Benzoyl peroxide radical polymerization

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. 

Thus values of az can be calculated for the benzoyl peroxide-initiated polymerizations from the known values of az (persulfate-initiated polymerizations), and the ratios of average particle volumes and polymerization rates. These values of az may then be substituted in Equations 3 and 4 to give values for f/k and fk which may then be combined to give values for f and The results of these calculations are shown (Table VIII-A). The values of az are all considerably less than 2. From Figure 13 for the case where the radicals are generated in, or enter,... 

WeVe seen only a few examples of radical reactions because they re 1< common than polar reactions. Those we have studied can be dassified as eith radical addition reactions or radical substitution reactions. Radical additio-such as the benzoyl peroxide-catalyzed polymerization of alkene monome (Review Table 1, reaction Ij), involve the addition of a radical to an unsaturated substrate. The reaction occurs through three kinds of steps, all of whicli involve odd-electron species (1) initiation, (2) propagation, and (3) termination. 

The reactivity of the initiating radicals toward the backbones can vary and this can also change the efficiency of grafting. Benzoyl peroxide initiated polymerizations of methyl methacrylate monomer, for instance, in the presence of polystyrene yield appreciable quantities of graft copolymers. Very little graft copolymers, however, form when di-r-butyl peroxide initiates the same reactions. Azobisisobutyronitrile also fails to yield appreciable quantities of graft copolymers. This is due to very inefficient chain transferring to the polymer backbones by r-butoxy and isobutyronitrile radicals. 

Free radical mechanisms have been inferred from product distributions in many pyrolyses and photolyses of alkyl transition metal compounds. For example, phenyltitanium(IV) alkoxides decompose at 80° C to give Ti(III) alkoxides, benzene, and biphenyl. The presence of phenyl radical was supported by the fact that C6HsTi(OBu)3 polymerized styrene at a rate comparable to the benzoyl peroxide-initiated polymerization 131). The formation of coupling products, as in the following reactions, has been taken to indicate a radical mechanism. 

Radical polymerization of vinyl compounds was performed under UV irradiation through slotted openings of the cavity shown in Fig. 48 b, and ESR spectrum measurements were made immediately after irradiation. The ESR spectra in Fig. 51 are for benzoyl-peroxide initiated polymerizations of MMA, isobutyl methacrylate (IBMA), benzyl methacrylate (BzMA), and TPMA at 30 °C. No ESR spectrum was obtained without an initiator, while an ESR spectrum of the initiating radical was found in the absence of methacrylates. 

Polymerization of styrene is carried out under free radical conditions often with benzoyl peroxide as the initiator Figure 1111 illustrates a step m the growth of a poly styrene chain by a mechanism analogous to that of the polymerization of ethylene (Sec tion 6 21)... 

When initiator is first added the reaction medium remains clear while particles 10 to 20 nm in diameter are formed. As the reaction proceeds the particle size increases, giving the reaction medium a white milky appearance. When a thermal initiator, such as AIBN or benzoyl peroxide, is used the reaction is autocatalytic. This contrasts sharply with normal homogeneous polymerizations in which the rate of polymerization decreases monotonicaHy with time. Studies show that three propagation reactions occur simultaneously to account for the anomalous auto acceleration (17). These are chain growth in the continuous monomer phase chain growth of radicals that have precipitated from solution onto the particle surface and chain growth of radicals within the polymer particles (13,18). 

The reaction rate of fumarate polyester polymers with styrene is 20 times that of similar maleate polymers. Commercial phthaHc and isophthaHc resins usually have fumarate levels in excess of 95% and demonstrate full hardness and property development when catalyzed and cured. The addition polymerization reaction between the fumarate polyester polymer and styrene monomer is initiated by free-radical catalysts, commercially usually benzoyl peroxide (BPO) and methyl ethyl ketone peroxide (MEKP), which can be dissociated by heat or redox metal activators into peroxy and hydroperoxy free radicals. 

Benzoyl chloride is an important benzoylating agent. In this use the benzoyl radical is introduced into alcohols, phenols, amines, and other compounds through the Friedel-Crafts reaction and the Schotten-Baumaim reaction. Other significant uses are in the production of benzoyl peroxide [94-56-0], benzophenone [119-61-9], and in derivatives employed in the fields of dyes, resins, perfumes, pharmaceuticals, and as polymerization catalysts. 

