Is it permissible for an employee to purchase materials and then sell them to his company at a higher price without the knowledge of its management, knowing that the company buys the same materials at a similar or lower price?

There is no harm for the questioner in selling materials to the company he works for, unless he is responsible for purchasing for the company. In that case, it is not permissible except with the permission of the company owner. It is also not permissible to sell in the name of another person if the company prohibits its employees from dealing with it. Selling at a price higher than the purchase price is permissible.