The polymer chains between entanglements may act as a network of entropic springs.  Hence, the mol. wt. between entanglements may be calculated from the number of active chains that contribute to the viscoelastic modulus.  The details, including the key formulae, should be given in any textbook of polymer rheology, such as that by Ferry (Viscoelastic properties of polymers. 3rd edn. John Wiley & Sons Inc. 1980). 

Please see the link below (2 pages )reference which gives an idea about calculating MW of  chain entanglement in a polymer.

When the link opens, press look inside.

http://link.springer.com/article/10.1007%2FBF00712317#page-1

The MW of an entanglement has no sinn, or it has more then  a sinn.

The polymer, glassy ot not, can be caracteriszed  'among others) by  two distributed quantities: the molecular-weright and the entanglement-length

The polymer chains between entanglements may act as a network of entropic springs.  Hence, the mol. wt. between entanglements may be calculated from the number of active chains that contribute to the viscoelastic modulus.  The details, including the key formulae, should be given in any textbook of polymer rheology, such as that by Ferry (Viscoelastic properties of polymers. 3rd edn. John Wiley & Sons Inc. 1980). 

Thank you all for the valuable suggestions.

Dear all;

to say the molecular weight of entanglements is something misleading since this a physical attachment of the polymer chains once it reaches a certain length or molecular weight, usually named the critical (or threshold) molecular weight for the onset or occurrence of entanglements.

It is measured by different means, the most used is to follow the change in the specific volume as a function of different MW, the abrupt change (positive) in the slop of the straight line is the point (or MW) at which the entanglements start to participate in the overall physical behavior of the material. Regards