Linear segmented polyurethanes. I. A kinetics study

Abstract

This work investigates the two-step polymerization between methylene diphenyl diisocyanate (MDI), two different poly(tetramethylene oxide) macrodiols, and 1,4-butanediol (BD) as chain extender. At the end of the prepolymerization, the reaction mixture contains MDI in excess and a prepolymer with isocyanate end group. Then, BD and a solvent (tetrahydrofuran) were added to start the finishing stage under nominal stoichiometric equilibrium. The reaction was analyzed by Fourier transform infrared spectroscopy, hydrogen nuclear magnetic resonance (1H-NMR), and size exclusion chromatography. 1H-NMR was employed to follow global concentrations of unreacted isocyanate end groups and internal urethane groups. This information enabled to estimate the following ?effective? rate constants: k1 = 1.07 × 10−3 L mol−1 s−1 for the prepolymerization; and k2 = 1.94 × 10−4 L mol−1 s−1 for the finishing stage. These values are subject to errors caused by biases introduced in the recipe, in the measurements, in the reaction conditions, in the quality of reagents, and in the reaction mechanism assumptions. Such errors also explain the dispersion of the published rate constants values. The 1H-NMR measurements also enabled to estimate the evolution (with extent of reaction) of the number-average number of structural units along the prepolymerization and finishing stages; and such estimates reasonably verify Flory´s classical expressions.