In 1999, in Bartlesville, Oklahoma, a plaque was placed on the site of the laboratory where the polymer polypropylene was invented, designating it a National Historic Chemical Landmark by the American Chemical Society. The first commercially successful use of the new material was in the Hula Hoop®. In 1951, J. Paul Hogan and Robert L. Banks, research chemists working for Phillips Petroleum Company, discovered the polymer unexpectedly during experiments with catalysts while trying to convert the natural gas components ethylene and propylene into compounds useful for gasoline. Later, they also developed a new catalytic process for making high-density polyethylene. Phillips soon invested in new plastic manufacturing plants.