SPONTANEOUS POLYMERISATION

The polymers of the 2-cyanoacryhc esters, more commonly known as the alkyl 2-cyaiioacrylates, are hard glassy resins that exhibit excellent adhesion to a wide variety of materials. The polymers are spontaneously formed when their Hquid precursors or monomers are placed between two closely fitting surfaces. The spontaneous polymerisation of these very reactive Hquids and the excellent adhesion properties of the cured resins combine to make these compounds a unique class of single-component, ambient-temperature-curing adhesives of great versatiUty. The materials that can be bonded mn the gamut from metals, plastics, most elastomers, fabrics, and woods to many ceramics. 

Figure 4c illustrates interfacial polymerisation encapsulation processes in which the reactant(s) that polymerise to form the capsule shell is transported exclusively from the continuous phase of the system to the dispersed phase—continuous phase interface where polymerisation occurs and a capsule shell is produced. This type of encapsulation process has been carried out at Hquid—Hquid and soHd—Hquid interfaces. An example of the Hquid—Hquid case is the spontaneous polymerisation reaction of cyanoacrylate monomers at the water—solvent interface formed by dispersing water in a continuous solvent phase (14). The poly(alkyl cyanoacrylate) produced by this spontaneous reaction encapsulates the dispersed water droplets. An example of the soHd—Hquid process is where a core material is dispersed in aqueous media that contains a water-immiscible surfactant along with a controUed amount of surfactant. A water-immiscible monomer that polymerises by free-radical polymerisation is added to the system and free-radical polymerisation localised at the core material—aqueous phase interface is initiated thereby generating a capsule sheU (15). 

Figure 4c also describes the spontaneous polymerisation ofpara- s.yX en.e diradicals on the surface of soHd particles dispersed in a gas phase that contains this reactive monomer (16) (see XylylenePOLYMERS). The poly -xylylene) polymer produced forms a continuous capsule sheU that is highly impermeable to transport of many penetrants including water. This is an expensive encapsulation process, but it has produced capsules with impressive barrier properties. This process is a Type B encapsulation process, but is included here for the sake of completeness. 

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. 

E Materials subject to spontaneous polymerisation (eg ethylene oxide) 50 to 75... 

A store of 250 m of acrolein detonated. This was probably due to the spontaneous polymerisation of the aldehyde. 

A terpene inhibitor is usually added to the monomer to prevent spontaneous polymerisation, and in its absence, the monomer will spontaneously explode at pressures above 2.7 bar. The inhibited monomer will explode if ignited [1]. Explosion under thermal initiation is now held to be a disproportionation, that to tetrafluo-romethane and carbon gives 3.2 kJ/g, the same energy as black powder [3], Liquid tetrafluoroethylene, being collected in a liquid nitrogen-cooled trap open to air, formed a peroxidic polymer which exploded [2]. 

Vinyl acetate is normally inhibited with hydroquinone to prevent polymerisation. A combination of too low a level of inhibitor and warm, moist storage conditions may lead to spontaneous polymerisation. This process involves autoxidation of acetaldehyde (a normal impurity produced by hy droly sis of the monomer) to a peroxide which initiates exothermic polymerisation as it decomposes. In bulk, this may accelerate to a dangerous extent. Other peroxides or radical sources will initiate the exothermic polymerisation. 

Nitrite salts inhibit the sometimes explosive spontaneous polymerisation of N-vinylpyridine derivatives . 

The key problem for physicists studying the interstellar medium is to establish the maximum length of linear carbon chains and then to find out from which length these chains tend to close up and assemble into plane rings and three-dimensional fullerenes by spontaneous polymerisation. 

Pseudomonas chloraphis cells were used first, and more recently Rhodococcus rhodochrus Jl. Cells are immobilised in polyacrylamide particles and used in column reactors operated at below 10°C. The acrylamide is produced in 100% yield, and is so pure that polymerisation inhibitors have to be added to prevent spontaneous polymerisation. Both acrylonitrile and acrylamide inhibit the nitrile hydratase the nitrile hydratase is extremely stable. Therefore acrylonitrile is fed to maintain a level of 6% resulting in the accumulation of acrylamide of 66% (w/v), after which is it simply decolourised and concentrated (Yamada and Kobayashi, 1996). 

Processes subject to spontaneous polymerisation or heating Processes which are exothermic... 

