Cross-linking a range of alkyl polymers: Some alkyl polymers, such as polyethylene, can be cross-linked by using peroxides or high-energy radiation or through the addition of a radical forming agent. Others, like polypropylene, are likely to undergo chain scission, and this process tends to be uncontrolled in the distribution of the cross-links. Lepage et al. developed a widely applicable approach using bis -diazirine molecules as cross-linking agents (see the Perspective by de Zwart et al. ). These molecules can be thermally or photochemically activated to form carbenes that readily insert into the polymer carbon-hydrogen bonds, thus leading to cross-linking. The bis -diazirine is nonexplosive, nonvolatile, and easily activated at relatively mild temperatures and thus could be used to fine-tune the properties of existing polymers through small chemical modifications. Science , this issue p. 875 ; see also p. 800

Electron-rich aryl diazirines have lower activation temperatures and a longer λ_max than electron-poor analogues, and undergo C–H insertion up to ten-fold more efficiently—suggesting improved performance for biological probes and polymer crosslinkers.