Polyisocyanide

Kros A, Jesse W, Metselaar GA, Comelissen JJLM (2005) Synthesis and self-assembly of rod-rod hybrid poly(gamma-benzyl L-glutamate)-block-polyisocyanide copolymers. Angew Chem hit Ed 44 4349 352... 

Helix is the most common higher-order structure of synthetic polymers such as peptides, polymethacrylates, polychloral, polyisocyanides, polyisocyanates, and polysilanes. Polyacetylenes bearing appropriate substituents also form a helix. Substituted helical polyacetylenes are promising candidates for enantioselective permeable materials, polarization-sensitive electro-optical materials, asymmetric electrodes, and hence their synthesis is currently under intensive research. This section overviews the synthesis and properties of helical polyacetylenes recently reported. 

Polyisocyanides Capable of Thermodynamic Mixing with Solvents. 132... 

As antidpated from their very inception, polyisocyanides have exhibited intriguing physical properties, and they merit study as model substances for the examination and development of present knowledge of polymer characterization. 

It is suggested by several equations describing the solution behavior of flexible polymers in good solvents that the second and third virial coefficients of the concentration dependence have a common parameter dependence 12-14). This behavior may be attributed to interchain entanglements for some polymers. In contrast, rigid rods can show completely linear concentration dependence in moderately dilute solution. Here the third virial coefficient is either negligibly small, or it is non-existent—in which case no relation exists between the second and third coefficient. Viscometry of some polyisocyanide solutions shows time-dependent transformations, reflected in pronounced changeovers from parabolic to linear concentration dependencies, and it would be of interest to define the transformations, and to detail their physical descriptions. 

The initial experiences with polyisocyanides, especially solubilities and some Debye-Scherrer X-ray data (3), and an consideration of the most probable molecular structure (4), led to an early tentative conclusion that poly(a-phenyl-ethyl isocyanide) and poly(conformational model of a tightly wound helix with an overall shape of a cylindrical rod of about 15 A diameter (3). 

A tentative reservation exists about this work. As reported, in a communication, it as yet gives no explanation for the unexpected solubility of poly(r t-butyl isocyanide) in chloroform, nor does it describe a safeguard against the mutual solubility, i.e. plasticization, of polyisocyanides, which is a possibility between the non-crosslinked, otherwise insoluble support medium and the mobile solute. Yet, the rotation data is compelling. 

The rotation of alkyl and aryl substituents about the N-R single bond may be severely restricted on steric grounds in most polyisocyanides, (which usually contain branching at the a-carbon of substituent R). This is consistent with the observation that the NMR spectrum of whole samples of J-poly(a-phenylethyl isocyanide) in tetrachloroethylene undergoes little change upon heating between room temperature and 128° C (4). 

Numerous homo- and copolymeric polyisocyanides have now been synthesized. Table 1 gives a select list of reported polyisocyanide preparations a more extensive list was published earlier (21). The entries included in Table 1 were chosen on the basis of the best reported yields or, preferably, in instances in which viscometric data is also reported for the polymer. 

Primary alkyl isocyanides are known not to comply simply to homopolymerization. It was recently reported that polyisocyanides prepared with nickel(II) compounds varied in color from yellow to black (22,23). The latter occurred at preparative temperatures in excess of 25° C, or when polymer non-solvents or acids were used. Further, the addition of acids to solutions or suspensions of the yellow polymers also led to black compounds. The NMR spectrum of a soluble polymer sample (i.e. MVPO = 1100) of black poly(ethyl isocyanide) shows methylene resonance shifts to values as occur in nitriles, which were interpreted as giving evidence of rearrangement to a polycyanide... 

One additional aspect of complexity in defining the molecular structure of polyisocyanides arises, especially wherein the substituent R group is aralkyl and contains hydrogen on the benzylic a-carbon atom, as in poly(a-phenylethyl isocyanide). The possibility of tautomeric shift of the hydrogen atom on the a-carbon is reasonably likely (5), whereby the 7t-electrons of the original... 

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