Polymeric diene complexes

The reaction of the polymeric diene complexes, [RuCl2(diene)]u [diene = cod (1,5-cydooctadiene), nbd (2,5-norbomadiene)], with allylic Grignard reagents gives white bis(allylic) complexes, Ru(allyl)2(diene) (allyl = C3H5, 2-methylallyl), which contain asymmetrically bonded allyl ligands (Eq. 5.2) [10],... 

Table 10. Polymerization of ethylene catalyzed by niobium-diene and tantalum-diene complexes/ MAOa...

The electron-rich diene system reduced the electrophilicity of the catalyst and favored the polymerization of ethylene at the expense of the propylene. The mast important role of diene complexes is this of shifting the catalyst ionicity toward more anionic character. 

This chapter illustrates that electron-rich transition metal-diene complexes can couple with carbon electrophiles and, thereby, provide unusual methods for carbon-carbon bond formation. These procedures are of interest from a synthetic viewpoint since normally uncomplexed dienes or polyenes are not reactive toward weak carbon electrophiles or, with strong electrophiles, undesirable reactions such as polymerization occur. Furthermore, the metal-mediated route often results in desirable regio- and/or stereo-selectivity. Important to the utility of these methods is the ability to free the organic ligand from the metal. In most instances efficient oxidative procedures have been developed for such cleavage reactions. 

Copolymerization of styrene with diolefins provides further support that monomer coordinates with the cationic site prior to addition. Korotkov (218) showed that in homopolymerizations styrene is more reactive than butadiene, but in copolymerization the butadiene reacted first at its homopolymerization rate and when it was exhausted the styrene reacted at its homopolymerization rate. This interesting result has been duplicated by Kuntz (245) and analogous results have been obtained by Spirin and coworkers (237) for the styrene-isoprene system. Presumably, the diene complexes more strongly than styrene with the lithium and excludes styrene from the site. That the complex occurs at a cationic site, rather than at the anion or the metal-carbon bond, is indicated by the fact that dienes form more stable complexes than styrene with Lewis acids (246). It should be emphasized that selective monomer coordination is not the only factor influencing reactivities in copolymerizations. Of greatest importance are the relative reactivities of the different polymer anions. The more resonance-stabilized anion is more readily formed and is less reactive for polymerizing the co-monomer. 

Stereospecific emulsion polymerization of butadiene has been achieved in the presence of soluble transition metal salts 350, 351). Polymer microstructure was controlled by varying the transition metal ion and its ligands. Although the initiation mechanism has not been determined, it is most likely to be of the coordinated radical type with steric control arising from the transition metal-diene complexes. 

They are based on various metals. Such as zirconium, complexed with cyclopentadienide anions. This type of compound is called a zirconocene and is used with organoalu-minum to make highly regular polymers. The catalyst has the ability to flip back and forth from making atactic to isotactic polypropylene in the same polymerization. The alternating tacticity of the polymer breaks up the crystallinity of the chains and yields an elastomer. Metallocene catalysts are currently very expensive and cannot yet polymerize dienes such as butadiene, so they have only enjoyed limited commercial success in elastomers. However, this is one of the most intense fields of polymer research and many new product breakthroughs are expected in the near future. 

Because of the stability of iron tricarbonyl diene complexes, conjugated dienals are protected from polymerization when complexed, while other reactions can be carried out at the aldehyde functionaUty. A number of synthetically attractive nucleophilic transformations of the aldehyde can be performed on these complexes. These include, aldol reactions, Michael additions, reactions with organozinc, -silicon, -boron, and -tin... 

Diene complexes of the so-called constrained geometry monocyclopentadienyl-amido titanium complexes have also been prepared. Interest in these molecules stems from their utility as catalyst precursors in olefin polymerization... 

Both of these complexes can be used in ADMET polymerizations at temperatures up to approximately 55 °C, although decomposition certainly occurs over the time scale of a typical ADMET polymerization (days). A structure-reactivity study was performed on complexes 1 and 2 that revealed a number of features of these complexes [68]. Notably, 2 will polymerize dienes containing a terminal and a 1,1-disubstituted olefin, but never produces a tetrasubstituted olefin. One of the substituents of the 1,1-disubstituted olefin must be a methyl group. In contrast, complex 1 will not react with a 1,1-disubstituted olefin. The tungsten complex is more reactive towards internal olefins than external olefins [23, 63] indicating that secondary metathesis, or trans-metathesis, probably dominates the catalytic turnovers in ADMET with complex 1. 

The value of the metal complexation results from control of the reaction, rather than any activation, Lewis acids being excellent catalysts for diene polymerization. Friedel-Crafts acylations of diene complexes have been used for the preparation of dienes, with decomplexation following carbonyl reduc-tion. 5 Decomplexation to afford dienones has been less explored. The intermediate cationic o -complex on treatment with triethyl phosphite or triphenylphosphine affords metal-free. y-unsaturated phospho-nates or phosphonium salts (Scheme 19). The initial s-cis conformation of the diene fragment of the... 

Trivalent lanthanocene complexes that are isoelectronic (14e, (P) to cationic group 4 metallocenes are also active for olefin polymerization, although their activity is much less than that of Ti and Zr [28,29]. Mashima et al. have reported monocyclopentadienyl rf-diene complexes of tantalum and niobium that are isoelectronic to group 4 metallocenes. These complexes catalyze ethylene polymerization, albeit with low activity [30]. 

Process (i) is a unimolecular process, while (ii) is a bimolecular process and the rate depends on monomer concentration. Frequent chain-transfer reactions bring about low molecular weight polyolefins. If chain transfer is negligible or very slow, the polymerization can be living , as observed in group 5 metallocene-diene complexes [30, 31]. j3-Methyl elimination is also reported in bis(pentamethylcyclopentadienyl)metallocene catalysts [32,33]. 

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