Observation of Resonances Associated with Stereo and Regio Defects in the Crystalline Regions of Isotactic Polypropylene: Toward a Determination of Morphological Partitioning

Abstract

We report defect-resonance patterns associated with two kinds of low-concentration defects typically found in metallocene-synthesized isotactic polypropylenes (iPP's). These defects are the simple mrrm stereo defect and the regio 2,1-erythro defect. This work is a critical part of our effort to determine the extent to which various defects, typically found in isotactic polypropylene (iPP) samples, are incorporated into the crystalline regions of this semicrystalline polymer. The relationship between defect concentrations and mechanical (as well as thermal) properties is quite dependent on the extent of incorporation of defects into the crystalline regions. Several melt-crystallized (at a cooling rate of 1 °C/min) iPP samples, whose concentrations of various stereo and regio defects are known from high-resolution NMR, have been examined in the solid state by 13C NMR. Using a method based on differences in the rotating-frame proton relaxation times of the crystalline (CR) and the noncrystalline (NC) regions, signals from the CR and the NC regions are separated. The resulting “CR” spectra, pertaining to the CR regions of the iPP, are examined for distinct resonances associated with such defects; relative integrals associated with these resonances are also determined. Definite defect-resonance patterns associated with both the simple mrrm stereo defect and the regio 2,1-erythro defect have been identified. One of our samples, having a rather low molecular weight, contained a substantial amount of the regio 1,3 defect. The corresponding CR spectrum had no sharper resonances that would indicate the presence of 1,3 defects in the CR lattice. Associated with each type of defect, “i”, we define a partitioning coefficient, PCR(i), as the ratio of the ith-defect concentration in the CR region to the overall ith-defect concentration. While we cannot, at this point, be absolutely sure about assignments which ultimately dictate the crucial correspondence between defect populations and defect intensities, we can make arguments or assumptions about this correspondence and then suggest PCR values for the stereo and regio defects. On the basis of the arguments and assumptions made herein, the following values are obtained: PCR(stereo: mrrm) = 0.48 ± 0.06 and PCR(regio: 2,1-erythro) = 0.28 ± 0.08. In principle, partitioning coefficients might depend on both on the crystallization kinetics and the crystal habit. Many of our samples possessed mixed amounts of α- and γ-crystallites. The few indications we have suggest that there is only a weak dependence, if any, on kinetics or crystal habit. The hypothesis is considered that those defects seen in the CR spectrum are highly concentrated at the CR/NC interface. On the basis of a modeling of the experimental proton polarization, including spin diffusion, it is concluded that the defects are not highly concentrated on the CR side of the interface; at the same time, we have no information about the possibility of defect concentration on the NC side of the interface. Finally, one of our samples was an iPP pseudofraction derived from an inhomogeneous Ziegler−Natta polymerization. PCR(stereo) for this sample was lower than for the metallocene iPP's, including one with a similar overall defect concentration. Our results support the notion that the Ziegler−Natta pseudofraction consists of a much more inhomogeneous distribution of defect concentrations per chain than that which typifies metallocene iPP's.