In this paper the thinning and breakup of filaments of semi-dilute solutions of polystyrene in diethyl phthalate is investigated with a capillary breakup extensional rheometer. The solutions show a long initial viscocapillary balance that can be fitted with 0-D Newtonian and power law models. The onset of the elastocapillary balance is molecular weight dependent and the coil stretch transition of the polymer is shifted close to the breaking point and to small filament diameters. However, the transition is even for concentrations up to c/$c^*$ ~ 30 observable due to a strong extension thinning in the Caber experiment. It is shown that the transition to the elasto-capillary balance takes place at local elasto-capillary number $Ec^*$ = 4.7 that incorporates the extensional relaxation time ${lambda}_E$ and the extension thinning viscosity ${eta}_{E,app}$.