Читать книгу Poly(lactic acid) - Группа авторов - Страница 72

The selective reactivity of oxazolines provides a possibility of performing dual linking processes with both diisocyanates and oxazolines. It has been shown that the order of addition of the linking agent affects both the reaction and the structure of the polymers, for example, the degree of branching. Simultaneous addition of 2,2′‐bis(2‐oxazoline) and 1,6‐hexamethylene diisocyanate results in a slower increase of the molecular weight than does sequential addition. This approach is of particular benefit in the linking of lactic acid prepolymers prepared by polycondensation of solely LA. Prepolymers with a higher molecular weight and lower acid number can be prepared without the addition of a diol. Linking with oxazoline will, in this way, both increase the molecular weight and further reduce the acid number, which will make the subsequent diisocyanate linking more successful and result in higher molecular weight, less thermal degradation, and shorter total reaction times [55]. The LA‐based polymers prepared by dual linking with diisocyanates and 2‐oxazolines exhibit properties of both poly(ester‐urethane)s and poly(ester‐amide)s. This, together with the fact that the dual linking process can be used for controlling the branching, opens a wider field of applications, for instance, in applications where the melt flow behavior or the hydrolytic degradation need to be tailored.