Radical polymerization dialkyl peroxides

Radiation-induced decomposition, 5,6-dihydrothymine, 930 Radiation stress, polymers, 685 Radical polymerization dialkyl peroxides, 707 peroxycarboxylic esters, 697 Radicals... 

Because high temperatures are required to decompose diaLkyl peroxides at useful rates, P-scission of the resulting alkoxy radicals is more rapid and more extensive than for most other peroxide types. When methyl radicals are produced from alkoxy radicals, the diaLkyl peroxide precursors are very good initiators for cross-linking, grafting, and degradation reactions. When higher alkyl radicals such as ethyl radicals are produced, the diaLkyl peroxides are useful in vinyl monomer polymerizations. 

At Van Sickle s conditions of low temperatures and low conversions, branching routes A and B appear to be dominant since there is little alkenyl hydroperoxide decomposition. In our work above 100°C., the branching routes are supported by the nearly linear initial portions at low conversions for alkenyl hydroperoxide and polymeric dialkyl peroxide curves (see Figures 2, 3, and 4). The polymeric dialkyl peroxides formed under our reaction conditions include those formed by the branching mechanism postulated by Van Sickle (routes A and B) and those formed by the reaction of the alkenoxy and hydroxy radicals from alkenyl hydroperoxide thermal decomposition reacting further and alternately with olefin and oxygen (step C). The importance and kinetic fit of the sequential route A to C appears to increase with temperature and extent of olefin conversion owing to the extensive thermal decomposition of the alkenyl hydroperoxides above 100°C. 

Few monomeric radicals are lost by coupling with polymeric radicals when dialkyl peroxides are used as the curative. Also, if the elastomer is properly... 

Few monomeric radicals are lost by coupling with polymeric radicals when dialkyl peroxides are used as the curative. Also, if the elastomer is properly chosen, the scission reaction is not excessive [82-88]. For dicumyl peroxide in natural rubber, the crossUnking efficiency has been estimated at about 1.0. One mole of crosslinks is formed for each mole of peroxide crossUnking is mainly by the coupling of two polymeric radicals [89,90]. One peroxide moiety gives two monomeric free radicals, which react with rubber to give two polymeric radicals that couple to form one crosslink. 

Dialkyl peroxides have the stmctural formula R—OO—R/ where R and R are the same or different primary, secondary, or tertiary alkyl, cycloalkyl, and aralkyl hydrocarbon or hetero-substituted hydrocarbon radicals. Organomineral peroxides have the formulas R Q(OOR) and R QOOQR, where at least one of the peroxygens is bonded directly to the organo-substituted metal or metalloid, Q. Dialkyl peroxides include cyclic and bicycflc peroxides where the R and R groups are linked, eg, endoperoxides and derivatives of 1,2-dioxane. Also included are polymeric peroxides, which usually are called poly(alkylene peroxides) or alkylene—oxygen copolymers, and poly(organomineral peroxides) (44), where Q = As or Sb. 

Methods for detecting whether peroxy compound have been used for cross-linking elastomers have been reviewed. An important application of dialkyl peroxides is as initiators of cross-linking and graft polymerization processes. The success of both processes depends on the ability of the peroxide to produce free radicals and the ability of the free radicals for H-abstraction from a relevant donor substrate. A method for evaluating this ability consists of inducing thermal decomposition of the peroxide dissolved in a mixture of cyclohexane and MSD (225). The free radical X" derived from the... 

Free radicals should initiate polymerization efficiently. Some peroxides such as dialkyl peroxides and peresters tend to abstract hydrogen from the monomer more readily than they react to initiate polymerizations. Consequently, their efficiency as initiators is reduced. 

Dialkyl peroxydicarbonates are used primarily as free-radical initiators for vinyl monomer polymerizations. Dialkyl peroxydicarbonate decompositions are accelerated by certain metals, concentrated sulfuric acid, and amines. Violent decompositions can occur with neat or highly concentrated peroxides. 

