Modification of the rates of chain cleavage of poly(ϵ-caprolactone) and related polyesters in the solid state

The rates of chain cleavage of poly(ϵ-caprolactone) (PCL), poly (l-lactic acid) (PLLA), and a 3:7 copolymer of glycolic acid and l-lactic acid (PGLA) as films in water, alcohols, and acidic and basic reagents were determined by measurements of intrinsic viscosities and molecular weights. Partial ethoxylation of the carboxylic acid end groups of PCL reduced the rate of chain cleavage in water, consistent with an autocatalytic mechanism. The firstorder rate constants for hydrolysis of PCL, PLLA, and PGLA were 7.5 × 10−5, 7.4 × 10−5, and 1.2 × 10−3 h−1, respectively. Ethanol, pentanol, 5% oleic acid, decylamine, and tributylamine increased the rate of chain cleavage of PCL but not that of PLLA. The rate of erosion but not the rate of chain cleavage of PGLA was enhanced by these reagents, with the exception of pentanol. Changes in the morphology of the polymers during degradation were characterized by differential scanning calorimetry. The origin of the more rapid rate of hydrolysis of PGLA, relative to PCL and PLLA, appears to be the intrinsic reactivity of the glycolate linkage.