Chirality analysis of helical polymers

Abstract

Optically active macromolecules, having a preferred- or single-handed helical conformation, play important roles in polymeric materials and in life. This article presents how helical polymers can be assessed from a view of chirality. These assessments, based on optical rotation (OR) and circular dichroism (CD) spectral measurements with associated information, include theoretical spectral calculations as well as chromatographic resolution. Specific applied examples are discussed for poly(9,9-dioctylfluorene-2,7-diyl) and derivatives, stereoregular polyolefins bearing centers of chirality in the side chain, isotactic poly(triphenylmethyl methacrylate), and π-stacked poly(dibenzofulvene). For more convincing establishment of a helix, it is important to correlate chiroptical properties with related information such as molar-mass effects, temperature effects, and chemical transformation effects on the properties. Helices of the polyolefins and poly(TrMA) were confirmed considering these aspects. In addition, comparison of chiroptical properties between the polymer in question and a monomeric unit model compound generally helps to confirm a helix. There are no general, reliable methods to quantify helical sense excess. On the other hand, absolute helical sense can be determined by comparing experimental and theoretical CD spectra as well as considering exciton coupling effects in CD spectra. The former method can be more generally applied for a wide range of polymers as far as a good model for calculations is designed, while the latter can be more conveniently and empirically applied for a certain class of polymer structures that are suited to exhibit exciton coupling without using computers.

Distinctive Chiral Conformations Induced to Poly (naphthalene‐1,4‐diyl) by Helix–sense–selective Polymerization and Circularly Polarized Light Irradiation

“…of the standard compound is unambiguously determined and that the chemical structure of the standard compound is similar enough to the units of polymer. [28] 1,1'-Binaphthyl as the standard compound fulfills both conditions, and the above results are hence reliable.…”

“…It should be noted that experimental determination of helix‐sense excess is generally difficult [27,28] . The CD–intensity based method employed here requires the conditions that the e.e.…”

“…It should be noted that experimental determination of helix-sense excess is generally difficult. [27,28] The CD-intensity based method employed here requires the conditions that the e.e. of the standard compound is unambiguously determined and that the chemical structure of the standard compound is similar enough to the units of polymer.…”