Free radical vinyl polymerization initiation

Allen, P. W., G. Ayrey, C. G. Moore and J. Scanlan Radiochemical studies of free-radical vinyl polymerizations. Part II. The polymerization of vinyl monomers in the presence of polyisoprenes use of C14-labeled initiators to determine the mechanism of graft-interpolymer formation. J. Polymer Sci. 36, 55 (1959). 

Neutral radical intermediates formed in the photo-oxidation of amines etc. by aryl ketones are useful in initiation of free radical vinyl polymerization and may afford technological advantages over more conventional photoinitiation systems (Ledwith and Purbrick, 1973 Ledwith, 1975). 

The initiation process, similar to other free-radical vinyl polymerizations, involves the chemical decomposition of unstable peroxides - azocompounds, or persulfates - into free radicals which can react rapidly with monomer to begin the propagation of polymer chains [4]. In the case of a water-soluble initiator, the radical concentration in polymer particles is related to the initiator concentration in water and the radical capture efficiency of latex particles. The radical capture efficiency of monomer droplets is very small and, therefore, their contribution to overall polymerization process is negligible. Thus, the small surface area of monomer droplets and/or high concentration of radicals in monomer droplets disfavor the growth events. Using an oil-soluble initiator, the radical concentration in particles and monomer droplets is related to the initiator concentrations in both phases. The initiator concentration between these phases is usually expressed in terms of an initiator partition coefficient. 

The following conclusions may be made about free-radical vinyl polymerization using a chemical initiator ... 

BenzUketals are another important class of photoinitiators (Table 4) developed for free-radical vinyl polymerization. Benzilketals exhibit higher thermal stability than benzoin compounds due to the absence of thermally labile benzylic hydrogen. The most prominent member of this class is the commercially used 2,2-dimethoxy-2-phenylacetophenone (DMPA). This initiator shows an excellent efficiency in photopolymerizations and is, at the same time, easy to synthesize. Other benzilketals are also suitable initiators but do not reach the price performance ratio of DMPA. 

The polymer-bound thixanthone derivatives 5 and 6 (Table 19) have been used in conjunction with amines for free-radical vinyl polymerization. Their initiation efficiency is similar to that of low-molecular-weight analogs. 

Figure 1 shows some examples of continuous stirred tank reactor systems for free-radical vinyl polymerization processes. In the bulk styrene polymerization process shown in Fig. la [2], styrene monomer, stripped of inhibitor added for transportation, is supplied to a prepolymerization reactor with an organic initiator. The monomer-polymer mixture then fed to a series of stirred tank reactors operating at higher temperatures than in the prepolymerization reactor. At low temperatures, the polymer s molecular... 

Polymerization reactions. There are two broad types of polymerization reactions, those which involve a termination step and those which do not. An example that involves a termination step is free-radical polymerization of an alkene molecule. The polymerization requires a free radical from an initiator compound such as a peroxide. The initiator breaks down to form a free radical (e.g., CH3 or OH), which attaches to a molecule of alkene and in so doing generates another free radical. Consider the polymerization of vinyl chloride from a free-radical initiator R. An initiation step first occurs ... 

Organic peroxides are used extensively for the curing of unsaturated polyester resins and the polymerization of monomers having vinyl unsaturation. The —O—O— bond is split into free radicals which can initiate polymerization or cross-linking of various monomers or polymers. 

As in the case of ceric and vanadium ions, the reaction of organic compounds with Co(III) proceeds via formation of an intermediate complex. Such a complex decomposes and produces free radicals capable of initiating vinyl polymerization. However, only a few reports on Co(IIl) ion-initiated grafting onto cellulose fibers are available [38]. 

