Studies on the ring-opening polymerization of benzoxazines: Understanding the effect of the substituents

A wide set of differently substituted 3-phenyl-3,4-dihydro-2H-1,3-benzoxazine derivatives (models of thermosetting benzoxazine resins) were synthesized and their polymerization temperature was studied using Differential Scanning Calorimetry (DSC). The effect of the electronic properties of different substituents in different positions of the alkoxyphenyl ring on the polymerization temperature was analyzed by the Hammett equation. A non-linear Hammett plot was obtained indicating a change in the polymerization mechanism. Based on these results, different ring-opening polymerization (ROP) mechanisms are proposed to operate depending on the electronic character of the substituents. Thus, the previously unpredictable dependence of the polymerization temperature with the electronic character of the substituents in the alkoxyphenyl ring can be now understood and even advanced for future developments.