Catalytic Ring-Opening Copolymerization of Limonene Oxide and Phthalic Anhydride: Toward Partially Renewable Polyesters

Catalytic ring-opening copolymerization of limonene oxide with phthalic anhydride was performed applying metal t-Bu–salophen complexes (t-Bu–salophen)­MX; M = Cr, X = Cl (1), M = Al, X = Cl (2), M = Co, X = OAc (3), M = Mn, X = Cl (4), t-Bu–salophen = N,N-bis­(3,5-di-tert-butylsalicylidene)­diimine. The chromium and aluminum catalysts performed best, and catalyst 1 was selected for further studies. Investigating the effect of different cocatalysts in the copolymerization of limonene oxide and phthalic anhydride revealed that the onium salt PPN⁺Cl^– showed the best activity while phosphines and N-heterocyclic-based amines showed a somewhat lower activity. ¹H NMR and MALDI-ToF-MS spectra of the copolymers formed confirmed the alternating microstructure. Applying various mono-, di-, and trifunctional chain transfer agents (CTA)­s such as water, (poly)­alcohols, acids, and diamines in all cases resulted in a decrease in molecular weight while the PDI remained low, characteristic for an immortal system. Catalyst 1 proved to be robust in the presence of these CTAs not showing any drop of catalytic activity, which allows the formation of polyesters with tunable molecular weights and narrow PDIs and varying numbers of functionalities.