Comments on “: Rationale for Uniform Terminology” by Darling et al.

Comments by Comments by Professor Michael Szwarc, F.R.S. Professor Takuzo Aida 1176 Santa Luisa Drive, Solana Beach, Department of and California 92075 Biotechnology, Graduate School of Engineering, , 7-3-1 One of the aims of nomenclature is to convey to Hongo, Bunkyo, Tokyo 113-8656, Japan readers or listeners an idea that arose in one’s mind, if possible in one word. Whatever the original definition, living poly- The novel characteristic of a living is merization is now the most convenient and widely its capacity to retain for a relatively long time its accepted term for clearly expressing that a work ability to grow whenever a suitable monomer is or an article is intended to control the molecular supplied. In these terms, living resem- weight of a polymer rather than the comonomer ble biologically living systems that grow when sequence, stereosequence, or other structural pa- food is available. The word living conveys this rameters. In a recent article,1 Szwarc stated that idea. a living polymer should retain its activity for Living polymerization is the polymerization chain growth whenever new monomer is supplied. that yields living polymers whether the initiation A problem, to be discussed here, is what we is fast or slow. Of course, if one desires to produce should call or how we should classify the following polymers of narrow molecular weight distribu- examples (extreme cases): tions, one has to rapidly initiate the polymeriza- tion and ensure that the initiation is completed in Case 1: Polymerization that can produce a a short time. However, this is a technical aspect of growing polymer with a very long lifetime the problem, and it should not be included in the but a broad molecular weight distribution definition of living polymerization. due to a slow initiation. In full agreement with the authors, I find the Case 2: Polymerization that can produce a word controlled unfortunate in the definition of polymer with a narrow molecular weight Ͻ living polymerization. It does not convey the char- distribution (Mw/Mn 1.1) and a desired acteristic feature of living polymerization, and, as molecular weight but involves an equilibra- pointed out by the authors, we control various tion between active and inactive species as a aspects of polymerization by various techniques. result of a rapid, reversible termination or For example, we control the temperature of exo- chain-transfer reaction. thermic polymerization by performing it in emul- sion and the viscosity of rubber or nylon by adding In Case 1, each polymer grows in a living fash- appropriate ingredients. For a more detailed dis- ion, but the polymerization should not be called cussion of the discovery of living polymerization living according to the original definition of living and my most recent definition, see ref. 1. polymerization, which requires the initiation to be faster than the propagation. In Case 2, the polymerization should not be called living if the original definition of living polymerization is rig- REFERENCES AND NOTES orously applied. However, the polymerization allows the formation of block and end- functionalized polymers and displays all the char- 1. Szwarc, M. J Polym Sci Part A: Polym Chem 1998, acteristics of the classical living anionic polymer- 36, ix. ization of styrene. Case 2 includes living radical polymerization, involving atom transfer radical polymerization (ATRP), reversible addition frag- Journal of Polymer Science: Part A: , Vol. 38, 1710–1752 (2000) © 2000 John Wiley & Sons, Inc. mentation chain transfer (RAFT), and nitroxide- 1710 LIVING OR CONTROLLED? 1711 mediated radical polymerization, and also immor- diene initiated by n-butyl lithium in a hydrocar- tal polymerization with aluminum . bon solvent shows most of the properties of a To my understanding, it may now be accept- living system, but, because the rate of initiation is able for the polymerizations in Case 2 to be called slow, the molecular weight distribution is rela- living polymerization. ATRP and RAFT are the tively broad and certainly not described as Pois- names that are given on the basis of the chemis- son-like. The polymerization, however, shows all try of the polymerization mechanisms. Immortal the other attributes of a living system. When dis- polymerization is a catalytic version of living po- cussing the nature of living polymerizations with lymerization. Although the polymerizations in my students, I am inclined to define a living sys- Case 2 involve inactive species such as alkyl ha- tem as one in which, once a propagating chain is lides and alcohols at the polymer termini, these created, the ability of that center to propagate is can, however, be reversibly converted into the retained in that particular polymer chain. Fur- corresponding active species when particular ac- thermore, if the polymerization is living, the spe- tivators (catalysts) are present, so these species cies will not enter into any other reactions that are not actually dead but dormant. The original will destroy the integrity of the chain and prevent definition of living polymerization was made it from participating in further propagation reac- without the consideration of such an equilibration tions. This then permits reactions to take place, between active and inactive species, but it in- provided they exist, that enable a dormant chain cludes a consideration of the reactivities of free to be converted to an active state and back again ions and ion pairs. It is likely that the equilibra- to the dormant state. After all, the anionic poly- tion between active and inactive species is merely merization of styrene in tetrahydrofuran initiated on an extreme of the equilibration between free by sodium compounds involves the establishment ions and ion pairs (particularly when the equili- of an equilibrium between free ions and solvent- bration is rapid). separated ion pairs. The activities of the species In conclusion, we should be more generous in differ markedly, and one could argue that the using (or use together with individual names) the propagation effectively takes place while any par- term living polymerization for Case 2. Otherwise, ticular chain is in the free-ion state and the ion we have to create many new, complicated names pair accounts for very little of the activity. The that would lower the efficiency of electronic equilibrium reaction for their interconversion ex- searching. ists, however.

