The molecular weights of standard polystyrenes (1000≤M ≤17000) and a series of polyesters containing 5-fluorouracil in the main chain (MM=1000 is possible．The second virial coeffident A2 of low molecular weight polystyrenes increases rapidly as the molecular weight decreases．The relationship between A2 and Mn is A2=const Mn1.40 This suggests that polymers of low molecular weights in dilute solution are fully extended．