Water initiator polymerisation

The reaction is considerably modified if the so-called emulsion polymerisation technique is used. In this process the reaction mixture contains about 5% soap and a water-soluble initiator system. The monomer, water, initiator, soap and other ingredients are stirred in the reaction vessel. The monomer forms into droplets which are emulsified by some of the soap molecules. Excess soap aggregates into micelles, of about 100 molecules, in which the polar ends of the soap molecules are turned outwards towards the water whilst the non-polar hydrocarbon ends are turned inwards (Figure 2.17). 

The use of monomer-polymer doughs has been largely confined to the production of dentures. A plaster of Paris mould is first prepared from a supplied impression of the mouth. Polymer powder containing a suitable polymerisation initiator is then mixed with some monomer to form a dough. A portion of the dough is then placed in the mould, which is closed, clamped and heated in boiling water. After polymerisation, which usually takes less than half an hour, the mould is cooled and opened. This technique could also be usefully employed for other applications where only a few numbers-off are required but does not seem to have been exploited. 

The in situ polymerisation consists of filling a capillary or a column with the prepolymerisation mixture containing the template, the functional monomer, the crosslinker, the initiator and the porogenic solvent (Fig. 11). Then the column is heated or submitted to UV radiation for polymerisation. In the in situ thermally initiated polymerisation process, the tube with the pre-polymerisation mixture is submerged in a controlled-temperature water bath, whereas for in situ photoinitiated polymerisation, a UV-transparent capillary or column is needed. The resulting continuous rod of polymer is washed with an appropriate solvent to remove the template and the excess of monomer. 

Plesch s group dso studied the polymerisation of styrene by the combination TiCl4—The internal order in monomer was found to be close to two and the external order, based on initial polymerisaticai rates, also two. The initial rate of polymerisation depended on the first power of water concentration for (H2O] < [H20]c, but became independent of it for [H2O] > [H20]c, (H20]c being an empirical value. Further monomer additions at the end of the first polymerisation resulted in a new reaction proceeding at the same rate as the first one. The effect of temperature on the initial polymerisation rate was unusual in that a decrease was observed between —30 °C and about —45 °C, and an increase below this range. All the above observa-... 

The above equflibrium would obviously lie strongly to the lefthand side. The presence of small amounts of water would kill the carbenium ions formed in the initiation reaction and no polymerisation would thus occur in a wet stem. As for the limited yields following rapid initial polymerisation, the authors postulated that the polymer might from a complex with the catalyst and so remove it from the above equflibrium. A firm conclusion on the real nature of the initiation process would however require further fundamental work on this system. 

The results in Table III show the effect of various amines of different ionisation potential on the photopolymerisation of 2-hydroxyethylmethacrylate in nitrogen saturated water initiated by the benzophenone with the structure 2. As found earlier for the water soluble thioxanthones ( ), the percentage photoconversion decreases with increasing ionisation potential of the amine. In all of these experiments, oxygen had a strong quenching effect on the photopolymerisation. It would appear, therefore, that in aqueous media the photoinduced polymerisation of the acrylate monomer occurs solely via the lowest excited triplet state of the benzophenone molecule to form an exciplex with the amine co-synergist (Schemel ). 

Traces of oxygen-containing materials such as water initiate cross-linking by providing bridging atoms, but poly-dichlorophosphazene (phosphonitrilic chloride polymer) decomposes slowly in contact with atmospheric moisture, and hydrolysis occurs rapidly and completely in water at 100°C. The tetramer polymerises more slowly than the trimer when heated, but eventually gives the same products. When the rubbery products are heated above about 350°C, depolymerisation begins (12.230). 

Figure 9. NMR study of the polymerisation of monomer (6) in water initiated by the Grubbs initiator...

For this set of three monomers we conclude that for (4) and (6), which are amphiphilic and form micelles in water, the polymerisation probably starts in the organic droplet in which the water-insoluble Grubbs initiator is introduced to the system. Alternatively the organic droplet may dissolve in the micelle carrying the initiator to the... 

The primary distinction between miniemulsion and conventional emulsion polymerisation is the nucleation mechanism. In miniemulsion polymerisation (104,132, 193, 361) radicals from the water phase enter the dispersed monomer droplets directly to initiate polymerisation (i.e., the droplets act as individual reactors). This nucleation mechanism is referred to as droplet nucleation. Because of the small size and large surface area of the miniemulsion droplets, they are competitive for radicals relative to the homogeneous and micellar nucleation mechanisms. The monomer droplets polymerise to become polymer particles (275). Miniemulsion latex particles are typically prepared in the size range of 50 to 500 nm in diameter. 