REFERENCES AND NOTES Comments by Dr. George G. Barclay 1. Szwarc, M. J Polym Sci Part A: Polym Chem 1998, Shipley Company, 455 Forest Street, 36, ix. Marlboro, Massachusetts 01752

Practical terminology is extremely important Comments by in scientific literature because it allows the defi- Professor Allan J. Amass nition and discussion of complex subject areas Specialty Polymer Research Group, without the need for elaborate descriptions. How- Aston University, CEAC, Aston Triangle, ever, care is required when defining nomencla- Birmingham B4 7ET, ture to avoid confusion. To date, the controversy surrounding the use of the term living polymer- Thank you for the opportunity to comment on ization in relation to the recent developments in the article by Darling et al. on the nature and nitroxide-mediated radical polymerization and definition of living polymerization. Personally, I atom transfer radical polymerization has been do not wish to see any reference to the breadth of extremely ambiguous. the molecular weight distribution of the product In organic chemistry, descriptive terms are in the definition of living polymerization. The for- given to general classes of reactions, for example, mation of narrow molecular weight distribution reduction, which covers the use of a broad spec- polymers is a special case of living systems, one in trum of reducing reagents such as metal hydrides which the rate of initiation is fast compared to the and catalytic hydrogenation. Depending on the rate of propagation. The polymerization of buta- reducing reagent and functional group, various 1712 LIVING OR CONTROLLED? reaction yields and chemoselectivities can be a prerogative and a pure and noble character to achieved. However, it is generally accepted that, this type of polymerization. independent of the yield and chemoselectivity, As for me, having only used radical polymer- the reaction being carried out is unambiguously a ization during my career (30 years), I immedi- reduction. ately admitted that rendering the transfer (I have Living polymerization, as discussed in this ar- published 150 articles about “dead” telomeriza- ticle, covers a general class of polymerization tion), or the termination, reversible was a suffi- techniques that undergo a chain-growth process cient contribution to call that living polymeriza- in which the end groups are controlled, enabling tion. Moreover, the term controlled is totally in- chain growth “whenever additional monomer is appropriate, just as stated in the article, because supplied.” This definition covers a number of tech- controlled means to me that one is able to foresee niques that provide end-group control, regardless the reaction by modeling and, consequently, is of the chemoselectivity, yield, and mechanism of able to give a kinetic scheme close to reality. That the chain-growth process. The controversy sur- is what I was able to do in dead telomerization rounding the use and broadening of the term liv- and what gave conclusive results. It is a control, ing polymerization concerns the degree to which but it is not a living character. the chain-growth process is controlled, in terms In addition, I would say that nothing is perfect of narrow polydispersity (1.0–1.1), molecular in this world, and if it is possible to achieve a weight control, and the elimination of irreversible living polymerization of styrene by an anionic termination processes. However, as the authors process, what about methyl methacrylate (MMA) and—a fortiori—acrylates? recognized, these stipulations are the ideal re- Thus, it is not possible to reserve a term exclu- sults of a perfect living polymerization, and such sively for one method of polymerization, the sty- idealism should not limit the use of the term rene one, which occurs in drastic conditions. living polymerization. The definition of living po- Finally, nobody has missed the similarity be- lymerization described in this article is consistent tween free or associated ion pairs and counter- with the accepted practice in organic chemistry of radicals, and it is clear that the small differences naming general classes of reactions, and as such, currently observed (i.e., at our level of knowledge) I strongly support the authors in their efforts to between ion pairs (anionic) from very weak en- relax and broaden the use of this terminology. ergy bonds (styrene–nitroxide), complexes with metals through halogens (atom transfer radical polymerization), or simply metal complexes Comments by (Ziegler–Natta or metallocene) will not take long Professor Bernard Boutevin to break into pieces when theoretical chemistry is Laboratory of Applied Chemistry, able to better describe the reaction intermediates. University of Science and Technics in In short, I completely agree with your article Languedoc, Superior National School of and your idea to resume using the single expres- Chemistry, 8 rue de l’Ecole Normale, sion radical living polymerization (particularly F-34053, Montpellier Cedex 1, France radical). Then, each researcher will need to eval- uate the percentage of living character for each The purpose of this article is to try to rational- system, that is, evaluate the amount of side reac- ize the terms used by researchers in the field of tions (and their nature) that decrease this per- polymerization. Particularly, the emergence of centage, but he should be conscious that today’s radical polymerization in the synthesis of con- imperfections may evolve quickly and become as trolled architecture polymers has been intensely close to perfection as cannot be imagined now. debated to know if it may be considered living. In For example, we may cite the anionic MMA France too, where I am involved in research with polymerization that, because of the relentless other laboratories interested in this field, we also work of some researchers (such as P. Teyssie), has have had lots of discussions, and since the begin- reached some outstanding results.1 ning (in 1994), I have always firmly defended the As far as I am concerned, I can tell the case of term living radical polymerization. Besides, I redox catalyzed telomerization, which remained a have noticed that the detractors are always (or dead process for 30 years and which, thanks to almost always) former anionists who want to keep the occurrence of an amine (bipyridine), became LIVING OR CONTROLLED? 