Azobis compounds, radical

Only free radical polymerisation, which requires the formation of reactive free radical species to initiate polymerisation, appears to have been used to form MIPs. Free radicals are produced by the decomposition of an initiator species by the action of heat or light. Commonly used initiators are benzoyl peroxide and azobis compounds such as azobisisobutyronitrile (AIBN) or 2,2 -azobis(2,4-dimethylvaleronitrile) (ABDV) Figure 6.20). 

Such reactions can be initiated by free radicals, derived from compounds (initiators) such as benzoyl peroxide, ammonium persulphate or azobis-isobutyronitrile or by ionic mechanisms... 

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. 

Generation of radicals by redox reactions has also been applied for synthesizing block copolymers. As was mentioned in Section II. D. (see Scheme 23), Ce(IV) is able to form radical sites in hydroxyl-terminated compounds. Thus, Erim et al. [116] produced a hydroxyl-terminated poly(acrylamid) by thermal polymerization using 4,4-azobis(4-cyano pentanol). The polymer formed was in a second step treated with ceric (IV) ammonium nitrate, hence generating oxygen centered radicals capable of starting a second free radical polymeriza-... 

The radical alkylation of ketones is achieved by their conversion into the desired N-silyloxy enamines 81 (Scheme 13). The reaction of 81 with diethyl bromomalonate in the presence of EtsB (0.5 equiv) in benzene was performed in open air and stirred at room temperature for 3h. With nitro compounds it is achieved by their conversion into the desired ]V-bis(silyloxy)enamines (82) (Scheme 13). When the reaction is carried out with 82 and alkyl iodides with an electron-withdrawing substituent at the a-position, using V-70 as radical initiator (2,2 -azobis(4-methoxy-2,4-dimethylvaleronitrile)), it underwent a clean radical alkylation reaction to yield an oxime ether. Successful radical alkylation of... 

The same research group has further performed radical carbonylation reactions on the same microreactor system [36]. First, alkyl halides were initiated and effectively reacted with pressurized carbon monoxide to form carbonyl compounds. The principle was subsequently successfully extrapolated to the multicomponent coupling reactions. 1-Iodooctane, carbon monoxide and methyl vinyl ketone were reacted in the presence of 2,2 -azobis(2,4-dimethylvaleronitrile) (V-65) as an initiator and tributyltin hydride or tris(trimethylsilyl)silane (TTMSS) as catalyst (Scheme 15). 

For perhaps a century, the primary interest in azo compounds has been in dye chemistry. Recently, aliphatic azo compounds, which are thermally less stable than their aromatic counterpart, have enjoyed attention as sources of free radicals for polymerization reactions. In this regard, an initiator such as a,a -azobis(isobutyronitrile) is important for two reasons ... 

It must be reiterated that, whereas aromatic azo compounds are relatively stable thermally and can be subjected to typical reactions of aromatic compounds [67, 68a, 88], the aliphatic azo compounds may be substantially less stable thermally. Aliphatic azo compounds, such as oc,a -azobis(isobutyro-nitrile), do decompose on heating and are used as free radical sources. Hence adequate safety precautions must be taken in handling them. This, by the way, does not mean that aliphatic azo compounds have not been subjected to distillation and to vapor phase chromatography. Many have been distilled and, as will be pointed out in a subsequent section, their preparation by isomerization of hydrazone depends on a distillation technique. 

In a similar way as has been described for syntheses of type al, the majority of examples of type b involve polycondensation of a,ea bifunctional, small molecule reaction partners. Some examples are the reaction of AIBN or AIBN derivatives with 1,4-cyclohexane bismethyl diamine78), 1,2-ethylene diamine78), 1,6-hexamethylene diamine 78-80 , bisphenol A 78,81 and mono-, di- and tetraethylene glycol 55-64 . In almost all case using the AIBN derivative 4,4 -azobis(4-cyano valeryl chloride), an interfacial polymerization was employed. These polymeric azo compounds could be used as initiators for radical block copolymerizations. 

Azo compounds such as the water-soluble 2,2 -azobis(2-methylpropionamidine) dihydrochloride slowly decompose at elevated temperatures (Table 2.5) already at 40 °C, thereby yielding molecular nitrogen and two radicals [reaction (56)] (Niki 1990 Paul et al. 2000). These are first formed within the solvent cage, where about half of them recombines [reaction (57)]. The other half escapes the cage, and in the presence of 02 these radicals are converted into the corresponding peroxyl radicals [reactions (58) and (59)]. 

The two most important classes of radical initiators are azo-compounds and peroxides (Fig. 10.1). The most commonly used azo-initiators are 2,2/-azobis(isobutyronitrile) (AIBN) and l,l/-azobis(cyclohexane-l-carbonitrile) (ACN). The shorter half-life of AIBN ( ti/2 = 1.24 h at 80°C in benzene) has led to international shipping restrictions and ACN (ti/2 = 29.61 h under the same conditions [l])isbecominganincreasinglyvaluablereplace-ment. As explained in Section 10.4, however, this appreciable difference in decomposition rate means that ACN cannot replace AIBN as an initiator for all radical reactions. Furthermore, AIBN is soluble in a wider range of solvents, polar as well as non-polar (including alcohols, acetonitrile and benzene), compared with ACN which is restricted to use in nonpolar solvents, such as benzene, toluene and cyclohexane. 

