Russian President Vladimir Putin cited fears that Ukraine would join NATO when he annexed the Crimean Peninsula in March last year.
I don't think that NATO will grant membership to Ukraine, even if Poroshenko applies to join the organization. If Ukraine attempts to join the NATO, then Putin will send real Russian troops with heavy weaponry to Kiev, and the battle will be over in a matter of days, if not a few hours. Also, a majority of the people in South and East Ukraine are opposed to NATO membership. They will try to secede from Kiev, if the latter applies for NATO membership.