Let's think about this carefully... there must be something wrong somewhere if the chain keeps breaking!!!
1... Is your rear sprocket centered perfectly on the axle as well as no side to side wobble?? (Spin the wheel without the chain on to see what the sprocket is doing!!)
2... How about the chain tensioner? Is it feeding the chain straight on to the rear sprocket without the chain snagging on the teeth?
3... Is the chain touching the tire at high speed & whipping from side to side?? (The wheel might need truing??)
4... Did the chain come off while riding & get damaged early on??? (The chain must be straight & undamaged to work right!)
Check carefully for these four problems & report back!