Fig. 3. Thermal homolysis of 2,2/-azobis(2-amidopropane) (ABAP) in the presence of oxygen leads to formation of peroxyl and alkoxyl radicals. This compound is usually available as mono- or dihy-drochlorode and then is abbreviated also as AAPH.

Radical initiators are thermally labile compounds, which decompose into radicals upon moderate heating or photolysis. These radicals initiate the actual radical chain through the formation of the initiating radical. The most frequently used radical initiators are azobis-isobu-tyronitrile (AIBN) and dibenzoyl peroxide (Figure 1.11). AIBN has has a half-life of 1 h at 80 °C, and dibenzoyl peroxide a half-life of 1 h at 95 °C. 

In this case it is not that the carbon-nitrogen bond is so weak rather, it is the formation of the strong nitrogen—nitrogen triple bond of the N2 product that enables the reaction to occur at relatively low temperatures. Azobis(isobutyronitrile), also known as AIBN, has been widely used as a radical source because it is commercially available. In addition, it undergoes bond homolysis at lower temperatures than other azo compounds (below 100°C) because the product radicals are tertiary and are stabilized by resonance. 

Bicyclic eight-membered ring phosphorus compounds are usually obtained by the free radical reactions of the corresponding alkenes with phosphines. In most cases, these reactions require the presence of such free radical initiators as azobis(isobutyronitrile) (AIBN) or azobis(isovaleronitrile) (VAZO). 

Jin and collaborators reported the antioxidant activity of cleomiscosins A (9) and C (7), isolated from the leaves and twigs of Acer okamotoatum (19). Compound 7 inhibited LDL oxidation mediated by either catalytic copper ions or free radicals generated with 2,2 -azobis(2-amidinopropane) dihydrochloride, in a dose-dependent manner. By means of electrophoretic analysis it was also observed that 7 protected apolipoprotein B-lOO against Cu fragmentation. Also, fluorescence analysis clearly indicated that both 7 and 9 protect against the oxidative modification of apoB-100, induced by either Cu + or HOCl. Compounds 7 and 9 could, therefore, be beneficial in preventing LDL oxidation in atherosclerotic lesions. 

Azo compounds. RN=NR. Compounds such as a,a azobis(isobu-tyronitrile) abbreviated as AIBN, decompose thermally to give nitrogen and two alkyl radicals. 

In the first experiment we will chlorinate 1-chlorobutane because it is easier to handle in the laboratory than gaseous butane and we will use sulfuryl chloride as our source of chlorine radicals because it is easier to handle than gaseous chlorine. Instead of using light to initiate the reaction we will use a chemical initiator, 2,2 -azobis-(2-methylpropionitrile). This azo compound (R—N=N—R) decomposes at moderate temperatures (80-100°C) to give two relatively stable radicals and nitrogen gas ... 

Regarding the initiation process of polymerization, it can be started by y-radiation. It is a method that has been used for the synthesis of hydrogels of PEO as well as hydrogels based on vinyl monomers " in this latter case, azo-compounds such as 2,2-azo-isobutyroni-trile (AIBN)f or 2,2 -azobis (2-amidine-propane) dihydrochloride or V-SO, and aqueous salt solutions such as aqueous ammonium peroxodisulfate are also used. Among the monomers most used in the preparation of hydrogels through free-radical polymerization are 2-hydroxyethyl methacrylate (HEMA) and A-vinyl-2-pyrrolidone (VP). ... 

Olive oil wastewaters are rich in antioxidant compounds, particularly in hydroxytyrosol derivatives (Visioli et al., 1999). Hydroxytyrosol strongly inhibited low-density lipoprotein oxidation stimulated by 2,2 -azobis(2-aminopropane) hydrochloride (Aruoma et al., 1998). Further investigations point out that hydroxytyrosol and oleuropein are potent scavengers of superoxide radicals (Visioli et al, 1998). Tyrosol and hydroxytyrosol are dose-dependently absorbed by humans and eliminated as their glucuronide conjugates, indicating a good bioavailability (Visioli et al., 2000). 

Although AIBN is a popular choice, other azo compounds can give superior results due to their varying half-life. Indeed, the nature of the substitution of a newly formed carbon radical plays an important role in their half-life, as can be seen from the following for 2, 2 -azobis(4-methoxy)-3,4-dimethylvaleronitrile (AMYN), with a t /2 in toluene of 1 h at 56 °C and 10 h at 33 °C. There are also hydrophilic azo compounds, such as 2, 2 -azobis(2-methylpropionamidine) dihydrochloride (APPH), with a t j2 in water of 10 h at 56 °C. Typically, 5-10 mol% of the initiator is added, either in one portion or via slow addition. There... 

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