Metathesis polymerisation coordination

Fig. 3.31 Steric control in alternating ROMP Tendencies of norbomene and cyclooctene to give productive olefin metathesis upon coordination are illustrated by a thick arrow (preferred monomer) or a thin arrow (less favoured monomer) (a) only minor steric hindrance SlMes greatly favours the polymerisation of the strained norbomene (b) the rotating phenylethyl-group induces a steiically more congested active site, leading to preferred incorporation of the smaller cyclooctene (c) the flexible and small cyclooctene derived polymer fragment permits the incorporation of the bulky norbomene...

Given the success of the Grubbs-type NHC-Ru catalysts in metathesis polymerisation (Chapter 3), it is somewhat surprising that more research has not been done on mid-transition metal carbene complexes for coordination-insertion polymerisation. At this stage however, there are only a few reported attempts with the metals Co, Fe and Ir. 

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... 

An interesting case is the coordination polymerisation of acetylene and higher alkynes. It may proceed by a mechanism quite similar to the metathesis polymerisation of cycloalkenes involving metal carbene and metallacycle (metallacyclobutene) species [45], The initiation and propagation steps in alkyne polymerisation (leading to a polymer of cis structure) in the presence of a catalyst with a diphenylcarbene initiating ligand are as follows ... 

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] ... 

This review summarises the most interesting approaches in the polymerisation of alkynes and the ring opening metathesis polymerisation of cyclic olefins catalysed by seven-coordinate tungsten(II) and molybdenum(II) compounds. Special attention is given to the catalytically active intermediate compound formed in the reaction of the metal complex and the organic substrate (alkyne or cyclic olefin) and to the reaction mechanism. 

Seven-coordinate bimetallic compounds are also active unicomponent catalysts in the ring-opening metathesis polymerisation (ROMP) of cyclic olefins. The first step in this reaction is the coordination of olefin to metal. However, only in the reaction with a chelating ligand such as norbornadiene (NBD) was it possible to isolate and characterise the reaction product. In the reaction with NBD chloride bridge splitting in the binuclear compound of tungsten(II) by a diene molecule occurs very easily and monomeric seven-coordinate compounds are formed (Eq. 6) [48]. 

Earlier attempts at emulsion polymerization of norbomenes in aqueous solution using iridium complexes as catalysts gave poor yields (10%). However, it was subsequently found that 7-oxaborane derivatives could be rapidly polymerised in aqueous solution in presence of air using some group VIII coordination complexes as catalysts giving a quantitative yield of a ring-opening metathesis polymerisation product (Scheme 116). ... 

Coordinatively unsaturated species can achieve saturation through partial bonding to the hydrogen or carbon atoms of organic ligands. Metal-hydride-metal and metal-hydride-carbon interactions in transition-metal complexes play an important role in catalytic reactions like carbon monoxide reduction, olehn metathesis, alkyne polymerisation and methylene transfer. 

The mechanism of norbornene polymerisation has been investigated using complex (44) as catalyst. Cis-coordination of the olefin is necessary for catalytic activity. The performance is improved by irradiation, by the solvent and especially by Lewis acids. A WCCOlg/CCli, mixture becomes a slow catalyst for 1-octene metathesis under photolytic conditions. ... 

---