Rapid synthesis of functional poly(ester amide)s through thiol-ene chemistry

Abstract

Poly(ester amide)s (PEAs) bearing various side chains were synthesized by post-polymerization modification of PA-1, a vinylidene containing PEA. The thiols 1-dodecanethiol (1A-SH), 2-phenylethanethiol (1B-SH), 2-mercaptoethanol (1C-SH), thioglycolic acid (1D-SH), furfuryl mercaptan (1E-SH) and sodium-2-mercaptoethanesulfonate (1F-SH) were reacted with PA-1 to form PEAs PA-1A through PA-1F respectively. PEAs containing non-polar thiol side chains (PA-1A, PA-1B, PA-1E), showed little change in solubility compared to PA-1, while PEAs with more polar side chains improved solubility in more polar solvents. PA-1F, functionalized with sodium-2-mercaptoethanesulfonate, became water-soluble. The introduction of pendant functional groups impacted the thermal behaviors of PEAs in a wide range. The PEAs were thermally stable up to 368 °C, with glass transition temperatures (T_g) measured between 117 to 152 °C. Moreover, to demonstrate the versatility of the PEAs, thermal reprocessable networks and polyurethanes were successfully fabricated by reacting with a bismaleimide (1,6-bis(maleimido)hexane, 1,6-BMH) and a diisocyanate (4,4'-diphenylmethane diisocyanate, 4,4'-MDI), respectively. This study paves the way for the facile synthesis of functional poly(ester amide)s with great potential in many fields.

Scheme 1. (a) Ring-opening polymerization for PEAs synthesis; (b) polycondensation for PEAs synthesis; (c) polyaddition for PEAs synthesis; (d) polyamides (including PEAs) prepared by step-growth polyaddition of bis(N-carbonyl aziridine)s; (e) unsaturated bio-based PEAs post-polymerization modification (this work).

Scheme 2. Step-growth polymerizations of TP-Az with itaconic acid for PA-1 synthesis, and thiol–ene click reactions based on PA-1 with various thiol reagents.

Fig. 1. ¹H NMR spectrum (500 MHz, DMSO-d₆) of poly(ester amide)s PA-1 (top, the template) and PA-1A (bottom, after modification).

Fig. 2. The DSC curves of PEAs with various pendant thiol groups (heating/cooling rate, 10 °C min⁻¹; under nitrogen). All samples were heated through 3 complete heating/cooling cycles, and the data from the 3rd heating cycle were used for plotting.

Fig. 3. Top: the retro-Diels–Alder reaction (rDA) based on PA-1E and 1,6-bis(maleimido)hexane (1,6-BMH) for reversible cross-linked polyamide organogel formation; bottom: digital photos of (1) PA-1E in DMF; (2) after added 1,6-BMH for 5 min at room temperature; (3) after heated for 1 min at 100 °C; (4) continue heated for 4 min at 100 °C; (5) cooling to room temperature.

Fig. 4. Comparison of FT-IR spectrum of (a) PA-1C; (b) 4,4′-methylenediphenyl diisocyanate (4,4′-MDI) and (c) polyurethane (PU) produced by a reaction between 4,4′-MDI and PA-1C. The characteristic peaks of the polymers were labeled in the FT-IR spectra.