Precursors for the Copolymerization of Ethene

The relative stability of the two structures seems to be determined by entropic factors. The increase in the size of the olefin substituent [32-35] or of the number of substituents, e.g., cychc olefins such as norbornene [36] or dicyclopentadiene [4], leads to the stabilization of the spiroketal structure, which can survive even in solution. However, a precise determination of the relative stabihty has not been reported. As far as the growing of the copolymer chain is concerned, the mechanistic role, if any, of the spiroketal structure is still not very clear [4, 30]. It is noteworthy that the copolymers are, for the most part, isolated in the spiroketal structure when the copolymerization reaction is regio- and stereoregular. 

Catalyst precursors modified with other types of hgands, such as bis(N-methyl-imidazole) 27 [56], N-heterocyclic carbene chelates 28 [57], and cahx[6]arene-derived 

Previous experiments carried out with ethene using 38 (M = K) [66] had revealed a low catalytic activity, possibly due the rather impure ligand [67]. In the presence of Bronsted acids, such as trifluoroacetic or p-lolucriesulforiic acid 38 (M = Na) showed turnover frequencies up to 7.6x10 mol (moi li) at 70 °C [68]. The anisyl systems 39 and 40, the former with turnover frequencies up to 2x10 rnol (mol h) , were even more active [69]. The productivity of 41 is somewhat lower ( 1.4x 1 o mol (mol h) at 85 °C). Depending on the reaction conditions, the activities of systems 42 and 43 may be even higher than that of 38 [44]. 

Copper catalysts have also been mentioned as active for the copolymerization of ethene [77]. 

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Scheme 8.4 Kinetic equation obtained for the copolymerization of ethene with CO using [Pd(dppp)(CF3COO)2] as the catalyst precursor.

Sen reported that (C6F5)2AlR (2) (generated in situ) is an ethene polymerization catalyst (precursor) [13]. Moreover, the system also catalyzes copolymerization of ethene and propene. This latter fact, in particular, is remarkable, since a j5-branched alkyl (formed after propene insertion) should undergo very easy jS-elimination and hydrogen transfer to ethene, as discussed above. Thus, for this system to work the intrinsic chain transfer barriers of (QF5)2AlR should be much higher than those of trialkylaluminium. 

As a consequence of the living nature of the copolymerization wifh fhis catalyst, palladium-capped block copolymers of norbornene and ethene as well as of norbornene, ethene, and styrene were synthesized [61]. Higher activities (up to ten times higher) were observed for a series of oxazohne-phosphine complexes (e.g., 31). Several complexes, modified with bisphosphine monooxide and monosulfide ligands (Scheme 8.8, 32 and 33), were also used as catalysts precursors. The best reported turnover frequency is 0.6x10 mol (molh) at 80°C [62, 63]. A slightly lower activity was observed for fhe ketophosphine containing catalyst precursor 34 [64]. The activity of catalyst precursors 35, 36, and 37 is even lower [65]. 

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