Cycloolefins coordination polymerisation

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. 

The same group of coordination polymerisations in which alkene undergoes re complex formation with the metal atom includes the copolymerisation of ethylene, a-olefins, cycloolefins and styrene with carbon monoxide in the presence of transition metal-based catalysts [54-58], In this case, however, the carbon monoxide comonomer is complexed with the transition metal via the carbon atom. Coordination bond formation involves the overlapping of the carbon monoxide weakly antibonding and localised mostly at the carbon atom a orbital (electron pair at the carbon atom) with the unoccupied hybridised metal orbitals and the overlapping of the filled metal dz orbitals with the carbon monoxide re -antibonding orbital (re-donor re bond) [59], The carbon monoxide coordination with the transition metal is shown in Figure 2.2. 

Cycloolefins, unlike acyclic internal olefins, undergo coordination polymerisation owing to the presence of ring strain. Loss of ring strain is an important contribution to the driving force of this polymerisation. 

Table 6.1 Commerically available polymers of cycloolefins, produced by coordination polymerisation, and their typical uses ...

Name and characterise the main types of coordination polymerisation of cycloolefins and give examples of representative catalysts for each type. 

Another type of metal-carbon bond, the metal carbene bond (with carbene of an electrophilic or nucleophilic character), appears to be the active bond in transition metal-based catalysts for the ring-opening metathesis polymerisation of cycloolefins. Such a bond, which is co-originated with metal by the sp2-hybridized carbon atom, possesses a a, n double bond character (Mt = C) [34,35], The enchainment of the coordinating cycloolefin at the active site... 

As regards stable metallacycle catalysts for cycloolefin polymerisation, catalysts based on Ti (titanacyclobutanes) and Ta (tantallacyclobutanes) are used [47,49]. The first demonstration of a living and well-controlled system concerned the polymerisation of norbornene in the presence of substituted biscy-clopentadienyltitanacyclobutane in which four-coordinate titanium possesses formally a d° 16-electron structure [97,98], The initial titanacycle undergoes a reaction with norbornene at 20 °C to yield the trisubstituted metallacyclobu-tane ... 

If one considers a general scheme of the ring-opening metathesis polymerisation of cycloolefins [scheme (22)], it involves complexation of the monomer molecule at the free coordination site, followed by an attack of the carbene carbon atom on the complexed monomer [46] ... 

The propagation reaction consists in repetitive interaction of the cycloolefin double bond with a metal alkylidene species formed via the initiation reaction, and differs from this reaction only in the rate constant value. The rate of the propagation reaction should be first order with respect to the monomer concentration. However, the kinetics of polymerisation of cycloolefins is complicated by the fact that any active catalytic species in the polymerisation system will be able not only to coordinate the monomer double bond to lead to a propagation step but also to coordinate a C=C bond from the polymer chain. If the coordination of the latter C=C bond proceeds intramolecularly, then a new... 

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