Poly isocyanide

Linear-difference effects in SHG have been observed from Lang-muir-Blodgett films of chiral poly(isocyanide)s.6... 

To fit the experimental results, it is necessary to fix the overall phase. This can be done, for example, by defining h as a real quantity (/ / = 0). The values found for the coefficients /, g, and h can then subsequently be used to calculate the values of the components of the second-order susceptibility, X(2). This is done in detail for a Langmuir-Blodgett film of a poly(isocyanide) in the following section. Note that both phase and magnitude of all tensor components are relative values. The absolute phase cannot be determined... 

The material system is a Langmuir-Blodgett film of the S enantiomer of a chiral polymer deposited on a glass substrate. The polymer is a poly(isocyanide)30 functionalized with a nonlinear optical chromophore (see Figure 9.14). In this particular system the optical nonlinearity and chirality are present on two different levels of the molecular structure. The chirality of the polymer is located in the helical backbone whereas the nonlinearity is present in the attached chromophores. Hence, this opens the possibility to optimize both properties independently. 

These poly(isocyanide) polymers form rigid, rodlike helices on the surface of water, which can be easily transferred to a solid substrate by means of the LB method. The resulting films have a Coo symmetry. 

Figure 9.14 Chemical structure of 5-enantiomer of the chromophore-functionalized poly(isocyanide).

A chiral TTF-substituted poly(isocyanide) 11 was prepared by Amabilino et al. 

Nolte et al. found that an amphiphilic block copolymer 83 composed of a hydrophobic tail of poly(styrene) and a hydrophilic head group of a charged, right-handed helical poly(isocyanide), which is referred to as a superamphiphile , self-assembles in a hierarchical fashion in water to form left-handed superhelices (Fig. 34) [ 158]. They suggest that this type of copolymer will serve as an experimental model for the theoretical study of the packing of helices due to their versatility and easy accessibility. 

Examples of helix-forming polymers are poly(isocyanides) and poly(glutamates). The repeat imits of these polymers are shown in Figure 7. For poly(isocyanides), each carbon atom of the polymer backbone was functionalized with a nonlinear optical chromophore (Figure 7, I). Because of large steric interactions between the... 

Figure 7. Structural formula of the repeat units of the functionalized I) poly(isocyanide) and II) poly(y-benzyl-L-glutamate) polymers that adopt a helical conformation in solution.

Millich, F. Rigid Rods and the Characterization of Poly isocyanides. Vol. 19, pp. 117-141. 

Inspired by the contribution of the carbene-like resonance structure, the homopolymerization of isocyanide giving rise to the formation of poly(iso-cyanide) has attracted much attention [3, 4]. On storage, or distillation, isocyanides that lack bulky AT-substituents tend to form solid materials, which had been supposed to be poly(isocyanide)s. However, this polymerization , (or resinification), largely depended upon the nature of the glass surface of the apparatus used for storage or distillation and, therefore, was poorly reproducible. Moreover, no structural information was provided for these materials, making the evaluation of the polymerization systems difficult. The historical background has already been overviewed by Millich in two reviews published in 1972 and 1980 [3, 4]. 

The significant contribution by the Nolte and Drenth group in this area was that, for the first time, they demonstrated the existence of a non-racem-ic helical structure for poly(isocyanide)s. Optical resolution using a chiral HPLC technique or asymmetric polymerization led to the isolation of optically active polymers, whose chirality was supposed to be solely due to the main chain helicity. Their effort, in conjunction with that of Novak s and Takahashi s significant contributions to asymmetric polymerization, will be discussed in the next section. Non-asymmetric and asymmetric polymerizations will be described separately in the following sections. 

As has already been described, the nickel-catalyzed-system is currently the most general protocol for the polymerization of isocyanides. An initial report [5] described that Ni(CO)4, Ni(CO)3(PPh3), Cp2Ni, and CpNi(CO)2 show high catalytic activity in the polymerization of cyclohexyl isocyanide in benzene, yielding poly(isocyanide) 2 as a white powder, although the nickel catalysts are a little less active than the corresponding cobalt catalysts (Scheme 7). In a typical experiment, the polymerization of cyclohexyl isocy-... 

The superior catalytic activity of Ni catalysts over other transition metal complexes was proven by a comparison of a series of metal acetylacetonates as catalysts for the polymerization of ethyl isocyanide in chloroform (Scheme 14) [15]. Fe(acac)3, Mn(acac)2, Zn(acac)2, and Cd(acac)2 showed either no, or an extremely low, catalytic activity, whereas Cr(acac)3 and Co(acac)3 afforded almost 10% yield of poly (isocyanide). Co(acac)2, Pd (acac)2, and Cu(acac)2 exhibited moderate activity, resulting in the formation of the polymer in 29-61% yields. Ni(acac)2 showed almost the same catalytic activity as Co2(CO)8, whose remarkable catalytic activity had already been established by Yamamoto et al. [5]. 

All the polymers had a yellowish color when prepared in polar solvents at temperatures between 0 and 25 °C. The yellow color of the poly(isocyanide)s derived from pr/ra-alkyl isocyanides changed to black on addition of an acid to the solution or suspension of the polymer [15]. This change in color was not observed for polymers derived from sec- and tert-alkyl isocyanides. The structure of the black polymer was assigned to poly(ethyl cyanide) from spectroscopic and conductivity measurements (Scheme 15). 

A nucleophile (X-) attacks one of the isocyano carbon atoms, generating the a-iminomethylnickel species B. Then, coordination of the fifth isocyanide to the nickel center (to form C) is followed by the migration of the imi-nomethyl group onto the neighboring isocyano carbon atom (C2), forming a dimeric intermediate D. This associative migration step is repeated to form the poly(isocyanide) E. Nolte and Drenth called this possible mechanism a merry-go-round mechanism, and it was also proposed as the key mechanism for the screw-sense selective polymerizations discussed later. 

In 1991, Deming and Novak reported the pronounced effect of molecular oxygen on the polymerization process [16]. Under anaerobic conditions, isocyanides underwent only slow polymerization in the presence of NiCl2, even in ethanol, affording poly(isocyanide)s in only moderate yields of 50-60% after 20 h. When the polymerization was carried out in air at 1 atm, the polymerization was completed within 1 h. Air at 1 atm was sufficient to main-... 

The existence of a non-racemic helical conformation in poly(isocyanide)s was first suggested by Millich et al. in 1969 [37]. They carried out polymerizations of optically pure d- and /-1-phenylethyl isocyanide 29 in the presence of an acidic ground glass catalyst (Scheme 26). The observation of a... 

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