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Incorporation of Stereo, Regio and Comonomer Defects Into the Crystalline Regions of Isotactic Polypropylene: Results From NMR, Molecular Dynamics and Chemical Shift Calculations

Published

Author(s)

David L. VanderHart, Marc R. Nyden, R G. Alamo, L Mandelkern

Abstract

We have shown that under conditions of crystallization during cooling at 1C/min, substantial amounts of 'defects' of the types a) stereo-mrrm, b) regio-2,1-erythro, c) ethylene-comonomer and d) butylene- comonomer can be found in the crystalline regions, the regio defect being most strongly rejected. We have also shown that the important resonance-assignment question can be addressed using computational approaches for 1) finding minimum-energy conformations in the crystal lattice and 2) doing ab initio chemical shift calculations to predict spectra associated with the defects. From the agreement in shifts between calculation and experiment we can also argue for certain general conformations near these defects. A corollary finding is that the one Ziegler-Natta pseudo-fraction, whose defect population was mainly stereo-mrrm, displayed a PCR value significantly less than a correponding metallocene iPP. We took this as support for the non-uniform distribution of defects/per/chain in Ziegler-Natta iPP's. We also conclude that these defects are not highly concentrated on the CR side of the CR/NC interface. At the same time, we obtain no information about the possibility that defects concentrate on the NC side of that interface. The different partitioning coefficients add another dimension to the control of iPP properties via a tailoring of defect concentrations. The inclusion of defects also suggests a reexamination of the interpretation of DSC melting endotherms in terms of crystallinity, since, in the presence of defects, one can not assume that the heat of fusion is a fixed value.
Proceedings Title
Preprints of the American Chemical Society, Polymer Science and Engineering Div.
Conference Dates
March 1, 2000
Conference Location
Undefined
Conference Title
Polymer Preprints

Keywords

assignment, c-13 NMR, chemical shifts, comonomer, computation, defects, isotactic polypropylene, partitioning, stereo

Citation

VanderHart, D. , Nyden, M. , Alamo, R. and Mandelkern, L. (2008), Incorporation of Stereo, Regio and Comonomer Defects Into the Crystalline Regions of Isotactic Polypropylene: Results From NMR, Molecular Dynamics and Chemical Shift Calculations, Preprints of the American Chemical Society, Polymer Science and Engineering Div., Undefined (Accessed April 20, 2024)
Created October 16, 2008