Calculation formula of degree of polymerazation is an important content in studying polycondensation theory.In this paper

the unified formula of number average degree of polymerization

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according to the definition of degree of polymerization were derived from reaction steps for linear polycondensation

including 2-2 type monomers of

a-A-a+b-B-b

2-2+ 1 type monomers of

a-A-a+b-B-b+C-b

and 2+1 type monomers of

a-A-b+C-b

.The characteristics of stepwise growth are fully reflected by this method

which is clear and easy to understand. Moreover

the implication of stoichiometric ratio "

r

" becomes more clear and specific in the mathematical derivation of calculation formula of degree of polymerization.