Using a mean-field theory with two order parameters and scaling laws, we study the properties of adsorbed polymer layers in contact with a pure solvent, at a fixed amount of polymers. We calculate the surface pressure of the polymer Langmuir monolayer, implicitly assuming that the monolayer is insoluble, i.e., that the polymer desorption time is much larger than any other characteristic time, in particular, the equilibrium relaxation time of the layer. This work was motivated by experiments done on poly(ethylene oxide)(PEO) and telechelic PEO adsorbed at the air-water interface that present an adsorbed layer structure at certain densities. Our theoretical results explain quite well the shape of the surface pressure isotherms and the influence of the chain length and of the hydrophobic ends.