High-Sensitivity Microcalorimetry and Gel Permeation Chromatography in Tandem Reveal the Complexity of the Synthesis of Poly-(2-isopropyl-2-oxazoline) Stars

Abstract

A series of star poly­(2-isopropyl-2-oxazolines) (S-PIPOZ) was prepared by two different approaches: “coupling-onto (CO)” and “core-first (CF)”. On the one hand, for “the core-first” approach, SCF-PIPOZ were prepared by direct cationic ring opening polymerization from a tetra tosylate-functionalized pentaerythrityl core. On the other hand, for the “coupling-onto” approach, SCO-PIPOZ were synthesized by coupling via copper alkyne-azide cycloaddition between L-PIPOZ-N3 and a tetrafunctionalized alkyne core. Taking advantage of the thermosensitive properties of PIPOZ, high-sensitivity microcalorimetry analyses (HS-DSC), in addition to commonly used 1H nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), and spectrophotometry (UV analyses), proved to be powerful tools. On the one side, microcalorimetry analysis facilitates purification of the polymers by setting the appropriate temperature for centrifugal fractionation, and on the other side, it helps in the resolution of their microstructures. SCO-PIPOZ demonstrated good correlation between the 1H NMR, GPC, and UV results and showed sharp unimodal HS-DSC endotherms. Contrary to the SCO-PIPOZ data, SCF-PIPOZ results revealed a discrepancy between the molecular weights measured by 1H NMR and GPC. This issue was resolved with the help of HS-DSC, which allowed detection of different populations of polymers to analyze them before and after isolation by isothermal centrifugal fractionation. Cleavage of the arms of the isolated SCF-PIPOZ showed incomplete/not uniform synthesis of S-PIPOZ by the “core-first” approach. These results highlight the importance of HS-DSC coupled with the commonly used analytical techniques for both refinement and elucidation of the microstructure of architecturally complex thermosensitive polymers such as star polymers.

Features of Solution Behavior of Polymer Stars with Arms of Poly-2-alkyl-2-oxazolines Copolymers Grafted to the Upper Rim of Calix[8]arene

“…Thermoresponsive poly-2-alkyl-2-oxazolines (PAlOx) have been actively studied in recent decades due to the wide potential of their application [20][21][22][23]. One of the ways for using this class of polymers is medicine due to their biocompatibility and non-toxicity, 2 of 21 stability in enzyme media [24], and lower critical solution temperature (LCST) near to the human body one.…”