Helix Sense-Selective Polymerization of Chloral with Chiral Alkoxide Initiators: Tuning the Chiroptical Properties of the Polymer

Abstract

The helix sense-selective polymerization of chloral with chiral lithium alkoxides derived from R(−), S(+)-2-octanol and four mono-hydroxy sterols afforded optically active polymer samples with high solid state optical rotations at the sodium D-line (589 nm). This chiroptical behavior stemmed from a predominance of a single screw direction along the polymer’s rigid helical backbone. The sign and magnitude of the optical rotation enhancements were found to depend upon both initiator structure and the conditions under which the polymerizations were carried out. By carefully manipulating experimental conditions, polychloral samples with specific rotations ranging from (+) 4000° to (−) 4000° were readily prepared. Model end-group studies confirmed that the presence of asymmetric initiator fragments in the polymer contributed little to the observed optical rotations reported here. Polychloral prepared with achiral or racemic initiators was optically inactive.