Poly autocatalysis

Kinetic studies on the bulk polyesterification of a,o-dicarboxy poly(hexamethylene adipate) with a,polymeric medium. Solomon s mechanism1 can be considered as reasonable. 

Even though the hydrolysis eventually produces an acid, polymer erosion rate is controlled by hydrolysis of the ortho ester bonds. The subsequent hydrolysis of the ester bonds takes place at a much slower rate so that the neutral, low molecular weight reaction products can diffuse away from the implant before hydrolysis to an acid takes place. Thus, unlike the poly (ortho ester) system I, no autocatalysis is observed and it is not necessary to use basic excipients to neutralize the acidic hydrolysis products. 

Polymer hydrolysis occurs as shown in Scheme 12 for a polymer prepared from 1,2,6-hexanetriol. Initial hydrolysis occurs at the labile ortho ester bonds to generate one or more isomeric monoesters of the triol (Wuthrich et al, 1992). This initial hydrolysis is followed by a much slower hydrolysis of the monoesters to produce a carboxylic acid and a triol. Thus, as with the poly (ortho ester) II, no autocatalysis is observed. 

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