You can read all about it here:
The Sixth Nicene Canon and the Papacy
"Let me, at the risk of being tedious, state, first of all, my understanding of the passage. The supremacy of the Bishop of Alexandria had been contested by the Meletian bishops. They had, asked him, if not in words at least in facts, upon what warrant he based his claim to rule over and depose his fellow-bishops. If he had a title let him produce it. Now the Alexandrian prelate had no written document of any kind to produce. The Council of Nicaea, therefore, came to his assistance, by decreeing that the Patriarch's [16] authority must be respected, and that for two reasons: first, because it was (archaia), immemorial, aboriginal; and second, because it was sanctioned by constant recognition on the part of the Roman Pontiff. Two very good reasons".......A quote from the above.
There was no "Pope". The Bishop of Rome did not have Primacy over the whole Church.
In fact The First Council of Nicæa (A.D. 325)was called by Constantine. If there was any Primacy over the whole Church it was his.