation in which the reaction kinetics are modified because the A are compartmentalised in small polymer particles [48, 49]. These particles are usually dispersed in water and reaction (78) occurs in the aqueous phase. Initiating radicals diffuse to the particles which are stabilised by surfactant material. Chain termination becomes retarded physically and a relatively high polymerisation rate is obtained. If chain transfer is not prominent, a high molecular weight polymer is produced. The polymerisation rate is given by the expression... 

The normal dispersants used for both kaolin and calcium carbonate pigments are aqueous solutions of sodium polyacrylate. These are prepared by free radical polymerisation using various combinations of initiators and terminators which may be proprietary to the manufacturer. Number average molecular weights are... 

The Cu(II)/Cu(I) redox system added as bromide has recently been used to prepare well-defined polymers (controlled molecular weight, reduced poly-dispersity, terminal functionalities). One of the most successful methods to make well-defined polymers is atom transfer radical polymerisation (ATRP)18 ... 

Using Alfin catalysts, butadiene polymers were already obtained in the 1940s. The Alfin catalytic system consists of three components (formed in statu nascendi from alkyl chloride, metallic sodium, alcohol and olefin) sodium salt of secondary alcohol (e.g. sodium isopropoxide), alkenylsodium (e.g. allyl-sodium) and finely dispersed sodium chloride (the name Alfin originates from a/cohol + olefin) [2,3], Since the molecular weight of polybutadiene obtained with Alfin catalysts is very high (it can reach a value of a few millions), 1,4-dihydronaphthalene is often added to the polymerisation system for the regulation of molecular weight [1],... 

Another possibility is represented by running suspension polymerisation in media other than water. For example, a suspension polymerisation may also involve a perfluorinated liquid alkane as the dispersing phase [33]. In other examples, a two-step swelling and polymerisation method was applied to prepare molecularly imprinted beads [34]. 

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