Vibrational spectroscopic investigation of stereoregularity effects on syndiotactic polypropylene structure and morphology

Vibrational spectroscopy is used to sensitively detect specific morphologies and microstructures present in metallocene-catalyzed syndiotactic polypropylenes (sPP). Six materials, ranging in racemic triad content from 26 to 96% rr, are studied. Changes in high-resolution infrared (IR) and Raman spectra of melt-slow-cooled films are observed as the degree of syndiotacticity varies. Three different types of peak behavior are observed: splitting, wavenumber shift, and change in peak intensity. An overall trend toward greater molecular order (e.g. ordered chain conformations, increased crystallinity) is observed as syndiotacticity increases. By combining results with supporting evidence from X-ray diffraction and IR linear dichroism experiments of highly syndiotactic sPP, new peak assignments are proposed for tacticity-sensitive vibrational bands. Some very interesting spectral behavior is observed for material of intermediate stereoregularity (49% rr). Previously unobserved peaks appear in X-ray diffractograms and IR absorbance spectra, suggesting the presence of an as-yet unidentified “transitional” structure—perhaps a disordered modification of crystalline Form I. This moderate level of syndiotacticty appears to be a critical point or threshold below which sPP chains are unable to adopt characteristic helical or planar zigzag conformations. Results from this work provide a more thorough understanding of stereochemical effects on vibrational spectra, which will be very useful in the interpretation of ongoing IR linear dichroism studies of newly available semi-syndiotactic (semi-sPP) materials.