Observation-of resonances associated with stereo and regio defects in the crystalline regions of isotactic polypropylene: Toward a determination of morphological partitioning

We report defect-resonance patterns associated with two kinds of low-concen tration defects typically found in metallocene-synthesized isotactic polypr opylenes (iPP's). These defects are the simple mrrm stereo defect and the r egio 2,1-erythro defect. This work is a critical part of our effort to dete rmine the extent to which various defects, typically found in isotactic pol ypropylene (iPP) samples, are incorporated into the crystalline regions of this semicrystalline polymer. The relationship between defect concentration s and mechanical (as well as thermal) properties is quite dependent on the extent of incorporation of defects into the crystalline regions. Several me lt-crystallized (at a cooling rate of 1 degrees C/min) iPP samples, whose c oncentrations of various stereo and regio defects are known from high-resol ution NMR, have been examined in the solid state by C-13 NMR. Using a metho d 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 t he simple mrrm stereo defect and the regio 2,1-erythro defect have been ide ntified. One of our samples, having a rather low molecular weight, containe d a substantial amount of the regio 1,3 defect. The corresponding CR spectr um had no sharper resonances that would indicate the presence of 1,3 defect s in the CR lattice. Associated with each type of defect, "i", we define a partitioning coefficient, P-CR(i), as the ratio of the ith-defect concentra tion in the CR region to the overall ith-defect concentration. While we can not, at this point, be absolutely sure about assignments which ultimately d ictate the crucial correspondence between defect populations and defect int ensities, we can make arguments or assumptions about this correspondence an d then suggest P-CR values for the stereo and regio defects. On the basis o f the arguments and assumptions made herein, the following values are obtai ned: P-CR(stereo: mrrm) = 0.48 +/- 0.06 and P-CR(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 pos sessed mixed amounts of alpha- and gamma-crystallites. The few indications we have suggest that there is only a weak dependence, if any, on kinetics o r crystal habit. The hypothesis is considered that those defects seen in th e CR spectrum are highly concentrated at the CR/NC interface. On the basis of a modeling of the experimental proton polarization, including spin diffu sion, it is concluded that the defects are not highly concentrated on the C R 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; Final ly, one of our samples was an iPP pseudofraction derived from an inhomogene ous Ziegler-Natta polymerization. P-CR(stereo) for this sample was lower th an for the metallocene iPP's, including one with a similar overall defect c oncentration. Our results support the notion hat the Ziegler-Natta pseudofr action consists of a much more inhomogeneous distribution of defect concent rations per chain than that which typifies metallocene iPP's.