Azobisisobutyronitrile free radical polymerization

Free-radical polymerization processes are used to produce virtually all commercial methacrylic polymers. Usually free-radical initiators (qv) such as azo compounds or peroxides are used to initiate the polymerizations. Photochemical and radiation-initiated polymerizations are also well known. At a constant temperature, the initial rate of the bulk or solution radical polymerization of methacrylic 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 methacrylic monomers initiated with 2,2 -azobisisobutyronitrile [78-67-1] (AIBN) have been deterrnined and are shown in Table 8. 

The same authors proposed an alternative methods for obtaining soluble poly(/i-vinylborazine) homopolymers and poly(styrene-co-B-vinylborazine) copolymers 28 In fact, gentle polymerization conditions in solution at 80°C using Azobisisobutyronitrile (AIBN) (1.6 mol%) as an initiator provided soluble homopolymers. The polymer displays typical Mw and Mn values of —18,000 and 11,000, respectively, whereas an increase in the AIBN concentration results in a decrease in the molecular weight, contrary to what is usually observed in free-radical polymerization. 

We have also investigated the kinetics of free radical initiation using azobisisobutyronitrile (AIBN) as the initiator [24]. Using high pressure ultraviolet spectroscopy, it was shown that AIBN decomposes slower in C02 than in a traditional hydrocarbon liquid solvent such as benzene, but with much greater efficiency due to the decreased solvent cage effect in the low viscosity supercritical medium. The conclusion of this work was that C02 is inert to free radicals and therefore represents an excellent solvent for conducting free radical polymerizations. 

Poly(styrene) and PMMA were synthesized from their respective monomers using azobisisobutyronitrile-initiated radical polymerization in benzene. Four freeze-pump-thaw cycles were used to degas the monomer solutions and polymerization was carried out for 48 hours at 60°C. The polymers were purified by multiple reprecipitations from dichloromethane into methanol. Films of these polymers were prepared and found to be free of any fluorescent impurity. 

Rasmussen and co-workers. Chapter 10, have shown that many free-radical polymerizations can be conducted in two-phase systems using potassium persulfate and either crown ethers or quaternary ammonium salts as initiators. When transferred to the organic phase persulfate performs far more efficiently as an initiator than conventional materials such as azobisisobutyronitrile or benzoyl peroxide. In vinyl polymerizations using PTC-persulfate initiation one can exercise precise control over reaction rates, even at low temperatures. Mechanistic aspects of these complicated systems have been worked out for this highly useful and economical method of initiation of free-radical polymerizations. 

Table 1. Field influence on free radical polymerization . Monomer styrene, catalyst azobisisobutyronitrile (0.02 mole/l), temp. 60°, polymerization time 30min...

Chemistry. Vinyl acetate is polymerized commercially using free-radical polymerization in either methanol or, in some circumstances, ethanol. Suitable thermal initiators include organic peroxides such as butyl peroxypivalate, di(2-ethylhexyl) peroxydicarbonate, butyl peroxyneodecanoate, benzoyl peroxide, and lauroyl peroxide, and diazo compounds such as 2,2 -azobisisobutyronitrile (205—215). The temperatures of commercial interest range from... 

The steps in free-radical polymerization reaction and the corresponding rate laws are summarized in Table 7-3. For the polymerization of styrene at 80°C initiated by 2,2-azobisisobutyronitrile the rate constants9 are... 

Free-radical polymerization is usually initiated by thermal or UV-initiated decomposition (usually homolytic dissociation) of a suitable radical initiator (Scheme 9.10). For example, thermal decomposition of AIBN (azobisisobutyronitrile) gives N2 and carbon radicals, which initiate polymerization of vinyl monomers. 

FIGURE 3.4 Polymerization steps and kinetic equations relevant in the chain growth polymerization exemplified for the free radical polymerization of styrene initiated with azobisisobutyronitrile (AIBN) denote the kinetic constants for... 

Under certain conditions, no free radical polymerization at all occurs with certain free radical initiators. For example, azobisisobutyronitrile polymerizes vinyl mercaptals, CH2=CH—S—CH2—S—R, to high-molar-mass compounds. But no polymer at all is produced under the same conditions by dibenzoyl peroxide the mercaptal groups induce dibenzoyl peroxide decomposition, producing benzoic acid and an unstable ester, CH2=CH—S— CH(00CC6H5)—S—R. Thus, the initiator is completely consumed by this side reaction. 

Thus, different free radical initiators may be preferred according to monomer and process. Dilauryl peroxide and diisopropyl peroxidicarbonate were mostly used previously for the polymerization of vinyl chloride, but bis(4-f-butyl cyclohexyl)peroxidicarbonate and bis(2,4-dimethyl)valeronitrile are presently being used in increasing amounts. Azobisisobutyronitrile and azobis(2,4-dimethyl)valeronitrile have also been partially replaced by peroxidicarbonates in Japan. Peroxidicarbonates are also being increasingly used for the free radical polymerization of ethylene. 

