In the title paper we reported the I3C NMR and X-ray diffraction spectra of poly[(S)-4-methyI-l -hexenel obtained in the presence of a catalytic system based on isopropylidene(cyclopentadienyl)(9-fluorenyl)zirconium dichloride and methylaluminoxane which promotes syndiotactic-specific polymerization of a-olefins. The polymer is optically active ([a]i5 = 219) and crystalline, and the I3C NMR and X-ray diffraction spectra are different from the corresponding spectra of isotactic poly[(S)-4-methyl-1 -hexenel. We concluded that the polymer was syndiotactic and interpreted the splitting of the resonances of the methylene carbons of the backbone observed even in solution at 40,4 and 40,6 ppm as arising from a possibly twofold asymmetric helix conformation of the chain stabilized by the asymmetric substituents. This could be the case but, during a discussion on the matter, Professor M. Farina pointed out that a syndiotactic polymer must have first neighbouring monomer units of opposite configuration '1. The truly syndiotactic structure cannot be obtained from homopolymerization of a single enantiomer of a chiral a-olefin but only from alternate copolymerization of the two enantiomers. The polymer structure we proposed is diastereomeric with respect to both isotactic poly[(S)-4-methyl-l -hexenel and syndiotactic alternating copoly[(R, S)-4-methyl-1 -hexenel. The methylene carbons of the first neighbouring monomer units are diastereotopic and, in principle, could have different chemical shifts, no matter what the conformation of the chain. This can be visualized by considering that the only symmetry element of the model cyclodimer reported in Fig. l(a) is the twofold rotational axis (C,) passing through the methylene carbons labelled C' and Cz, and that by this symmetry operation C' and C2 do not interchange to each other.

Syndiotactic poly[(S)-4-methyl-1-hexene]

ZAMBELLI, ADOLFO;GALIMBERTI, MAURIZIO STEFANO;
1992-01-01

Abstract

In the title paper we reported the I3C NMR and X-ray diffraction spectra of poly[(S)-4-methyI-l -hexenel obtained in the presence of a catalytic system based on isopropylidene(cyclopentadienyl)(9-fluorenyl)zirconium dichloride and methylaluminoxane which promotes syndiotactic-specific polymerization of a-olefins. The polymer is optically active ([a]i5 = 219) and crystalline, and the I3C NMR and X-ray diffraction spectra are different from the corresponding spectra of isotactic poly[(S)-4-methyl-1 -hexenel. We concluded that the polymer was syndiotactic and interpreted the splitting of the resonances of the methylene carbons of the backbone observed even in solution at 40,4 and 40,6 ppm as arising from a possibly twofold asymmetric helix conformation of the chain stabilized by the asymmetric substituents. This could be the case but, during a discussion on the matter, Professor M. Farina pointed out that a syndiotactic polymer must have first neighbouring monomer units of opposite configuration '1. The truly syndiotactic structure cannot be obtained from homopolymerization of a single enantiomer of a chiral a-olefin but only from alternate copolymerization of the two enantiomers. The polymer structure we proposed is diastereomeric with respect to both isotactic poly[(S)-4-methyl-l -hexenel and syndiotactic alternating copoly[(R, S)-4-methyl-1 -hexenel. The methylene carbons of the first neighbouring monomer units are diastereotopic and, in principle, could have different chemical shifts, no matter what the conformation of the chain. This can be visualized by considering that the only symmetry element of the model cyclodimer reported in Fig. l(a) is the twofold rotational axis (C,) passing through the methylene carbons labelled C' and Cz, and that by this symmetry operation C' and C2 do not interchange to each other.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11311/999423
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