The chain conformation and the crystal packing of syndiotactic poly(4-methyl-1-pentene) have been studied by solid state C-13 NMR cross polarization/magic angle spinning (CPMAS) spectroscopy. The presence in the CPMAS spectrum of two distinct resonances separated by approximate to 5 ppm (one gamma-gauche effect) for the methyl carbons indicates that the two methyl carbons belonging to the monomeric unit are nonequivalent. This has been explained on the basis of the conformation of the lateral groups proposed in the literature. A single resonance of the backbone methylene carbons, although broad and with various shoulders, is present. The absence of the splitting of approximate to 10 ppm for this resonance, typical of other syndiotactic polymers having TTGG helical conformation, like polypropylene and poly(1-butene), is still fully consistent with the helical TTGG conformation of the chains of syndiotactic poly(4-methyl-1-pentene), since the conformational assignment of the lateral groups makes the two methylene carbons in conformational environments TG.GT and GT.GT, nearly equivalent. The narrow splitting of the resonance of the backbone methine carbons indicates that the helical TTGG conformation of the chains, approximately described with a s(12/7)2 symmetry, should be better described with a complex nonuniform helix with slightly different values of the backbone torsion angles of different structural units. In this nonuniform helix, backbone methine carbons of different structural units are nonequivalent since they experience different amounts of the gamma-gauche shielding effect resulting in the asymmetric splitting of the corresponding resonance. Both resonances of the methyl carbons present narrow splittings, which can be traced back to packing effect. These splittings are easily interpreted on the basis of the model of packing proposed in the literature.

Solid state C-13 nuclear magnetic resonance spectrum of syndiotactic poly(4-methyl-1-pentene)

GRASSI, Alfonso;
1998

Abstract

The chain conformation and the crystal packing of syndiotactic poly(4-methyl-1-pentene) have been studied by solid state C-13 NMR cross polarization/magic angle spinning (CPMAS) spectroscopy. The presence in the CPMAS spectrum of two distinct resonances separated by approximate to 5 ppm (one gamma-gauche effect) for the methyl carbons indicates that the two methyl carbons belonging to the monomeric unit are nonequivalent. This has been explained on the basis of the conformation of the lateral groups proposed in the literature. A single resonance of the backbone methylene carbons, although broad and with various shoulders, is present. The absence of the splitting of approximate to 10 ppm for this resonance, typical of other syndiotactic polymers having TTGG helical conformation, like polypropylene and poly(1-butene), is still fully consistent with the helical TTGG conformation of the chains of syndiotactic poly(4-methyl-1-pentene), since the conformational assignment of the lateral groups makes the two methylene carbons in conformational environments TG.GT and GT.GT, nearly equivalent. The narrow splitting of the resonance of the backbone methine carbons indicates that the helical TTGG conformation of the chains, approximately described with a s(12/7)2 symmetry, should be better described with a complex nonuniform helix with slightly different values of the backbone torsion angles of different structural units. In this nonuniform helix, backbone methine carbons of different structural units are nonequivalent since they experience different amounts of the gamma-gauche shielding effect resulting in the asymmetric splitting of the corresponding resonance. Both resonances of the methyl carbons present narrow splittings, which can be traced back to packing effect. These splittings are easily interpreted on the basis of the model of packing proposed in the literature.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11386/3136629
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