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Abstract: Composite materials of isotactic polypropylene and graphene nanoplatelets were synthesized by in situ polymerization in liquid propylene with the use of metallocene catalyst system. Graphene nanoplatelets were prepared by chemical oxidation of graphite and subsequent reduction. Preliminary ultrasonic treatment of GNP suspensions in toluene results in better dispersion of particles, both in liquid polymerization medium and in polymer matrix. GNP particles consist of 3–5 graphene layers and have aspect ratio 40. In composites with pristine GNP particles their aspect ratio is 110, whereas ultrasonic processing reduces aspect ratio to 40–50. Ultrasonic processing provides, apparently, a more uniform distribution of GNP particles, which in nanocomposite are largely individualized. This change of aspect ratio of filler particles and their aggregates results in different properties of composites with pristine and sonicated GNP. Percolation threshold for composites with pristine GNP is 0.25 vol.%. In composites with sonicated GNP percolation threshold is 2–3 vol.%. This is due to reduction in the size of filler particles aggregates and much more uniform distribution of particles in polymer matrix after ultrasonic treatment. [Copyright &y& Elsevier]