NOVELTY - Composition comprises at least one polyamide polymer derived from at least one reactive polyamide prepolymer comprising at least one chain extender (I). The polyamide polymer is prepared at a temperature T1 greater than or equal to the melting temperature or glass transition temperature of the polymer and has a number average molecular weight Mn1. The composition has a viscosity in the molten state modulated according to the temperature at which the composition is subjected, in which the temperature is T2 to T3, and the temperatures T2 and T3 are greater than T1.    USE - The composition is useful for: manufacturing the thermoplastic composite material for making a mechanical or structural part; and extrusion, injection or molding for manufacturing mechanical or structural parts made of the composite material, where the mechanical parts are useful in the field of automotive, railway, ocean or sea, wind, photovoltaic, solar including solar panels and components of solar power plants, sports, aeronautics, road transport concerning trucks, construction, civil engineering, recreation panels, electrical or electronics, and the part is a vehicle part post-treated by cataphoresis, a part for a wind turbine, and a part for aeronautics (all claimed).    ADVANTAGE - The composition: is free of fiber reinforcement as well as polymeric chain extender; and provides easy impregnation of reinforcing fibers for manufacturing composite materials (all claimed).    DETAILED DESCRIPTION - Composition comprises at least one polyamide polymer derived from at least one reactive polyamide prepolymer comprising at least one chain extender of formula (I). The polyamide polymer is prepared at a temperature T1 greater than or equal to the melting temperature or glass transition temperature of the polymer and has a number average molecular weight Mn1. The composition has a viscosity in the molten state modulated according to the temperature at which the composition is subjected, in which the temperature is T2 to T3, and the temperatures T2 and T3 are greater than T1. The viscosities n12 or n13 in the molten state observed at the temperature T2 or T3 respectively are less than the viscosities n1 or n2 in the molten state of the polyamide polymer, devoid of chain extender and with the same mean molecular weight Mn1 observed at the same temperatures T2 or T3. The composition comprises at least one polyamide including random or block copolyamides which are polymers including different amide units A1, B1 (r times) and C1 (s times). The A1 amide unit is a repeating amide unit comprising lactams, 6-14C amino acid, or amide units of formula (II). The B1 (r times) and C1 (s times) amide units are different repeating amide units, optionally present at depending on the value of r and s, in which r and s denote 0 or 1. The B1 and C1 amide units are selected from the same constituents as for the amide unit A1, with the proviso that when r and s are 1, then B1 and C1 are different. The sum of the units A1, B1 and C1 is 100 wt.%.    PA1 = polyamide;    A12 = polymeric chain extender;    X1 = at least one diamine comprising aliphatic, cycloaliphatic and/or aromatic diamine; and    Y1 = at least one carboxylic diacid comprising aliphatic, cycloaliphatic or aromatic diacid, where the diamine and the diacid have 4-36 C atoms, preferably 6-18 C atoms.    INDEPENDENT CLAIMS are also included for:    (1) the polyamide polymer derived from the polyamide prepolymer comprising the chain extender;    (2) use of at least one polyamide polymer for modulating the viscosity of the molten state composition comprising the polyamide polymer and the average molecular weight of the polyamide polymer; and    (3) manufacturing the thermoplastic composite material for making a mechanical or structural part, comprising at least one step of molding the composition at a temperature for obtaining a desired viscosity.