Cycloolefins ring-opening polymerisation

A characteristic feature of cycloolefin ring-opening metathesis polymerisation is alteration of the metal-carbon active bonds from the metal carbene a, n bond into metallacycle a bonds, and vice versa, as polymerisation progresses. It is worth mentioning, in this connection, that metallacyclobutanes can be successfully used as catalysts for this polymerisation [36,37]. 

The stereoregularity of polymers relates not only to the configuration of four substituents attached to saturated carbon atoms in the polymer chains but also to the geometric isomerism, resulting from the presence of unsaturated carbon atoms in the polymer chains. Such isomerism appears in chains of polymers formed in the 1,4 polymerisation of conjugated dienes [scheme (19)] and the polymerisation of acetylenes [scheme (20)] as well as the ring-opening polymerisation of cycloolefins [scheme (16)] ... 

The first ring-opening polymerisation of cycloolefin was reported shortly after discoveries by Ziegler and Natta Anderson and Merckling [40] claimed that the polymerisation of norbornene could be promoted by the TiCU—MgEtBr catalyst. The structure of the product, however, was not recognised immediately, but is now known to have been poly(l-vinylene-3-cyclopentylene) ... 

Confirmation that the ring-opening polymerisation of cycloolefins also proceeds by complete scission of the C=C bond (transalkylidenation) was provided soon after by Dall Asta and Motroni [44]. 

Cycloolefins can be ring-open polymerised with a high degree of stereospecificity. Many poly(l-alkenylene)s prepared by metathesis polymerisation of cycloolefins are stereoregular polymers with presumably a cis or Irons configur-... 

Hocker, H., Ring-opening Polymerisation of Cycloolefins by Means of Metathesis Catalysts Kinetic and Thermodynamic Effects on the Product Distribution , Makro-mol. Chem. Macromol. Symp., 6, 47-52 (1986). 

A book on olefin metathesis and the ring-opening polymerisation of cycloolefins has appeared.175 Collections of the papers given at the 6th International Symposium on Olefin Metathesisl76 and at the 5th International Symposium on Ring-Opening Polymerisationl77 have been published. 

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... 

The most strained cycloolefins, which are substituted cyclopropenes, e.g. 3,3-dimethylcyclopropene or 3-methyl-3-ethylcyclopropene, appeared to be polymerised readily to respective substituted poly(l,2-cyclopropene)s in the presence of Pd-based catalysts containing very bulky non-labile ligands. Such catalysts are characterised by reduced activity in order to prevent ring opening of the cyclopropene monomer [23],... 

There exists a wide variety of cyclic olefins capable of being polymerised via a ring-opening metathesis reaction From high-strained cycloolefins (cyclobutene and homologues, norbornene and homologues) up to low-strained (cyclopentene) and unstrained cycloolefins (cycloheptene, cyclooctene) [45]. 

The above examples show that the ring-opening metathesis polymerisation of cycloolefins, even simple substituted bicyclic olefins, gives rise, in principle, to polymers with a very wide range of microstructures defined by the frequency and distribution of cis and trans vinylene units, m and r diads and h-h, t-t or h-t arrangements of cycloaliphatic units. 

There is a wide variety of transition metal compounds, ranging from group 4 (Ti) to group 8 metals (Ir), that can be applied as catalysts or catalyst precursors for the ring-opening metathesis polymerisation of cycloolefins. However, the most commonly used are W, Mo, Re and Ru compounds tungsten-based catalysts appeared to be the most effective. Other transition metal compounds such as Nb and Ta compounds have also often been used as catalysts, but especially for mechanistic studies [45]. 

Although this view is oversimplified and borderline metal carbene complexes have been isolated, this approach is convenient for discussing the activity of metal carbene species in the ring-opening metathesis polymerisation of cycloolefins. Calculations have predicted [81,82] and recent results have shown [83] that, in some systems, metal alkylidene reactivity is competitive with metal carbene reactivity, i.e. olefin metathesis is competitive with olefin cyclopropanation. 

It is interesting that, to promote the ring-opening metathesis polymerisation of cycloolefins, metal carbyne complexes can also be used in such a case, the carbyne complex is rearranged to form the actual metal carbene complex [scheme (9)] capable of initiating the polymerisation [95] ... 

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] ... 

Copolymerisation, which is often carried out in order to gain a better insight into the nature of polymerisation initiating and/or propagating species and to modify the properties of the polymeric products formed, has also been satisfactorily carried out in the case of cycloolefins. The ring-opening metathesis... 

The products of the ring-opening metathesis polymerisation of cycloolefins, poly(l-alkenylene)s, are known as polyalkenamers according to the nomenclat-... 

Commercially available polymers, produced from cycloolefins by ring-retaining polymerisation and ring-opening metathesis polymerisation of cycloolefins, and their typical uses are listed in Table 6.1 [12,14,144-147, 150,167-171,177-186],... 

Give the structure and stereochemical designation of all possible cycloolefin polymers that might, in principle, be obtained by stereospecific ring-opening metathesis polymerisation. 

Which cycloolefins (monocyclic, bicyclic or policyclic) will undergo secondary metathesis reactions during ring-opening metathesis polymerisation Give reasons why. 

Explain why ring-opening metathesis polymerisation of cycloolefins is accompanied with backbiting reactions leading to cyclic oligomers more readily in dilute solution than in bulk. 

As in the case of the ring-opening metathesis polymerisation of cycloolefins, an important matter is the control of polymerisation to prepare acetylenic polymers having precise structures. A living polymerisation is of practical importance in the synthesis of monodisperse polymers, such as terminally functionalised polymers and block copolymers. The metathesis catalysts that promote the living polymerisation of acetylene [42] and acetylenic monomers include M0OCI4 SnBu EtOFkNbCls and Ta, Mo and W alkylidenes [84, 133, 152, 153]. 

Acyclic a, co-diene metathesis is a method for the synthesis of poly(l-alkeny-lene)s, which are also available from the ring-opening metathesis polymerisation of cycloolefins [13-15]. 

Metathetical polycondensation of acyclic dienes has not been successful with conventional catalysts used for the ring-opening metathesis polymerisation of cycloolefins, which is due to the fact that Lewis acids are usually present, and produce deleterious side reactions [13,16,17]. Only Lewis acid-free, well-defined catalysts have been successfully applied for acyclic diene metathesis polycondensation the key success has been to choose catalysts that obviate other pathways not involving the metathesis mechanism [18-20]. It was Wagener et al. [16,21] who first were able to convert an acyclic a, co-diene (1,9-decadiene), by using an acid-free metal alkylidene catalyst, to a high molecular weight... 

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