Organo-Catalyzed Cationic Ring-Opening Copolymerization of Cyclic Anhydrides with Oxolanes: Access to Structurally Diverse Polyesters

The
cyclic ether candidates for ring-opening copolymerization (ROCOP)
are mainly three-membered oxiranes with high ring tension, while ROCOP
of cyclic anhydrides with low ring strain oxolanes was seldom thoroughly
investigated. Herein, we report rapid cationic copolymerizations of
tetrahydrofuran (THF) with anhydrides to facilely synthesize alternating
polyesters. These processes are realized by a highly active organic
Lewis acid trityl tetrakis(pentafluorophenyl)borate ([Ph3C]+[B(C6F5)4]−) that is almost inactive for THF homopolymerization. Poly(ester-co-ether)s with AB, ABB, and ABBB-type repeating units (A
= anhydride, B = THF) are formed during copolymerization. The alternating
enchainment of anhydride and THF can produce AB-type units, while
the coupling reactions between trityl ether and oxonium chain ends
will generate ABB- and ABBB- type units. The model reactions suggests
that AB-, ABB-, and ABBB-type repeating units are interconvertible,
and their relative content is determined by reaction time, temperature,
and monomer feed ratio. Alternating polyesters are finally formed
by feeding equimolar THF and cyclic anhydride. [Ph3C]+[B(C6F5)4]−-catalyzed ROCOP is further extended to other biomass-derived oxolanes
and 14 commercially available cyclic anhydrides, thus allowing for
the fabrication of polyesters with structural and functional diversity.
This methodology provides an alternative approach to some “AB”-type
alternating polyesters that are usually obtained by traditional step-growth
polycondensation.