  We study the force-induced unfolding of a homopolymer on the three
dimensional Sierpinski gasket. The polymer is subject to a contact energy
between nearest neighbour sites not consecutive along the chain and to a
stretching force. The hierarchical nature of the lattice we consider allows for
an exact treatment which yields the phase diagram and the critical behaviour.
We show that for this model mean field predictions are not correct, in
particular in the exact phase diagram there is {\em not} a low temperature
reentrance and we find that the force induced unfolding transition below the
theta temperature is second order.
