Radical initiator, AIBN

In contrast to /3-PCPY, ICPY did not initiate copolymerization of MMA with styrene [39] and AN with styrene [40]. However, it accelerated radical polymerization by increasing the rate of initiation in the former case and decreasing the rate of termination in the latter case. The studies on photocopolymerization of MMA with styrene in the presence of ICPY has also been reported [41], /8-PCPY also initiated radical copolymerization of 4-vinylpyridine with methyl methacrylate [42]. However, the ylide retarded the polymerization of N-vinylpyrrolidone, initiated by AIBN at 60°C in benzene [44]. (See also Table 2.)... 

It is important to emphasize that the hydroxy dithioketal cyclization can be conducted under mild reaction conditions and can be successfully applied to a variety of substrates.15 However, the utility of this method for the synthesis of didehydrooxocane-contain-ing natural products requires the diastereoselective, reductive removal of the ethylthio group. Gratifyingly, treatment of 13 with triphenyltin hydride and a catalytic amount of the radical initiator, azobisisobutyronitrile (AIBN), accomplishes a homolytic cleavage of the C-S bond and furnishes didehydrooxocane 14 in diastereo-merically pure form (95 % yield), after hydrogen atom transfer. 

Some of the more remarkable examples of this form of topologically controlled radical polymerization were reported by Percec et cii.231 234 Dendron maeromonomers were observed to self-assemble at a concentration above 0.20 mol/L in benzene to form spherical micellar aggregates where the polymerizable double bonds are concentrated inside. The polymerization of the aggregates initiated by AIBN showed some living characteristics. Diversities were narrow and molecular weights were dictated by the size of the aggregate. The shape of the resultant macroniolecules, as observed by atomic force microscopy (ATM), was found to depend on Xn. With A, <20, the polymer remained spherical. On the other hand, with X>20, the polymer became cylindrical.231,232... 

Experimental conversion-time data, obtained from the literature, on the bulk free radical polymerization of MMA initiated by AIBN at several temperatures and initiator concentrations, were described by the model. However, the expressions for the rate of conversion and gel effect index were first simplified and rearranged. ... 

Figure 5, Variation of the model parameters Cj (O) and Cb (X) u)ith temperature for the bulk free radical polymerization of MM A initiated by AIBN...

The radical addition of bromoform to ketensilylacetals has been described, initiated with AIBN or Et3B (ref. 12). The reaction yields polyfunctional silicon-containing compounds of CHBr2C(R)CBr(OR )OSiR type or products of their conversions (hydrolysis, fragmentation of R etc.). 

Ring-opening polymerization of 2-methylene-l,3-dioxepane (Fig. 6) represents the single example of a free radical polymerization route to PCL (51). Initiation with AIBN at SO C afforded PCL with a of 42,000 in 59% yield. While this monomer is not commercially available, the advantage of this method is that it may be used to obtain otherwise inaccessible copolymers. As an example, copolymerization with vinyl monomers has afforded copolymers of e-caprolactone with styrene, 4-vinylanisole, methyl methacrylate, and vinyl acetate. 

Driscoll [67], Lorimer and Mason [79] and Price [65[ have also obtained inverse Arrhenius temperature dependencies for reactions performed in the presence of ultrasound. For example Driscoll has investigated the polymerisation of styrene and methyl methacrylate in the presence of their respective homopolymers and observed that the lower the reaction temperature the faster was the reaction rate and the higher the final polymer yield (Figs. 5.38 and 5.39). Price on the other hand using a non polymer system has sonicated methyl butyrate (MeOBu) and compared the rates of radical production in the absence and presence of the initiator azobisisobutyronitrile (AIBN) (Tab. 5.18). 

By sonically inducing the polymerisation of methyl methacrylate (MMA), Price [65] has extended the work of Kruus and studied the effect of the absence and presence of the initiator azobisisobutyronitrUe (AIBN). Similar conversions to Kruus (2-3% per hour) were obtained in the absence of initiator at 25 °C. However considerable improvements in the polymerisation rate were observed when 0.1% of initiator were used (Fig. 5.40), the reaction appearing to become autocatalytic. This no doubt is due to the faster production of polymer in the initiated system (faster initiation due to enhanced initiator breakdown) which is then available for degradation to produce more free radical entities. 

Photochemically generated radicals in chain reactions are less familiar to synthetic chemists [8,21]. The above mentioned peroxides have been used in the presence of light to initiate radical chain reactions at room or lower temperatures. Azo compounds are also known to decompose photo-lytically to afford alkyl radicals. AIBN has rarely been used under such conditions. 

The initiation process appears more complicated than described above, although data are not available in more than a few systems. The benzoyl peroxide initiated polymerization of styrene involves considerable substitution of initiator radicals on the benzene ring for polymerizations carried out at high conversions and high initiator concentrations. About one-third of the initiator radicals from t-butyl peroxide abstract hydrogen atoms from the a-methyl groups of methyl methacrylate, while there is no such abstraction for initiator radicals from benzoyl peroxide or AIBN. 

The yield of initiating radicals is, however, generally smaller than would be expected. In the case of AIBN this is because a certain amount of tetramethylsuc-cinic acid dinitrile is formed by combination of the primary radicals, while some methacrylonitrile and iso-butyronitrile are formed by disproportionation of the primary radicals. Azo compounds are especially suited as initiators for polymerization in bulk or in organic solvents. 

H202 decomposes to free radicals in 2-propanol by the action of H+. Free radicals are also produced by the reaction between tert-BuOOH and Br in 1-propanol. The HCOf ions inhibit the oxidation of cyclohexanol initiated by AIBN, destroying many oxyperoxide radicals—i.e., HCOf is a negative catalyst. Appropriate reaction schemes and rate equations are proposed. 

