Use of Cobalt for Radical Initiation

Polymerization, recently reviewed" ) only pertains to the use of cobalt and is not spedfic for the reversible deactivation mechanism vide infra). The term OMRP covets aU metals. The use of this aaonym was initially limited to the reversible deactivation mechanism outlined in Figure 1. However, it has recently been shown that organometallic compoimds may also act as transfer agents for the controlled radical polymerization that follows the degenerative transfer prindple, as outlined later in Section 3.11.4. In this chapter, both these two controlled polymerization methods, which may in certain cases interplay, will be outlined. When addressing each specific mechanism, an additional qualifier will be added to the acronym, OMRP-RT for reversible termination and OMRP-DT for degenerative transfer, whereas the OMRP term will be used in a more general situation. 

Application. Polyesters are cured by free radicals, most commonly produced by the use of peroxides. A wide range of peroxide initiators (qv) are available for use in curing polyesters. Most peroxide initiators are thermally decomposed into free radicals, and the common initiators used at room temperature requke the use of a promoter such as dimethylaniline or cobalt octoate. 

Use of CD30D or methyl tetrahydrofuran solvents to encourage electron capture, resulted in a complex set of reactions for methyl cobalamine. Initial addition occurred into the w corrin orbital, but on annealing a cobalt centred radical was obtained, the e.s.r. spectrum of which was characteristic of an electron in a d z.y orbital (involving the corrin ring) rather than the expected d2z orbital. However, the final product was the normal Co species formed by loss of methyl. Formally, this requires loss of CH3 , but this step seems highly unlikely. Some form of assisted loss, such as protonation, seems probable. 

PEER polymers can be cured with traditional radical initiators such as methyl ethyl ketone (MEK) peroxides and benzoyl peroxide (BPO). Curing can be carried out either at room temperature or at elevated temperature. A PEER polymer containing 30 % maleic anhydride can be cured at room temperature with MEK peroxides in 10 to 60 min, depending on the type of peroxide used (Table 22.2). To cure a PEER resin with MEK peroxides at room temperature, a co-catalyst is needed. The commonly used cobalt naphthenate works very well in this case, while another co-catalyst, dimethyl aniline, is very efficient for the BPO system. 

The cyclization reactions of organocobalt complexes are very useful, and they offer an excellent alternative to the tin hydride method when reduced products are not desired. Most cobalt cyclizations have been conducted with nucleophilic radicals. Precursors are prepared by alkylation of cobalt(I) anions, and are usually (but not always) isolated. One suspects that alkylcobalt precursors should be useful for slow cyclizations because there are no rapid competing reactions that would consume the initial radical (coupling of the initial radical with cobalt(II) regenerates the starting complex). 

Vinylene carbonate is one of the few 1,2-disubstituted ethylenes that is known to undergo facile radical initiated homopolymerization. Initiation may be by oxygen, peroxides or cobalt-60 y-radiation. Such polymers are reportedly useful as coatings and films. Vinylene carbonate also copolymerizes with ethylene under high pressure to yield a material with about 10% vinylene carbonate content. This polymer, when blended with polyvinyl chloride, is suitable for injection molding. 

Steady-state, free-radical methods of LCoD generation were developed.197 The methods are versatile and work for LCo like cobalt porphyrins that are not readily reduced by borohydrides. The use of tribu-tyltin hydride has also been reported.235 The initial approach employed AIBN-cfo. Using this deuterated radical source, cis addition of the resulting LCoD was demonstrated to be the predominant mode of reaction for maleic anhydride and other cyclic olefins such as cyclohexene and 2,5-dihydrofuran. Selectivity depended upon temperature, and this important feature will be discussed below. Unfortunately, AIBN has a limited thermal operating window of 50—70 °C. Lower or higher temperatures would require the nontrivial synthesis of different deuterated azo initiators. To circumvent this problem, a second steady-state free-radical approach was developed. 

Ions of transition metals (homogeneously or in some cases supported on polymers [5]) also effectively catalyze the autoxidation. Salts of cobalt, manganese, iron, copper, chromium, lead, and nickel are used as catalysts that allow the reactions to be carried out at lower temperatures, therefore increasing the selectivity of the oxidation (see, for example, [6]). However, it is more important that the catalyst itself may regulate the selectivity of the process, leading to the formation of a particular product. The studies of the mechanism of the transition metal salt involvement have shown their role to consist, in most cases, of enhancing the formation of free radicals in the interaction with the initial and intermediate species. 

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