Back-biting metathesis

As with all polycondensation reactions, the formation of cychc ohgomers by ADMET is possible and has been demonstrated in a variety of cases [31-33]. This occurs by intramolecular back-biting metathesis of an active metal carbene with an internal olefin of the polymer (Scheme 6.5) to hberate cyclooctadiene, for example, from ADMET polybutadiene, although larger cychcs have also been observed. A related undesired cyclization is the intramolecular cyclization of the monomer by RCM. 

Scheme 6.5 (a) Back-biting metathesis of ADMET polybutadiene to produce cyclooctadiene (b)... 

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. 

Data from sonicated polymerizations suggest that power ultrasound promotes also the back-biting metathesis route leading to formation of oligomers from the metallacarbene propagating species Pn+1 (Eq. 3). 

In ROMP these by-products arise from back-biting processes, because already formed polymer chains as well as monomers may approach the propagating carbene species with their double bonds. If these secondary metathesis reactions proceed intramolecularly cyclic products are formed as shown in structure (31). The cyclic structure of such oligomers has been proven by mass spectroscopy [261,262]. 

Table VII summarizes the products and activities of the various catalysts mentioned versus the types of substrates. With the metathesis mechanism, both polymer and cyclotrimer product are possible for each monomer since the two proposed pathways are very similar [65, 111, 115]. In a metathesis mechanism, after several insertions the propagating chain end can swing around and coordinate to the metal center again instead of inserting another monomer (Scheme XXII). If an intra-molecular olefin metathesis takes place at this point, the product will be cyclic (e.g., cyclotrimer). Due to this possibility of back-biting, sterics play an important role in the production of cyclotrimer. For Mo- and W-...

Scheme XXIL A proposed back-biting mechanism for the metathesis polymerization of alkynes. Substituents have been omitted for clarity.

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