Hydrogen bonding effects on aspartate ester reactions

Abstract

Michael addition reactions were utilized to form AB2 type monomers and esterified to form polyaspartate esters (PASPE). FTIR and NMR spectroscopic methods indicate that a relatively linear architecture was produced that contrasts with an expected hyperbranched (HB) structure. Linear chains formed due to a reactivity difference between the hydrogen bonded (H-bonded) and non-H-bonded esters. The esterification reaction was directed toward the H-bonded ester as shown by 2D NMR correlation spectroscopy. A model is proposed to explain the observed increase in reactivity of the H-bonded ester. Hydrogen bonding (H-bonding) in both the monomeric and polymeric aspartate (ASP) esters was analyzed by 2D NMR spectroscopy. The difference in chemical shifts of the methylene protons before and after reaction was attributed to the geometrical effects of the five-membered ring formed from H-bonding. Monomeric aspartate esters (ASPE) were found to have the greatest difference in chemical shift, while the in-chain H-bonded protons of the PASPE were observed to have the least difference in chemical environments. Internal H-bonding in the ASPE affected its reaction with an aliphatic isocyanate (NCO) due to varying reactivities of the primary hydroxyl (OH) compared to the secondary amine (NH). Internal hydrogen bonding of the OH groups with the amine may also explain unexpected relative reactivities with isocyanates. Both NMR and FTIR spectroscopy indicated that the OH group was exclusively consumed with no NH reaction product detected. A lack of hydantoin ring formation upon further heating the NCO/ASPE reaction product proved that the OH group was more reactive. In an equimolar reaction of ASPE with cyclohexyl isocyanate, NMR and FTIR measurements showed quantitative formation of the urethane adduct instead of the expected urea.