1713 living (see the work of Matyjaszewski,2 particularly true because most of the alternative Sawamoto,3 etc.). terms do not convey a more specific or succinct Thus, it may be concluded that when a poly- subbranch or meaning. merization takes place with an increase in the One modification that I would like to see is to chain length with the monomer conversion, it pos- have the article end in a clear, concise conclusion sesses a living character and may be labeled liv- that states the authors proposed definition for the ing radical polymerization. term living polymerization. A big effort remains to be made on a kinetic level to give true models to evaluate the percent- age of living character; for example, the work of Comments by Fukuda may be cited.4 In the case of nitroxides, Professor William J. Brittain he evaluated the purity of the obtained polymers. Department of Polymer Science, University of Akron, Akron, Ohio 44325- 3909 REFERENCES AND NOTES I appreciate the concerns of Darling et al. in 1. Jerome, R.; Teyssie´, Ph.; Vuillemin, B.; Zundel, T.; raising the use of the word living in polymer Zune, C. J Polym Sci Part A: Polym Chem 1999, science meetings and publications. Among syn- 37, 1. thetic polymer chemists, this word is probably 2. Wang, J.-S.; Matyjaszewski, K. J Am Chem Soc used (and perhaps, misused) in the greatest 1995, 117, 5614. variety of connotations. When teaching gradu- 3. Kato, M.; Kamigaito, M.; Sawamoto, M.; Higa- ate students, I feel the obligation to follow the shimura, T. Macromolecules 1995, 28, 1721. use of the word living with a definition of how a 4. Fukuda, T.; Goto, A. Macromol Rapid Commun 1997, 18, 683. particular author applied the term. At the end of the day, this may provide the resolution to the issues raised by Darling et al. As practitio- Comments by ners of the art of polymer synthesis, we have an Professor Rebecca Braslau obligation to the field to justify our use of the word living by substantiating this claim with a Department of Chemistry & set of experimental measurements. If an author Biochemistry, University of California at claims a living process, it is the assessment of Santa Cruz, Santa Cruz, California 95064 the proof offered that determines synthetic In regard to the article entitled “Living Poly- value. merization: Rationale for Uniform Terminology,” I feel that a multiplicity of terms such as pseu- I find the authors’ arguments for generalizing the do-living, quasi-living, controlled, and living ob- terminology of living polymerization to be scure results more than they illuminate them. thoughtful and practical. I need to caution you, However, I do not agree with the analogy that a the editors, that I am a newcomer to the field of living polymerization is analogous to a named polymerization and, therefore, do not feel that I organic reaction. Named organic reactions are have the overview and experience in the field to more specific. Living polymerizations can poten- take part in your experiment in contributing to tially refer to a variety of bond-forming and bond- the printed discussion on this nomenclature is- breaking reactions. A difference between organic sue. Nonetheless, I find the arguments made in and polymer chemistry is that the former is con- this article to be thoughtful, reasonable, and ra- cerned primarily with the success of single-step tional. I agree with the argument that a process reactions. The goal of polymer chemistry is high- or reaction name should not be dependent on the yield, sequential reactions. The use of living is a side reactions or on an analytical measure (e.g., measure of the success of conducting a series of polydispersity). I also agree that there is already sequential reactions rather than a specific moni- a general understanding by chemists of the term ker for a bond-forming mechanism. living polymerization, and I support the applica- For me, there are three influential articles in tion of Occam’s razor in trimming back the foliage the field of living polymerization. The first is that of newly sprouting terminology that is appearing of Szwarc,1 in which the term living was first as alternatives to living polymerization. This is coined. The anionic polymerization of styrene re- 1714 LIVING OR CONTROLLED? mains one of the best polymerization systems that in which there is not one, but a multiplicity of can be described as living. However, the reality of experimental measurements that characterize most polymer syntheses is that chain-transfer the stability of the chain end. It is incumbent on and chain-termination reactions limit achievable all polymer chemists who author scientific litera- molecular weights and monomer conversions. ture and review the work of their peers to care- Radical polymerizations are among the most chal- fully use and assess the applicability of the word lenging systems because spontaneous, bimolecu- living. lar termination is omnipresent. The legacy of physical organic chemistry is the elucidation of mechanisms through the kinetics REFERENCES AND NOTES and identification of intermediates and side reac- tions. In this sense, I appreciate the article by 1. Szwarc, M.; Levy M.; Milkovich, R. J Am Chem Soc Matyjaszewski2 that divides polymerizations into 1956, 78, 2656. 