Optically Active Silicon-Containing Polymers

Abstract

Optically active chiral macromolecules have been around since the dawn of time and indeed our whole universe, from atoms upwards, is chiral [1] In biological systems, at least, it is not the presence of optical activity which is remarkable, but rather its absence. DNA is a classic example of a chiral macromolecule, its chirality deriving from two features: (i) the incorporation of chiral sugars (to which are attached chromophoric bases such as adenine, guanine, cytosine and thymine) and (ii) the macromolecular helical conformation arising from base stacking in hydrogen-bonding solvents (a helix is a chiral motif). The task of covalently linking small molecules to form well defined, single screw sense, rigid helical rod polymers with a single molecular weight is a longstanding issue in modern polymer stereochemistry [2]. Such polymers are usually produced only during the course of precisely controlled polymerisation reactions using very specialised monomers and stereospecific catalysts [3]. The synthesis and quantitative conformational analysis by direct spectroscopic characterisation of such ideal polymers, therefore, are very challenging [4]. Synthetic polymers are non-ideal, however, comprising a mixture of molecular weights and stereoisomers and the most prominent properties of the ideal polymer remain a challenge. Synthetic polymers containing enantiopure chiral side groups including polyisocyanides [5], polyisocyanates [6], polyacetylenes [7], polythiophenes [8] poly(p-phenylenevinylene)s [9] and polysilanes, [10] may also adopt preferential screw sense (PSS) helical backbone conformations because of side group interactions. Concerning the analysis of optical active materials, there are several techniques available: optical rotation (rotation of the plane of linearly polarised light on passing through the sample), ellipticity (almost never measured directly), single crystal X-ray crystallography (when crystals can be grown) and circular dichroism (CD; differential absorption of left and right circularly polarised light). For the purposes of structure elucidation, the last two techniques provide the most information, but in the case of most macromolecules, X-ray crystallography is not feasible due to the lack of suitable crystals. Thus, the most appropriate technique for the analysis of optically active polymers is CD spectroscopy, which permits the direct analysis of chiral backbone physical and electronic structures.