Effect of Acetic Anhydride

Due to its oxygen atoms, acetic anhydride is an electron pair donor in very much the same fashion as THF. The donor number DN (defined as the negative enthalpy of reaction for a 1 1 adduct formation between antimony pentachloride and the electron pair 

The reaction of acetic anhydride with a growing PTHF chain leads to the attachment of an acetate end group on the existing chain, and to a new chain embryo consisting of an acetate starter group bonded to a THF molecule by trivalent oxygen. This reaction is shovra below, where for the sake of clarity the hydrogen atoms bonded to the carbon atoms are now omitted. 

As can be seen, with the acetic anhydride being split, the old chain is terminated while a new chain starts growing. Such a process is called chain transfer , and the agent causing it, in this case the acetic anhydride, is known as the polymerization modifier . 

From the mechanism described, it is readily realized that the average molecular weight of the end product must decrease with increasing modifier concentration. However, it must be noted that the average molecular weight also depends on the yield. At a low yield, the final ratio of monomer concentration to modifier concentration is greater than in the case of a 

In the resulting reaction shown, the oxygen Oa forms a bond with the carbon atom Ca and abandons its previous bond with the acid site A , while Ca abandons its previous bond with Op. The outcome is a freed PTHF chain with an acetyl end group, and a CH3COO group attached to an acid site. The mechanism is analogous to a normal acetylation reaction such as ethanol + acetic anhydride — ethyl acetate + acetic acid (A) 

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Acetic anhydride is often encountered as a solution in ace tie acid Figure 30-b shows the effect of acetic anhydride concentration im the liquid viscosity at 15 C and T75C- ... 

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