What is the term for a chemical reaction in which a catalyst promotes the combination of simple molecules into a chain of molecules?

The term that describes a chemical reaction in which a catalyst facilitates the combination of simple molecules into a chain is known as polymerization. In this process, monomers, which are simple molecules, undergo a reaction in the presence of a catalyst to form larger, more complex molecules called polymers. This is a key process in the production of many synthetic materials, including plastics and rubbers, where the properties of the resulting polymer are significantly different from those of the individual monomers.

Synthesis refers more broadly to the formation of compounds from simpler substances and does not specifically denote the creation of chains or polymers. Decomposition describes reactions where a compound breaks down into simpler components, which is the opposite of what is occurring in polymerization. Oxidation refers to a type of chemical reaction that involves the transfer of electrons, often resulting in an increase in oxidation state, but this does not necessarily pertain to the combination of molecules into chains. Hence, polymerization is the most precise term for the type of reaction described in the question.