Cyclooctene, cyclic oligomers

When cyclic alkenes are utilized as starting materials the metathesis reaction will lead to long chain polymers and/or cyclic oligomers [103, 104, 107, 108]. If a strained cyclic alkene is employed the reaction is effectively irreversible. Industrially cyclooctene (polymer Vestenamer), 2-norbornene (polymer Norsorex), and dicyclopentadiene (polymer Telene, Metton, Pentam) are used as monomers. Upon polymerization cyclooctene and 2-norbornene yield straight chain polymers while dicyclopentadiene also allows cross-linking (Scheme 5.56). 

Fig. 12.4 The proportion of extractable cyclic oligomers as a function of the reciprocal initial monomer concentration [M]o for the ROMP of (A) cw-cyclooctene, and (B) cis,cis-cycloocta-1,5-diene. Catalyst WCl6/EtAlCl2/EtOH (1/4/1) in benzene (Scott 1969)...

Table 12.3 ROMP of cyclic trienes, tetraenes, and pentaenes (obtained as cyclic oligomers by the ROMP of cyclooctadiene or cyclooctene)...

It was first observed in 1969 that the ROMP of cycloocta-1,5-diene and cyclooctene produced not only linear polymer but also a series of cyclic oligomers, detectable by GC [67]. These are the products of back-biting metathesis reactions in competition with the propagation reaction. Eqn. (10) is an example of the formation of cyclic tetramer during the ROMP of cyclopentene, [68]. The back-biting reaction is thus the reverse of the propagation step for the cyclic oligomer itself. 

Detailed investigation on ring-chain equilibria for the polymerization of cyclooctene [183] showed that the polymer obtained in the absence of acyclic olefins consists of 15-20% cyclic fraction and 80-85% linear fraction. Hocker et al. [184] correlated the product distribution as a function of initial cyclooctene concentration. They found that below a threshold monomer concentration of 0.1 mol, almost no polymer was present at higher monomer concentrations, however, formation of cyclic oligomers and linear polymers was observed. 

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