Azodi-isobutyronitrile

The initiation stage may be activated by free-radical or ionic systems. In the following example a free-radical system will be discussed. In this case a material which can be made to decompose into free radicals on warming, or in the presence of a promoter or by irradiation with ultraviolet light, is added to the monomer and radicals are formed. Two examples of such materials are benzoyl peroxide and azodi-isobutyronitrile, which decompose as indicated in Figure 2.13. 

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. 

Polymerisation in bulk of styrene with azodi-isobutyronitrile as initiator. This initiator evolves nitrogen as it decomposes so that expansion and polymerisation occur simultaneously. This method was amongst the earliest suggested but has not been of commercial importance. There has, however, been recent resurgence of interest in this process. 

Oxidation of methanol in the liquid phase is slow. At 81—145° C with azodi-isobutyronitrile and t-butyl peroxide as initiators, the oxidation products are formaldehyde, formic acid, hydrogen peroxide, and methyl formate [2]. 

The styrene and acrylonitrile monomers used were freshly distilled. Azodi-(isobutyronitrile) (Porofor N) (PN), benzoyl peroxide (BPO), and tert-butyl peroctoate (f-BO) with a purity higher than 99% were used as initiators. 

Grafting Efficiency. In all the batches mentioned above, polymerization yields exceeded 90%. The graft products contained 10-45 wt % polystyrene. When the grafting efficiencies attained with the various peroxides under otherwise identical conditions were compared, the following activity scale was established azodi(isobutyronitrile) (I) < benzoyl peroxide (10, II, 12) < lert-butyl peroctoate (Figure 4). 

Polyethylene (also known as polythene) was synthesized by accident in 1932 when scientists at Imperial Chemical Industries (ICI) investigated the reaction between ethylene and various compounds at high pressure. Polyethylene is generally commercially polymerized from monomer ethylene gas under high pressure (1000—3000 atmospheres) and at temperatures of 80-300°C (Brydson, 1999). A free radical initiator such as benzoyl peroxide, azodi-isobutyronitrile or oxygen is added. The reaction is exothermic and must be cooled to control the rate of polymerization and molecular weight. 

PMMA was first produced commercially in the UK and in Germany in the early 1930s. Monomer methyl methacrylate polymerizes readily under ambient conditions and is therefore supplied with an inhibitor (up to 0.1 per cent of hydro-quinone). After removal of the inhibitor, free radical pol) merization with peroxides or azodi-isobutyronitrile at 100°C is employed commercially. Oxygen slows the rate of polymerization and leads to the formation of peroxides so is excluded by allowing nitrogen into the reaction vessel. Shrinkage between monomer to polymer is high at around 20 per cent. 

Styrene is polymerized at 60 C using an initiator concentration of 1 X 10 g mole/L (azodi-isobutyronitrile). The fraction of radical formed that initiates the chain reaction is 0.6, while ki is 0.85 X 10" s Termination kt value is 1.8 X 10 L/mole s. 

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