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

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-N-3 and a tetrafunctionalized alkyne core. Taking advantage of the thermosensitive properties of PIPOZ, high-sensitivity microcalorimetry analyses (HS-DSC), in addition to commonly used H-1 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 H-1 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 H-1 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.

Automated summary

Two different approaches, coupling-onto (CO) and core-first (CF), were used to prepare a series of star poly(2-isopropyl-2-oxazolines) (S-PIPOZ). High-sensitivity microcalorimetry analyses (HS-DSC) were shown to be powerful tools in combination with commonly used analytical techniques for refinement and elucidation of the microstructure of architecturally complex thermosensitive polymers such as star polymers. The results highlighted a discrepancy in synthesis completeness between the core-first and coupling-onto approaches, with the latter showing better correlation and consistency in the results.