The free-radical carbonylation of iodoalkanes in SCCO2 initiated by AIBN (0.2-0.3 equiv.) with (TMS)3SiH (1.5 equiv.) was studied for both intermolecular reactions and intramolecular reactions (Scheme 42). For example, the carbonylative addition of 1-iodooctane 304 to acrylonitrile was carried out at 80°C and 50 atm of CO in SCCO2 under a total pressure of 310 atm to give 4-oxododecanenitrile 305 in 90% yield. Also, the intramolecular carbonylation of 6-iodohexyl acrylate 306 under similar conditions afforded 11-membered macro-lide 307 in 68% yield. 

Polymer-bound dinitrogen complexes containing Mn have been prepared [8] directly from the polymer bound (n5-vinylmethyl-cyclopentadienyl) tricar-bonylmanganese (VCM) and molecular nitrogen in THF/benzene. The metal-containing base polymer may be readily prepared by AIBN initiated radical polymerization of VCM with styrene or /V-vinylpyrrolidone (Fig. 4). 

A series of internal alkynes has been investigated, revealing that the presence of an alkyl or aryl group does not appear to change the course of the reaction. Internal alkynes, however, do not undergo germylformylation in this system. In the case of tin, two reports exist of the stannylformyla-tion of unsaturated carbon substrates, but both proceed by a free radical mechanism initiated by AIBN and do not require a transition metal catalyst.128... 

AIBN, (67) forms in high yield because the initial radical abstracts the hydrogen a to the carbonyl (with concomitant elimination of BusSn) more rapidly than it cyclizes. However, blocking this pathway results in smooth ring expansion, as illustrated by the conversion of (66) to (68). The alkene stereochemistry is dictated by the configuration of the precursor (see Scheme 49). 

A multi-MIP array has been fabricated photolithographic ally for determination of an albuterol broncholidator (Table 6) [185]. 20-pm diameter acrylic MIP beads have been synthesized by co-polymerization of the benzyl methacrylate functional monomer, MAA functional monomer and HEMA cross-linker in the propylene glycol monomethyl ether acetate porogenic solvent. Thermo-radical polymerization on a Pt electrode was initiated by AIBN. Albuterol was recognized in the... 

Non-activated double bonds, e.g. in the allylic disulfide 1 (Fig. 10.2) in which there are no substituents in conjugation with the double bond, require high initiator concentrations in order to achieve reasonable polymerisation rates. This indicates that competition between addition of initiator radicals (R = 2-cyanoisopropyl from AIBN) to the double bond of 1 and bimolecular side reactions (e.g. bimolecular initiator radical-initiator radical reactions outside the solvent cage with rate = 2A t[R ]2 where k, is the second-order rate constant) cannot be neglected. To quantify this effect, [R ] was evaluated using the quadratic Equation 10.5 describing the steady-state approximation for R (i.e. the balance between the radical production and reaction). In Equation 10.5, [M]0 is the initial monomer concentration, k is as in Equation 10.4 (and approximately equal to the value for the addition of the cyanoisopropyl radical to 1-butene) [3] and k, = 109 dm3 mol 1 s l / is assumed to be 0.5, which is typical for azo-initiators (Section 10.2). The value of 11, for the cyanoisopropyl radicals and 1 was estimated to be less than Rpr (Equation 10.3) by factors of 0.59, 0.79 and 0.96 at 50, 60 and 70°C, respectively, at the monomer and initiator concentrations used in benzene [5] ... 

Thermal decomposition of 29 in refluxing benzene or toluene or azo-bis-isobutyronitrile (AIBN) initiated radical chain reactions employing either Bu,SnH or f-BuSH as chain propagating agents also can be used. 

A radical initiator (UV, AIBN) is needed for the homolytic bond cleavage of Br2 ... 

Allylation of these P-alkoxy-a-halo esters with allyltributyltin initiated with AIBN at 60° also shows good to impressive stereoselectivity, which can be improved by use of triethylborane as initiator at —78°. However, these reactions are generally slower than radical reductions. 

Hydrostannylation This reaction with a tin hydride normally requires an initiator, usually AIBN but also B(C2H5)3 (14, 314). It can also be initiated with high intensity ultrasound, and such reactions show large rate acceleration (100-600 times) and take place even at temperatures of -50°. Sonication is also effective for radical reductions and cyclizations (last example). 

Polymer and copolymer. Polymer and copolymer were prepared in our laboratory for strict control of purity, branching content and molecular weight. All monomers and solvents were carefully purified. PVC samples were prepared as powders in bulk using free radical photoinitiation with either AIBN or uranyl nitrate as initiators. The AIBN/UV initiation was used for photopolymerization at temperatures of 50°, 25°, and 0°C. 

Radical homopolymerization and copolymerization with MMA initiated by AIBN in benzene solution or in bulk led to high MW graft (co)polymers. 

Highly stereoregular PMMA macromonomers, 14, prepared by Hatada and coworkers, have recently been fractionated by supercritical fluid chromatography into completely uniform fractions with no structural distribution [20,21]. They have been oligomerized with a radical (AIBN) or an anionic initiator (3,3-dimethyl-1,1-diphenylbutyllithium). After a new fractionation by SEC comb or star polymers of completely uniform architecture are obtained. No doubt, these samples will be most promising to investigate the branched structure-property relationship. 

Addition reactions with Mode B are not popular, but are occasionally useful. Eq. 4.7 indicates the reaction of ethyl bromoacetate and sugar vinyl ether with Bu3SnH initiated by AIBN. The ethyl acetate radical is electrophilic and it reacts with electron-rich sugar vinyl ether through SOMO-HOMO orbital interaction to form a ribosyl anomeric radical, as shown below. Then, the formed ribosyl anomeric radical abstracts... 

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