Reversible-addition
fragmentation chain transfer (RAFT) polymerization
has been widely explored since its discovery due to its structural
precision, versatility, and efficiency. However, the lack of tunability
of the polymer backbone limits some applications. Herein, we synergistically
combine RAFT and step-growth polymerization mechanisms, by employing
a highly selective insertion process of a single monomer with a RAFT
agent, to achieve RAFT step-growth polymerization. A unique feature
of the RAFT step-growth polymers is that each backbone repeat unit
bears a pendant RAFT agent, which can subsequently graft side chains
in a second polymerization step and afford molecular brush polymers.
Enabled by cleavable backbone functionality, we demonstrate transformation
of the resulting brushlike polymers into linear chains of uniform
size upon a stimulus.
Here, commercially available N-aromatic substituted bismaleimides were used in RAFT step-growth polymerization for the first time. In our initial report (J. Am. Chem. Soc. 2021, 143 (39), 15918-15923), maleimide precursors...
RAFT step-growth polymerization was previously demonstrated with monomers that bear low rate of homopropagation to favor the chain transfer process; by contrast, acrylates are known to be fast homopropagating monomers, thereby posing serious challenges for RAFT step-growth. Here, we identified a chain transfer agent (CTA) that rapidly yields single unit monomer inserted (SUMI) CTA adducts with a model acrylate monomer. Using a bifunctional reagent of this CTA, we successfully demonstrated RAFT step-growth polymerization with diacrylates, yielding linear polymer backbones. Furthermore, we achieved inclusion of functionality (i.e., disulfide) into RAFT step-growth polymer via a disulfide incorporated bifunctional CTA. Grafting from this backbone resulted in molecular brush polymers with cleavable functionality in each repeat unit of the backbone, allowing selective degradation to afford well-defined unimolecular species of two polymeric side chains. Given the wide selection of commercially available diacrylates, RAFT step-growth polymerization of diacrylates will further enable facile synthesis of complex architectures with modular backbones.
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