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

In this contribution, we thoroughly
investigated the ring-opening
alternating copolymerization (ROAC) of cyclic anhydride and epoxide
by using commercially available alkali metal carboxylates (AMCs) as
the simple and green initiators. The idea of our work is based on
the coordination effects of epoxide on an AMC and the formation of
AMC–epoxide adducts, which will weaken the interaction between
metal cation and its carboxylate counterion and therefore render the
carboxylate to feasibly attack epoxides in a nucleophilic manner at
high temperature. The coordination effects of epoxide on the AMC could
be proved by Fourier transform infrared (FT-IR) spectroscopy and density
functional theory (DFT) calculations. AMCs could effectively catalyze
the copolymerization of phthalate anhydride (PA) and cyclohexene oxide
(CHO) in bulk at 110 °C, affording polyesters with perfectly
alternating structure. Potassium acetate (KOAc) was able to copolymerize
some common cyclic anhydrides and epoxide, allowing for the preparation
of polyesters with structural diversity. Of note, KOAc could mediate
ROAC of PA with propylene oxide (PO) with a high molar feed ratio
of [KOAc]/[PA]/[PO] = 1:20 000:150 000, affording poly­(PA-alt-PO) with high molecular weight (>100 kDa). Finally,
two different polymerization mechanisms, including anionic polymerization
and cooperative catalysis, have been proposed according to the interaction
strength between metal cation and carboxylate anion. In the “cooperative
catalysis” mechanism, the alkali metal cation served as the
Lewis acid to activate epoxide and cyclic active species were generated.