Structural control during the cyclopolymerization of unconjugated dienes

Various attempts to get highly cyclized polymers from bifunctional compounds, mainly from 1,6-dienes, are summarized along with those for regulating the ring sizes formed during their polymerizations. One way of getting polymers with a high degree of cyclization can be achieved by suppressing the intermolecular reaction leading to pendant functional groups. Based on this consideration it has been proposed that bifunctional monomers, the monofunctional counterparts of which do not polymerize, are likely to give rise to highly cyclized polymers, if they can be polymerized at all. This is because the use of such monomers inevitably reduces the rate of the intermolecular addition reaction that forms pendant functional groups. The cyclopolymerization behavior of various bifunctional compounds proved the concept. However, the problem was the low polymerizability of the monomers designed by applying the concept. A detailed examination of the properties of the functional groups with no homopolymerizability led to the new proposal that unconjugated dienes with not only high cyclization tendency but also high polymerizability could be obtained by the use of functional groups with no homopolymerization tendency along with a high conjugative nature. The cyclopolymerizabilities of various unconjugated dienes are discussed in terms of the polymerization tendency of their monofunctional counterparts to show the validity of the proposal. The other approach for getting highly cyclized polymers involves the extensive enhancement of the rates of the intramolecular cyclization reaction over that of the intermolecular propagation reaction. Some examples for this procedure are also described. Another problem related to structural control during cyclopolymerizations is how to regulate the ring sizes formed. Little success has been achieved on this problem. This is because the mechanism of the intramolecular cyclization reactions during the cyclopolymerizations of 1,6-dienes is rather complicated. The mechanistic aspect of the intramolecular cyclization reactions has been discussed in some detail. (C) 2000 Elsevier Science Ltd. All rights reserved.