Initially developed to overcome the limited low-temperature impact resistance of polypropylene (PP) and allow its application in subzero temperatures, the PP/elastomer blends became a very important and industrially relevant material class. Within the last 40 years, a whole range of PP impact copolymers from multireactor processes have been developed that are capable of covering wide application areas. This review summarizes their developmental history and presents options for the in-reactor design of these polymers and their modification by compounding with elastomers and fillers or reinforcements after the polymerization. An overview of the application range is also presented together with a glimpse into the future of this material class.

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Five PP homopolymers were selected and their molecular structure was thoroughly characterized to determine the effect of molecular architecture on their annealing behavior and on the ultimate stiffness achieved by heat treatment. Molecular mass and its distribution were characterized by rheological measurements, while chain regularity was determined by calorimetry, by the stepwise isothermal segregation technique (SIST). The samples were annealed in two different ways. Tensile bars were treated in an oven at 165 °C for increasing times to determine changes in stiffness. Various defects developed during the annealing of tensile specimens that did not allow the reliable determination of modulus by direct measurement. On the other hand, the second approach, the annealing of small samples in a DSC cell clearly showed the changes occurring in crystalline structure and also the effect of nucleation and molecular architecture on them. The large molecular weight fraction used to facilitate nucleation hinders crystal perfection, while the presence of a heterogeneous nucleating agent increases overall crystallinity, but does not influence recrystallization during annealing. Melting traces were transformed into lamella thickness distributions, from which average lamella thickness was derived. Lamella thickness and crystallinity, the independent variables of the empirical equation used for the calculation of modulus, were extrapolated to infinite annealing time to predict maximum stiffness. The Full paper submitted for publication in Journal of Applied Polymer Science 2 value obtained, 3.5 GPa, is very far from the theoretically predicted 40 GPa of oriented crystals, which cannot be achieved under practical conditions.

Polypropylene (PP) random copolymers are an important class of commercial materials that can be tuned for a wide range of different applications. The structure-property relations are mainly discussed in this article focusing on two major influencing factors, the catalyst system and the type and amount of comonomer. Ziegler-Natta (ZN) catalyst systems still dominate the production of polypropylene and todays sixth-generation ZN catalysts are based on postphthalate donor systems. Metallocene (MC) catalysts offer possibilities to exactly control the microstructure of the polymer even better. Mostly, ethylene is used as comonomer being randomly distributed for up to 7.3 mol% when using ZN catalysts. For single site catalysts, homogenous (single phase) materials can be achieved up to ethylene contents of 20.8 mol%. The incorporated comonomer reduces the crystallization speed and the lamellar thickness of the polymer thus affording soft and highly transparent materials. Random copolymers and terpolymers with higher 𝜶-olefins only play a minor role but offer an additional possibility for property adjustments. PP random copolymers can also be varied in their performance through multistage polymerizations, altering the comonomer content and or the molecular weight in the different polymerization reactors.

The chain regularity of isotactic polypropylene (iPP) homo-and random copolymers was characterized by different thermal fractionation methods in this study. Different stepwise temperature programs were applied in a calorimeter (DSC), in order to develop a method which is significantly faster than SIST and provides reliable information about the chain regularity of iPP. Our studies prove that self-seeding accelerates the crystallization process during annealing in SSA-DSC experiments (successive self-nucleation and annealing). Consequently, the time of isothermal steps can be shortened significantly in the SSA-DSC method. On the other hand we found that step time should not be too short if the goal of the measurement is the determination of average chain regularity. Our results clearly indicate that both the experimental conditions and the evaluation technique influence the obtained results. A standard experimental procedure is proposed for reliably determining the average chain regularity of iPP. The length of the SSA-DSC temperature program developed in this study is much shorter compared to that of the conventional SIST (stepwise isothermal segregation technique) measurements used recently for such experiments. The proposed SSA-DSC program makes the reliable characterization of a large number of samples on an acceptable timescale possible.