if i buy iphone 5 unlocked and use it on t-mobile will it be locked to them ?

I want to buy an iphone 5 unlocked so i can use t-mobile but i might leave t-mobile so i was wondering if t-mobile will lock it to them

and if its a yes would anyone think it could be use on t-mobiles 30$ plan of 100 min and unlimited talk and data up to 5gb