Polypropylene molar mass distribution

A suitable system to consider initially is polypropylene, which has a hydrogen atom on the tertiary carbon atom that is available for abstraction to form a polymer-backbone radical. This is also of practical significance since controlled scission of polypropylene has been used to produce polymers with controlled molar-mass distribution and rheological properties (Brown, 1992). 

FAB-MS analysis also has been used to characterize low mass commercial polyethylene- and polypropylene-glycols, by detecting the oligomer ion abxmdances in the mass spectra of the samples. However, the relative peak intensities of the lower mass oligomers are distorted by the fragmentation processes of the oligomers at higher molar masses. Therefore it is not possible to estimate molar mass distributions by FAB-MS. ... 

Molar mass distribution is a dominant microstracture parameter that, in copolymers, needs to be measured with additional information to account for long chain branching, comonomer incorporation, or ethylene propylene combinations (in the case of EP copolymers). The combination of GPC and IR spectroscopy has been shown to be of great value in the characterization of copolymers. The importance of automation and sample care, especially in the case of polypropylene, has been discussed as well as the significant improvement in sensitivity by the use of IR MCT detectors. There are big expectations for the analysis of ultrahigh molar mass polyolefins by the new AF4 technology. 

Keywords Activation ansa-Zirconocene catalysts Constrained-geometiy catalysts ethene/propene rubbers Isotactic polypropylene Linear low-density polyethylene Molar-mass distribution Olefin polymerization Reaction... 

Until about 40 years ago, the determination of the molar mass distribution was the prime objective of polymer fractionation by phase separation. Since the advent of size exclusion chromatography the need for such time consuming and essentially ineffective classic fractionation methods has disappeared. However, today chromatographic techniques are still incapable of producing large-size fractions of the order of 100 g and phase separation methods continue to be needed for the preparation of large amounts of narrow-distribution polymers that cannot be obtained by direct synthesis. The most important polyolefins, polyethylene and polypropylene, fall into this category and a discussion of fractionation by distribution between two... 

The previous sections of this chapter have been concerned with methods for determination of average molar masses, molar mass distributions and molecular dimensions. In many instances this information is all that is necessary to characterize a homopolymer when its method of preparation is known. However, for certain homopolymers (e.g. polypropylene, polyisoprene) knowledge of molecular microstructure is of crucial importance. Additionally, for a copolymer it is necessary to determine the chemical composition in terms of the mole or weight fractions of the different repeat units present. It is also desirable to determine the distribution of chemical composition amongst the different copolymer molecules which constitute the copolymer (Section 3.17.6), and to determine the sequence distribution of the different repeat units in these molecules. Furthermore, when characterizing a sample of an unknown polymer the first requirement is to identify the repeat unit(s) present. Thus methods for determination of chemical composition and molecular microstructure are of great intportance. 

Lederer, K., and Mingozzi, I. (1997) Molecular characterization of commercial polypropylene with narrow and broad distribution of molar mass. Pure Appl. Chem., 69, 993-1006. 

Amer and van Reenen [39] fractionated isotactic polypropylenes by TREE to get fractions with different molar masses but similar tacticities. The DSC results of the fractions indicated that the crystallization behaviour is strongly affected by the configuration (tacticity) and the molar mass of the PP. Soares et al. [40] proposed a new approach for identifying the number of active catalyst sites and the polymer chain microstructural parameters produced at each active site for ethylene/l-olefin copolymers synthesized with multiple-site catalysts. This method is based on the simultaneous deconvolution of bivariate MMD/CCD, which can be obtained by cross-fractionation techniques like SEC/TREE or TREE/SEC. The proposed approach was validated successfully with model ethylene/1-butene and ethylene/ 1-octene copolymers. Alamo and co-workers [41] studied the effects of molar mass and branching distribution on mechanical properties of ethylene/1-hexene copolymer film grade resins produced by a metallocene catalyst Molar mass fractions were obtained by solvent/non-solvent techniques while P-TREE was used for fractionation according to the 1-hexene content. 

Figure 1 Effect of multiple site types and mass and heat transfer resistances on the microstructure of polypropylene made with heterogeneous Ziegler-Natta and metallocene catalysts. The overall MWD and CCD are assumed to result from the superposition of individual MWDs and CCDs for three site t)rpes (T = temperature, M = number average molecular weight, = hydrogen, CjH = propylene, C2H4 = ethylene, Fj = molar fraction of propylene in copolymer, /(F,) == copolymer composition distribution, r = chain length, wix) = weight chain length distribution).

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