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Controlling the structure and reactivity of the chain-end group is a central objective in modern polymer chemistry. Here, we introduce 3,6-anhydrogalactal as a single-addition monomer that enables efficient and versatile chain-end functionalization of metathesis polymers. Readily synthesized from biomass-derived galactal, 3,6-anhydrogalactal exhibits excellent single-addition reactivity, allowing precise chain-end modifications even when introduced simultaneously with the propagating monomer. Theoretical calculations provide mechanistic insights into the unique reactivities governing its single-addition behavior. Its broad functional group compatibility facilitates diverse applications, including block copolymer synthesis, polymer-polymer coupling, and bioconjugation, demonstrating significant potential for advancing polymer materials and bioconjugation strategies. 

Abstract Heterologous tRNAs used for noncanonical amino acid (ncAA) mutagenesis in mammalian cells typically show poor activity. We recently introduced a virus‐assisted directed evolution strategy (VADER) that can enrich improved tRNA mutants from naïve libraries in mammalian cells. However, VADER was limited to processing only a few thousand mutants; the inability to screen a larger sequence space precluded the identification of highly active variants with distal synergistic mutations. Here, we report VADER2.0, which can process significantly larger mutant libraries. It also employs a novel library design, which maintains base‐pairing between distant residues in the stem regions, allowing us to pack a higher density of functional mutants within a fixed sequence space. VADER2.0 enabled simultaneous engineering of the entire acceptor stem ofM. mazeipyrrolysyl tRNA (tRNA^Pyl), leading to a remarkably improved variant, which facilitates more efficient incorporation of a wider range of ncAAs, and enables facile development of viral vectors and stable cell‐lines for ncAA mutagenesis.