NOVELTY - Reactive molding composition, comprises a reactive precursor composition of a semicrystalline thermoplastic polymer which is a semicrystalline polyamide and optionally at least one fibrous reinforcement made from long fibers. The thermoplastic polyamide polymer comprises: (a) at least one thermoplastic polyamide prepolymer carrying n identical terminal reactive functional units (X1) comprising amino or carboxy, preferably carboxy with n being 1-3, preferably 2; and (b) at least one chain extender (I).    USE - The reactive molding composition is useful: for manufacturing a mechanical or structural part of a thermoplastic composite material, which is 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, protective panels and leisure, electrical or electronics; and in a molded part, preferably a part of the composite material (all claimed).    ADVANTAGE - The reactive molding composition: provides a glass transition temperature of the polyamide polymer of at least 80 degrees C, preferably at least 90 degrees C, in the field of wind power; provides a glass transition temperature of the polyamide polymer of at least 100 degrees C, in the automotive field; provides a glass transition temperature of the polyamide polymer of at least 120 degrees C, in the field of aeronautics (all claimed); and is easy to use.    DETAILED DESCRIPTION - Reactive molding composition, comprises a reactive precursor composition of a semicrystalline thermoplastic polymer which is a semicrystalline polyamide and optionally at least one fibrous reinforcement made from long fibers. The thermoplastic polyamide polymer comprises: (a) at least one thermoplastic polyamide prepolymer carrying n identical terminal reactive functional units (X1) comprising amino or carboxy, preferably carboxy with n being 1-3, preferably 2; and (b) at least one chain extender of formula (I). The semicrystalline polyamide polymer is derived from the polyaddition reaction between the components (a) and (b) of the precursor composition having a glass transition temperature of at least 80 degrees C, preferably at least 90 degrees C and a melting temperature of less than or equal to 280 degrees C. The polyamide polymer and prepolymer (a) include, in their structure, amide units (A1 and B1) which are different, and optionally amide units (C1 and D1). The polymer and polyamide prepolymer comprise in their structure of the various amide units (A1 and B1) and optional amide units (C1 and D1).    Y1 = 2-terminal reactive epoxy functional units;    A11 = single bond linking the functional units (Y1) or nonpolymeric bivalent hydrocarbyl structure, based on 2-terminal reactive epoxy functional units (Y1), reactive by polyaddition with at least one X1 unit of the prepolymer (a), preferably chain extender (b) having a molecular weight less than 500, preferably less than 400;    A1 = amide units majorly comprising x1x T units, and present at a molar rate of 55-95%, preferably 55-80%;    x1 = linear aliphatic diamine with 9-18C, preferably 9C, 10C, 11C or 12C;    T = terephthalic acid;    B1 = amide units different from unit A1 and comprising x2x T, and present at a molar rate of 5-45%, preferably 20-45%, based on the melting temperature of the polyamide based on units A1;    x2 = branched aliphatic diamine carrying a single branch (preferably methyl or ethyl) (and having a different main chain with a length of at least two C atoms with respect to the length of the main chain of the diamine x1 of the units A1, where x2 is a linear aliphatic diamine of 4-18C (when x1 is a linear aliphatic diamine of 11-18C) or 9-18C diamine (when x1 is 9C or 10C), preferably 2-methyl pentamethylene diamine, meta-xylylenediamine, or linear aliphatic diamine;    C1 = amide units (which are different from units A1 and B1) based on a cycloaliphatic and/or aromatic structure, made of x2x T, where x2 is different from x2 of units B1; and    D1 = amide units (which are different from units A1, B1 and C1) comprising aliphatic amide units derived from amino acids or lactams with 6-12C, preferably 6C, 10C and/or 12C, and derived from the reaction of a linear aliphatic diacid 6-18C, preferably 6-12C and a linear aliphatic diamine with 6-18C and/or 6-12C.    Provided that the sum of the molar proportions of units A1+B1+C1+D1 is 100%. INDEPENDENT CLAIMS are also included for:    (1) manufacturing a mechanical or structural part of a thermoplastic composite material, based on at least one composition, comprising carrying out an impregnation step in the molten state of at least one fiber reinforcement with the reactive precursor composition, in an open or closed mold or outside the mold; and    (2) use of the precursor composition (in the absence of fiber reinforcement) for the melt impregnation of the fibrous reinforcement, as precursor of the thermoplastic polymer matrix, and for the manufacture of mechanical or structural parts of composite material.