2. Matyjaszewski, K. Macromolecules 1993, 26, 1787. Classes 1–6 according to the ratios of ktr/kp and 3. Quirk, R. P.; Lee, B. Polym Int 1992, 27, 359. kt/kp. Matyjaszewski’s definition follows the clas- sic tenets of physical organic chemistry by recog- nizing the important role of kinetics in mechanis- tic studies. This kinetic ranking of polymeriza- Comments by tions also acknowledges the fact that no Professor William K. Busfield and polymerization is devoid of chain transfer or ter- Dr. Ian D. Jenkins mination for an indefinite period of time. The School of Science, Griffith University, original definition of Szwarc may have been too Nathan, Brisbane, Queensland 4111, demanding. Few polymerizations will satisfy the Australia most rigorous definition of living; it is the time- scale of termination and chain transfer that dis- We are in agreement with the majority of the tinguishes polymerizations. Unfortunately, the concerns and discussion presented in the article. paucity of reliable kinetic data for polymeriza- Of the two suggested names, controlled and liv- tions limits the utility of Matyjaszewski’s ranking ing, we certainly favor the latter, as do the au- system. thors of the article. A common problem with both The article by Quirk and Lee3 describes a set of these names, however, is that they have everyday experimental criteria for living polymerizations. meanings that do not coincide exactly with what This article has been the most influential on the they represent in this context. Thus, their use can experimental work in my research group. The be confusing to those not expert in the field. For goals of resumptive monomer consumption, block example, we have occasionally asked students, in formation, and chain-end functional- a third-year undergraduate examination on poly- ization (among other experimental criteria), as mer science, to write “a short account” or “notes described by Quirk and Lee, are practical and on” living polymerization. Several students over relevant metrics of a living polymerization. Be- the years have given a free-radical polymeriza- cause polymer chemistry is strongly connected to tion (with normal termination) as an example, applied science and technology, it is important citing the growing polymer radical as the living that living polymerizations produce a high per- polymer. Admittedly, these are students who centage of chains that are capable of further have not listened carefully or read widely, but it is growth. Their article describes experiments that difficult to deny them marks for their misconcep- measure the contribution of termination and tion. In the article, the authors contrast the cur- chain-transfer reactions. The quantification of rent confusion regarding nomenclature in the liv- side reactions follows the spirit of physical or- ing radical field with the comparative serenity in ganic chemistry, where a reaction is not only the field of Diels–Alder reactions. A major reason characterized by its yield but also by the nature surely relates to the choice of a specific and there- of the side reactions that consume starting fore nonconfusing name. materials. We propose that if it is possible to change the We should not relax the traditional standards name, it would be entirely appropriate to honor used to characterize a living polymerization. My the pioneering work of Szwarc and call the reac- recommendation to the polymer community is to tion Szwarc polymerization. It may be helpful to use the word living to describe a polymerization have the interim name Szwarc-living polymeriza- LIVING OR CONTROLLED? 1715 tion to provide a link with traditional practice become particularly relevant in recent years over the past 40 years, with the intention of drop- thanks to developments in cationic, radical, and ping the living tag in the longer term. Szwarc was metal-catalyzed processes. the first to recognize that the combination of fast The first point necessary to make is the clear propagation with no termination step in anionic distinction between living polymerization and liv- systems led to what he called polymer architec- ing macromolecule. The former refers to the pro- ture. With his coworkers, he convincingly demon- cess, the latter to the polymer chain: strated that polymers with very narrow molecu- lar weight distributions, specific end groups, and 1. A living process is observed when no ter- planned blocks of different monomer units could mination reactions exist but does not ex- be made by the anionic living polymer technique. clude transfer reactions; this is typically His work spawned a flurry of research activity on observed in transition-metal-catalyzed anionic and, eventually, cationic systems during polymerization. As a consequence, the the 1960s. The objectives were little different amount of polymer increases after a second from those of what we might call the second phase addition of monomer, but the molecular of research activity of the late 1980s and 1990s weight does not. into living polymerization utilizing cleverly de- 2. If both termination and transfer reactions signed free-radical systems. are lacking, both the process and the poly- In summary, our proposal is that any polymer- mer chain are living, and this is the case ization designed to minimize the occurrence of that should be called living polymerization. irreversible polymer chain termination and In this case, the addition of monomer thereby produce polymers with predictable molec- brings an increase in molecular weight, ular weights, narrow polydispersities, and capa- block copolymer can be formed, and the bilities for chain extension should be named a polymer is monodisperse in its molecular Szwarc polymerization. weight. 