Divorcing a Barren Woman - Meishiv Davar 4:8

A case was brought to the Netziv concerning a man whose wife had not given birth and was now past her childbearing years. While we have a tradition that Sarah gave Hagar to Avraham ten years after they entered Israel and still did not have children, and that this is a template for how long a couple should try having children before the man is to look elsewhere, we also now have to contend with the חרמות דרבינו גרשום which prohibit both polygamy and forced divorce. In this case, the woman refused to accept a get, and thus the question arose as to whether the man had any hope of ever having children.

The Netziv ruled that this was a clear-cut case where the man was entitled to make use of the היתר מאה רבנים, under which he could divorce his wife or at least take on another wife provided that he obtained the signatures of 100 Rabbis from at least three distinct communities. He notes that since the woman has not done anything wrong, the היתר מאה רבנים will allow the man to take on a second wife, and at whatever time that the first wife agrees to accept a get she will receive the full compensation for her ketubah.

The Missing Wife - Igrot Moshe Even HaEzer 4:2

A man from Kovno who emigrated to America in 1939 had planned to send for his wife as soon as was feasible. In 1940 he sent a coded message to her to join him, and she replied via telegram that she was on her way. After that, he never heard from her nor saw her again. In 1962, Rav Moshe ruled that it is a fair assumption that she was murdered along with so many others in the Holocaust, and that since the only reason that he could not remarry is the cherem of Rabbeinu Gershom that forbids polygamy (which thus ranks below a Rabbinic prohibition), he could remarry.

However, Rav Moshe does add that if the woman would show up at some point that he should write her a get immediately and pay her her ketubah.