Defining Alternating Sequences in Polyurethanes: Sequence-Controlled Photo-Degradation in Step Polymerization

Abstract

Programmable degradation is an important functionality for sustainable polymer materials; however, in conventional step polymerization of AA and BB type monomers, polymer sequence itself has long lacked a clear and actionable definition. Polymers synthesized from AA and BB type monomers enable the incorporation of functional units directly into the polymer main chain, while inevitably featuring AA-BB connectivity; yet the concept of an "alternating" sequence has remained ambiguous. To address this fundamental limitation, we redefine sequence control in step polymerization by focusing on the connectivity unit rather than monomer composition. Using polyurethane as a representative model polymer, an AB-type monomer framework combined with dimeric species enables the explicit definition and practical implementation of (partially) alternating sequences. Incorporation of a photo-degradable monomer unit directly into the polymer backbone reveals pronounced sequence-dependent photo-degradability via main chain scission, which is drastically enhanced in alternating polymers, while thermal properties are likewise strongly influenced by polymer sequence. This work establishes polymer sequence as an additional and actionable design parameter in polyurethane as an example of step polymerization and provides a conceptual framework potentially applicable beyond polyurethanes to a broad range of AA/BB-based polymers. The utility of this framework for functional material development is demonstrated through sequence-controlled photo-degradation on the polymer main chain, offering a promising strategy toward sustainable polymer design.