Previously, Millich had found that acid-coated glass can act as a catalyst for the polymerization of isocyanides, although not very efficiently [33, 46]. Metselaar et al. recently reported that the acid (TFA)-initiated polymerization of isocya-nopeptides leads to extremely long polymers with lengths up to 14 mm (Fig. 12.10D) [85, 90]. For the polymerization of 35a at a TFA concentration of 1 mM, kinetic studies revealed a large entropy of activation (—170 J mol—1 K_1), which indicates a very high degree of organization in the transition state [82]. At higher acid concentrations instead of polymerization, conversion of the monomer to the corresponding formamide was observed. Based on this result a polymerization mechanism was proposed in which first a helical oligomer is formed acting as a template for the incorporation of subsequent monomer units through a supramo-lecular complex (Fig. 12.11). In the case of high acid concentrations the template is disrupted and destroyed. The reaction is highly stereospecific since the addition of the enantiomeric monomer 35b completely blocked propagation of the polymerization of 35a even when present in only 1%. The diastereomer 34a, but not 34b, could be incorporated into the growing polymer, although 34a itself without 35a present could not be polymerized with TFA. These subtle differences demonstrate the critical effect of the configuration of the first chiral center of the monomer on the polymerization reaction and the high stereospecificity of the transition state. The fact that 34a itself cannot be polymerized with TFA was attributed to the inability of 34a to form a helical template due to larger steric repulsion between the monomeric units in the helix. When a nickel catalyst was used all monomer combinations could be readily polymerized.

Wezenberg et al. showed that polyisocyanides 36 and 37 derived from b-amino acids also form well defined rod-like polymers (Scheme 12.7) [91]. The kinetically

Fig. 12.11 Mechanism of the acid initiated polymerization of 35a, showing the helical template formation and the subsequent polymerization (Route A). The side reaction to the corresponding formamide, which occurs at high TFA concentrations is shown in Route B.

formed polymer, however, turned out to be unstable and transformed into a more stable structure, which possessed a better defined hydrogen bonding pattern as was concluded from IR and temperature-dependent CD studies. The precise structure of the transformed polymer remained unclear.

0 0

Post a comment