The effects of molecular structure on sol-to-gel transition of biodegradable poly(depsipeptide-<i>co</i>-lactide)-<i>g</i>-PEG copolymers

<div><p>We report on the effects of number and length of PEG chains in poly(depsipeptide-<i>co</i>-dl-lactide)-<i>g</i>-poly(ethylene glycol) (P(DG-dl-LA)-<i>g</i>-PEG) copolymers on their sol-to-gel transition behavior. The graft-type copolymer is suitable for the systematic study of the effects of molecular structure and hydrophilic/hydrophobic balance on its sol-to-gel transition. We prepared various P(DG-dl-LA)-<i>g</i>-PEG copolymers through coupling reactions between the pendant carboxylic acid groups of P(GD-dl-LA) and the end hydroxyl group of MeO-PEG having various molecular weights. Temperature-responsive sol-to-gel transition of the obtained copolymer solution in phosphate-buffered solution (pH 7.4, ionic strength = 0.14) was investigated by the test tube inverting method and rheological measurements. P(GD-dl-LA)-<i>g</i>-PEG copolymer prepared from higher molecular weight PEG showed higher sol-to-gel transition temperatures compared with the copolymers prepared from lower molecular weight PEG, although these copolymers have similar weight content of PEG (23–24 wt.%). Similar trends were observed for groups of copolymers whose PEG contents were 27 or 30 wt.%. These results are informative for providing strategies on rational design of thermo-gelling polymers.</p></div>