Architectural effects on the thermoresponsive behaviour of PEGylated hyperbranched polyesters and their linear analogues

Abstract

Peripherally clickable hyperbranched polyesters carrying numerous allyl ester (HPB-AL) or propargyl ester (HBP-PG) terminal groups were transformed into amphiphilic core–shell systems by clicking with either polyethylene glycol monomethyl ether (MPEG)-azides or MPEG-thiols; whereas the former, when used with HBP-PG, quantitatively installed a single MPEG segment via an azide–alkyne click reaction, the latter installed two MPEG segments onto each propargyl group, under thiol–yne reaction conditions. These PEGylated HBPs were water soluble and exhibited a cloud point; which was seen to increase both with the increase in the length and number density of PEG segments. To reveal the effect of topology, compositionally identical linear polyesters bearing propargyl units were similarly clicked to install either one or two MPEG segments per repeat unit; the cloud points of the linear PEGylated polyesters were consistently lower than those of their HBP counterparts. Furthermore, the PEGylated polymers with a triazole linker, formed by virtue of an azide–alkyne reaction, exhibited a higher cloud point when compared to analogous polymers carrying a thioether linkage; the latter were prepared by a thiol–ene reaction of HBP-AL with MPEG-thiols. Taken together, these observations revealed that the MPEG segments are more exposed and better solvated in the HBPs than in their linear counterparts, suggesting that the hydrophilic–lyophilic balance (hlb) of amphiphilic polymers cannot be gauged using the chemical composition alone, but it depends strongly on their structure and topology.