Initiators in free-radical polymerization

Peroxides of the type ROOR are often used as radical initiators in free radical polymerization reactions. According to our suggested structures, compound B is a peroxide of the type ROOR. Vinyl chloride will undergo a free radical polymerization to form polyvinylchloride (PVC). Polymerization occurs in a head to tail fashion the growing end of the polymer chain is the most stable radical possible. Hence the growing end of the PVC chain will be... 

Thus, the initiator free radical becomes an end group of the polymer. Consequently, initiators in free radical polymerizations are not catalysts since they are consumed in the start reaction. 

Lauroyl peroxide [CH3(CH2)ioC(0)-]2. A peroxide used as an initiator in free-radical polymerizations of styrene, vinyl chloride, and acrylic monomers. 

Kreft T, Reed WF. Direct monitoring of the cross-over from diffusion controlled to decomposition controlled initiation in free radical polymerization. Macromol Chem Phys 2008 209 2463-2474. 

FIGURE 13.1 Residuals for fits of decomposition limited and diffusion limited initiation for Am conversion during free radical homopolymeiization. The two fits from which the residuals are computed are shown in the inset. Adapted with permission from Kreft T, Reed WF. Direct monitoring of the cross-over from diffusion-controlled to decomposition-controlled initiation in free radical polymerization. Macromol Chem Phys 2008 209 2463-2474. 

The reversible addition of sodium bisulfite to carbonyl groups is used ia the purification of aldehydes. Sodium bisulfite also is employed ia polymer and synthetic fiber manufacture ia several ways. In free-radical polymerization of vinyl and diene monomers, sodium bisulfite or metabisulfite is frequentiy used as the reduciag component of a so-called redox initiator (see Initiators). Sodium bisulfite is also used as a color preventative and is added as such during the coagulation of crepe mbber. 

The theory of radiation-induced grafting has received extensive treatment [21,131,132]. The typical steps involved in free-radical polymerization are also applicable to graft polymerization including initiation, propagation, and chain transfer [133]. However, the complicating role of diffusion prevents any simple correlation of individual rate constants to the overall reaction rates. Changes in temperamre, for example, increase the rate of monomer diffusion and monomer... 

Addition of phosphonyl radicals onto alkenes or alkynes has been known since the sixties [14]. Nevertheless, because of the interest in organic synthesis and in the initiation of free radical polymerizations [15], the modes of generation of phosphonyl radicals [16] and their addition rate constants onto alkenes [9,12,17] has continued to be intensively studied over the last decade. Narasaka et al. [18] and Romakhin et al. [19] showed that phosphonyl radicals, generated either in the presence of manganese salts or anodically, add to alkenes with good yields. 

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. 

Moore and Hemmens [119] studied the photosensitization of primaquine and other antimalarial agents. The drugs were tested for in vitro photosensitizing capability by irradiation with 365 nm ultraviolet light in aqueous solutions. The ability of these compounds to photosensitize the oxidation of 2,5-dimethylfuran, histidine, trypotophan, or xanthine, and to initiate the free radical polymerization of acrylamide was examined in the pH range 2 12. Primaquine does not have significant photochemical activity in aqueous solution. 

Living radical polymerizations have received considerable attention because they provide a convenient alternative for synthesizing block copolymers, polymers of narrow polydispersity and complex polymer structures (1-5). Because of their ability to initiate living free radical polymerizations, iniferters have been examined extensively after Otsu et al. (6) introduced them in 1982. In particular, dithiocarbamate derivatives have been studied more closely by several researchers. Lambrinos et al (7) have examined the molecular weight evolution during the polymerization of n-butyl acrylate using p-x ylylene bis(A,A-diethyl... 

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. 

