Effect of ring size and substitution on thermodynamic polymerizability

1 Effect of ring size and substitution on thermodynamic polymerizability 

The failure of cyclohexene to give high polymer is due to its low strain energy, — AH being too small to offset the positive value of — TAS. However, at low T and high [M] it is possible to obtain a small proportion of oligomer see Section 12.4.1. The reverse reaction, namely the elimination of cyclohexene from molecules in which two double bonds are separated by five single bonds, occurs readily, for example, in the intramolecular metathesis of octa-1,7-diene and of deca-2,8-diene. 

It is also eliminated from the polymer made by the ROMP of cw,fra 5-cyclodeca-1,5-diene, initiated by WCl6/EtAlCl2 (Hocks 1975b) eqn. (1). When the monomer adds to the propagating metal carbene, it may form either [Mt]=CH(CH2)4CH==CH(CH2)2CH=CHP , which may immediately split out cyclohexene, or [Mt]=CH(CH2)2CH=CH(CH2)4CH=CHP , which may split out cyclohexene in a secondary reaction of the polymer. 

Some thermodynamic data for the ROMP of cycloalkenes are summarized in Table 11.2. For cyclohexene the thermodynamic fimctions of the polymer must be estimated from the data for related polymers. Note that AC° is negative for the polymerization of all monomers save cyclohexene. 

In the ROMP of norbomadiene and the dicyclopentadienes, the opening of the second double bond is thermodynamically less favourable than the opening of the first double bond, but nevertheless occurs if the concentration of the repeat units is somewhat above 0.2 M (see Sections 13.4.1.1 and 13.4.3). 

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