Are there not many cases in history where the Pope has granted a divorce and permission to re-marry for various reasons?

The Catholic Answers

You would find it very difficult to prove one such case. Many decrees of nullity have been issued, but they are not divorces. Yet even supposing that you could prove that some individual Pope had granted such a divorce, that would be no argument against the doctrine of the Catholic Church. It would but prove that such an individual Pope acted against his conscience and against the teaching of the Church. An appeal to the lapse on the part of an individual Pope proves nothing against the Church. You cannot disprove a law by pointing to a criminal who has broken it. The Catholic Church has always taught that divorce of a true marriage with the right to re-marry is not allowed.

Radio Replies Volume 1 by Rev. Dr. Leslie Rumble MSC and Rev. Charles Mortimer Carty