Solid-state 13C nuclear magnetic resonance spectra of four crystalline forms of isotactic poly(4-methyl-1-pentene)
Solid-state 13C nuclear magnetic resonance spectra of four crystalline forms of isotactic poly(4-methyl-1-pentene)
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DOI：
10.1021/ma970706m
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发表时间：
1997-12-29
期刊：
影响因子：
5.5
通讯作者：
Cosco, S
中科院分区：
文献类型：
--
作者：
De Rosa, C;Capitani, D;Cosco, S
本文用固态C-13 NMR CP-MAS谱研究了四种全同立构聚（4-甲基-1-戊烯）（i-P4 MP）的晶型（晶型I、II、II和IV）。在C-13 NMR CP-MAS光谱中观察到的共振的归属由偶极移相实验进行。在所有光谱中存在两个不同的共振，分开约3 ppm，甲基碳表明，属于单体单元的两个甲基是不等价的。这已经根据γ-左屏蔽相互作用以及聚合物链和侧基的构象来解释。在III型的光谱中观察到的共振与其它形式的共振相比明显去屏蔽，而I型的共振与IV型的共振相比去屏蔽。这种去屏蔽一直被解释的假设，即γ-gauche屏蔽效应的量取决于主链的扭转角的值。实际上，I型7/2螺旋构象的主链扭转角平均偏离，与60度和180度的精确交错值相差12-15度（扭转角的平均值为θ（1）= 72度和θ（2）= 195度），对于4/1螺旋构象的形式III（θ（1）= 82度和θ（2）= 206度）。当偏转角接近60度时，γ-偏转角屏蔽参数通常为-5ppm，当扭转角偏离60度接近20度时，γ-偏转角屏蔽参数降低到-2至-2.5ppm之间的值。在IV型中，与I型和III型相比，共振的高场位移表明碳原子经历了完全的γ-交错屏蔽相互作用，因此，主链扭转角θ（1）和θ（2）分别接近60度和180度，这是典型的螺旋3/1构象。该分析使我们能够提出i-P4 MP的IV型的特征在于3/1螺旋构象的链。四种形式的i-P4 MP的甲基碳的共振显示出窄的分裂（1-2 ppm），这应该追溯到堆积效应。这为检验文献中提出的I型和III型堆积模型提供了机会。在I型的光谱中，骨架次甲基碳的共振也分裂成双峰。这种分裂是由于构象效应，而不是堆积效应，并证实，根据I型晶体结构的细化，链呈现从均匀的7/2螺旋稍微扭曲的螺旋构象。扭转角沿着主链的顺序是... TGT'G“T“G“...而不是（TG）（n）。因此，不同单体单元的主链次甲基碳在构象上是不等价的，因为它们由于不同单体单元的不同的扭曲角θ（1）值而经历不同量的γ-扭曲屏蔽效应。
Four crystalline forms of isotactic poly(4-methyl-1-pentene) (i-P4MP) (forms I, II, II, and IV) have been studied through solid-state C-13 NMR CP-MAS spectroscopy. The assignment of the resonances observed in the C-13 NMR CP-MAS spectra was made by dipolar dephasing experiments. The presence in all spectra of two distinct resonances, separated by approximate to 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 gamma-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 gamma-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 degrees from the exact staggered values of 60 degrees and 180 degrees (the average values of the torsion angles are theta(1) = 72 degrees and theta(2) = 195 degrees) they deviate by 22-26 degrees for the 4/1 helical conformation of form III (theta(1) = 82 degrees and theta(2) = 206 degrees). The gamma-gauche shielding parameter, normally -5 ppm when the gauche angle is nearly 60 degrees, decreases to a value between -2 and -2.5 ppm when the torsion angle deviates by nearly 20 degrees from 60 degrees. In the form IV, the upfield shift of the resonances, compared to forms I and III indicates that the carbon atoms experience a full gamma-gauche shielding interaction and, hence, the main chain torsion angles theta(1) and theta(2) are near 60 and 180 degrees, 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 gamma-gauche shielding effects owing to different values of the gauche torsion angle theta(1) of different monomeric units.