Achiral polymers

Crystallization of polymers in chiral crystals, even in the case of achiral polymers, is quite frequent and strictly related to the occurrence of helical conformations of the chains. The crystallizable polymer consists of a regular sequence of a chemical repeating unit which can be chiral if it presents an asymmetric center or achiral. On the contrary, helical conformations assumed by the polymer chains in the crystalline state are intrinsically chiral, even though the chemical repeat is achiral. Three possible cases can be distinguished ... 

The use of a chiral polymer instead of the achiral polymers in XXXIX and XXXX allows an asymmetric synthesis. An example is the stereoselective reduction of acetophenone to (I )-l-phenylethanol in 76-97% enantiomeric excess by using the indicated chiral support (Eq. 9-69) [Itsuno et al., 1985] ... 

Among the different approaches to immobilization, main chain chiral polymer catalysts are different from the traditional polymer catalysts prepared by anchoring monomeric chiral catalysts to an achiral polymer backbone (Pu, 1998). The three classes of synthetic main chain chiral polymers include ... 

A noncovalent chiral amplification based on the sergeants and soldiers effect between chiral and achiral polymers in the film state has been developed by Fujiki et al., which results in the formation of helix bundles (Fig. 35) [ 159]. An optically active, helical polysilane 84 with an almost single-handed screw-sense was chemically bonded or spin-coated on the surface of a quartz plate, followed by further spin-coating of an optically inactive polysilane 85 to give... 

In a related system Wittung et al. [181] reported chiral amplification involving achiral polymers composed from glycine repeating units to which a cytosine nucleobase has been attached. These molecules can form complementary base-paired helical duplexes that are analogous to those of DNA and... 

In this chapter, we mainly discuss and deal with photoresponsive, chiral biorelated and synthetic polymers bearing a configurational and/or conformational chirality in the polymer main chains. The photocontrol of the chiral recognition ability of chiral polymers and chirality induction on achiral polymer films by circular polarized light (CPL) are also briefly reviewed. 

The imprinting of polymer supports is an exciting development in the immobilization of transition metal complexes. The process involves the copolymerization of an inorganic or an organic template into a crosslinked polymer network. In a subsequent step, the template is chemically removed leaving an imprint of molecular dimensions in the resin. Ideally, the imprint retains chemical information related to the size and shape of the template. This approach has been used to prepare chiral imprints in otherwise achiral polymer networks. The method is outlined in Scheme... 

N. N. Reed, T. J. Dickerson, G. E. Boldt, K. D. Janda, Enantioreversal in the Sharpless asymmetric epoxidation reaction controlled by the molecular weight of a covalently appended achiral polymer, J. Org. Chem. 70 (2005) 1728. 

Achiral polymers synthesized from achiral monomers have been modified after polymerization using asymmetric catalysts to yield optically active polymers. For example, enantioselective ketone reduction, hydrogenation, olefin epoxida-tion, and olefin hydroxylation have been carried on the functional groups of achiral polymers [111, 112]. Such functionalizations, however, are often incomplete or occur with a low degree of asymmetric control. 

Treatment of a number of covalent polymers substituted with molecular recognition capability with suitable guests leads to chiral induction. The same crown ether-amino acid complementary pair described for the rosettes above was employed in the form of crown-ether pendant cis-transoidai poly(phenylacetylene). When the achiral polymer is treated with amino acids (in the form of their hydroperchlorate salts in acetonitdle) a large induced CD signal is observed in the backbone of the polymer. The polymer is sensitive to small enantiomeric excesses in the amino acid, as little as 0.005% enantiomeric excess of alanine can be detected. In a similar vein, c/5-transoidal poly(carboxyphenylacetylene) shows induced circular dichroism when treated with nonracemic chiral amines In addition, the system displays chiral memory, in that treatment of the complex with achiral amino alcohols results in retention of the chiral polymer backbone. 

In addition, it was also reported that circular dichroism and circular birefiingence can be induced in thin films of achiral polymer liquid crystals containing 7 to 15 mol % azobenzene chromophores by irradiation with circularly polarized light The polymer can be illustrated as follows ... 

Link, D.R., Clark, N.A., Ostrovskii, B.I., Soto Bustamante, E.A. Bilayer-by-bilayer antiferroelectric ordering in freely suspended films of an achiral polymer-monomer liquid crystal mixture. Phys. Rev. E 61, R37-R39 (2000)... 

Keith, C. Reddy, R. A. Tschierske, C. The first example of a hquid crystalline side-chain polymer with bent-core mesogenic units ferroelectric switching and spontaneous achiral symmetry breaking in an achiral polymer. Chem. Commun. 2005, 7, 871-873. 

In the case of the achiral polymer (MP2, MU2), by the open SLC mechanism is again precluded by single chain flexibility. Moreover, even growth due to the HG mechanism is mled out since no cooperative helix formation was observed. DP should therefore be close to the value predicted above by the MSOA mechanism and liquid crystallinity should be related to intrinsic characteristics of the P-U assembly (soft anisotropic interactions or the ability to form diskUke assemblies such as those documented for the supermolecule 19 described below). 

Fig. 2 Molecular models Illustrating the structural and conformational changes during the transition from (a) cisoid to (b) transoid conformatitni. Optical microscopy images show an extruded fiber sample of the achiral polymer shown in Fig. la. Corresponding wide-angle XRD patterns for the cisoid and transoid conformations are shown below. Reproduced with permission from [50], Copyright 2008 American Chemical Society...

Fig. 3 Experimental setup that demonstrates the macroscopic expansion of the oriented fiber by lifting of a dime on the inclined plane of a Mettler hot stage (top). Expanded images collected at 25°C bottom left) and at 80°C bottom right) of the oriented fiber generated from the achiral polymer in Fig. la during lifting of 250-times its weight via thermally fueled unwinding of its helix at the cisoid-to-transoid transition. Reproduced with permission Irom [50]. Copyright 2008 American Chemical Society...

Toda et al. recently reported quite interesting results for the achiral polymers polyethylene [99, 100] and poly(vinylidene fluoride) [112]. Special crystallization cells enabled the determination of growth rate G for banded spherulites at large undercoolings. In addition to G, the maximum lamellar width and the band spacing S were measured as a function of T. As in the analysis of unbanded spherulites described in Section 3.4.3, it was found that the morphological sizes scaled with the square root of the diffusion length ... 

---