= Solution
In a dilute solution, solvent-mediated <hydrodynamic interactions> couple distant chain segments. The <Zimm model> describes the resulting cooperative dynamics: an intermediate-time segment mean-square displacement scales as $t^{2/3}$, while whole-chain translation has $D_Z\sim k_BT/(\eta_sR)$. Once internal modes relax, each tagged segment follows the chain's ordinary long-time <diffusion>, with mean-square displacement asymptotic to $6D_Zt$.
In a concentrated solution or <polymer melt>, hydrodynamic coupling is screened. The unentangled <Rouse model> has an intermediate $t^{1/2}$ segment regime and whole-chain diffusivity proportional to $N^{-1}$. Long chains with <polymer entanglement> are additionally confined by neighbouring chains: <reptation> gives a slower intermediate $t^{1/4}$ regime and an asymptotic chain diffusivity scaling approximately as $N^{-2}$. The time windows depend on segment, entanglement and terminal relaxation times, so these powers do not all describe the same time interval.
After sufficiently long times both dilute and concentrated chains undergo ordinary translational <diffusion>. The main long-time difference is the coefficient and its chain-length dependence, with entanglement strongly reducing mobility when the chains are long enough; concentration alone does not guarantee an entangled regime.
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