Coordination polymerisation conjugated dienes

The origin of the stereoregulation in conjugated diene coordination polymerisation was recognised by Porri et al. [7,41] who explained the different modes of formation of the isotactic and the syndiotactic polymers in the presence of various transition metal-based catalysts. 

Aspects of mechanism of coordination polymerisation of conjugated dienes... 

Conjugated dienes, 11 Conjugation, 11, 368 Conrotation, 345 Coordination polymerisation, 322 Copolymerisation, 322 Cope reaction, 268 Cope rearrangement, 354 Cracking, 112, 305 Cram s rule, 235 Cross-linking, 323... 

Coordination polymerisation via re complexes comprises polymerisation and copolymerisation processes with transition metal-based catalysts of unsaturated hydrocarbon monomers such as olefins [11-19], vinylaromatic monomers such as styrene [13, 20, 21], conjugated dienes [22-29], cycloolefins [30-39] and alkynes [39-45]. The coordination polymerisation of olefins concerns mostly ethylene, propylene and higher a-olefins [46], although polymerisation of cumulated diolefins (allenes) [47, 48], isomerisation 2, co-polymerisation of a-olefins [49], isomerisation 1,2-polymerisation of /i-olcfins [50, 51] and cyclopolymerisation of non-conjugated a, eo-diolefins [52, 53] are also included among coordination polymerisations involving re complex formation. 

It is clear that each polymerisation step involves the insertion of the coordinated conjugated diene into the active metal-carbon a bond, becoming a n bond owing to this insertion, which by subsequent monomer coordination reverts to a a bond. 

However, no method of polymerisation known before 1954 allowed one to obtain polymers with a high regularity of structure from the most common conjugated dienes. A true breakthrough in the development of conjugated diene rubbers took place after the discovery of stereospecific polymerisation with transition metal-based coordination catalysts. From the late 1950s, a rapid development of industrial production of solution types of polybutadiene by means of polymerisation with Ziegler-Natta catalysts was observed. 

Figure 5.3 Schematic presentation of the formation of a ciy-1,4 diad (isotactic in the case of the terminally monosubstututed dienes) versus a 1,2 diad (syndiotactic) in the polymerisation of conjugated dienes. The last inserted monomeric unit (as an r 3-allylic ligand) is below the plane, the incoming coordinating monomer is above the plane and the transition metal atom is on the plane...

An interesting example of high chemoselectivity in the coordination polymerisation of conjugated dienes is the polymerisation of 1,3-cyclohexadiene in the presence of Ziegler-Natta [186] and re-allylnickel [187] catalysts, which leads to polymers consisting mainly of 1,4 monomeric units. 

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