  We propose the quantitative mean-field theory of mechanical unfolding of a
globule formed by long flexible homopolymer chain collapsed in poor solvent and
subjected to extensional deformation. We demonstrate that depending on the
degree of polymerization and solvent quality (quantified by the Flory-Huggins
$\chi$ parameter) the mechanical unfolding of the collapsed chain may either
occur continuously (by passing a sequence of uniformly elongated
configurations) or involves intra-molecular micro-phase coexistence of a
collapsed and a stretched segment followed by an abrupt unraveling transition.
The force-extension curves are obtained and quantitatively compared to our
recent results of numerical self-consistent field (SCF) simulations. The phase
diagrams for extended homopolymer chains in poor solvent comprising one- and
two-phase regions are calculated for different chain length or/and solvent
quality.
