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This work investigates the 2-steps synthesis of linear segmented thermoplastic polyurethanes (STPUs). The reactions involved methylene diphenyl diisocyanate (MDI), two different poly(tetramethylene oxide) (PTMO) macrodiols, and 1,4-butanediol (BD) as chain extender. The syntheses were carried out in two stages. In the first (Prepolymerization) stage, a low molar mass polymer was obtained by reaction between the macrodiol and an excess of diisocyanate. In the second (Finishing) stage, the system was diluted with THF, and the chain extender was added to increase chain length. The reaction was analyzed by FTIR, 1H-NMR and size exclusion chromatography (SEC). Measurements by 1H-NMR enabled to follow the global concentrations of isocyanate and urethane groups, the extent of reaction and the number-average number of structural units along the Prepolymerization and Finishing stages. This information was employed to estimate the kinetic constants in each reaction stage, yielding k1 = 1.07×10-3 Lmol-1s-1 for the Prepolymerization; and k2 = 1.94×10-4 Lmol-1s-1 for the Finishing stage. The adjusted kinetic constants are in reasonable agreement with those determined for similar reagents and reaction conditions carried out in the bulk [1] and in THF [2]. In order to estimate the global number-average number of structural units as a function of the extent of reaction along the Prepolymerization and Finishing stages, Flory?s expressions [3] were adapted, taking into consideration the use of a macrodiol in the Prepolymerization, and the presence of two different types of repeating units along the Finishing stage. The predictions by Flory?s equations are in agreement with the 1H-NMR measurements. In the second part of this sequel [4], the SEC measurements of molar mass distributions (MMDs) and averages are compared to predictions from a new comprehensive polymerization model.