Über die Autorin bzw. den Autor

E.T. DENISOV is Principal Researcher in the Institute of Problems of Chemical Physics at the Russian Academy of Sciences.

T.G. DENISOVA and T.S. POKIDOVA are Senior Research Fellows in the Institute of Problems of Chemical Physics at the Russian Academy of Sciences.

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A comprehensive, one-of-a-kind reference on free radical initiators

Free radical initiators–chemical molecules which easily decompose into free radicals–serve as reactive intermediates in synthetic methodologies such as organic and polymer synthesis as well as in technological processes, oligomerization, network formation, and kinetic research. The Handbook of Free Radical Initiators presents an up-to-date account of the physicochemical data on radical initiators and reactions of radical generation.

The authors divide their study into three sections. Part I is devoted to initiators of free radicals; coverage includes different mechanisms of decomposition of initiators, the cage effect that accompanies decomposition in liquids and solid polymers, and methods of study of initiator decomposition. Part II addresses bimolecular reactions of free radical generation, including those of retrodisproportionation, of paraffins and olefins with haloid molecules, and bimolecular and trimolecular reactions of RH with dioxygen, ozone, and nitrogen dioxide. Part III collects data on the mechanism and rate constants of reactions of free radicals formed from initiators. Individual chapters include:

• Dialkyl Peroxides and Hydroperoxides
• Diacyl Peroxides, Peresters, and Organic Polyoxides
• Azo-Compounds
• Bimolecular Reactions of Free Radical Generation by Ozone, Dioxygen, Hydroperoxides, and Haloid Molecules
• Free Radical Abstraction Reactions
• Free Radical Addition Reactions
• Free Radical Recombination and Disproportoination Reactions

Professionals and academic researchers in chemical engineering, pharmaceuticals, biotechnology, plastics, and rubbers will find the Handbook of Free Radical Initiators to be a distinguished, vital resource.

Aus dem Klappentext

A comprehensive, one-of-a-kind reference on free radical initiators

Free radical initiators–chemical molecules which easily decompose into free radicals–serve as reactive intermediates in synthetic methodologies such as organic and polymer synthesis as well as in technological processes, oligomerization, network formation, and kinetic research. The Handbook of Free Radical Initiators presents an up-to-date account of the physicochemical data on radical initiators and reactions of radical generation.

The authors divide their study into three sections. Part I is devoted to initiators of free radicals; coverage includes different mechanisms of decomposition of initiators, the cage effect that accompanies decomposition in liquids and solid polymers, and methods of study of initiator decomposition. Part II addresses bimolecular reactions of free radical generation, including those of retrodisproportionation, of paraffins and olefins with haloid molecules, and bimolecular and trimolecular reactions of RH with dioxygen, ozone, and nitrogen dioxide. Part III collects data on the mechanism and rate constants of reactions of free radicals formed from initiators. Individual chapters include:

• Dialkyl Peroxides and Hydroperoxides
• Diacyl Peroxides, Peresters, and Organic Polyoxides
• Azo-Compounds
• Bimolecular Reactions of Free Radical Generation by Ozone, Dioxygen, Hydroperoxides, and Haloid Molecules
• Free Radical Abstraction Reactions
• Free Radical Addition Reactions
• Free Radical Recombination and Disproportoination Reactions

Professionals and academic researchers in chemical engineering, pharmaceuticals, biotechnology, plastics, and rubbers will find the Handbook of Free Radical Initiators to be a distinguished, vital resource.

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Handbook of Free Radical Initiators

John Wiley & Sons

Chapter One

1.1 INTRODUCTION

A lot of organic molecules, dealing with technique, technological processes, and organic synthesis, are stable at moderate (~300-400 K) and elevated (>400 K) temperatures. Atoms of these compounds are connected by sufficiently strong chemical bonds with bond dissociation energy (BDE) ~350-500 kJ [mol.sup.-1]. Radical initiators are molecules bearing one or several weak bonds with BDE ~100-200 kJ [mol.sup.-1]. When the temperature of the reaction is sufficiently high, the initiator decomposes with homolysis of the weakest bond and produces free radicals. These free radicals initiate a chain or nonchain free radical reaction.

What are the factors that influence the BDE of any chemical bond? First, there are atoms forming the bond. Here are a few examples of the types of bonds in various compounds:

[ILLUSTRATION OMITTED]

The following bonds have sufficiently low values of BDE:

[ILLUSTRATION OMITTED]

Organometallic compounds have weak metal-carbon bonds:

[ILLUSTRATION OMITTED]

Atoms surrounding the atom with the bond being split also influence the BDE. Here are a few examples:

[ILLUSTRATION OMITTED]

A [pi]-bond in the [alpha]-position has a strong influence on the dissociating bond. This influence is clearly seen for several alkylaromatic hydrocarbons:

[ILLUSTRATION OMITTED]

This dependence is the result of stabilization of the formed radical due to the interaction of an unpaired electron with [pi]-electrons of the benzene ring.

