 Effective-sphere polymer drag

Approximating a dilute polymer coil as a hydrodynamic object of size $R$ gives $D\sim k_{B}T/(\eta R)$. If a stipulated model uses the theta-chain end-to-end distance $R=2a\sqrt{N}$ as a literal solid-sphere radius, Stokes law gives $D=k_{B}T/(12\pi \eta a\sqrt{N})$. The scaling matches theta-solvent hydrodynamic coupling, but a permeable coil's hydrodynamic radius is not generally equal to its end-to-end distance.