  Diffusion properties of a self-avoiding polymer embedded in regularly
distributed obstacles with spacing a=20 and confined in two dimensions is
studied numerically using the extended bond fluctuation method which we have
developed recently. We have observed for the first time to our knowledge, that
the mean square displacement of a center monomer $\phi_{M/2}(t)$ exhibits four
dynamical regimes, i.e., $\phi_{M/2}(t) \sim t^{\nu_m}$ with $\nu_m\sim 0.6$,
3/8, 3/4, and 1 from the shortest to longest time regimes. The exponents in the
second and third regimes are well described by segmental diffusion in the
``self-avoiding tube''. In the fourth (free diffusion) regime, we have
numerically confirmed the relation between the reptation time $\tau_d$ and the
number of segments $M, \tau_d\propto M^3$.
