Uncatalyzed reactions of 4,4′-diphenylmethane-diisocyanate with polymer polyols as revealed by matrix-assisted laser desorption/ionization mass spectrometry†
The uncatalyzed reactions of polyols including polypropylene glycol (PPG, Mn = 2000 g mol−1), polytetrahydrofuran (PTHF, Mn = 1000 g mol−1), poly(ε-caprolactone)-diol (PCLD, Mn = 2000 g mol−1) and polypropylene glycol glycerol triether (PPG_GL, Mn = 1000 g mol−1) with 4,4′-diphenylmethane-diisocyanate (MDI) were studied using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). The reactions between these polyols and MDI were monitored in time and first-order consecutive reaction kinetics for the formations of polyols end-capped with MDI units were established. The fractional MALDI-TOF MS intensities obtained for the different polymer series at various reaction times were converted into molar fractions versus time data by means of the estimated relative MALDI-TOF MS response factors from which the corresponding rate constants and the relative reactivities of the hydroxyl groups of polyols were determined. It was found that the pseudo first-order rate constants for the polyol-MDI reaction decreased in the order of PCLD > PTHF > PPG ≈ PPG_GL. It was also ascertained that the reactivity of the unreacted hydroxyl groups of the diols does not change significantly after the first one has reacted. On the contrary, in the case of PPG_GL it was found that the reactions of the hydroxyl groups with MDI proceed faster after any of the three hydroxyl groups has reacted, suggesting a positive substitution effect for this system.