聚酸酐的合成

Explain why the polyanhydrides were synthesized by melt polycondensation of mixed anhydrides of dicarboxylic monomers and acetic acid, rather than direct dehydration of dicarboxylic acids?
大致意思是 为什么聚酸酐是用二元羧酸单体和乙酸熔融缩聚反应合成,而不是直接二元羧酸脱水?

There are main three factors influence the synthetic process.
The dicarboxylic acids always have a high melting point, when we adopt the melt polycondensation, a quite high temperature will be reqiured which is a disadvantage for engineering. While the anhydrides’ melting point is much lower, so the polymerization could be process at a lower temperature.
The dicarboxylic acid is easily carbonized by removing the carboxyl as carbon dioxide when at high temperature, when transfers the dicarboxylic acids to anhydrides they will be much stabiler.
The activities of the anhydrides is much higher than the dicarboxylic acids in the polymerization. W hat’s more, removing the byproduct as acetic anhydride is much easilier than as water under vacuum. It improves the polyanhydride’s molecular weight a lot.
以上是我的答案 楼下的貌似跟我们老师出题的思路不一致啊 不过还是谢谢

这是为了减小副产物的量，提高聚合物的产率；如果直接用二元羧酸脱水聚合，会形成很多的小聚合单位，如二聚体，三聚体，虽然有较大分子量的高分子聚合物生成，但不能排除低聚体内部环化形成环状低聚物等副产物的形成。
如果使用二元羧酸单体与乙酸熔融缩聚反应合成就不同了，这个反应是属于连锁聚合，它有特定的反应活性中心，全过程包括链的引发，链增长，和链终止等基元反应。这儿少量的乙酸作为引发剂，一旦反应发生就可以一直进行下去，一个二元羧酸分子接一个二元羧酸分子不断进行下去，环状低聚体的生成量大为减少,即副产物少。产量产率都可达到要求。

（c4h2o3）n