Sequential Polymerization from Complex Monomer Mixtures: Access to Multiblock Copolymers with Adjustable Sequence, Topology, and Gradient Strength

Abstract

Switchable polymerization shows considerable potential for simulating the molecular precision of natural biopolymers, such as nucleic acids or proteins, to synthesize highly sequence-controlled macromolecules but is mainly limited to three- and four-component systems. To expand the scope to systems with up to five components, we established a reactivity gradient among 12 monomers, including cyclic anhydrides, cyclic esters, and epoxides. Highly selective competitive anhydride/epoxide, self-switchable cyclic anhydride/epoxide/cyclic ester, and competitive cyclic ester/trimethylene carbonate copolymerizations were achieved using a simple alkyl metal carboxylate catalyst. Anhydrides gave access to gradient copolymers with reactivity ratios of 2 < r1 < 5, 0.7 > r2 > 0.3 giving medium-gradient copolymers and r1 > 400, r2 < 0.03 giving block copolymers. Further, the anhydride reactivity was predicted using 13C NMR chemical shifts. This comonomer library will allow more complex copolymer structures with adjustable sequence, topology, and gradient strength to be predicted and prepared.