shared ownership and the freehold

I know that if you are buying a property that is leasehold, then after you have paid it fully you can obtain the freehold.

My question is this, if you are buying say 50% on a leasehold and you have paid it fully, can you obtain the freehold for your share, or is this nonsense?
I am asking this because I am aware that it is possible to get evicted from shared ownership and have your share seized by the landlord, but if you can get the freehold for your share, this wouldn't be possible. Or will the only way to obtain the freehold on a leasehold property be by gaining full ownership and fully paying the mortgage off?
thanks