Case 1 appears to accurately predict the observed dependence on persulfate concentration. Furthermore, as [Q]+otal approaches [KX], the polymerization rate tends to become independent of quat salt concentration, thus qualitatively explaining the relative insensitivity to [Aliquat 336]. The major problem lies in explaining the observed dependency on [MMA]. There are a number of circumstances in free radical polymerizations under which the order in monomer concentration becomes >1 (18). This may occur, for example, if the rate of initiation is dependent upon monomer concentration. A particular case of this type occurs when the initiator efficiency varies directly with [M], leading to Rp a [M]. Such a situation may exist under our polymerization conditions. In earlier studies on the decomposition of aqueous solutions of potassium persulfate in the presence of 18-crown-6 we showed (19) that the crown entered into redox reactions with persulfate (Scheme 3). Crematy (16) has postulated similar reactions with quat salts. Competition between MMA and the quat salt thus could influence the initiation rate. In addition, increases in solution polarity with increasing [MMA] are expected to exert some, although perhaps minor, effect on Rp. Further studies are obviously necessary to fully understand these polymerization systems. 

Aqueous PolyNIPAAM Homopolymer (PolyNIPAAM). To 20 mg NIPAAM dissolved in phosphate buffered saline, 2.3 mg of ammonium persulfate and 9.3 mg of N,N,N, N -tetramethylethylenediamine (TEMED) was added to initiate the free radical polymerization. The mixture was then incubated for 3 hours at room temperature. The polyNIPAAM was isolated by precipitation in 14.3%, by volume, saturated (NH4)2S04 After removal of residual (NH4)zS04 by ion exchange chromatography (Bio-Rad AG501-X8D), polyNIPAAM was stored as the lyophilized solid. 

The emulsion polymerization methodology is one of the most important commercial processes. The simplest system for an emulsion (co)polymerization consists of water-insoluble monomers, surfactants in a concentration above the CMC, and a water-soluble initiator, when all these species are placed in water. Initially, the system is emulsified. This results in the formation of thermodynamically stable micelles or microemulsions built up from monomer (nano)droplets stabilized by surfactants. The system is then agitated, e.g., by heating it. This leads to thermal decomposition of the initiator and free-radical polymerization starts [85]. Here, we will consider a somewhat unusual scenario, when a surfactant behaves as a polymerizing comonomer [25,86]. 

Teixeira D, Lalot T, Brigodiot M et al (1999) P-Diketones as key compounds in free-radical polymerization by enzyme-mediated initiation. Macromolecules 32 70-72... 

In free-radical polymerization in bulk, solution, and aqueous suspension the initiator is dissolved in the monomer. Aqueous suspension polymerization is considered as bulk polymerization in droplets. Some polymers, like poly (vinyl chloride) or polyacrylonitrile, are insoluble in the monomers and precipitate during bulk polymerization. The growth of the precipitated chain, which depends on the number of trapped radicals, is... 

More recently, Schuster [25] has demonstrated that cyanine dyes, i.e., cyanine borates or cyanine dye-borate mixtures, provide visible light activated initiation of free radical polymerization [26]. The photoexcited cyanine dye oxidizes alkyltriphenylborates by PET to produce the bleached reduced cyanine along with an alkyl radical. The alkyl radical can then initiate free radical polymerization [27], This visible light activated PET bond cleavage is of considerable importance in photoimaging and photocuring [28]. 

When polysilanes are used in free radical polymerization, silyl-type radicals formed by chain scission are the primary initiating radicals [61,62], However, in the presence of pyridinium ions these radicals undergo oxidation to yield reactive cations capable of initiating the polymerization of appropriate monomers (Scheme 14). 

The triphenyl methyl or trityl radical behaves as a radical trap and favors the polymerization-termination which is thermoreversible and thus allows the insertion of a new polymeric sequence. In 1982, Otsu et al. [49,213,214] proposed an interesting example involving phenylazotriphenylmethane as Initer (initiator-terminator) able to initiate a free radical polymerization from the phenyl radical. Alternatively, the trityl end-capped polymer can be utilized as an original macroiniter for the polymerization of a second monomer and yields block copolymers as follows ... 

Cellulose can also be modified by introducing long-chain polymer(s) onto its main chain. The preparation of a graft copolymer requires the formation of a reactive site on cellulose in the presence of a polymerizable monomer. The principal techniques frequently used are (1) grafting initiated by free radical polymerization, (2) grafting initiated by ionic polymerization (3)... 

As in free radical polymerization, there are initiation and propagation steps. Various initiators, such as organometallic compounds, alkali metals, Grignard reagents, or metal amides, like sodium amide, shown in Figure 3-31, can be used. Propagation proceeds in the usual manner, but there is no termination... 

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