Which of the following best describes initiation in addition polymerization?

Initiation in addition polymerization refers specifically to the stage where the polymerization process begins. This stage involves the formation of reactive species known as radicals or ions that set the chain reaction of the polymerization in motion.

In this context, the correct response signifies that initiation occurs precisely at the moment when a radical attacks a weak bond within the monomer. When this attack happens, it generates a new radical, which can further react with other monomers, leading to the growth of the polymer chain. This is a critical step because it marks the transition from individual monomers to the beginning of a polymer structure.

The other options do not accurately capture the essence of the initiation phase. For instance, saying it's the phase where monomers fully bond relates more to the polymer's growth rather than its initiation. The reference to heat initiating the process does not apply universally, as polymerizations can also begin through other means, such as light or chemical reactions. Lastly, the final step to terminate the reaction pertains to the end of the polymerization process rather than the initiation phase, which is aimed at starting the reaction rather than concluding it.