In the context of polymerization mechanisms, the process by which two free radicals collide and form a stable molecule, effectively stopping the growth of the polymer chain, is known as __________.

The process is called termination by combination. In this step two growing polymer chains, each ending with a free‑radical center, meet and their unpaired electrons pair up, forming a covalent bond that creates a stable, non‑reactive molecule. This reaction removes the radicals from the system, stopping further chain growth. For example, if two radicals R• and R′• combine, they produce a single molecule R–R′, ending both chains. This mechanism is common in radical polymerization of vinyl compounds.