Direct Observation of Tacticity

These 7-substituted derivatives are also important because much of the above mechanistic interpretation depends upon the assumption that attack on the norbomene molecule occurs at the exo face. The result of ring-opening polymerisation of mixtures of syn- and ann-7-methylnorbomene [61] shows that this assumption is valid. Thus, only poly(anri-7-methylnorboraene) was obtained from the polymerisation of syn/anti mixtures, although a small proportion of syn isomer was incorporated in some cases. With particularly active catalysts the syn isomer could be homopolymerised. More recently, and in relation to the regioselectivity studies discussed above, 7-methylnorbomadiene was prepared and polymerised [62]. 

The importance of these polymers (for NMR analysis) lies in the excellent resolution of the NMR spectra which may be achieved and the fact that ring 

Figwel.l6 C NMR spectra of poly(anti-7-methylnorbomene) (a) an intermediate os-tactic polymer prepared using the W(mesit)(CO)3/EtAlCl2 catalyst system, and (b) an atactic polymer of similar cis content prepared using the WCl JBu Sn catalyst system. Reproduced by kind permission of Elsevier Science Publishers from [61] 

In keeping with the general principle that polymerisation of monomers that have a pair of double bonds capable of chelation at the catalyst site leads to the formation of high cis polymer [66], polymers formed from 7-methylnorbor-nadiene were generally high cis. Resolution of the various microstructural features is also observed in the spectra of these polymers, but paradoxically it 

An important consequence of the foregoing discussion is that it is impossible to predict which resonance will be split by any of the possible microstructural features, and one must therefore be careful not to assume that a polymer is, for example, tactic simply because no fine structure is resolved. Also, spectra of polymers taken on modem high field instruments (125 MHz for may show up fine structure not resolved on lower field instruments. A most apposite example of this was observed recently in the NMR spectroscopy of polymers 

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