Different mechanisms of free radicals formation as a result of initiator decomposition are known. Most initiators decompose with dissociation of the weakest bond, for example,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

Initiators that decompose with simultaneous dissociation of two or more bonds are known, for example,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

This decay is known as concerted fragmentation (see Section 1.3).

Some ortho-substituted benzoyl peroxides decompose with formation of unstable intermediates. These intermediates are the result of formation of an additional bond between the oxygen atom of the benzoyloxy radical and ortho substituent, for example,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

This decay was called anchimerically assisted peroxide cleavage (see Section 1.4).

The decomposition of initiators very often proceeds homolytically with dissociation of the molecule to free radicals only. However, there are compounds that decay simultaneously to free radicals and molecular products. For example, peresters decay to free radicals with dissociation on the O-O bond and their isomerization to aryloxyester proceeds in parallel:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

Free radicals formed from the initiator react with the reactant or recombine. Free radicals formed from the initiator or reactant also can react with the initiator. If this reaction proceeds intensely, the initiator is decomposed by free radicals, and this decreases its effectiveness as an initiator (see Section 1.5).

1.2 NONCONCERTED UNIMOLECULAR DECOMPOSITION

Before decomposition, a molecule of the initiator (I) should be activated. Its activation is the result of collisions of the initiator molecule with other molecules M in the gas or liquid phase. The energized molecule may undergo deenergization by collision with a normal molecule, or it may undergo a unimolecular reaction to form products. These three processes are quite distinct and the situation may be represented as:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where [I.sup.*] is the activated molecule and [I.sup.#] is the activated complex. The activated molecule [I.sup.*] passes through the top of the activation barrier. The energized molecule [I.sup.*] has acquired all the energy it needs to become the activated molecule [I.sup.#]. The full description of the activation process and reaction is given by the Rice-Ramsperger-Kassel-Marcus (RRKM) theory of unimolecular reactions. This theory describes the dependence of the rate constant for unimolecular decay of molecules on gas pressure. When the pressure is growing, the rate constant of decomposition [k.sub.d] -> [k.sub.[infinity]], and

[k.sub.[infinity]] = [k.sup.#][e.sup.-E/RT] (1.1)

where [k.sub.#] is the rate constant of [I.sup.#] decay. The expression for initiator decay in solution, where the frequency of molecular collisions is extremely great, is the same. According to transition state theory, the rate constant of unimolecular reactions at high pressure is the following:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1.2)

When a polyatomic molecule, for example, peroxide ROOR, decomposes to two free radicals RO, the following changes in the energy distribution are observed:

1. One stretching vibration along the O-O bond disappears,

2. One inner rotation of the O-O bond disappears,

3. Two C-O-O angles vibrations disappear.

As a result, the activation entropy of unimolecular decomposition [DELTA]S.sup.# > 0 and the preexponential factor [A = eRT (Lh).sup.-1] exp([DELTA][S.sup.#]/R)] is sufficiently higher than eRT (Lh).sup.-1 [approximately equal to] [10.sup.13] [s.sup.-1]. For many unimolecular reactions [DELTA][S.sup.#] [approximately equal to] 20-80 J[mol.sup.-1] [K.sup.-1].

Due to the elongation of the dissociating bond (e.g. O-O in peroxide), the volume of the transition state [I.sup.#] > I. As a result, the difference in the volumes V ([I.sup.#]) - V (I) = [DELTA][V.sup.#] is positive. The study of decomposition of initiators with one bond dissociation under pressure gives evidence that [DELTA][V.sup.#] is positive and helps us to evaluate [DELTA][V.sup.#] according to the following dependence of k on pressure p:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1.3)

where [k.sub.0] = k at p = 0 and b = 9.2 [10.sup.-9] [Pa.sup.-1]. The value of [DELTA][V.sup.#] depends on the pressure p:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1.4)

1.3 CONCERTED FRAGMENTATION OF INITIATORS

Peroxides have the weak O-O bond and usually decompose with dissociation of this bond. The rate constants of this decomposition of ROOR into RO radicals demonstrates a low successibility of the BDE of the O-O bond to the structureof the R fragment (see Chapter 4). Bartlett and Hiat studied the decay of many peresters and found that rate constants of their decomposition covered a range >[10.sup.5] [s.sup.-1]. The following mechanism of decomposition was proposed:

[RC(O)OOR.sup.1] [right arrow] R + C[O.sub.2] + [R.sup.1]O

parallel with a simple dissociation of one O-O bond. This decay of the molecule to fragments was named concerted fragmentation. The energy needed for the activation of the molecule is concentrated simultaneously on the two bonds: R-C(O) and O-O. Among other products, C[O.sup.2] is formed as the result of perester fragmentation. The formation of the [pi] bond of formed carbon dioxide partially compensates for the energy of dissociation of the C-C and O-O bonds of perester. The decay of perester into four fragments is known:

[Me.sub.3]COOC[Me.sub.2]C(O)OOC[Me.sub.3] [right...