The synthesis and characterization of (carboxylic acid)-telechelic epsilon-caprolactone and L, L-lactide based degradable polyester ionomers

John Walter Sherman

Abstract

Two novel methods of generating (carboxylic acid)-telechelic poly(ε-caprolactone model compounds were developed. Both methods produced succinic acid terminated materials in the melt without the need to isolate a hydroxy-telechelic polymer intermediate. A one-step method was developed where the succinic anhydride along with stannous octoate catalyst, glycolic acid initiator, and ε-caprolactone monomer, was added to the polymerization mixture initially and allowed to react with the hydroxy-functional polymer chain ends created during polymerization. This method produced 100% (carboxylic acid)-telechelic materials with broad molecular weight distributions (MWD's): polydispersity indices ranged from 2.41 to 2.78. In addition, there was poor agreement between theoretical and experimental molecular weights. A two-step method was developed in response to the unacceptable MWD's and poor molecular weight targeting seen in the one-step method. This was accomplished by changing the initiator from glycolic acid to ethylene glycol and withholding the addition of succinic anhydride until high monomer conversion was reached. A series of kinetic experiments was conducted on the stannous octoate catalyzed polymerization of ε-caprolactone to find the time at which high monomer conversion was reached. Our results showed that the rate of monomer consumption was not dependent on the amount of added alcohol initiator, but only on the amount of stannous octoate provided there is a 2-fold molar excess of alcohol with respect to stannous octoate concentration. A succinic anhydride addition delay of 60min was found to be sufficient to allow high monomer conversion for each targeted molecular weight, provided the monomer/catalyst ratio remained equivalent. Telechelic ionomers derived from (carboxylic acid)-telechelic poly(ε-caprolactone) were generated by the simple base neutralization of the carboxylic acid end groups. 13 C NMR analysis of each ionomeric material showed that base hydrolysis of the backbone does not occur to any appreciable extent during neutralization. Low molecular weight (carboxylic acid)-telechelic poly(L,L-lactide) was synthesized by first generating a hydroxy-telechelic pre-polymer, and in a second, solvent-based step, reacting the end groups with succinic anhydride. (Abstract shortened by UMI.)