Using light scattering spectroscopy, we demonstrate that a transition from small molecular behavior to Rouse dynamics in poly(dimethylsiloxane) (PDMS) is a smooth function of molecular weight, M-w. The asymptotic Rouse behavior is achieved when the chain begins to display Gaussian behavior. Surprisingly, the molecular weight dependence of physical properties such as T-g and density also level off at the same molecular weight. A comparison of PDMS and polystyrene (PS) demonstrates that neither molecular weight between entanglement (M-e) nor Kuhn segment length l(K) is important for T-g and density. We speculate that there may be a third important parameter of molecular weight, in addition to M-e and l(K), which characterizes chain statistics and determines the molecular weight dependence of some physical properties. We introduce the molecular weight of a random step, m(R), as such a parameter. m(R) is estimated to be similar to560 for PDMS and similar to5100 for PS. The molecular weight dependence of Tg in PDMS and PS appears to be the same when presented as a function of M-w/m(R). Moreover, our analysis shows that a molecule becomes a polymer (approaches Gaussian coil) at M-w similar to 10-20m(R).