Polymerisation Stabilisers

Escudero Sanz FJ, Lahitte JF, Remigy JC. Membrane synthesis by microemulsion polymerisation stabilised by commercial non-ionic surfactants. Desalination 2006 199 127-129. 

About 0-1 per cent, of hydroquinone should be added as a stabiliser since n-hexaldehyde exhibits a great tendency to polymerise. To obtain perfectly pure n-/iexaldehyde, treat the 21 g. of the product with a solution of 42 g. of sodium bisulphite in 125 ml. of water and shake much bisulphite derivative will separate. Steam distil the suspension of the bisulphite compound until about 50 ml. of distillate have been collected this will remove any non-aldehydic impurities together with a little aldehyde. Cool the residual aldehyde bisulphite solution to 40-50 , and add slowly a solution of 32 g. of sodium bicarbonate in 80 ml. of water, and remove the free aldehyde by steam distillation. Separate the upper layer of n-hexaldehyde, wash it with a little water, dry with anhydrous magnesium sulphate and distil the pure aldehyde passes over at 128-128-5°. 

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. 

The ionic nature of the radicals generated, by whatever technique, can contribute to the stabilisation of latex particles. Soapless emulsion polymerisations can be carried out usiag potassium persulfate as initiator (62). It is often important to control pH with buffets dutiag soapless emulsion p olymerisation. 

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. 

For both suspension and mass polymerisations at less than 2% conversion, PVC precipitates from its monomer as stable primary particles, slightly below 1-p.m dia (4,10—12). These primary particles are stabilised by a negative chloride charge (4,13). Above 2% conversion, these primary particles agglomerate. Sectioning the PVC grains of either suspension or mass resins readily shows the skins primary particles at 1-p.m dia, and agglomerates of primary particles at 3—10-pm dia (4,7,8,14). 

As reaction proceeds the micelles become swollen with monomer and polymer and they eject polymer particles. These particles which are stabilised with soap molecules taken from the micelles become the loci of further polymerisation, absorbing and being swollen by monomer molecules. 

Formaldehyde is a gas with a boiling point of -21 °C. It is usually supplied as a stabilised aqueous solution ( 40% formaldehyde) known as formalin. When formalin is used as the source of the aldehyde, impurities present generally include water, methanol, formic acid, methylal, methyl formate and carbon dioxide. The first three of these impurities interfere with polymerisation reactions and need to be removed as much as possible. In commercial polymerisation the low polymers trioxane and paraformaldehyde are convenient sources of formaldehyde since they can be obtained in a greater state of purity. 

Care has to be taken in the polymerisation of aldehyde polymers in order to achieve reproducible results. It is also difficult to stabilise most of the products since thermodynamics frequently favour depolymerisation at temperatures a little above or at room temperature. 

Emulsion polymerisation is initiated using a water-soluble initiator, such as potassium persulfate. This forms free radicals in solution which may initiate some growing chains in solution. These radicals or growing chains pass to the micelles and diffuse into them, which causes the bulk of the polymerisation to occur in these stabilised droplets. 

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... 

An old bottle (1 year) containing glycolonitrile with phosphoric acid used as a stabiliser showed the appearance of tars. It detonated during handling. The detonation was probably due to the polymerisation of nitrile that was made possible by the fact that phosphoric acid was isolated by tars. Besides, the cap was cemented by tars that had already formed round the cap. A similar accident happened thirteen days after distilling the same nitrile. 

Place 25 g. of methyl methacrylate polymer (G.B. Diakon (powder). Perspex (sheet) U.S.A. Lucite, Plexiglass) in a 100 ml. Claisen flask, attach an efficient condenser e.g., of the double surface type) and distil with a small luminous flame move the flame to and fro around the sides of the flask. At about 300° the polymer softens and undergoes rapid depolymerisation to the monomer, methyl methacrylate, which distils over into the receiver. Continue the distillation until only a small black residue (3-4 g.) remains. Redistil the hquid it passes over at 100-110°, mainly at 100-102°. The yield of methyl methacrylate (monomer) is 20 g. If the monomer is to be kept for any period, add 0 -1 g. of hydro-quinone to act as a stabiliser or inhibitor of polymerisation. 

As the alkene monomers can absorb oxygen from the air, forming peroxides (c/. p. 329) whose ready decomposition can effect autoinitiation of polymerisation, it is usual to add a small quantity of inhibitor, e.g. quinone, to stabilise the monomer during storage. When subsequent polymerisation is carried out, sufficient radical initiator must therefore be added to saturate the inhibitor before any polymerisation can be initiated an induction period is thus often observed. 

A year-old bottled sample, containing syrupy phosphoric acid as stabiliser and which already showed signs of tar formation, exploded in storage. The pressure explosion appeared to be due to polymerisation, after occlusion of inhibitor in the tar, in a container in which the stopper had become cemented by polymer [1], A similar pressure explosion occurred when dry redistilled nitrile, stabilised with ethanol [2], polymerised after 13 days [3], The spontaneous and violent decomposition of the nitrile on standing for more than a week is usually preceded by formation of a red polymer [4],... 

---