The most common chain reaction polymerization is free-radical polymerization. A free radical is merely a molecule with an unpaired electron, which has a tendency to add a supplementary electron in order to form an electron pair which makes it extremely reactive. These molecular complexes could be produced by heat or irradiation, or formed by the addition of a compound, named the initiator (I), for example, dialkyl peroxides (R compounds (R — N = N — R), which are not strictly, catalysts, since they are chemically altered during the reaction [196],... 

Dialkyl peroxides (1), R-O-O-R (R and R are = or primary, secondary, tertiary alkyl, cycloalkyl, aralkyl and heterocyclic radicals Homolytic decompn when heated or irradiated with prodn of free radicals for org synthesis difficult to hydrolyze and reduce rearrangement crosslinking and polymerization polymeric peroxides are thick liqs or amorph wh powds used as polymerization catalysts Primary radicals are unstable, lowest members such as dimet peroxide are shock sens and dangerous expls sensitivity lessens with increasing mw polymeric peroxides (copolymers of olefins and Oj) explode on heating... 

The initiator decomposes thermally to give the initiating radicals R - with an efficiency f. This is typically between 0.1 and 0.8, depending on the type of initiator and the viscosity of the polymerizing medium, since it is reduced by radical recombination. The initiators such as AIBN shown have a higher efficiency than, say, dialkyl peroxides, since they liberate nitrogen to produce two radicals that are more widely separated. The rate coefficient, k, will be described by an Arrhenius temperature dependence. 

Peroxides are the most important initiators for free-radical polymerization. Among them, dialkyl or diaryl peroxides, peroxyesters, diacyl peroxides, and per-oxycarbonates are of particular relevance. For a large number of such peroxides, the temperature and pressure dependence of the decomposition rate coefficient. 

Organic peroxides act through the splitting of the —0—0— bond into free radicals, thereby initiating the polymerization or crosslinking of monomers or polymers. Their exceptionally broad line includes diacyl peroxides, dialkyl peroxides, hydroperoxides, ketone peroxides, peroxyketals, peroxydicarbonates, and peroxyesters. The last two are particularly important in PVC resin manufacture as initiators in the polymerization of vinyl chloride monomer. 

Most frequently the polymerization process is initiated by free radicals obtained through the decomposition of hydroperoxides, alkyl peroxides, dialkyl peroxides, acyl peroxides, carboxylic ester peracids, salts of (tetraoxo)sulphuric acid, hydrogen peroxide, aliphatic azo compounds and bifunctional azobenzoin initiators. The rate of decomposition of different initiators into free radicals depends on their stmcture and on temperature. A measure of the efficiency of the initiator in the pol5mierization process is the half-decomposition period. 

Catalysis (initiation) by a free radical, on the other hand, is fairly conclusive evidence of a radical reaction, provided it is known that the catalyst is indeed a free radical and that it does not have pronounced polar properties as well. Many classes of compound once thought to decompose exclusively into ions or exclusively into radicals are now known to do both. Peroxides are one well-known example, AT-halo-amides are another. Catalysis by benzoyl peroxide probably does indicate a radical reaction since there is no evidence that this particular peroxide tends to give ions even under the most favorable conditions. But many other peroxides are known to decompose into ions, or at least ion pairs, as well as into radicals. The decomposition of azo compounds can also be either radical or ionic, the dialkyl azo compounds tending to give radicals, the diazonium compounds either radicals or ions. Catalysis by a borderline example of an azo compound would therefore be dubious evidence of either kind of mechanism. The initiation of the polymerization of octyl vinyl ether by triphenylmethyl chloride in polar... 

Gregory patented Ultraviolet Curable Resin Compositions Having Enhanced Shadow Curing Properties in 2001. This patent has the same idea as Dixon s patent, that is, use photopolymerization at the surface of a filled resin to trigger a thermal front. Gregory went beyond using peroxide-cured vinyl resins. He used dialkyl iodonium salts, sensitized by a-hydroxy ketones, that produced Lewis acids upon UV irradiation. The Lewis acid triggered cationic polymerization of epoxy resins and vinyl ethers. The heat from the photoinitiated process decomposes peroxides into radicals that react with the iodonium salts to produce Lewis acids. 

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