1,4-Butadiene, -metal coordination compounds

Cycloaddition reactions have also been used to prepare a range of [CpCo(ri -cyclopentadienonesuperphane)] complexes The [CpCo(CO)2]-mediated oligomerisation of ethynylazulenes has been described and shown to result in the preparation of [CpCo(ri -butadiene-like)] complexes as well as other metal-coordinated and free oligomers. Compound 88 has been prepared, which on treatment with Li in THF afforded the cyclobutadiene dianion 89 which essentially fulfils the criteria for aromaticity ... 

Slcreospecific solution polymerization has been emphasized since the discovery of the complex coordination catalyses that yield polymers or butadiene and isoprene having highly ordered microstructures. The catalysts used are usually mixtures of organometallic and transition metal compounds. An example of one of these polymers is cis- 1.4-polybutadiene. 

The two coordinated COD molecules in Am[Co(COD)2] (27b) and the four bonded ethylenes in Am[Co(C2H4)4] (27a) are readily displaced by CO or butadiene at low temperatures as in Eq. (32) (42, 43). The diphos-phane (CH2)2[P(C6HU)2]2 displaces only two ethylene molecules from 27a at 0°C giving 30. These latter compounds (Am = Li, K) react with hydrogen to yield novel alkali metal-containing hydridocobalt-phosphane complexes (31) which have 2H2 per AmCo (47). 

As the first transition series is ascended the metal—carbon bond becomes weaker and the alkyls of iron, cobalt and nickel are stable at ambient temperatures only when coordinated with strong it donors such as 1,1 bipyridyl [12], Organic groupings which form delocalized bonds with the metal are more stable and many tt complexes or 7r-allylic compounds of transition metals are known. The metal—cyclopentadienyl bond is too stable for these compounds to initiate polymerization, but some TT-allyl compounds will polymerize monomers such as butadiene where a comparable structure will be retained in the propagating chain. 

In addition to the binary catalysts from transition metal compounds and metal alkyls there 2ire an increasing number which are clearly of the same general type but which have very different structures. Several of these are crystalline in character, and have been subjected to an activation process which gives rise to lattice defects and catalytic activity. Thus, nickel and cobalt chlorides, which untreated are not catalysts, lose chlorine on irradiation and become active for the polymerization of butadiene to high cis 1,4-polymer [59]. Titanium dichloride, likewise not a catalyst, is transformed into an active catalyst (the activity of which is proportional to the Ti content) for the polymerization of ethylene [60]. In these the active sites evidently react with monomer to form organo-transition metal compounds which coordinate further monomer and initiate polymerization. 

The polymerization mechanisms for vinyl chloride and acrylonitrile (and also styrene) with coordination catalysts are also uncertain [222] and the copolymerization of butadiene/acrylonitrile (q.v.) also shows some features suggesting the formation of free radicals (or possibly radical-ions from charge transfer complexes). As these polar monomers can react, or form strong complexes, with the organo-metal compound it is likely that the kinetic schemes will be complex. As with styrene there is a good deal of scatter in the experimental kinetic data with these monomers which detracts from the certainty of the deductions, and much work will be required to put their polymerization by coordination catalysts on a sound mechanistic and kinetic basis. 

Lanthanides, which are d-electron deficient metals similar to early transition metals such as Ti and Zr, polymerize ethylene and acrylic esters. Although studies of coordination polymerization of 1,3-butadiene catalyzed by lanthanide compounds started as early as 1964 by discovery of the Ce catalysts... 

Polybutadiene containing a high content of trans-, 4 structure has been prepared with [(i7 -allyl)NiX]2 (X = Cl, Br, I). Since [(Tj -allyl)NiX]2 exists in the dimer state, it breaks into the monomer state only after coordination with butadiene. Butadiene can be polymerized with (17 -allyl) Ni compounds without base metal alkyls being present . 

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