On melting the crystals, a rapid decarboxylation takes place with the formation of a low molecular weight polypeptide. These spontaneous polymerisations were first noticed by Leuchs (7, 2), and were subsequently thoroughly studied by Katchalski and his co-workers (6) who carried out these reactions in vacuum at elevated temperatures. Under these conditions the conversion of the anhydride into polypeptide appears to be quantitative. 

The kinetics of the relatively fast polymerisation of the non-N-substituted NCA s initiated by LiCl in dimethyl formamide was thoroughly studied by Ballard, Bamford and Weymouth (44). Addition of this salt enormously speeded, by a factor of at least 1000, the extremely slow spontaneous polymerisation of glycine NCA. (In the absence of salt no reaction was detectable even after a few hours.) Other NCA s behaved similarly, although in the absence of the salt their spontaneous polymerisations were slightly faster than that of glycine NCA. The initial... 

TV-terminal sections of these three chains, which are close in evolutionary terms. The a and 3 chains are terminated by a short peptide sequence called a fibrinopeptide that ordinarily prevents the spontaneous polymerisation of fibrinogen. 

Some useful gel mixtures The monomers are most conveniently made up as concentrated 15-50% stock solutions, containing both acrylamide and bis in the required ratio, in water. They are stable for long periods, often months, though, ultimately, evidently due to factors that have not been investigated, partial spontaneous polymerisation occurs. 

Methylfuran [534-22-5] M 82.1, b 62.7-62.8 /731mm, d 0.917, n 1.436. Washed with acidified satd ferrous sulfate soln (to remove peroxides), separated, dried with CaSOa or CaCl2, and fractionally distd from KOH immediately before use. To reduce the possibility of spontaneous polymerisation, addition of about one-third of its volume of heavy mineral oil to 2-methylfuran prior to distn has been recommended. 

I. Fenoglio, M. Tomatis and B. Fubini (2001) Chemical Communications, p. 2182 - Spontaneous polymerisation on amphibole asbestos relevance to asbestos removal . 

D Certain substances which are susceptible to spontaneous polymerisation or decomposition are generally placed on the market in a stabilised form. It is in this form that they are listed in Part 3. However, such substances are sometimes placed on the market in a non-stabilised form. In this case, the supplier must state on the label the name of the substance followed by the words non-stabilised . 

Spontaneous polymerisation of some monomers such as styrene can occur in the presence of sunlight and heat, conditions in which free radicals may be generated. Such an uncontrolled initiation is a dangerous hazard. As polymerisation is an exothermic process, temperature increases in the bulk of the monomer and the process can lead to a blast. 

Both head- and tail-type cationic vinyl monomers with long methylene chains in THF or chloroform had the maximum adsorption on colloidal silica on the respective adsorption isotherm. Above the monomer concentration giving maximum adsorption, the monomer formed micelles or clusters in bulk solution with removal of adsorbed water molecules from the silica surface. At the monomer concentration giving maximum adsorption, the adsorbed monomer was spontaneously polymerised on the silica surface in THF or chloroform without an initiator at 40C or 60C. The polymerisation mechanism was discussed. Adsorption of an anion-type monomer with a carboxyl group on delta-alumina which showed a positive zeta potential in neutral aqueous solution was higher than that on colloidal silica, but spontaneous polymerisation did not occur on alumina. 19 refs. lAPAN... 

Kricheldorf et al. (1996) reported spontaneous polymerisation of TMC which was performed in bulk at temperatures of about 100°C. A zwitterion intermediate with tri-oxocarbenium ion and an alkoxide ion are formed in this reaction, which can initiate chain growth according to the anionic mechanism. 

Styrene is able to undergo spontaneous polymerisation by heat. Styrene can generate enough free radicals when ample heat is applied. These radicals then participate in the propagation steps with an excess amount of styrene monomers to form high molecular weight polymers at high conversion rates. 

Acrylic monomers are known to readily polymerise and will do so in an imcontrolled maimer if an unintentional introduction of trace quantities of species which cause initiation is allowed to happen. In order to prevent such events, and the related risks and hazards, it is usual for aqueous monomer solutions to be stabilised against spontaneous polymerisation through the use of an inhibitor . Compoimds such as paramethoxyphenol (PMP) are widely used in this regard, since they work most effectively in the presence of a small amount of dissolved oxygen which is usually present in water. 

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