The melting point is determined as the temperature at which a polymer transits from the crystalline state into the viscous flow state. In contrast to low-molecular substances, in which this process proceeds in a jump-like manner, in the case of polymers melting is observed in a temperature range. This happens due to polydispersity of polymeric chains, their branching and imperfection of crystallites formed. The equilibrium and experimental melting points are distinguished. The equilibrium melting point = ?H _m/ ?S _m, where ?H _m is the melting enthalpy, ?S _m is the melting entropy. The equilibrium melting point is determined by the point of the phase equilibrium between a monocrystal of the polymer and its melt. Since perfect monocrystals are difficult to obtain from the polymer, the equilibrium melting point is determined by methods of extrapolation, for example, by extrapolation of the dependence of the experimental melting point on the size of crystallites or on the molecular mass of the polymer.

The melting point T _m is a physical characteristic and is most difficult to calculate. The case in point is calculation based on the chemical structure of the repeat unit of the polymer. Let us discuss two approaches to solution of this problem. One of them is based on estimation of the relation between the glass transition temperature T _g and the melting point T _m. It should be noted that, according to the Beaman rule [132], T