Functionalized Polyisocyanides

As was discussed in the previous sections, polyisocyanides possess a unique polymer backbone that can adopt a very stable rigid conformation when bulky side chains or additional stabilizing interactions between the repeat units, like p-p stacking and hydrogen bonding are present. Polyisocyanides, therefore, are very attractive scaffolds to arrange functional groups in well defined arrays, thus creating new materials with special properties. Many of the functionalized polyisocyanides already have been described in excellent reviews, for example by Cornelis-sen et al. and very recently by Suginome et al. [8, 9, 11, 14], and will not be discussed here.

Bioinspired sugar (38-40) [51, 92, 93] and cholesterol (41) [94] derived polyiso-cyanides and polymers from isocyanides functionalized with ionic side groups (42-45) [54, 95-97] have all been reported (Scheme 12.8). Amphiphilic isocya-nide 42 forms vesicles in water and the isocyanide functions could be polymerized leading to cross-linking of the bilayer [96]. Iyoda et al. have shown that it is also possible to polymerize isocyanides with extremely bulky groups using Pd-Pt complex 5a as the catalyst. The dendronized polyisocyanides 46 were synthesized with a polymerization degree exceeding 100 [98]. Using such an approach, polyisocyanides bearing even polystyrene side-groups (47) could be synthesized with a polymerization degree of ca. 50 [99].

Polyisocyanides also have been used as well-defined templates in catalysis, crystal growth and for the transfer of chirality (Scheme 12.9). Polymeric catalysts have been prepared from polyisocyanide 48, after coordination of Rh-catalysts to the phosphor ligands in the side chains [100-102]. The stability and catalytic activity of the rhodium complex of 48 towards the hydrogenation of cyclohexene was found to be better than that of the monomeric rhodium complex. The catalytic activity was also better than the activity of analogous polymers with a flexible polystyrene backbone. The latter result was ascribed to the rigidity of the polyiso-cyanide, which makes the catalytic centers more accessible.

The water-stable peptide-derived polyisocyanide 49a was used as a supramolec-ular template for the crystallization of CaCO3. The formation of unusually shaped calcite was found to be controlled by nucleation and adsorption processes involving 49a [89, 103]. The fact that less control over the crystallization process was obtained when polymer 49b was used, demonstrated the specificity of the interaction between 49a and the growing crystal. Single polymer chains of 49a could be visualized by AFM by complexing the polymer to amino alkanes [104]. By varying

Polymer Crystallization Afm
0 0

Post a comment