movement plus difficulty in finding adhesives that adhere to plastics. You could consider widening the crack to reach to stiffer, non bending portions of the sole, then fill the resulting trench with shoe goo, so that the shear forces will fall on homogeneous goo...or accept that boots, like most things in life, are not forever.
_________________________
Dance like you have never been hurt, work like no one is watching,love like you don't need the money.