3. A complex situation may arise if two mech- anisms are occurring in the system, one Comments by Dr. Stanley Bywater producing living macromolecules and the National Research Council of Canada, other dead macromolecules. In this case, Ottawa, Canada K1A 0R6 the degree of living should be evaluated, but the product formed is different from I do not wish to make formal comments, but in that expected from the real living situation my view the Szwarc definitions of living polymer- indicated in the previous paragraph. ization, which mean that each individual polymer grows until the monomer is exhausted without I hope this very short comment can be useful in transfer or termination, should only be used if some way. rigorous experiments show that this is true (in my Ͼ 5 view, to Mn 10 ). No quasi-living, controlled- living, or other hyphenated varieties or “living” Comments by terms should be used, but more specific names Professor James V. Crivello such as atom transfer radical should be used for Department of Chemistry, Rensselaer these systems. Polytechnic Institute, Troy, New York 12180

Comments by The comments of the authors of this article Professor Francesco Ciardelli concerning the current fragmented terminology Department of Chemistry and Industrial used in the literature with respect to living Chemistry, University of Pisa, Via free-radical polymerizations are entirely appro- Risorgimento, 35, 56126 Pisa, Italy priate. As a teacher, I find the present nomen- clature in this field to be especially confusing to I find the article by Darling et al. very useful, students who see the literature replete with a as some confusion about the real significance of multiplicity of highly specialized terms applied living polymerization has always existed and has to seemingly very similar types of polymeriza- 1716 LIVING OR CONTROLLED? tions. The development of any new field of sci- should really be addressed by the scientific com- ence also brings with it an evolutionary aspect munity actively engaged in this field of research. of the nomenclature that is used to describe it. There are currently a considerable number of Initially, the terminology applied is complex, meetings and conferences on living polymeriza- but it becomes simplified with time as general tions that could serve as forums for the polymer concepts and unifying principles emerge to de- research community to come together and decide scribe the entire field. It appears that this is the this issue. present situation with living free-radical poly- However, rarely are such matters decided by merizations. consensus. Ultimately, the question of terminol- One is reminded of a similar situation in the ogy will be solved, as it has in the past, by the early 1970s in which the classical/nonclassical future authors of textbooks of polymer chemistry. carbenium ion controversy raged in the ranks of In the meantime, the multiplication of terms used organic chemists. Then as now, specific terms in this field may well continue. were being invented and applied to the extreme cases of what turned out in the end to be a con- tinuum of species present in these reactions. Dar- Comments by ling et al. correctly reminded us that there is no Professors Takeshi Endo1 and such a thing as a perfect living polymerization. Ikuyoshi Tomita2 This is because in polymer chemistry, we are al- 1Research Laboratory of Resources ways dealing with a large assembly of molecules Utilization, Tokyo Institute of undergoing reaction. Statistically, if a side reac- Technology, 4259 Nagatsuta-cho, tion can occur that results in the permanent ter- Midori-ku, Yokohama 226-8503, Japan mination of a growing polymer chain, it will take 2 place. I would agree that if a given polymer form- Department of Electronic Chemistry, ing a reaction meets the minimal criteria, such as Interdisciplinary Graduate School of very low levels of chain transfer and termination, Science and Engineering, Tokyo Institute narrow molecular weight distribution, and the of Technology, 4259 Nagatsuta-cho, ability to chain-extend, then it should be charac- Midori-ku, Yokohama 226-8503, Japan terized as simply a living polymerization. This designation should be applied regardless of We think that it is a good idea to have an occa- whether the mechanism involves free-radical, cat- sion to discuss the terminological aspect in the jour- ionic, anionic, or coordination intermediates. It nal. We have had a feeling that our definition of should also be pointed out that living polymeriza- living polymerization is a little different from that of tions are observed only in those systems in which many researchers who are interested in or are chain termination and transfer are minimized ei- working in this field. Within our understanding, the ther by the inherent low reactivity of the active definition of living polymerization seems to be too species or by the deliberate suppression of its strict, which may cause confusion among the re- reactivity by one means or another. Generally, searchers. the specialized terms that Darling et al. cited in If we want to satisfy the proposed definition of their article are used by various authors to de- living polymerization, we must be very careful to scribe the specific manner in which the reactivity use the term living polymerization. The research- of a free-radical species has been suppressed. ers should devote a lot of time to determining For these reasons, this commentator would ad- whether their systems follow a living polymeriza- vocate