Molecular Structure of Isotactic Polypropylene Formed from Homogeneous Solution. Gelation and Crystallization

The molecular structure and morphology grown from isotactic polypropylene (iPP)/o-dichlorobenzene solution were investigated by thermal analysis and high-resolution solid-state 13C NMR. The morphology varied depending on incubated temperature. A gelation occurred below 70°C, while the crystallization took place over 80°C. The crystal obtained was identified with the α-form, which is the most stable crystalline modification of iPP. In CP/MAS 13C NMR spectrum, the chemical shift of methylene, methine, and methyl carbons provided almost the same shift for both the gel and α-form, but the characteristic doublet for methylene and methyl signals of the gel was different in intensity ratio with the α-form. These results indicate that the gel-forming crystal is identified with α1-form, being the disordered α-form. Indeed, the lower melting temperature and smaller melting enthalpy than the crystal were observed by thermal analysis. This result connects to the incompleteness of crystal formed in the gel.