Cocatalysis

The above scheme cannot explain some experimental facts such as effect of cocatalysis, chain transfer termination, etc. 

Medvedev, the Grand Old Man of polymer science in the USSR, to communicate it to the Doklady [33]. I had no doubt, such were the ethics of science at the time, that he would do this, although we were claiming a disproof of his own theory. Of course he did, and when I met him subsequently at a conference he dismissed my thanks briefly with the remark But that was the natural thing to do. That is what a scientist owes to his colleagues . As the Romans used to say O si sic omnes (if only everyone were like that ). The pre-cocatalysis theories were reviewed by me in 1949 [34]. Various other theories of direct initiation proposed subsequently by various authors have also proved to be untenable in the face of hard evidence. 

Sims (40) has studied the effects of water on PF5 initiation of THF polymerization (Section IIIB2b). Possibly, in addition to cocatalysis and destruction of catalyst, his results are complicated by transfer. 

Fig. 7. Schematic illustration of cocatalysis processes group transfer and ligation reactions occuring within the supramolecular complex formed by the binding of substrates to the two macrocyclic subunits of a macrotricyclic coreceptor molecule.

Fig. 8. Cocatalysis pyrophosphate synthesis by phosphoryl transfer mediated by macrocycle 38 via the phosphorylated intermediate 81 [5.12].

Bond-making processes such as those described above extend supramolecular reactivity to cocatalysis, mediating synthetic reactions within the supramolecular entities formed by coreceptor molecules. 

Another version of the Conia-ene reaction is the focus of this research. Here the reaction of y-alkynic (3-ketoesters was tested.48 The screening resulted in a copper / silver cocatalysis system. It remains unclear why the authors did not at least run some reactions with gold catalysts, too, especially since they even cite the work on the gold-catalyzed Conia-ene reaction in the introduction (Section 12.1). 

In order to generate the active Nd-species the Nd precursors are activated by cocatalysts or activators . Some aspects about cocatalysis were already addressed in the previous Sects. 2.1.1.1-2.1.1.10. In this work qualitative aspects on cocatalyst components and cocatalysis are summarized from a general point of view. Quantitative aspects and the impact of cocatalysts on polymerization rate and polymer properties are reviewed in Sect. 2.1.4. 

In parallel the cocatalysis of cobalt compounds with A7-hydroxyphthalimide 398 was developed extensively for oxidative radical reactions [434]. Ishii and colleagues showed that these conditions can be used in radical additions (Fig. 92). Ketyl radicals generated by hydrogen abstraction from secondary alcohols 396 add to a.p-unsaturated esters 397 affording 2,4-dihydroxy esters, which cyclized to lactones 399 under the reactions conditions [435]. Using 0.1 mol% of Co(OAc)2, 1 mol% of Co(acac)3, and 10 mol% of 398 under 1 atm of oxygen, the cyclic products 399 were isolated in 14—90% yield. As observed for similar reactions, Co(III) alone needs an induction period (see below). 

The problem of cocatalysis in Friedel-Crafts reactions in general and in cationic polymerizations in particular is still unresolved. In 1936 Ipatieff and Grosse (30) made the significant observation that pure ethylene and aluminum chloride will not polymerize except in the presence of added water or hydrogen chloride. This can be regarded as the discovery of cocatalysis. Ten years later British scientists started a systematic investigation in this area and proposed the concept of cocatalysis which is now generally accepted (31—34). Early development was excellently surveyed (35) and will not be discussed here. 

The study of cocatalysis in cationic polymerizations is extremely complicated by the fact that Lewis acids can participate in a variety of ill defined reactions (91). Satchell (91, 92) regards the hydrogen exchange reaction between Bronsted acids and aromatics catalyzed by Lewis acids as a prototype for Friedel-Crafts catalysis. He postulates that cocatalytic efficiency is determined by the stability of the complex anion B -f HX + SnCl4 - BH SnCl4Xe. A simple enhancement of conventional acidity by B + HX -> BH Xe as proposed by Plesch (93) and Russel (94) is considered to be unimportant. The stability of the complex and its catalytic activity are determined by the electron density... 

Evidence for this suggestion was obtained by NMR spectroscopy of reacting mixtures which showed that triphenylmethane formation was almost eliminated and a trityl ether unit formed when the reaction systems contained propylene oxide. While this interpretation is consistent with most of the experimental work published, Kuntz and Melchior (25) also report that cocatalysis by propylene oxide can be observed when the latter is added several hours after the start of polymeriza-... 

Cocatalysis with copper salts has proven its efficiency in the enhancement of the reaction rate, particularly in polar solvents. Discovered by Liebeskind and Fengl , and largely exploited, the cocatalytic effect of Cu(I) depends on the reaction conditions in two... 

In addition to protonic acids, Lewis acids are the most common initiators of carbocationic polymerizations. Two mechanisms are possible. Direct initiation is rare and usually slow. The more prevalent mechanism is by cocatalysis in binary systems, with the Lewis acid acting as a coinitiator or catalyst rather than as initiator. Cationating or protonating species are the true initiators, which are therefore the species incorporated at the polymer s end group. The most common initiator is adventitious water in insufficiently dried systems. Thus, mechanistic studies should be performed under stringently dry conditions or in the presence of proton traps such as hindered pyridines. In addition to water, the protonating reagent may be an alcohol, carboxylic acid, amine, or amide [Eq. (28)]. 

In the absence of cocatalysis zwitterion formation is a reasonable postulate. 

This is often verified both in systems giving direct initiation and those operating through cocatalysis. 

The use of labelled molecules to clarify a reaction mechanism has often been -plied to initiation of cationic polymerisation. The most successfiil results have been obtained by Russell et al. in the context of water cocatalysis in the polymerisation of isobutene by stannic chloride. 

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