Effect of non-stoichiometric composition

In Section 7.2.1, the polymerization of A -type monomers was considered. In AB systems, equal numbers of functional end-groups of both types are guaranteed due to structure of the monomer (unless functional impurities or additives are present in the reaction mixture). 

However, for AA and BB systems, like those shown in Table 7.1, stoichiometric imbalance can occur, with serious consequences for the polymerization. The molar ratio of the two types of functional end-groups (A and B) that are available for polymerization is determined by the initial molar ratio of the two monomers in a batch reactor, and by any monomers or oligomers that might escape from the reacting mixture during the polymerization. Note that escape of volatile monomers with the resulting influence on the ratio of functional groups is a serious practical problem for some industrial polymerizations that use volatile monomers (e.g., HMD in nylon 6,6 production and diphenyl carbonate in polycarbonate production). 

Suppose there is a shght excess of BB monomers at the beginning of an AA and BB polymerization (and that no molecules with functional groups leave the reacting mixture). To see how the average molecular weight of the polymer can be calculated, define  

The total number of chain ends after reaction progresses to conversion p is total = 2Nao(1—p) + (Nbo — Nao) 

Since the final number of molecules at conversion p is Ntotai/2, the number-average chain 

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