  Increasing attention is currently given to carbyne, the sp1 hybridized
one-dimensional carbon allotrope, because of its predicted outstanding
mechanical, optical, and electrical properties. Although recently substantial
progress has been reported on confined carbyne synthesized inside double-walled
carbon nanotubes (DWCNTs), its formation mechanism and precursors for growth
remain elusive. Here, we show a rational design of isotope engineered
ultra-clean DWCNTs as tailored hosts to trace the growth of carbyne, which
allows to identify the precursor and unravel the formation mechanism of carbyne
during high-vacuum annealing at high-temperatures. Using this approach,
ultra-clean DWCNTs with 80.4% 13C-enriched inner walls and outer tubes of
naturally abundant served to unambiguously prove that only the carbonaceous
materials inside the DWCNTs can act as precursors. The exchange of C atoms
between inner and outer tubes happens without any growth of carbyne. After
applying a secondary oxidation step, it is possible to produce the carbonaceous
precursors from the partially oxidized DWCNTs. In this manner, not only carbyne
with a record of ~28.8% 13C enrichment is grown, but concomitant healing,
reorganization and regrowth of the DWCNTs occurs. This work enables to identify
the precursor and trace the growth mechanism of confined carbyne with
engineered properties. This is a crucial step, towards accessing the full
application potential of confined carbyne hybrids by tailoring not only the
isotopic fillers, but also the inner and outer tubes of the DWCNT hosts.