the use of the uniform term living free- tion by definition. Apparently, we cannot use the radical polymerization in the titles and keywords term living polymerization before a detailed study of articles in this field. As always, investigators if we pay much attention to the minor aspects have the freedom to provide detailed mechanistic specified in the definition. Even if we carry out a descriptions in their articles, in which they can careful study and detect no symptoms of unfavor- point out the peculiarities of a specific system able side reactions and mechanisms, still we can- that serve to distinguish it from other types of not say that further studies with more advanced living polymerizations. instruments might not expose any proofs to re- These considerations aside, the arguments phrase them as nonliving systems. that the Darling et al. advance for the establish- We believe that the important aspects of liv- ment of a uniform terminology are valid and ing polymerization are molecular weight con- LIVING OR CONTROLLED? 1717 trol, narrow polydispersity, and a capacity for guity. However, the frequent misuse (abuse) of perfect end functionalization and block copoly- an existing definition is not a reason to com- merization. From this viewpoint, a mechanistic pletely abandon the definition. discussion of whether they imply reaction steps It is striking that the precise definition of liv- unfavorable for living polymerization might be ing polymerization—chain polymerization pro- a secondary concern if the systems satisfy the ceeding in the absence of irreversible chain- aforementioned requirements. Most probably, breaking reactions (transfer and termination)—is the definition has been fixed through the quite not recited in the article. “Significantly faster (?) extensive research related to this field that may propagation in comparison with termination” and have, however, built up a tower of ivory. We “polydispersity of living polymerization should be might be allowed to use the term in a more less than 1.1” are not necessary or sufficient cri- relaxed way, although we should discuss this teria. This is exactly what leads to many misuses more carefully before modifying the definition of in the literature. Proper tools to diagnose living living polymerization. polymerization have been reported and analyzed Because the problem of searching Chemical in terms of their accuracy. Contrary to the opinion Abstracts is not specific to this field, it may not of the authors, living polymerization is not an be necessary to make haste in fixing the border impossible ideal in ionic and some coordination polymerization, but it certainly does not mean of living polymerization. Further discussion by 1 many researchers from different fields is immortality, as was pointed out by Szwarc. needed before we redefine living polymeriza- The analogy of name reactions is faulty. tion. However we may compare the Diels–Alder re- action to nitroxide-mediated polymerization, atom transfer radical polymerization, revers- Comments by ible addition fragmentation chain transfer, and Professor Rudolf Faust so forth, these are indeed named on the basis of their different mechanisms, regardless of their Department of Chemistry, University of yields. Additionally, they may be called living Massachusetts at Lowell, One University when they satisfy the criteria (the absence of Avenue, Lowell, Massachusetts 01854 irreversible transfer and termination) of living polymerization. It is true that the absence of The article entitled “Living Polymerization: chain-breaking reactions is dependent on our Rational for Uniform Terminology” by Darling detection limit. Therefore, it is more appropri- et al. has been kindly provided by the editors of ate to state that irreversible transfer and ter- the Journal of Polymer Science to allow the mination could not be detected from the diag- views of other polymer chemists on the subject nostic plots instead of saying that the absence to be stated and published together with the of termination and transfer was determined original article. from the diagnostic plots. It is important to Following the discovery of living anionic, recognize, however, that increasing the mono- ring-opening, cationic, and coordination poly- mer/initiator ratio, that is, the theoretical mo- merizations, new methods have been reported lecular weight, can facilitate the detection. This recently for the synthesis of well-defined mac- has been discussed frequently, and a critical romolecules with controlled molecular weight evaluation has been given. Unfortunately, there and relatively low polydispersity by radical po- seems to be a general lack of interest in deter- lymerization. The article discusses the termi- mining the limitations of any given technique. nology of living polymerization in light of these The proposed relaxation of the term living to new developments and proposes that these rad- include processes that yield living polymers ical polymerizations should be called living would lead to confusion because, on the basis of (even when chain-breaking reactions undoubt- the new definition (if the yield is unimportant), edly occur) instead of controlled. I strongly dis- almost all chain polymerizations could be called agree with this proposition. It is certainly true living. The following example illustrates this that a lack of common language may hinder the point. Cationic polymerization dominated by a progress of any scientific field. Therefore, it is chain transfer via ␤-proton elimination in the important to seek a nomenclature that precisely absence of irreversible termination yields poly- defines terms and conditions without any ambi- mers with unsaturated