Metal‐Free Ring‐Opening Alternating Copolymerization of Epoxides and Cyclic Anhydrides Mediated by a Ternary Phosphazene Base and Carboxylic Acids

Abstract

Organocatalytic, metal‐free ring‐opening alternating copolymerization (ROAP) of epoxides and cyclic anhydrides to produce polyesters with well‐defined alternating structure remains a challenge despite several organocatalysts having been reported. Herein, the ROAP of cyclohexene oxide (CHO) with phthalic anhydride (PA) using a series of carboxylic acids as initiator in the presence of an organic cyclic trimetric phosphazene base (CTPB) as catalyst is reported. The molar ratio of CTPB to carboxylic acid is proven to be crucial to obtain polyester with well‐defined alternating structure without formation of polyether homopolymer. The effects of carboxylic acid, temperature and feed molar ratio on monomer conversions and resultant molecular weights of poly(PA‐alt‐CHO) are systematically investigated. This work enriches the opinion of initiator for the ROAP of epoxides and cyclic anhydrides.

Ring-Opening Copolymerization Using Simple Fe(III) Complexes and Metal- and Halide-Free Organic Catalysts

“…DOSY NMR analysis confirmed that the copolymers isolated were one polymeric species and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) analysis provided evidence that there were ester and ether linkages present. However, the systems were complicated and a range of potential end groups were proposed relating to the auxiliary ligands (X = OAc or Cl) or hydrolysis and chloro-functionalization indicative of chain-transfer reactions occurring upon termination and isolating the polymers with the acidified MeOH/HCl work-up (see the Supporting Information, Figures S37–S42), or possibly cyclohexane/cyclohexene from potential Meerwein–Ponndorf–Verley reduction/Oppenauer oxidation side reactions. ,,, IR analysis was consistent with the literature (Figures S23 and S24). …”

“…However, the systems were complicated and a range of potential end groups were proposed relating to the auxiliary ligands (X = OAc or Cl) or hydrolysis and chlorofunctionalization indicative of chain-transfer reactions occurring upon termination and isolating the polymers with the acidified MeOH/HCl work-up (see the Supporting Information, Figures S37−S42), or possibly cyclohexane/cyclohexene from potential Meerwein−Ponndorf−Verley reduction/Oppenauer oxidation side reactions. 35,52,62,69 IR analysis was consistent with the literature S23 and S24). 33 ROCOP of Cyclohexene Oxide and PA in Neat Cyclohexene Oxide.…”

“…This provides hope that certainly other more reactive organic compounds may be efficient initiators for ROCOP. Additionally, there are few examples of organocatalysts in the literature, and these tend to rely on two component systems (e.g., Lewis pairs) and harsher, air-sensitive chemicals based on phosphine and boron. ,,− ,,− However, here, there is only one component with no extra additives. Of course, as was the case for North, this further highlights the need for ligand control reactions throughout all catalysis studies .…”

Organo-catalyzed/initiated ring opening co-polymerization of cyclic anhydrides and epoxides: an emerging story

“…In the same vein, the presence of alkoxide species in the reaction mixture, due to the use of strong bases as initiators, may lead to intra-or intermolecular transesterifications which are detrimental to polymerization control and thus materials properties. The homopolymerization of epoxides, attributed to excessive activation by strong Lewis acids 48 or the use of strong bases, 56 is also an obstacle to a well-behaved alternating ROCOP of anhydrides and epoxides. The comparison of the efficiency of the organocatalysts used in the Fischer-Inoue-Coates reaction is not straightforward given the diversity of the monomers employed.…”

Alkali Metal Carboxylates: Simple and Versatile Initiators for Ring-Opening Alternating Copolymerization of Cyclic Anhydrides/Epoxides

“…Organometallics with a trivalent metal center (such as Co, Cr, and Al) have prevailed in ROAC for a long time. − However, the toxic metal residuals played a detrimental role in the final product performance, to some extent, limiting their potential applications in microelectronics, food packing, and biomedical devices. To address the concern of metal contaminants and realize a relatively green synthetic process, metal-free catalysis has been developed in recent years, including quaternary onium salts, , phosphazenes, − and other nucleophilic bases. , Most of the organic catalysts showed low catalytic activity even at a high temperature and limited monomer adaptability. For example, succinic anhydride (SA) and maleic anhydride (MA), both of which are recognized as top value added chemicals from biomass and produced in a large scale, can not be copolymerized smoothly with epoxides by most organocatalysts because of the deprotonation of the active α-proton.…”