The method of choice for determining the molecular weight of a polymer is:

Osmotic pressure method is used to determine the molecular weight of polymers and proteins by using the equation PV=nRT and also because  of  the reason

1. Heating is not required but for other colligative properties heating is required. But at high temperature bio molecules are not stable.
2. Even though Osmotic pressure is very small, but it is measurable.  For other colligative properties, the difference is very small and can not be measured.