Oxygen Polymerisation

Tetrafluoroethylene. Emulsion polymerisation of tetrafluoroethylene, catalysed by oxygen, yields polytetrafluoroethylene (Tejlon) as a very tough horn-hke material of high melting point. It possesses excellent electrical insulation properties and a remarkable inertness towards all chemical reagents, including aqua regia. 

Acrylic acid derivatives. Acrylic esters polymerise readily under the influence of oxygen, peroxides, light or heat to give colourless, glass-Uke plastics. 

Group V. This group includes all the water-insoluble hydrocarbons and oxygen compounds that do not contain N or S and are soluble in cold concentrated sulphuric acid. Any changes—colour, excessive charring, evolution of gases or heat, polymerisation and precipitation of an insoluble compound— attending the dissolution of the substance should be carefully noted. 

Acetaldehyde is a highly reactive compound exhibiting the general reactivity of aldehydes (qv). Acetaldehyde undergoes numerous condensation, addition, and polymerisation reactions under suitable conditions, the oxygen or any of the hydrogens can be replaced. 

PVC. Poly(vinyl chloride) (PVC), a very versatile polymer, is manufactured by the polymerisation of vinyl chloride monomer, a gaseous substance obtained from the reaction of ethylene with oxygen and hydrochloric acid. In its most basic form, the resin is a relatively hard material that requites the addition of other compounds, commonly plasticisers and stabilisers as well as certain other ingredients, to produce the desired physical properties for roofing use. The membranes come in both reinforced and nonreinforced constmctions, but since the 1980s the direction has been toward offering only reinforced membranes. The membrane thickness typically mns from 0.8—1.5 mm and widths typically in the range of 1.5—4.6 m. 

Tetraneopentyltitanium [36945-13-8] Np Ti, forms from the reaction of TiCl and neopentyllithium ia hexane at —80° C ia modest yield only because of extensive reduction of Ti(IV). Tetranorbomyltitanium [36333-76-3] can be prepared similarly. When exposed to oxygen, (NpO)4Ti forms. If it is boiled ia ben2ene, it decomposes to neopentane. When dissolved ia monomers, eg, a-olefins or dienes, styrene, or methyl methacrylate, it initiates a slow polymerisation (211,212). Results from copolymerisation studies iadicate a radical mechanism (212). Ultraviolet light iacreases the rate of dissociation to... 

Bulk Polymerization. The spontaneous polymerisation of VDC, so often observed when the monomer is stored at room temperature, is caused by peroxides formed from the reaction of VDC with oxygen. Very pure monomer does not polymerize under these conditions. Irradiation by either uv or y-rays (26,28) also induces polymerization of VDC. 

Reaction between oxygen and butadiene in the Hquid phase produces polymeric peroxides that can be explosive and shock-sensitive when concentrated. Ir(I) and Rh(I) complexes have been shown to cataly2e this polymerisation at 55°C (92). These peroxides, which are formed via 1,2- and 1,4-addition, can be hydrogenated to produce the corresponding 1,2- or 1,4-butanediol [110-63-4] (93). Butadiene can also react with singlet oxygen in a Diels-Alder type reaction to produce a cycHc peroxide that can be hydrogenated to 1,4-butanediol. 

When the ratio MO/SiOj is a little less than 2/1, silica dimers form (Fig. 16.4b). One oxygen is shared between two tetrahedra it is called a bridging oxygen. This is the first step in the polymerisation of the monomer to give chains, sheets and networks. 

With decreasing amounts of metal oxide, the degree of polymerisation increases. Chains of linked tetrahedra form, like the long chain polymers with a -C-C- backbone, except that here the backbone is an -Si-O-Si-O-Si- chain (Fig. 16.4c). Two oxygens of each tetrahedron are shared (there are two bridging oxygens). The others form ionic bonds between chains, joined by the MO. These are weaker than the -Si-O-Si- bonds which form the backbone, so these silicates are fibrous asbestos, for instance, has this structure. 

In the above examples the polymerisation takes place by the opening of a carbon-carbon double bond. It is also possible to open carbonyl carbon-oxygen double bonds and nitrile carbon-nitrogen triple bonds. An example of the former is the polymerisation of formaldehyde to give polyformaldehyde (also known as polyoxymethylene and polyacetal) (Figure 2.3). 

Since impurities can affect both the polymerisation reaction and the properties of the finished product (particularly electrical insulation properties and resistance to heat aging) they must be rigorously removed. In particular, carbon monoxide, acetylene, oxygen and moisture must be at a very low level. A number of patents require that the carbon monoxide content be less than 0.02%. 

The close structural similarities between polychloroprene and the natural rubber molecule will be noted. However, whilst the methyl group activates the double bond in the polyisoprene molecule the chlorine atom has the opposite effect in polychloroprene. Thus the polymer is less liable to oxygen and ozone attack. At the same time the a-methylene groups are also deactivated so that accelerated sulphur vulcanisation is not a feasible proposition and alternative curing systems, often involving the pendant vinyl groups arising from 1,2-polymerisation modes, are necessary. 

Bulk polymerisation is heterogeneous since the polymer is insoluble in the monomer. The reaction is autocatalysed by the presence of solid polymer whilst the concentration of initiator has little effect on the molecular weight. This is believed to be due to the overriding effect of monomer transfer reactions on the chain length. As in all vinyl chloride polymerisation oxygen has a profound inhibiting effect. 

Free-radical polymerisation techniques involving peroxides or azodi-isobutyronitrile at temperatures up to about 100°C are employed commercially. The presence of oxygen in the system will affect the rate of reaction and the nature of the products, owing to the formation of methacrylate peroxides in a side reaction. It is therefore common practice to polymerise in the absence of oxygen, either by bulk polymerisation in a full cell or chamber or by blanketing the monomer with an inert gas. 

The polymer may be prepared readily in bulk, emulsion and suspension, the latter technique apparently being preferred on an industrial scale. The monomer must be free from oxygen and metallic impurities. Peroxide such as benzoyl peroxide are used in suspension polymerisations which may be carried out at room temperature or at slightly elevated temperatures. Persulphate initiators and the conventional emulsifying soaps may be used in emulsion polymerisation. The polymerisation rate for vinylidene chloride-vinyl chloride copolymers is markedly less than for either monomer polymerised alone. 

The monomer, styrene, is a derivative of benzene, vinyl benzene (1.2). It is a colourless, mobile liquid that polymerises readily. The first report of the polymerisation reaction came in 1839, when E. Simon described the transformation of what was then called styrof. He believed he had oxidised the material and called the product styrol oxide. Later, when it was realised that it contained no oxygen, the product became known as metastyrene. 

So styrene, which is not stabilised by an inhibitor at ambient temperature, can, especially from 40°C onwards and in the presence of oxygen or air, give rise to polymerisation as well as oxidation, which leads to a polyperoxide. If this polyperoxide is isolated, it detonates spontaneously. However, its solubility in monomer limits the risks. This property probably caused a detonation that... 

With aromatic compounds that have benzylic sites, the peroxidation is easy. An apparatus in which tetrahydronaphthalene was distilled detonated. It was assumed that this accident was linked to the concentration of peroxide formed in contact with oxygen. When using phenols as inhibitors of oxidation (and polymerisation too) for all these compounds, this avoids these riste. 

The acidic properties of methanesulphonic acid that have just been mentioned have been responsible for two other accidents. When this acid is contact with methyl and vinyl oxide, this caused the latter to polymerise violently. The electrolysis of methanesulphonic acid with an aqueous solution of hydrogen fluoride gives rise to a violent detonation that was put down to the formation of oxygen difluoride that is explosive. 

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