Solid-State13C Nuclear Magnetic Resonance Spectra of Four Crystalline Forms of Isotactic Poly(4-methyl-1-pentene)

Four crystalline forms of isotactic poly(4-methyl-1-pentene) (i-P4MP) (forms I, II, II, and IV) have been studied through solid-state 13 C NMR CP-MAS spectroscopy. The assignment of the resonances observed in the 13 C NMR CP-MAS spectra was made by dipolar dephasing experiments. The presence in all spectra of two distinct resonances, separated by ≈3 ppm, of the methyl carbons indicates that the two methyls belonging to the monomeric unit are nonequivalent. This has been explained on the basis of the γ- gauche shielding interaction and of the conformations of polymer chains and lateral groups. The resonances observed in the spectrum of form III are markedly deshielded compared to those of other forms, whereas the resonances of form I are deshielded compared to those of form IV. This deshielding has been consistently interpreted on the hypothesis that the amount of the γ- gauche shielding effect depends on the value of the torsion angles of the main chain. Indeed, the torsion angles of the main chain for the 7/2 helical conformation of form I deviate, on average, by 12−15° from the exact staggered values of 60° and 180° (the average values of the torsion angles are θ 1 = 72° and θ 2 = 195°); they deviate by 22−26° for the 4/1 helical conformation of form III (θ 1 = 82° and θ 2 = 206°). The γ- gauche shielding parameter, normally −5 ppm when the gauche angle is nearly 60°, decreases to a value between −2 and −2.5 ppm when the torsion angle deviates by nearly 20° from 60°. In the form IV, the upfield shift of the resonances, compared to forms I and III indicates that the carbon atoms experience a full γ- gauche shielding interaction and, hence, the main chain torsion angles θ 1 and θ 2 are near 60 and 180°, respectively, typical of a helical 3/1 conformation. This analysis allows us to suggest that the form IV of i-P4MP is characterized by chains in 3/1 helical conformation. The resonances of the methyl carbons of the four forms of i-P4MP show narrow splittings (1−2 ppm) which should be traced back to packing effects. This has provided a chance of testing the packing model of form I and form III proposed in the literature. In the spectrum of form I, the resonance of backbone methine carbons is also split in a doublet. This splitting is due to conformational effects, rather than packing effects and confirms that, according to the refinement of the crystal structure of form I, the chains assume a helical conformation slightly distorted from the uniform 7/2 helix. The succession of torsion angles along the main chain is ...TGT‘G‘T‘‘G‘‘..., instead of (TG) n . Therefore, backbone methine carbons of different monomeric units are conformationally nonequivalent because they experience different amounts of γ- gauche shielding effects owing to different values of the gauche torsion angle θ 1 of different monomeric units.