The reaction between benzoyl peroxide and A,A-dimethylaniline has been the subject of many examinations over the years. The following mechanism of initiation is fairly well accepted in the polymerization of styrene. It seems likely that a similar mechanism is followed for other free-radical polymerizations (Scheme 5). 

In the presence of radical initiators such as benzoyl peroxide (BPO), azobisisobutyronitrile (AIBN), persulfates (S208 ), etc., grafting of vinyl monomers onto polymeric backbones involves generation of free radical sites by hydrogen abstraction and chain transfer processes as described below ... 

I Initiation The polymerization reaction is initiated when a few radicals are generated on heating a small amount of benzoyl peroxide catalyst to break the weak 0-0 bond. A benzoyloxy radical then adds to the C=C bond of ethylene to generate a carbon radical. One electron from the C=C bond pairs up with the odd electron on the benzoyloxy radical to form a C-O bond, and the other election remains on carbon. 

The polymerization of vinyl monomers on the surface of silica can be induced also by free radical initiators such as azo-bis-isobutyronitrile (AIBN), di-tert-butylperoxide, benzoyl peroxide etc. The selection of initiator type and method of its introduction in polymerizable systems are determined by the nature of monomers and tasks of investigations. Usually, the following procedures are used ... 

Vinyl Acetate CH3COOCH=CH2 OH compds, HCN, Halides, Halogens, Mer-cap tans, Amine, Silanes Oxygen Vap in Air 2.6 to 13.4% > Ambient > Ambient Inhibitor—Methyl Ether of Hydroquinone or 3-5ppm Diphenylamine. Store in a dry, cool place shield from light impurities 20.9-21.5 402 427 Free-radical polymerization initiated by Benzoyl Peroxide... 

Few CIDNP studies on free radical reactions with olefins and related unsaturated molecules have been reported, and relatively little chemically useful information seems to have been derived, despite the potential relevance in polymerizing systems. Thus CIDNP has been reported in the decomposition of benzoyl peroxide in the presence of styrene and... 

Slow free radical polymerization at 80°C in presence of large amounts of peroxide (benzoyl) ... 

An impressive number of substances capable of generating free radicals have been shown to be potent accelerators for the polymerization of typical vinyl monomers such as styrene, methyl methacrylate, butadiene, and vinyl acetate. The most commonly employed initiators (often referred to inaccurately as catalysts) are organic peroxides, such as benzoyl peroxide. These are known to decompose slowly at temperatures of 50° to 100°C with release of free radicals as follows... 

Isopropenyl acetate and allyl chloride behave similarly. In the polymerization of the latter monomer degradative chain transfer occurs more readily by removal of the chlorine atom to yield the unsubstituted allyl radical CH2—CH—CH2, which manages to add monomer occasionally. This is indicated by the formation of about three polymer molecules, having an average degree of polymerization of six units, for each molecule of benzoyl peroxide decomposing. 

An alternative method of preparing the saturated cyclic amines via cyclopolymerization of diallylamine or diallylammonium chloride was unsuccessful. Common free radical initiators such as 2,2 -azobisisobutyronitrile, ammonium persulfate, benzoyl peroxide were found to be ineffective. Several procedures reported in the literature were followed, and unfortunately all of them have resulted only a small amount of low molecular weight oligomers. Further research for polymerization conditions and types of initiation is still required. 

Polymerization of vinyl chloride occurs through a radical chain addition mechanism, which can be achieved through bulk, suspension, or emulsion polymerization processes. Radical initiators used in vinyl chloride polymerization fall into two classes water-soluble or monomer-soluble. The water-soluble initiators, such as hydrogen peroxide and alkali metal persulfates, are used in emulsion polymerization processes where polymerization begins in the aqueous phase. Monomer-soluble initiators include peroxides, such as dilauryl and benzoyl peroxide, and azo species, such as 1,1 -azobisisobutyrate, which are shown in Fig. 22.2. These initiators are used in emulsion and bulk polymerization processes. 

The accepted kinetic scheme for free radical polymerization reactions (equations 1-M1) has been used as basis for the development of the mathematical equations for the estimation of both, the efficiencies and the rate constants. Induced decomposition reactions (equations 3 and 10) have been Included to generalize the model for initiators such as Benzoyl Peroxide for... 

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