Cobalt complexes catalytic chain transfer

Enikolopyan et al.til found that certain Co11 porphyrin complexes (eg. 87) function as catalytic chain transfer agents. Later work has established that various square planar cobalt complexes (e.g. the cobaloximes 88-92) are effective transfer agents.Ij2 m The scope and utility of the process has been reviewed several times,1 lt>JM ns most recently by Hcuts et al,137 Gridnev,1 3X and Gridnev and Ittel."0 The latter two references1provide a historical perspective of the development of the technique. 

The most important side reactions are disproportionation between the cobalt(ll) complex and the propagating species and/or -elimination of an alkcnc from the cobalt(III) intermediate. Both pathways appear unimportant in the case of acrylate ester polymerizations mediated by ConTMP but are of major importance with methacrylate esters and S. This chemistry, while precluding living polymerization, has led to the development of cobalt complexes for use in catalytic chain transfer (Section 6.2.5). 

Thus these cobalt complexes function as extremely efficient catalytic chain transfer agents, with each polymer chain having an olefinic end group. 

Finally, this mechanistic section cannot possibly be concluded without referring to catalytic chain transfer (CCT) this is the most probable side reaction of CMRP,and involves hydrogen abstraction by the cobalt complex, with the release of cobalt hydride ([Co "—Hj) and imsaturated polymer chains (Equation 4.4). Although CCT is mainly used for the preparation of macromonomers, in a process referred to as catalytic chain transfer polymerization (CCTP) [25], the reaction must be minimized in CMRP. 

Catalytic Chain Transfer. A highly useful variant of chain transfer was discovered in the 1970-1980s in the Soviet Union (216). A number of reviews have been published in recent years (217-221) on this synthetic method which has acquired the nomenclature of either catalytic chain transfer (CCT) or special chain transfer (SCT). The most commonly adopted catalysts are based on low spin cobalt macrocycles, although other metal-containing complexes have also been suggested in the patent and scientific literature. Some typical catalyst structures are shown as 12 and 13. 

In some cases where a reaction involving a radical species occurred within cobalt porphyrin complexes, it has been possible to trap transient cobalt porphyrin hydride species. This was indeed observed during the synthesis of organocobalt porphyrin that resulted from the reaction of cobalt(n) porphyrin and dialkylcyanomethylradicals with alkenes, alkynes, alkyl halides, and epoxide. A transient hydride porphyrin complex was also involved in the cobalt porphyrin-catalyzed chain transfer in the free-radical polymerization of methacrylate. The catalytic chain transfer in free-radical polymerizations using cobalt porphyrin systems has been extensively investigated and will not be treated in this section. Gridnev and Ittel have published a comprehensive overview of the catalytic chain transfer in free-radical polymerizations. ... 

Olefin-terminated poly(methyl methacrylate) (PMMA) 4 can be prepared by free radical polymerization of MMA using cobalt complexes as catalytic chain transfer agents (3). 

Most catalytic systems used in industrial production yield polyalkenes with very long chains which are unsuitable for current processing procedures and applications. For regulating molecular mass, H2 is preferred to organometal-lics. Hydrogen is not a suitable transfer agent in diene polymerizations on cobalt complexes [67] because it reduces the Co (II) zr-allylic centre to inactive Co (I) particles. 

In terms of the influence of the olefin structure, CCT activity is especially high for methacrylates and styrene, while acrylates tend to yield more efficient OMRP trapping by cobalt complexes. A study of H transfer from CpCr(CO)3H to a variety of olefins yields the rate constants and relative rates are shown in Figure 30, but no equivalent information is apparently available on the catalytically more relevant H transfer from the chain-carrying radical to the transfer agent, which is usually the rate-determining step in CCT. 

To achieve low radical concentrations, most radical reactions are traditionally performed as chain reactions. Atom or group transfer reactions are one of the two basic chain modes. In this process the atom or group X is the chain carrier. A metal complex can promote such chain reactions in two ways. On one hand, the catalyst acts only to initiate the chain process by generating the initial radical 29A from substrate 29 (Fig. 10). This intermediate undergoes the typical radical reactions, such as additions or cyclizations leading to radical 29B, which stabilizes to product 30 by abstracting the group X from 29. A typical example is the use of catalytic amounts of cobalt(II) salts in oxidative radical reactions catalyzed by /V-hydroxyphthalimide (NHPI), which is the chain carrier [102]. 

Hiatt et a/.34a-d studied the decomposition of solutions of tert-butyl hydroperoxide in chlorobenzene at 25°C in the presence of catalytic amounts of cobalt, iron, cerium, vanadium, and lead complexes. The time required for complete decomposition of the hydroperoxide varied from a few minutes for cobalt carboxylates to several days for lead naphthenate. The products consisted of approximately 86% tert-butyl alcohol, 12% di-fe/T-butyl peroxide, and 93% oxygen, and were independent of the catalysts. A radical-induced chain decomposition of the usual type,135 initiated by a redox decomposition of the hydroperoxide, was postulated to explain these results. When reactions were carried out in alkane solvents (RH), shorter kinetic chain lengths and lower yields of oxygen and di-te/T-butyl peroxide were observed due to competing hydrogen transfer of rm-butoxy radicals with the solvent. 

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