No. 17, 24th Aug.1999, p.5707-11 HETEROGENEOUS CATALYTIC INITIATION BY CUO COLLOIDAL PARTICLES OF WATER-DISPERSION POLYMERISATION... 

An alternative approach for producing latex with a wide particle size distribution is microsuspension polymerisation. In this process, an initiator such as lauroyl peroxide is used, which is highly soluble in the VCM, but is essentially insoluble in water. Thus, polymerisation takes place within the dispersed VCM droplets. The water insolubility of the initiator also helps to stabilise the VCM droplet, and it may be possible to use lower levels of emulsifier compared with the batch emulsion and continuous emulsion processes. Lower levels of emulsifier can be advantageous, for example for applications coming into contact with food, where water absorption or clarity is important, and also for the environmental impact of the proccess. Such latexes produce polymers which give very low plastisol viscosities, but tend to be dilatant in nature. This can be overcome by modifying the process to have a secondary particle size distribution alongside the primary one. 

The process of emulsion polymerisation begins when the free radicals derived from the, usually water-soluble, polymerisation initiator enter the monomer-saturated micelles where they find a sufficient number of solubilised molecules to start a rapid chain reaction (Elgood and Gilbekian, 1973). Each polymer radical first exhausts the monomer contained in the micelle and then captures additional supplies from 50 or more other micelles before the chain reaction is terminated. Some of the depleted micelles then break up and the released emulsifier molecules are adsorbed at the surface of the newly formed primary polymer particles (Dunn, 1971). The remainder are replenished by diffusion from the emulsified monomer droplets, which act essentially as reservoirs. 

Harkins did not explicitly state how the water soluble initiator would be able to initiate the monomer swollen, and therefore oil-rich , soap micelles. This detailed mechanism was somewhat unclear at the time (maybe stiU is), but it has been assumed that the initial polymerisation takes place within the aqueous phase. How these polymers (oligomers) would be capable of going into the micelles was not discussed. Harkins based his theory both on earlier opinions, as described above, and on experimental evidence. Building on the Harkins theory, the Smith-Ewart theory, which appeared in 1948, was a major leap forward in emulsion polymerisation. This is described further in Section 1.2.2. 

A radical polymerisation can be carried out with a range of polymerisation techniques. Those with only a single phase present in the system are bulk and solution polymerisations, involving the monomer, a solvent if present and the initiator. By definition, the formed polymer in a bulk or solution polymerisation remains soluble (either in the monomer or the solvent). A precipitation polymerisation is one in which the system starts as a bulk or solution polymerisation, but the polymer precipitates from the continuous phase to form polymer particles which are not swollen with monomer. A precipitation polymerisation when the polymer particles swell with monomer is called dispersion polymerisation apart from polymerisation in the continuous phase, the polymer particles have an additional locus of polymerisation, and the particles in these systems are colloidally stabilised. Precipitation polymerisation is often performed in an aqueous medium (e.g. acrylonitrile polymerisation in water). Dispersion polymerisation is usually performed in organic solvents that are poor solvents for the formed polymer (supercritical or liquid carbon dioxide may also be used as a continuous medium for dispersion polymerisation). 

This section provides an overview of the major ingredients in emulsion polymerisation. A laboratory scale recipe for an emulsion polymerisation contains monomer, water, initiator, surfactant, and sometimes a buffer and/or chain transfer agents (CTAs). Commercial emulsion polymerisation recipes are usually much more complicated, with 20 or more ingredients. The complexity of components, and the sensitivity of the system kinetics, mean that small changes in recipe or reaction conditions often result in unacceptable changes in the quality of the product formed. 

Until the early 1950s, the major method of emulsion polymerisation involved water-soluble initiators, such as potassium persulphate, being used to initiate polymerisation in an emulsion system stabilised by a fatty acid soap. Molecular weight was controlled by the use of a mercaptan and polymerisation proceeded at about 50 °C until approximately 72% of the monomer had been converted into polymer. This process yielded the so-called hot rubbers. Today, the bulk of SBR materials are prepared using so-called redox initiators which comprise a reductant such as ferrous sulphate with sodium formaldehyde sulphoxylate in combination with an oxidant such as /7-menthane hydroperoxide. In this case, the polymerisation temperatures are as low as 5 °C and conversion of monomer to polymer is only about 60%. Both the hot and cold rubbers are taken to number average molecular masses (molecular weights) of about 100 000, unless they are being used for oil extension (see later). 

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. 

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