chain ends and uncon- 1718 LIVING OR CONTROLLED? trolled (in the sense that we do not have control) classic picture, there is no (or only truly negligi- molecular weights. It is clear that this conven- ble) termination by the bimolecular self-reaction tional polymerization should not be called liv- of the transient propagating species, and all ing. According to the new definition, however, chains start simultaneously at the beginning of this process yields living polymers because of the process. Such a process is also called polymer- the following equilibrium at complete monomer ization without termination.1 I wish to emphasize consumption: Pϭ ϩ Hϩ 43Pϩ (where P rep- strongly that the classic picture cannot be used to resent the polymer chain). explain a living radical polymerization. The rea- In conclusion, I cannot support the proposed son is very simple: In a radical polymerization, change in the terminology of living polymeriza- the conversion rate of the monomer is propor- tion. Rather, scientists should be urged to use the tional to the concentration of the transient prop- old one rigorously. agating radicals. Whenever there are transient radicals, there will be bimolecular termination, and the radical concentration depends on the ter- REFERENCES AND NOTES mination rate constant, so this rate constant also influences the rate of conversion. 1. Szwarc, M. J Polym Sci Part A: Polym Chem 1998, Second, it must be recognized that there are 36, ix. two very different kinetic principles that lead to living radical polymerizations. The authors do not distinguish between them, but they have mecha- Comments by nistically little in common, and both do not follow Professor Hanns Fischer the classic picture. One is the persistent radical Institute of Physical Chemistry, effect.2–4 It operates in polymerizations mediated University of Zuerich, by persistent radicals, such as nitroxides, or by Winterthurerstrasse 190, metal complexes that bind transient radicals re- CH 8057 Zuerich, Switzerland versibly (cobalt complexes) and in the atom trans- fer radical polymerization technique. From the Important inventions often result from intrigu- beginning of a polymerization and continuously ing but imprecise suppositions, and the recently thereafter, the bimolecular self-termination of the invented radical polymerizations yielding poly- transient (propagating) radicals by coupling or mers which are “able to grow whenever additional disproportionation causes a buildup of persistent monomer is supplied,” that is, living polymers, species with time. These react reversibly with the confirm this rule. In my opinion, an initially low propagating growing transient radicals to the liv- level of understanding of the underlying basic ing (dormant) chains. As the concentration of the kinetics has caused the introduction of the con- persistent species increases, the cross reaction of fusing multitude of adjectives for the polymeriza- the transient propagating radicals with the per- tion process that the authors now correctly de- sistent species becomes faster than the bimolecu- plore. Such adjectives should characterize the de- lar termination of the transients. Hence, the liv- tails, but the synonymous use of controlled, ing (dormant) products accumulate more than the living, pseudo-living, living/controlled, regulated, dead termination products. The self-reaction of and so on is indeed anything but helpful. I agree the propagating radicals itself sets this scenario that a uniform terminology is needed. If one ac- for control and causes the simple chemical self- cepts the aforementioned definition of a living regulation. Therefore, it is kinetically important polymer, it is quite natural to call a radical pro- and cannot be called negligible, as the authors do, cess leading to such a polymer a living polymer- although the products are often minor fractions, ization, as the authors suggest. To distinguish it at least for successful experimental conditions. from other living (anionic, cationic, etc.) polymer- Moreover, when the monomer is exhausted, the izations, the extension to living radical polymer- propagation automatically ceases, but the termi- ization is needed. At this point, my comment nation does not, so at sufficiently long times, all could end, were it not for some points raised in the polymer chains die. However, the initiation must rationale that deserve further clarification. be fast compared to the total duration of the pro- First, the authors refer to the classical picture cess, and in this respect one feature of the classic of a living polymerization (ref. 1 of the article) and picture of a living polymerization is retained. The use its terminology repeatedly in the text. In this other process involves a degenerate transfer of LIVING OR CONTROLLED? 1719 atoms or groups between propagating radical and significantly faster than any process for irrevers- dormant chains. This homogenizes the chain- ible termination.” If this were true, the new phe- length distribution, and reversible addition frag- nomenon of living radical polymerization would mentation chain transfer is a very successful ex- have been observed long ago. I am not an experi- ample. In contrast to the case of the persistent enced polymer scientist, but I do teach my stu- radical effect, the initiation is not fast compared dents that the much higher rate (not rate con- to the overall reaction but is maintained at a very stant) of propagation compared to termination is low and constant level during the whole polymer- the essential feature of all successful polymeriza- ization, again at variance with the classic picture. tions. Am I mistaken, or do the authors wish to Yet the radical concentration is kept very low. call now all polymerizations living? Therefore, there is always a little bimolecular self-reaction of the transient propagating radi- cals, but the polymerization rate still depends on REFERENCES AND NOTES the termination constant. Obviously, in both cases one should never speak of a negligible ter- 1. Young, R. J.; Lovell, P. A. Introduction to Polymers, mination reaction and at best of a negligible ter- 2nd ed.; Chapman & Hall: London, 1991; p 78. mination product formation. Certainly, in due 2. Fischer, H. J Am Chem Soc 1986, 108, 3925. time adjectives specifying the two quite different 3. Kothe, T.; Marque, S.; Martschke, R.; Popov, M.; mechanisms of living radical polymerization will Fischer, H. J Chem Soc 1998, 2, 1553. emerge, and other mechanisms may be invented. 