Living polymerization: Rationale for uniform terminology

We do not think it appropriate to provide an answer here to all the comments made by the reviewers. It is clear that a majority of the reviewers agree on the principal issue addressed in our letter and recognize that nothing is perfect and that the degree of perfection should not be a criterion in the naming of chemical reactions or processes. Others vehemently disagree with this position. One reviewer noted that our statement, “The phenomenon of living polymerization will be observed whenever propagation is significantly faster than any process for irreversible termination,” would mean that all polymerizations could be classed as living. Although it may be argued that all polymerizations are living at some stage of conversion, it was not our intention to claim that all polymerizations are living. The current recommendation of the IUPAC Commission on Macromolecular Nomenclature is that living polymerization is “a chain polymerization from which chain transfer and termination are absent” (Pure App Chem 1996, 68, 2287). The term absent is absolute. Even in the best systems, chain transfer, termination, or both are likely to occur to a finite extent. Szwarc suggested the use of the term negligible. The definition also does not reflect that living polymerizations often involve the transient formation of dormant species. This could be addressed by the insertion of the word irreversible before chain transfer and termination. One reviewer pointed out that the IUPAC recommendation is currently under review. We prefer a definition that is based on an observable characteristic rather than one based on a requirement to prove that certain processes (termination and chain transfer) are absent. In our letter, we suggested the use of an alternate definition: “Living polymerizations are processes that yield living polymers.” Living polymers are those that are able to grow whenever additional monomer is provided.