Inimer mediated synthesis of hyperbranched polyglycerol via self-condensing ring-opening polymerization

摘要

A series of hyperbranched polyglycerols (HPGs) have been synthesized using glycol as an initiator in the presence of potassium counterion ([K ~+]_0/[-OH]_0 = 0.75) and employing batch monomer addition (BMA) to obtain insight into the kinetics of the polymerization. The first-order time-conversion plots show that the polymerization is fast up to ～200 min, and the rate decreases substantially with increasing reaction time. Size exclusion chromatography of the HPGs during the polymerization indicates the presence of two living species in the reaction: a large fraction that grows into oligomers (<3000 g/mol) and becomes stabilized at higher conversion and a small fraction, growing faster and able to sustain a larger degree of polymerization (>140 000 g/mol). ~(13)C NMR of the oligomer HPG shows signals corresponding to epoxy ring headgroup at 45.1 and 52.0 ppm and confirms the formation of epoxy anion, an inimer, via intermolecular proton transfer from glycidol. Self-condensing ring-opening polymerization of epoxy inimer produces ill-defined hyperbranched inimer-oligomers in high yield along with a small fraction of high molecular weight HPG that propagates without significant transfer to glycidol. The differential scanning calorimetry analysis shows the HPG exhibited two distinct T_gs (<-50 and >-20 C) indicating the oligomer and high molecular weight fractions are immiscible, which is attributed to conformational constraint of two different types of branching. A mechanism of the formation of HPGs is proposed involving inimer-mediated equilibrium between oligomers and high molecular weight HPGs. The slow monomer addition (SMA) protocol was employed to reveal the existence of inimers during the reaction, supporting the proposed mechanism.