when a molecule has to enter the cell it first has to interact with the polar part of the lipid membrane and then with the inner non-polar part of the membrane to pass through, hence if it is a polar molecule, it will face repulsions and will not be able to pass through without the assistance of transmembrane proteins, but, in the case of non-polar molecules, there is no repulsion from the fatty acid part of the phospholipids and hence it can pass through the membrane. When the non-polar molecules are too big some other mechanisms may be utilized (like receptor-mediated endocytosis).