This technique is based in the fact that when cellulose is oxidized by ceric salts such as ceric ammonium nitrate Ce(NH4)2(N03)6 free radicals capable of initiating vinyl polymerization are formed on the cellulose. However, the possibility remains that the radical formed is an oxygen radical or that the radical is formed on the C-2 or C-3 instead of the C-6 carbon atom. Another mechanism, proposed by Livshits and coworkers [13], involves the oxidation of the glycolic portion of the an-hydroglucose unit. Several workers [14,15], however, have found evidence for the formation of some homopolymer. In the ceric ion method free radicals are first generated and are then capable of initiating the grafting process [16-18]. 

Azoperoxydic initiators are particularly important due to their capacity to decompose sequentially into free radicals and to initiate the polymerization of vinylic monomers. The azo group is thermally decomposed first to initiate a vinyl monomer and to synthesize the polymeric initiator with perester groups at the ends of polymer chain (active polymer) [31,32]. 

Polyacrylics are produced by copolymerizing acrylonitrile with other monomers such as vinyl acetate, vinyl chloride, and acrylamide. Solution polymerization may be used where water is the solvent in the presence of a redox catalyst. Free radical or anionic initiators may also be used. The produced polymer is insoluble in water and precipitates. Precipitation polymerization, whether self nucleation or aggregate nucleation, has been reviewed by Juba. The following equation is for an acrylonitrile polymer initiated by a free radical ... 

Grafting of Vinyl Monomer on Radiation-Peroxidized Polymer. The polymer is irradiated in the presence of air or oxygen and then immersed in monomer. The peroxides in the irradiated polymer are decomposed by heat or catalysts to form free radicals, capable of initiating graft polymerization. 

Vinyl acetate was polymerized in a free-radical reaction. The initial monomer concentration was 1 mol/liter and its concentration after I h was 0.85 mol/liter. Chloroform was present as a chain transfer agent, with concentrations 0.01 mol/liter at time zero and 0.007 mol/liter after I h. What is the chain transfer constant C in this case ... 

Benzil has frequently been used as a means of generating free radicals in polymerization systems subjected to ultra-violet irradiation 11, 16, 56—58). In studies of the benzil-photoinitiated polymerizations of methyl methacrylate, and vinyl acetate, Melville (16) assumed that initiation was brought about by fragmentation of photoexcited benzil into two benzoyl radicals. However a survey of the photochemistry of benzil 34) indicates that such a cleavage does not in fact take place in solution studies of the products formed on irradiation of benzil in cyclohexane (59), cumene and isopropanol (60) can be rationalised on the basis of initial hydrogen abstraction from solvent by photoexcit i benzil, e.g. 

Free radical polymerization offers a convenient approach toward the design and synthesis of special polymers for almost every area. In a free radical addition polymerization, the growing chain end bears an unpaired electron. A free radical is usually formed by the decomposition of a relatively unstable material called initiator. The free radical is capable of reacting to open the double bond of a vinyl monomer and add to it, with an electron remaining unpaired. The energy of activation for the propagation is 2-5 kcal/mol that indicates an extremely fast reaction (for condensation reaction this is 30 to 60 kcal/mol). Thus, in a very short time (usually a few seconds or less) many more monomers add successively... 

Generalized methods of initiating the polymerization of these monomers have recently been reviewed in detail [9], and were also mentioned briefly earlier in this Chapter. As with vinyl monomers initiation can be efficient and rapid, with the production of a fixed number of active centres. Propagation appears to be much slower, however, and rates of polymerization are comparable to those in free radical addition polymerizations. Techniques such as dilatometry, spectrophotometry etc. are therefore convenient for kinetic investigation of this type of cationic reaction. 

Derivation Polymerization of vinyl chloride by free radicals with peroxide initiator. May be copolymerized with up to 15% of other vinyls. 

Addition polymerization takes place for unsaturated monomers. In the presence of a catalyst, such as a free radical, a pi bond in the monomer is disturbed, and the resulting molecule is. itself, a chemically active free radical. This first step of the process is called initiation. The process may then continue, with the new molecule bonding with additional monomers in the same manner, thus forming a chain. Following this propagation step, free radicals may combine, thus forming a more stable polymer chain. This final step is called termination. Peroxides, such as benzoyl peroxide, are common agents that, when heat is applied, form free radicals that can initiate the polymerization process. An example of addition polymerization is shown below for the monomer vinyl chloride, which forms polyvinyl chloride. 

