Directed backbiting as a tool for controlling copolymer sequence in ring-opening polymerization

• Copolyesters with microstructure control were synthesized by directed backbiting in ring-opening polymerization. • The reaction conditions: monomer concentration, reaction temperature, and system thermodynamics effect microstructure. • A simplified equation was derived that relate reaction conditions to microstructure. Controlling monomer sequence during sequential ring-opening copolymerization is imperative to the final polymer properties, but often challenging due to transesterification reactions that scramble the sequences of repeating units of the copolymer. With a deliberate choice of catalyst, intermolecular transesterification can be avoided and instead directed to intramolecular transesterification (backbiting), with predictable frequencies. In this work, we use exactly this feature to derive a relationship to relate the synthesis of block, random, and block-gradient copolymers, to the choice of reaction conditions. The relationship considers the relative rates of propagation (k 2 ) and backbiting (k 1 ) to enable a correlation between sequences or homo dyads in the copolymers to concentration, free energy of polymerization, and temperature. Experimental results are in good agreement with the expected trends and copolymers of varying homo dyads were prepared for pentadecalactone (PDL), ε-caprolactone (εCL), and δ-valerolactone (δVL) mixtures. The relative rates of propagation and backbiting were related to the polymer conformation-based Jacobson-Stockmayer theory. The explored approach towards controlling polymer architecture under conditions dictated by the ring-chain equilibria (RCE) may aid the development of new tailored copolymers.