  We study the force-induced unfolding of random disordered RNA or
single-stranded DNA polymers. The system undergoes a second order phase
transition from a collapsed globular phase at low forces to an extensive
necklace phase with a macroscopic end-to-end distance at high forces. At low
temperatures, the sequence inhomogeneities modify the critical behaviour. We
provide numerical evidence for the universality of the critical exponents
which, by extrapolation of the scaling laws to zero force, contain useful
information on the ground state (f=0) properties. This provides a good method
for quantitative studies of scaling exponents characterizing the collapsed
globule. In order to get rid of the blurring effect of thermal fluctuations we
restrict ourselves to the groundstate at fixed external force. We analyze the
statistics of rearrangements, in particular below the critical force, and point
out its implications for force-extension experiments on single molecules.
