Azo-compounds as initiators

Example 10.3. Free-radical polymerization of styrene. Styrene is a highly reactive monomer. If not stabilized, it polymerizes slowly even without an initiator [33]. Commercial polystyrene is produced with peroxy or azo compounds as initiators. The mechanism of polymerization initiated by 2,2 -azo-hw-isobutyronitrile (AIBN) is as follows ... 

In initial experiments we investigated the oxidation of HB in the presence of added hydroperoxide and azo compounds as initiators at different temperatures. Oxidation reactions were performed using neat CHB with bar of pure oxygen. The results in this case were not satisfactory selectivity decreased rapidly with... 

Most suitable initiators are organic peroxides (Table 10.2). In certain cases, it is advisable to use azo compounds as initiators (33). 

By careful removal of oxygen and other impurities from the system by passing the monomer over finely divided copper at 50°C prior to use, another patent claims the production of a 13% nonvolatile latex with particle diameter of 0.36//m using a water-soluble azo compound as initiator at 60°C-85 C and 40-60 kg/cm (0.4-0.6 MPa) of pressure [56]. 

In actual lipid oxidation, one cannot overlook the critical role of trace metals, which complicate the kinetic sequences of initiation and decomposition of lipid hydroperoxides. These metals catalyse both initiation of free radicals and decomposition of hydroperoxides, which become particularly significant with polyunsaturated lipids containing more than two double bonds. With these polyunsaturated lipids, although the yields of hydroperoxides are reduced in the presence of metals, they produce volatile decomposition products that have a serious impact on flavor deterioration. In foods and biological systems, the mixture of trace metals and hydroperoxides is the most important initiator that plays a key part in the development of free radical oxidation and rancidity. The use of artificial azo compounds as initiators to study free radical oxidation is therefore not relevant. 

Low-molecular weight azo compounds have frequently been used in cationic polymerizations producing azo-containing polymers. Thus, the combination of ionically and radically polymerizable monomers into block copolymers has been achieved. Azo compounds were used in all steps of cationic polymerization without any loss of azo function as initiators, as monomers and, finally, as terminating agents. 

The kinetics and mechanism of the thermal and photochemical decomposition of dialkyldiazenes (15) have been comprehensively reviewed by Engel. The use of these compounds as initiators of radical polymerization has been covered by Moad and Solomon2 and Sheppard.50 The general chemistry of azo-compounds has also been reviewed by Koga et cr/./11 Koenig,3 and Smith.3J... 

Such important materials as organic peroxides, azo compounds, as well as many other types of materials, are described by the first-order process and as such follow the general development given in this work. The efficiency will be assumed constant and the same for all initiators with 0.5. 

Polymerization of the quarterpolymers took place in a stirred reactor in an aqueous or a nonaqueous reaction medium. Preferred organic solvents were fluoroalkanes or chlorofluoroalkanes and their mixtures with water. Polymerization reaction could be carried out by bulk, solution, suspension, emulsion, or vapor phase polymerization methods. The reaction conditions depended on the selected polymerization process. The reaction temperature was in the range of 20°C to 100°C. An organic solvent medium required the use of organic peroxy or azo compounds as the reaction initiator. In aqueous media, water-soluble initiators such as ammonium persulfate could be employed. The best initiators for aqueous media were acids of manganese and their salts, such as potassium permanganate. The total pressure was in the range of 0.2-10 MPa. 

Most systems described in the literature make use of immobilized azo compounds as the initiating species [18-20]. These monolayers are in many cases... 

Numerous reviews on the photochemistry and photophysics of ketones [18,23,25,78,79] and azoalkanes [38,43,80,81] are already available. This chapter focuses on intermolecular photoreactions of azoalkanes, which are compared with known data for ketones (see Structure 3.1). Unimolecular reactions such as the Norrish type-1 a-cleavage reaction of ketones [17,82-87] and their Norrish type-II reactions [20,24,73,83,88-91] as well as denitrogenation [8,9,43,68,92,93] and cis-trans isomerization of azoalkanes [43,92,93] are not discussed. The anphasis lies, besides data compilation, on mechanistic understanding, such that classical applications of azo compounds as dyes [94] or more recent apphcations of azo compounds in photochromic materials [10-12], or ketones as radical initiators in polymerization [95], are omitted as well. 

Usually, free-radical initiators such as azo compounds or peroxides are used to initiate the polymerization of acrylic monomers. Photochemical (72—74) and radiation-initiated (75) polymerizations are also well known. At a constant temperature, the initial rate of the bulk or solution radical polymerization of acrylic monomers is first order with respect to monomer concentration and one-half order with respect to the initiator concentration. Rate data for polymerization of several common acrylic monomers initiated with 2,2 -azobisisobutyronittile (AIBN) [78-67-1] have been determined and are shown in Table 6. The table also includes heats of polymerization and volume percent shrinkage data. 

The type of initiator utilized for a solution polymerization depends on several factors, including the solubiUty of the initiator, the rate of decomposition of the initiator, and the intended use of the polymeric product. The amount of initiator used may vary from a few hundredths to several percent of the monomer weight. As the amount of initiator is decreased, the molecular weight of the polymer is increased as a result of initiating fewer polymer chains per unit weight of monomer, and thus the initiator concentration is often used to control molecular weight. Organic peroxides, hydroperoxides, and azo compounds are the initiators of choice for the preparations of most acryUc solution polymers and copolymers. 

Initiators, usually from 0.02 to 2.0 wt % of the monomer of organic peroxides or azo compounds, are dissolved in the reaction solvents and fed separately to the kettie. Since oxygen is often an inhibitor of acryUc polymerizations, its presence is undesirable. When the polymerization is carried out below reflux temperatures, low oxygen levels are obtained by an initial purge with an inert gas such as carbon dioxide or nitrogen. A blanket of the inert gas is then maintained over the polymerization mixture. The duration of the polymerization is usually 24 h (95). 

Initiators (1) and (2) have 10-h half-life tempeiatuies of 237°C and 201°C, respectively. It has been reported that, unlike organic peroxides and ahphatic azo compounds, carbon—carbon initiators (1) and (2) undergo endothermic decompositions (62). These carbon—carbon initiators are useful commercially as fire-retardant synergists in fire-resistant expandable polystyrenes (63). 

Suspension Polymerization. This method (10) might be considered as a number of bulk polymerizations carried out simultaneously in the monomer droplets with water acting as a heat-transfer medium. A monomer-soluble initiator, eg, a peroxide or azo compound, and a protective coUoid like poly(vinyl alcohol) or bentonite, are requited. After completion of the polymerization, the excess of monomer(s) is steam stripped, and the beads of polymer are collected and washed on a centrifiige or filter and dried on a vibrating screen or by means of an expeUer—extmder. 

Azo compounds having functional groups that stabilize the radical products are especially reactive. The stabilizing effect of the cyano substituent is responsible for the easy decomposition of azobis(isobutyronitrile) (AIBN), which is frequently used as a radical initiator. 

The addition of halogenated aliphatics to carbon-carbon double bonds is the most useful type of carbon-carbon bond forming synthetic method for highly halogenated substrates Numerous synthetic procedures have been developed for these types of reactions, particularly for the addition of perfluoroalkyl iodides to alkenes using thermal or photolytic initiators of free radical reactions such as organic peroxides and azo compounds [/]... 

Besides being used as initiators and monomers, azo compounds may also be used for terminating a cationic polymerization. Thus, the living cationic polymerization... 

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