Free radical addition features

Like other alkenes, conjugated dienes undergo addition not only by electrophilic reagents but also by free radicals. In free-radical addition, conjugated dienes show two special features they undergo 1,4-addition as well as 1,2-addition, and they are much more reactive than ordinary alkenes. We can account for both features—orientation and reactivity—by examining the structure of the intermediate free radical. 

The key feature of the use of a dormant species may be seen in the following general scheme (Scheme 1.32) that involves complexation of the propagating species by means of a stable nitroxide radical (Hawker et al, 2001). The P -0 bond of the alkoxy amine P -0-NR is thermally labile at the polymerization temperature, so this becomes the site for the insertion of monomer. Propagation then occurs at a rate that is much slower than for a simple free-radical addition reaction since the propagating radical concentration (which is governed by the position of the equilibrium with the alkoxy amine... 

An attractive and powerful feature of free-radical addition sequences is their ability to create multiple carbon-carbon bonds in a single reaction [5], In order for this methodology to be successful in the synthesis of repetitive molecular fragments such as those noted above, two general considerations must be addressed (1) control of stereochemistry, and (2) control of the number of iterative units which become incorporated. 

Figure 8.22 I Addition polymerization often proceeds by a free radical mechanism, featuring initiation, propagation, and termination steps. Conditions are chosen so that the initiation and termination steps occur infrequently. Each propagation step adds one monomer unit to the growing polymer chain.

All attempts to obtain cyclized products from the 4-pentenyl radical using the same conditions under which the 5-hexenyl radical cyclizes readily failed. This was early recognized and confirmed later. Only in special cases, as by the use of vibrationally excited radicals in the gas phase or carbene triplets has cyclization been observed. In these instances, only (Cy5) and no (Cy4) products were obtained. In solution, cyclized products have been observed only from 4-pentenyl radicals possessing special features, e.g., the radical (A ) which results from intermolecular free radical addition to cis cis-1,5-cyclooctadiene (Scheme 15). 

In this section we shall describe some reactions which present many features similar to intramolecular free radical additions, according to the mechanistic probes discussed in the preceding sections. These examples will permit a critical discussion of the use of the free radical intramolecular cycliza-tion probe. 

Work in our laboratory in the past few years has been concerned with the use of acetylene chemistry, both to aid the processing of aromatic heterocyclic polymers and to pro-, vide such materials a method by which they could become tougher and more durable in structural applications. The most attractive feature of the acetylenic carbon carbon triple bond is its capability to undergo various ionic and free radical addition reactions, leading to highly fused thermally stable aromatic systems. This paper will review our work on acety-lene containing aromatic heterocyclic polymers with respect to synthesis and characterization, as well as some already determined mechanical properties as composites and adhesives. 

Table V shows the relative rates for addition at 150° and most of the data comes from Tables III and IV. However two radicals (CH2C1 and CBr3. ) are included in this Table for which no accurate temperature variation is available. The general pattern of results confirms the picture built up from the tables of relative Arrhenius Parameters. The relative rates are the results of direct measurement and therefore probably represent a more accurate summary. A very noticeable feature of Table V is the low rates of addition of heptafluoro-iso-propyl radicals to CHF- and CF2- sites. This strongly suggests that classical "steric hindrance" plays a significant role in free radical addition.

Styrene-butadiene rubber is prepared from the free-radical copolymerization of one part by weight of styrene and three parts by weight of 1,3-butadiene. The butadiene is incorporated by both 1,4-addition (80%) and 1,2-addition (20%). The configuration around the double bond of the 1,4-adduct is about 80% trans. The product is a random copolymer with these general features ... 

We begin our discussion of copolymers by considering the free-radical polymerization of a mixture of two monomers. Mi and M2. This is already a narrow view of the entire field of copolymers, since more than two repeat units can be present in copolymers and, in addition, mechanisms other than free-radical chain growth can be responsible for copolymer formation. The essential features of the problem are introduced by this simpler special case, so we shall restrict our attention to this system. 

In the period 1910-1950 many contributed to the development of free-radical polymerization.1 The basic mechanism as we know it today (Scheme 1.1), was laid out in the 1940s and 50s.7 9 The essential features of this mechanism are initiation and propagation steps, which involve radicals adding to the less substituted end of the double bond ("tail addition"), and a termination step, which involves disproportionation or combination between two growing chains. 

One of the present authors (31) has developed a series of additives which combine the features of both free radical inhibitors and flame retardants of the tetrabromophthalimide or chlorendic imide type with hindered phenol antioxidant structures such as the following compounds ... 

Nitroxyl radicals as alkyl radical acceptors are known to be very weak antioxidants due to the extremely fast addition of dioxygen to alkyl radicals (see Chapter 2). They retard the oxidation of solid polymers due to specific features of free radical reactions in the solid polymer matrix (see Chapter 19). However, the combination of two inhibitors, one is the peroxyl radical acceptor (phenol, aromatic amine) and another is the alkyl radical acceptor (nitroxyl radical) showed the synergistic action [44-46]. The results of testing the combination of nitroxyl radical (>NO ) (2,2,6,6-tetramethyl-4-benzoylpiperidine-l-oxyl) + amine (phenol) in the autoxidation of nonene-1 at 393 K are given here ([>NO ]o + [InH]o = 1.5 x 10 4mol L 1 p02 98 kPa) [44]. 

On the one hand, this particular feature makes it more difficult to distinguish between reactions involving radical cations, free radicals or carbenium ions, but on the other hand the chemist acquires an additional tool to control the course of the intended reaction. Some illustrative examples of cyclization reactions that utilize cleavage of the radical cations, primarily generated by single-electron oxidation, will be given in the following sections. 

Free radical reactions are proving to be synthetically useful altern-atives for producing carbon-carbon bonds. Recently, Stork has shown that vinyl radicals are valuable in ring forming reactions since they place a double bond in a predictable position. Their compatibility with many unprotected functional groups and their ability to form quaternary centers are additional features which make vinyl radical cyclization an attractive synthetic method. 

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