Ring-opening of cyclooctatetraene

The application of metathesis catalysis to polyene synthesis may be viewed as following a progressive path from polymerizing acetylene itself, a two-membered ring , to ring-opening a 

When well-defined, less Lewis-acidic metathesis polymerization catalysts are used to polymerize COT, a lower level of detectable sp defects are formed. Also, although the polyacetylene produced is still insoluble, the reaction proceeds slowly enough to allow manipulation of the liquid reaction solution before hardening. In this way, one can obtain films in a desired shape and location, e.g., on a semiconductor [123]. This procedure was found to result in better electrical contact than can be obtained when a free-standing film prepared via the Shirakawa route is simply pressed against an electrode. 

A number of different high-molecular-weight (M of 10 -10 ) substituted polycyclooctatetraenes (poly(RCOT)s) have been prepared (see Thble 10-3), and they may be coarsely divided into two sets those which are soluble in organic solvents in the tmns configuration, and those which are not. Polymers containing n-alkyl side-chains are mostly soluble in the cis form, but insoluble in the tnms form (except for a very small fraction of the material). In contrast, poly(trimethylsilylcyclooctatetraene) and poly(5Cc-butylcyclooctatetraene) are completely soluble in both the cis and trans forms. Moreover, these two soluble polymers are 

These polymers are soluble in the cis form, but aggregate in the trans form. The A ax reported for the trans isomers of these polymers are for visually homogeneous, dilute ( 10 M) solutions, although they presumably contain aggregated material since filtration results in filtrates which absorb at higher energies = 580 10 nm) and contain low-molecular-weight polymer (Af 5000). 

An examination of the polymers solubility suggests that it is the steric bulk of the side-chain at the position adjacent (a) to the double bond which determines solubility. Thus, poly(neopentylCOT) and poly(2-ethylhexylCOT) (R(,= -CH2-), much like the poly-n-alkylCOT polymers (Ra = -CH2- also), are soluble in the cis form but not in the trans. Similar behavior is observed for alkoxy- (Ra = -0-) and phenyl-substituted derivatives. Polymers containing a secondary (R = -CHR R ) or tertiary (R, = -CR R R ) substituent adjacent to the double bond are soluble in both the predominantly cis and predominantly trans forms. 

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