For example, compositions of 10 ml of a 3.75 gm per 100 ml aqueous solution of sodium dioctylsulfosuccinate (Aerosol OT or Manoxol OT) and 1.25 ml of vinyl acetate with small additions of sodium dihydrogen phosphate, after exclusion of oxygen by at least four degassings, were sealed in glass ampoules. The samples were exposed, at 15°C, to a Co source (100 curie). Conversion was nearly 75% after 30 min. Polymers of MW as high as 10 were formed. The polymers contained trapped free radicals capable of initiating graft polymerization with methyl methacrylate after removal from the radiation source [185]. 

Additional well-defined side-chain liquid crystalline polymers should be synthesized by controlled polymerizations of mesogen-ic acrylates (anionic or free radical polymerizations), styrenes (anionic, cationic or free radical), vinyl pyridines (anionic), various heterocyclic monomers (anionic, cationic and metalloporphyrin-initiated), cyclobutenes (ROMP), and 7-oxanorbornenes and 7-oxanorbornadienes (ROMP). Ideally, the kinetics of these living polymerizations will be determined by measuring the individual rate constants for termination and... 

Vinyl acetate is polymerized in dispersion form using various initiators. Exanples of ionic initiators commonly used for free-radical emulsion polymerizations are ammonium, sodium or potassium persulfate. Topical nonionic hydrophobic initiators include 2,2 -azobis(isobutyronitrile) (AIBN) and benzoyl peroxide. Water-soluble nonionic initiators such as tertiary-butyl hydroperoxide are also employed. The initiator 4,4 -azobis(4-cyanovaleric acid) in its acid state is oil soluble, while neutralization causes it to become water soluble providing for further diversity in initiators. 

An ultraviolet light photoinitiator, diphenyliodonium 9-acridinecarbo-xylate shows different absorption and fluorescence profiles and photochemical properties when irradiated with near-UV light. The anion absorbs the radiation and sensitizes the photolysis of the iodonium cation and formation of a cationic photoinitiator. At the same time, the free radicals thus formed initiated polymerization of vinyl monomers The structure of ion pairs influences the rate and efficiency of the intra-ion-pair electron transfer and the polymerization. 

The most common restoratives comprise a mineral filler mixed with a vinyl monomer (often called a resin), which undergo photoinitiated free radical Chain polymerization. Typical components inclnde the adduct of bis-phenol A and glycidy methacrylate (bis-GMA) or nrethane dimethacrylate (UDMA), camphorquinone initiator (activated by a visible bine light sonrce of 480 nm) and a filler such as zirconia or borosilicate glass, which can comprise 70% by mass of the system and provides the strength . The whole is referred to as a composite . 

Monomers containing rings or double bonds can be polymerized by chain polymerization, which is also known as addition polymerization. (It should be contrasted with Step polymerization.) The chain reaction involves the sequential steps of initiation, propagation and termination. Initiation is the process by which active centres are formed these may be free radicals, anions or cations. The free radical chain polymerization of a vinyl monomer is illustrated below. 

The resulting N-radicals, which are all attached to substrate chains, initiate free-radical graft polymerization of suitable vinyl monomers. The method has been applied to synthesize hydrophilic grafts on poly(ether-urethanes)... 

Anionic polymerization of 1,3-butadiene allows for a wide range of heterotactic vinyl enchainment levels. The structures obtained in hydrocarbon solvents with alkali metal initiators are shown in Table 9. Lithium polybutadiene maintains the lowest vinyl, highest 1,4 content possible via anionic polsrmerization. The microstructure produced via a free-radical emulsion polymerization is provided as a reference. 

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