  Recently it has been shown that a two-dimensional model of self-attracting
polymers based on attracting segments displays two phase transitions, a
theta-like collapse between swollen polymers and a globular state and another
between the globular state and a polymer crystal. On the other hand, the
canonical model based on attracting monomers on lattice sites displays only
one: the standard tricritical theta collapse transition. Here we show that by
considering both models with the addition of stiffness the two models display
the same generic phase diagram. In fact we claim that any two-dimensional model
of a self-attracting single polymer in solution based upon a fully
volume-excluded backbone with isotropic short range attraction should show this
same phase structure. We point to the model of hydrogen bonded polymers to
demonstrate this observation. In three dimensions we note that more than one
crystalline phase may occur.
