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 ( 1 H-NMR), and size exclusion chromatography. 1 H-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: k 1 5 1.07 3 10 23 L mol 21 s 21 for the prepolymerization; and k 2 5 1.94 3 10 24 L mol 21 s 21 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 1 H-NMR measurements also enabled to estimate the evolution (with extent of reaction) of the numberaverage number of structural units along the prepolymerization and finishing stages; and such estimates reasonably verify Flory's classical expressions.

Linear segmented polyurethanes. II. A mathematical model for the prepolymerization stage

“…Linear segmented thermoplastic polyurethanes (STPUs) are multi‐block copolymers typically obtained in two‐steps polymerizations . In the prepolymerization, a macrodiol reacts with an excess of diisocyanate to produce a low molar mass prepolymer.…”

“…The synthesis of STPUs is still a matter of controversy, due to difficulties introduced by: (1) the presence of intramolecular and/or secondary reactions; (2) the use of macrodiols; (3) the use of asymmetric diisocyanates; (4) the effect of chain length on reaction rate; (5) the effect of solvents on reaction rate; and (6) the presence of system heterogeneities . Consider each of the mentioned difficulties.…”

“…Intramolecular reactions (yielding molecular cycles), and secondary reactions (yielding allophanates and biuret crosslinking) tend to broaden MMD with respect to the base step reaction mechanism, and their presence depends on the temperature and nature of the reagents . The use of polydisperse macrodiols instead of low molar mass diols introduces difficulties for attaining stoichiometric ratios . Asymmetric diisocyanates may exhibit substitution effects and/or differing reactivities in their functional groups.…”

“…Table compare many of the articles that contain mathematical models for the synthesis of STPU's, with the aim of predicting MMDs and averages. The models are grossly classified into two main groups, according to whether they simulate the synthesis of oligomers or high polymers . The reason for such classification is the greatly increased number of different molecular types in high polymers.…”

“…In the first part of this sequel, five polymerizations were carried out at 60°C between MDI, two different PTMO macrodiols, and BD. The prepolymerizations were in bulk, and the finishing stages were in THF solution.…”