No abstract

A 13C NMR method is presented for a quantitative determination of the respective comonomer distributions, the triad distributions, average sequence lengths, and run numbers in ethylene-l-hexene copolymers. Complete 13C NMR chemical shift assignments were formulated after observation of two reference systems, a poly(l-hexene) and an ethylene-l-hexene copolymer containing only 1.9 mol % 1-hexene. A second copolymer having 17.3 mol % 1-hexene allowed an observation of all the intermediate connecting sequences between the extremes of the two reference systems. A subsequent quantitative procedure, which avoids errors from peak overlap and uncertainties in configurational assignments, was developed by using relative areas from well-spaced 13C NMR spectral regions, defined uniquely in terms of contributing triad sequences. The method is designed to obtain a triad distribution with the highest possible accuracy, after which, structural information meaningful to the polymer chemist is extracted.

The microstructures of two poly(propylene)s with matched molar masses and overall defect concentrations are inferred from the crystallization behavior of their narrow molar mass fractions. One poly(propylene) was produced with a MgCl2-supported Ziegler-Natta catalyst and the other with a metallocene catalyst. The fractions obtained from the metallocene isotactic poly(propylene) display a range in molar masses but each has the same defect concentration indicating a uniform intermolecular concentration of defects in the parent metallocene isotactic poly(propylene). These fractions provide direct evidence of the "single site" character of the metallocene catalyst. The variations of crystallization rates with molar mass reflect different chain diffusion/transport phenomena that are governed by the remnant entanglement state of the melt during crystallization. The molar mass fractions obtained from the ZN-iPP confirm that the interchain distribution of the nonisotactic content is broad in this polymer. The stereodefects are more concentrated in the low molar mass fractions. Furthermore, the invariance of the linear growth rates among the ZN fractions and the lack of formation of any significant content of the γ polymorph, even in the most defected fraction, is consistent with a nonrandom, blocky intramolecular distribution of defects in the ZN-iPP molecules. In contrast to the growth rates, the overall crystallization rates are a direct function of the primary nucleation density, which varies among the fractions and the unfractionated iPPs. Hence, the measured overall crystallization rates would be correlated with nucleation density and not necessarily with the microstructure of the iPP molecules. The crystallization data are also interpreted in light of results from pentad/heptad distributions predicted by two-state and threestate statistical models. Parameters from the models allow the prediction of sequence distribution curves that could be used to evaluate each of the models as to their consistency with the crystallization rate data.

The microstructures of highly isotactic polypropylenes, prepared with magnesium chloride supported Ziegler−Natta catalyst systems, have been determined experimentally through 13C NMR measurements and also predicted through Markovian statistical models. Polypropylene molecules are seldom 100% isotactic but possess long isotactic sequences interrupted by stereo- and regio-irregularities called chain defects that ultimately define the crystallinity of the polypropylene. The structures and numbers per 10 000 repeat units of the various interrupting stereo-irregularities, termed “stereo-defects”, are determined in this study. It is shown that different families of stereo-defects in highly isotactic polypropylenes can be associated independently with symmetric and asymmetric chains. The single-parameter-zero order Markovian statistical approach typically utilized in the Doi two-state asymmetric and symmetric chain statistical models for characterizing polypropylene sequence distributions has been extended to first-order Markov for symmetric chains. First-order Markovian statistics for asymmetric chains naturally reduce to zero order for the isotactic polypropylenes examined in this study, whereas the symmetric chain components do not. Syndiotactic blocks of steadily increasing sequence lengths are predicted by first-order Markovian statistics for highly isotactic symmetric chains possessing a meso diad content of 0.99. The average sequence lengths of these syndiotactic blocks are predicted to become shorter as the polypropylenes become more isotactic. Finally, it was observed that the sequence distributions favored asymmetric chains with the addition of increasing amounts of an electron donor to the supported Ziegler−Natta catalyst system.

A carbon-13 NMR method is presented for a quantitative determination of the comonomer distribution, the triad and tetrad sequence distributions, average sequence lengths, and run numbers in ethylenel-butene copolymers. A series of hydrogenated polybutadienes and a poly(1-butene) served as reference systems for establishing chemical shift assignments and for determining optimum conditions for a quantitative development. A dependence of relative spectral areas upon concentrations was noted a t polymer sample weight percents of 30 and higher. The method presented should be free of errors associated with differences in spectral line widths, peak overlap, difficulties in detailed assignments, and complexities introduced through a configurational sensitivity.