Most of free radical polymerizations are started by free radicals that have been produced by the thermal decomposition of suitable free-radical-forming agents. Compounds whose bonds are easily broken make suitable free-radical-forming agents for example, radicals are formed by the decomposition of azo compounds such as azobisisobutyronitrile [reaction (20-3)] or by the degradation of compounds such as peroxides, peresters, peracids or hydroperoxides. Benzoyl peroxide can decompose into benzoyl-oxy radicals, and also, in certain solvents, into phenyl radicals ... 

Although the free-radical polymerization of MMA typically exhibits a syndiotactic bias (rr triad content = 60%-70%), it has long been known that the stereochemical interactions between the chain-end radical and vinyl monomers in free-radical polymerization can be modified by using chiral protecting groups. For example, the 80 °C AIBN-initiated polymerization (AIBN = 2,2 -azobisisobutyronitrile) of oxazolidine acrylamides based on valine and t rt-leucine ultimately yields highly isotactic (92% m dyad content) poly(acrylic acid) and PMA after chemical modification (Scheme 23.23). " ... 

Free-radical initiators such as azobisisobutyronitrile (AIBN) or peroxides readily polymerize NVK. The reaction rate of free-radical polymerization in cyclohexanone at 70 °C using 0.01 mol% AIBN as initiator has been determined [412]. NVK polymerizes 10 times faster than styrene. 

In free radical polymerization, initiation may be brought about by light or heat most commonly, however, it is achieved by the addition of a material which, on heating, decomposes into free radicals (which may be defined as organic molecules containing an unpaired electron). Examples of frequently used initiators are benzoyl peroxide and azobisisobutyronitrile which give rise to free radicals as follows ... 

This is in general a heterogeneous free radical polymerization that involves the emulsification of the relatively hydrophobic monomer in water and sometimes an organic phase-in-water emulsifier, followed by the initiation reaction with either a water soluble initiator e.g. sodium persulfate (NaPS)) or an oil-soluble initiator e.g. 2-20-azobisisobutyronitrile (AIBN)) [266]. Typical monomers used in the emulsion polymerization include butadiene, styrene, acrylonitrile, acrylate ester and methacrylate ester, vinyl acetate, and vinyl chloride, but also biopolymers are now obtained by this versatile technique in several mesodimensionate morphologies [267]. 

Free Radical Polymerization. In situ polymerization reactions of the monomers added to metal salt solution other than Pechini process were proposed. They utilize free radical polymerization of acrylamide (Gotor, 1993 Rao, 1995 Sin, 2000, 2002) or acryUc add (Mani, 1992). The gelation in a usual synthesis occurs due to the reaction between acrylamide and N- N= methylene-bis-acrylamide. Free radicals initiating the polymerization are created by hydrogen peroxide or azobisisobutyronitrile. Just solvated copper ions strongly inhibit polymerization and sufficient amount of EDTA (Sin, 2000) or dtric add (Gotor, 1993 Rao, 1995) should be added to chelate copper and possibly other metals. The clear advantage of this method is that in contrast to Pechini-type process, which enploys reversible polyesterification reaction, the polymer formation by free radical mechanism is irreversible process that can be conducted, in addition, at low temperatures. 

A RAFT polymerization system consists of initiator, monomer, chain transfer agent, solvent, and temperature. RAFT polymerization can be performed by simply adding a chosen quantity of an appropriate RAFT agent (thiocarbonylthio compounds) to a conventional free radical polymerization. Usually the same monomers, initiators, solvents and temperatures can be used. Because of the low concentration of the RAFT agent in the system, the concentration of the initiator is usually lower than in conventional radical polymerization. Radical initiators such as Azobisisobutyronitrile(AIBN) and 4,4 -Azobis(4-cyanovaleric acid)(ACVA) are widely used as the initiator in RAFT. RAFT polymerization is known for its compatibility with a wide range of monomers compared to other controlled radical polymerizations. These monomers include (meth)acrylates, (meth)... 

Cho and Lee [6] used three different initiators, potassium persulfate, 2,2 -azobisisobutyronitrile, and 4,4 -azobis(4-cyanovaleric acid) (water-soluble, but less hydrophilic than potassium persulfate) to investigate their effects on the emulsion polymerization of styrene in the presence of polymethyl methacrylate seed latex particles. Inverted core/shell latex particles were observed when 2,2 -azobisisobutyronitrile or 4,4 -azobis(4-cyanovaleric acid) was used to initiate free radical polymerization. The use of potassium persulfate resulted in various morphological structures of latex particles, which were largely determined by the initiator concentration and polymerization temperature. 

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