Introduction.Recently, syndiotactic polypropylene (sPP) with high stereoregularity was synthesized by Ewen et al. 1 Whereas early investigations carried out by using less syndiotactic samples had difficulties in precise characterization, the success of highly stereoregular polymerization made the detailed analysis of the crystal structure possible. A large number of investigations on the molecular chain conformation and molecular packing in crystals have also been reported for sPP. As for the polymorphs, four crystalline forms are known: the planar zigzag, 2-5 t 2 g 2 t 6 g 2 , 6-8 and two forms with the t 2 g 2 conformation. 9-22 Of these, the crystals with the planar zigzag conformation have been believed to be formed only by cold drawing after quenching from the melt into an ice-water mixture. [2][3][4][5] Very recently, we have found that the crystallization of the planar zigzag form is spontaneously induced at 0 °C without any mechanical stress, although the growth rate is very slow. In this communication, we report briefly the first results to confirm such spontaneous crystallization of the form at 0 °C, which have been obtained by CP/MAS 13 C NMR and wide-angle X-ray scattering methods.Experimental Section. A highly syndiotactic polypropylene sample with an rr fraction of 0.96, provided by Sumitomo Chemical Co. Ltd., was used without further purification. CP/MAS 13 C NMR spectra were measured at room temperature on a Bruker MSL 200 spectrometer under a static magnetic field of 4.7 T. Magic angle spinning at 3 kHz was achieved with the double air bearing system. The contact time for the cross polarization process was 1 ms and the pulse delay time was 4 s. 13 C chemical shifts were expressed as values relative to tetramethylsilane (Me 4 Si) by using the CO carbon line at 176.03 ppm of glycine crystals as an external reference. Wide-angle X-ray diffraction patterns were obtained at room temperature on an automatic RIGAKU diffractometer with Ni-filtered Cu KR radiation.Results and Discussion. Figure 1 shows CP/MAS 13 C NMR spectra of sPP films quenched in ice-water from the melt at 170 °C, which were measured at room temperature. The times shown in this figure indicate the periods for which each film was kept in ice-water. As shown in Figure 1a, the film taken out immediately after quenching in ice-water gives a spectrum similar to the previously reported one; 23 two resonance lines assignable to the methylene carbons clearly appear at 47.7 and 39.1 ppm. Although the downfield line at 47.7 ppm seems to be composed of multiple components

The spontaneous crystallization process of the planar zigzag form (form III) of syndiotactic polypropylene has been investigated at 0 °C for samples quenched from the melt by real-time wide-angle X-ray diffractometry and high-resolution solid-state 13 C NMR spectroscopy. The X-ray diffraction profile just after quenched from the melt is in good accord with that of the melt, indicating that the sample is in the noncrystalline state. With increasing crystallization time, broad diffraction peaks assignable to form III are found to evidently appear for the diffraction profile obtained after the subtraction of the noncrystalline contribution. The spontaneous crystallization of form III is also confirmed by high-resolution solid-state 13 C NMR spectroscopy; for example, the CH3(I) line assigned to the tt conformation really increased in intensity with increasing crystallization time in concomitancy with the decrease of the CH3(II) line ascribed to the tg conformation. The line shape analysis of these CH3 lines reveals that the degree of crystallinity rapidly increases up to about 0.10 in 5 h, but this value is still as low as 0.13 even after 4500 h. The additional crystallization of form I with the ttgg conformation is also found to be highly hindered for the sample kept at 0 °C for a longer period when it is annealed at room temperature, although this form is readily produced for the sample held at 0 °C for a shorter period. On the basis of these results, a structural model for the crystallization of form III at 0 °C is proposed by considering the possible formation of aggregates composed of the segmental chains with the trans-rich conformations in the noncrystalline region.

Crystal transformation behavior and structural changes for the planar zigzag form (form III) of syndiotactic polypropylene, which is spontaneously crystallized at 0 °C from the melt, have been investigated in the heating process by DSC, wide-angle X-ray diffractometry, and high-resolution solidstate 13 C NMR spectroscopy. The DSC curve for the sample containing only form III crystallites, which is crystallized at 0 °C for 5000 h, shows an evident endothermic peak at about 20 °C and an additional exothermic peak at about 100 °C prior to melting at about 150 °C. The endothermic peak, which is shifted up to about 40 °C by annealing at room temperature, is found to be ascribed to the crystal transformation from form III with the planar zigzag conformation to form II composed of isochiral helices with the ttgg conformation by wide-angle X-ray diffractometry. Moreover, the CH 3 resonance lines of the dipolar decoupling/MAS 13 C NMR spectra measured at different temperatures for the sample with almost the same structure as for DSC and WAXD measurements are well resolved into the components with the tt and tg conformations for the CH2-CH(CH3)-CH2 bond. As a result, it is found that about 16% form III is allowed to be transformed into form II, whereas most of form III is melted gradually in the transition temperature region and very rapidly above that temperature. Such melting of form III should induce the crystallization of form I above about 60 °C, and the final degree of crystallinity of form I attains to almost the same level as for the original form III. This process is really reflected on the appearance of the exothermic peak at about 100 °C in the DSC curve described above. An annealing effect of form III crystallites has also been examined just below the crystal transformation temperature from form III to II.