For cis-polyisoprene in a variety of solvents, published dielectric relaxation measurements(1) have determined the concentration dependence of the magnitude (R-2) and collective relaxation time tau(n) of the polymer end-to-end vectors. It is here shown for the first time that the measured concentration dependence of(R-2) accounts quantitatively for the transformation of a formal simple-exponential concentration dependence of a polymer transport coefficient into the observed stretched-exponential concentration dependence of that coefficient, as seen in the hydrodynamic scaling model of polymer dynamics.(2) Specifically, it is here shown that R and tau(n) are correlated by tau(n) = tau(n0) exp(ac(1)(R-2)(3/2)), a being a constant independent of c and R, the nonexponential concentration dependence of tau(n), arising from the concentration dependence of R in accord with expectations based on the ansatz of ref 2.