Cis/trans reactivity ratios in the ring‐opening polymerization of bicyclo[2.2.1]hept‐2‐ene; the nature of the propagating species in olefin metathesis

Abstract

AbstractThe olefinic regions of the 13C NMR spectra of numerous ring‐opened polymers of bicyclo[2.2.1]hept‐2‐ene (norbornene) having cis double bond contents in the range σc = 0,32–0,86, have been analysed in detail. At least six peaks are resolved corresponding to olefinic carbons defined in terms of the double bond sequences tcc, tct + ccc, cct, ttc, ctc + ttt, ctt; for polymers of intermediate cis‐content tct and ccc are also resolved. The relative intensities of the cis‐centred triads may be accounted for in terms of a single parameter, rc, derived from the observed dyad probabilities. The relative intensities of the trans‐centred triads, however, show that at higher cis‐contents, the sequence cttc occurs with greater frequency than expected, requiring interpretation in terms of two trans‐parameters, rtt and rtc. These results show that the generally noted feature of cis/trans blockiness at moderately high cis‐content is sometimes associated with the presence of three kinetically distinct propagating species rather than two. The first is postulated to be the highly cis‐directing olefin‐metal‐carbene complex 4 in which the previously formed cis double bond remains coordinated to the metal centre; this may have both chiral and achiral forms. The second and third are postulated to be isomeric species formed by decoordination of the previously formed trans double bond from the metal centre but in which the initial ligand geometry is governed by the nature of the penultimate double bond: 3tc and 3tt are postulated to have permanent ligands with square pyramidal and octahedral symmetry, respectively. It is shown how the present results and those from related work may be rationalised in terms of this hypothesis.