Prevailing mechanisms of the hydrolytic degradation of oligo(d,l-lactide)-grafted dextrans

The predominant mechanism of the hydrolytic degradation of oligo(d,l-lactide)-grafted dextrans in phosphate buffer was followed by quantifying both released dextran and lactic acid from the copolymers. The studied amphiphilic copolymers, with well-defined structure, exhibited various oligo(d,l-lactide) weight fractions (F^OLA) while having a quite high extent of free hydroxyl groups (>90%). Depending on their F^OLA, oligo(d,l-lactide)-grafted dextrans were soluble either in water or in organic solvents (THF, toluene, …) and different prevailing mechanisms of hydrolytic degradation were observed. The copolymer soluble in THF, with longer oligo(d,l-lactide) grafts and higher F^OLA, was found to degrade via a particular mechanism by which the greatest part of dextran was released into buffer medium during the first two weeks of degradation. During the initial stage of degradation, the hydrophilicity of dextran backbone was considered to be the main driving force for the hydrolytic cleavage of the ester linkage between backbone and grafts. Released oligo(d,l-lactide) grafts were found to be degraded via chain-end degradation or random degradation depending on their solubility in buffer medium. In case of water-soluble copolymers with shorter oligo(d,l-lactide) grafts and lower F^OLA, the chain-end degradation was exclusively observed.