  Using self-avoiding walk model on three-dimensional Sierpinski fractals (3d
SF) we have studied critical properties of self-interacting linear polymers in
porous environment, via exact real-space renormalization group (RG) method. We
have found that RG equations for 3d SF with base b=4 are much more complicated
than for the previously studied b=2 and b=3 3d SFs. Numerical analysis of these
equations shows that for all considered cases there are three fixed points,
corresponding to the high-temperature extended polymer state, collapse
transition, and the low-temperature state, which is compact or semi-compact,
depending on the value of the fractal base b. We discuss the reasons for such
different low--temperature behavior, as well as the possibility of establishing
the RG equations beyond b=4.
