Title

Synthesis and Characterization of Biodegradable Polyesters Derived From Carboxylic Acid-Terminated Polyesters

Date of Award

1997

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Polymers and High Performance Materials

First Advisor

Robson F. Storey

Advisor Department

Polymers and High Performance Materials

Abstract

Biodegradable carboxylic acid-terminated polyesters have been utilized as precursors to polymers with different end group chemistries such as anhydride, metal carboxylate, and glycidyl ester. These carboxylic acid-terminated polyesters were synthesized from various mono- and multifunctional hydroxyl-terminated polyesters, which were synthesized from the stannous octoate catalyzed, mono- or polyol-initiated ring-opening polymerization of $\varepsilon$-caprolactone, {\it D,L\/}-lactide, glycolide, and/or combinations of these monomers. These polyesters were converted to their corresponding carboxylic acid-terminated analogues by reaction with succinic anhydride. To aid in the structural elucidation of the end-functionalization chemistries, low molecular weight hydroxyl-terminated poly($\varepsilon$-caprolactone) was subjected to end group analysis using gated decoupling (Decgate) $\sp{13}$C NMR spectroscopy, and assignments for characteristic end groups were developed. In addition, since stannous octoate was the catalyst used in all ring-opening polymerizations, the effect of water and stannous octoate catalyst on the composition, molecular weight, and molecular weight distribution of alcohol-initiated poly($\varepsilon$-caprolactone) was studied. The anhydride end group chemistry involved the synthesis of linear poly($\varepsilon$-caprolactone)s containing one and variable numbers of anhydride functions along the polymer backbone. The inclusion of a single anhydride unit involved the coupling of monofunctional carboxylic acid-terminated polyesters using diphenyl chlorophosphate, and this reaction proceeded to nearly full conversion. The synthesis of polymers containing a variable number of anhydride linkages per chain was carried out in a similar manner using difunctional carboxylic acid-terminated polyesters as chain extending segments, and monofunctional, carboxylic acid-terminated polyesters as end-capping units. However, less than full conversion of end-functionalized polymers was achieved since the reaction proceeded through a condensation mechanism. The presence of anhydride units in the poly(ester-anhydride) chains was confirmed using $\sp{13}$C NMR and FT-IR spectroscopies. Gel permeation chromatography (GPC) confirmed coupling, chain extension, and subsequent degradation of these polymers. The metal carboxylate end group chemistry involved the synthesis of difunctional poly({\it D,L\/}-lactide-{\it co\/}-glycolide-{\it co\/}-$\varepsilon$-caprolactone)s terminated with sodium carboxylate groups. Carboxylic acid-terminated terpolyesters were titrated with aqueous NaOH to produce sodium carboxylates. Molecular weights and degree of conversion of terpolyesters were evaluated using GPC. The degree of randomization of the terpolyesters was determined from the polydispersities measured by GPC, and greater randomization of the terpolyester backbone rendered them water soluble. DSC was used to probe variation in glass transition temperatures with terpolyester composition. Additional information obtained from DSC, showed that the carboxylic acid end groups were thermally unstable, and their stabilization was achieved by converting to sodium carboxylates. Lastly, the glycidyl ester end group chemistry involved the synthesis of multifunctional glycidyl ester-terminated poly($\varepsilon$-caprolactone)s. Carboxylic acid-terminated polyesters were converted to their potassium carboxylate analogues by titration with methanolic KOH; subsequently, the potassium carboxylate end groups were converted to glycidyl esters by reaction with epibromohydrin. Results of HBr titration of the glycidyl ester end groups indicated the end-functionalization reaction to be less than quantitative. The prepolymers were cured with succinic anhydride, and the corresponding networks were evaluated using network extraction, equilibrium swelling, differential scanning calorimetry (DSC), and thermal gravimetric analysis. The results of network extraction and equilibrium swelling indicated the networks to have high sol contents. DSC showed no residual curing exotherms which suggested the high sol contents to be solely a result of inefficient end-functionalization rather than incomplete curing.