In the present paper, a theory of free induction decay in linear flexible chain polymers is developed. Theoretical and experimental dependences of the spin-spin relaxation time T2 on the average molecular weight M w and temperature are compared. It is shown that, with increasing Mw, the topological structure of linear polymers changes, and at Mw > 105, a physical network entanglements (quasi-network) is formed. A correlation function of molecular motions is obtained, which indicates whether the quasi-network is formed in polymer melts. © Pleiades Publishing, Ltd., 2014.