Hauptkettenchiralität und optische Aktivität von Polymeren aus C‐C‐Ketten

Abstract

Hauptkettenchiralitat ist die Chiralitat eines Polymers, die allein auf der konfigurativen oder konformativen Anordnung in der Hauptkette beruht. Durch systematische Symmetriebetrachtungen werden die wichtigsten Strukturtypen auf Chiralitat untersucht. Das uberraschende Ergebnis ist, daB auch bei Polymeren aus 1-substituierten und unsymmetrisch 1,l-disubstituierten Olefinen -den technisch wichtigsten Polymeren -zahlreiche chirale Strukturtypen auftreten, die bei Vorliegen eines einheitlichen Enantiomers auch optische Aktivitat erwarten lassen. Durch eine asymmetrische Cyclopolymerisation konnten bestimmte Strukturtypen als optisch aktive Co-oder Homopolymere realisiert werden, z. B. Copolymere von Styrol-Methylmethacrylat oder auch Polystyrol. Eine weitere neue Gruppe von optisch aktiven Polymeren sind die atropisomeren, helicalen Polyisocyanide, Polytritylmethacrylate oder Polychlorale. -Optisch aktive Polymere werden bereits als Adsorbentien fur die chromatographische Racemattrennung eingesetzt ; weitere Anwendungsmoglichkeiten zeichnen sich ab.

Transfer of Chirality from (−)‐Sparteine Zinc(II) (Meth)acrylate Complexes to the Main Chains of Their (Meth)acrylate Polymer Derivatives

“…Surprisingly, however, when dichloroethane (DCE) was used as the polymerization solvent (Table 2), even after 20 h of reaction the reaction mixtures remained homogeneous and the products 2 a , 4 a , 6 a , and 8 a had to be recovered by precipitation in excess methanol. These copolymers were readily resoluble in DCE and chloroform which, together with the absence of any signals from pendent vinyl groups in the 1 H NMR and FTIR spectra, is most easily explained in terms of the ZMS and ZAS comonomers undergoing selective cyclopolymerization of their two vinyl groups, as elegantly demonstrated by Wulff with his covalently templated bis‐styrylboronate comonomer 1. Whether or not some ZMS‐ or ZAS‐derived segments are incorporated with insertion of one or more styrene segments between the (meth)acrylate segments is impossible for us to say.…”

“…In general, symmetry operations are more difficult to recognize in a long polymer chain made up of uniformly repeating units than in low‐molecular‐weight compounds. Wulff, however, recognized that only one specific sequence of eight possible hexad sequences of a homopolymer of a monosubstituted vinyl monomer is chiral 1. Not surprisingly it is extremely difficult to control a vinyl polymerization to introduce such a chiral hexad sequence in a polymer backbone.…”

“…One of the methods proposed by Wulff to reduce this difficulty was to introduce a second repeat unit in the polymer backbone to help break down the overall symmetry. For such copolymers, triad sequences (Figure 1) in which a syndiotactic (or racemic) diad sequence of one comonomer is spaced apart by a repeat unit of another comonomer would be chiral 1. Indeed Wulff et al went on to complete some extremely elegant syntheses that involved the use of covalently bound chiral templates to confirm that his rationalizations were correct 1.…”

Synthesis of a series of soluble main-chain chiral nonracemic poly(alkyl-aryl ketone)

“…In order to achieve such goals materials must be stable and possess functional groups. Considerable attention has been focused on the synthesis of optically active polymers with chirality in the main-chain because the polymers typically exist or have the tendency to form helical and rigid-rod configuration, which offers enhanced capability in chiral applications [5][6][7][8]. Although it has constantly been great synthetic challenges in constructing mainchain chiral polymers, there are a number of strategies developed to build chirality into the polymer backbones.…”

“…The asymmetric polymerization requires a careful design of catalysts and laborious screening for the right chiral ligands. Direct polymerization of chiral monomers usually results in polymers of decreased processibility, which has been overcome by attaching long alkyl substituents [5,6]. The drawback is the alteration of rheological behavior of the polymers.…”