Degree of Polymerization

The degree is the number of times a monomer unit is repeated in a chain. As in the case of the molecular weight, it is not an exact value but an average: xn, xv, xw, xz, or xz+1. The degree is calculated by dividing the corresponding average molecular weight between the weights of monomer unit (M0) with the formula of the polymer, calculated as explained in the next section. Obviously, the polydispersity index could be calculated with the average degree of polymerization as:

 

r = xw / xn

 

Formula weight of monomer unit

 

The following explains how to calculate the weight of the monomer unit of some polymers with the formula commonly used for them:

Weight of the monomer unit of polystyrene = sum of atomic masses of all the atoms that compose it = (number of carbon atoms x atomic mass of carbon) + (number of hydrogen atoms + atomic mass of hydrogen) = (8 x 12.01) + (8 x 1.01) = 104.16 g / mol.

 

Therefore, the average degree of polymerization by weight of a sample of PS with a molecular weight Mw = 5.4 106 g / mol, is:

 

xw = 5.4 x 106 / 104 = 5.2 x 104.

Weight of the monomer unit of polyethylene = sum of the atomic masses of all the atoms that compose it = (number of carbon atoms x atomic mass of carbon) + (number of hydrogen atoms x atomic mass of hydrogen ) = (2 x 12.01) + (4 x 1.01) = 28.06 g /mol