4. Fischer, H. J Polym Sci Part A: Polym Chem 1999, Third, the analogy with the Diels–Alder reac- 37, 1885. tion terminology is not really correct or complete. Organic chemists normally isolate their stable products and correctly call a reaction leading to a Comments by typical Diels–Alder product a Diels–Alder reac- Professor Takeshi Fukuda tion, admittedly even regardless of the yield. Institute for Chemical Research, Kyoto However, the living (better dormant) product University, Uji, Kyoto 611-0011, Japan fraction of a living radical polymerization is sel- dom isolated or exactly determined. Also, as So-called living radical polymerization now pointed out previously, the living (or dormant) seems to have acquired a good, general concept. It product prepared by mechanisms involving the includes some kind of reversible activation of po- persistent radical effect is inherently unstable, at tentially active or dormant species, whereas other least in principle and without special posttreat- features of radical polymerization are essentially ment. Its fraction first increases but decreases unchanged (or at least there has been no clear continuously at long times because of the lasting evidence against this). Namely, chain-breaking unavoidable radical self-termination. In practice, reactions such as termination and irreversible it may even drop to zero during the monomer chain transfer can occur as they do in conven- conversion, whereas the polydispersity remains tional radical polymerization. This is why many narrow and the molecular weight is controlled by authors, including myself, when preparing an ar- the converted monomer/initiator ratio. Hence, I ticle or a formal lecture, hesitate or even object to agree with the authors that the term living rad- using the term living radical polymerization. Now ical polymerization may be used regardless of the that its nature is generally well understood, I, yield of the process because under unfavorable being tired of the multiple terminology, would circumstances a living radical polymerization like to welcome a unified name, living, “living”, may (paradoxically) give only final dead polymer. controlled, controlled/“living”, or whatever, pro- However, besides the other important criteria for vided that it is generally approved. living polymerizations, such as low polydispersity I find the proposal of Darling et al. particularly and control of the molecular weight, workers us- agreeable. The Szwarc definition of living poly- ing the term should always establish that their mers, that they are “able to grow whenever addi- living radical polymerization has produced a liv- tional monomer is supplied,” would smoothly ap- ing polymer at least at some stage of conversion. ply to the growable species in the radical system, Finally and astonishingly, the authors state which are in either an active or a dormant form. boldly that “the phenomenon of living polymeriza- Then, their proposal that living polymerizations tion will be observed whenever propagation is “be defined as those processes that yield living 1720 LIVING OR CONTROLLED? polymers” would also smoothly apply to the radi- further polymerization of monomers or can par- cal system because it states nothing about the ticipate in chemical reactions that enable their yield. (To rule out from this definition those pro- ends to be functionalized. Polymer chemists take cesses that yield living polymers only at the final advantage of these features to prepare block co- stage, for example, conventional chain-transfer polymers, tapered copolymers, polymers with processes yielding terminally functional poly- star, radical block, and dendritic architectures, mers, it might be slightly modified to something polymers with narrow molecular weight distribu- like “living polymerizations are defined as those tions, polymers with useful end-group functional- processes that are mediated by and yield living ity, and so forth. In very favorable situations, polymers.”) The nature of radical polymerization reactant concentrations, solvents, temperatures, is mentioned in their proviso, “It should, however, counterions, catalysts, and modifiers can be var- also be recognized that . . . are negligible.” Living ied to control the structures of the polymers pro- radical polymerization defined in this way would duced, but this can be true whether or not a invite little confusion. Above all, it is simple. polymerization system exhibits the pragmatic I hope that this experiment by the editors will features of living polymerizations. For example, bring about a good yield, in any form. molecular weight can be controlled in some living polymerization processes by varying the mono- mer/initiator ratios but not in all of them. I do not Comments by believe that initiation rates must equal or prefer- Professor H. James Harwood ably exceed propagation rates for a system to be considered living. Furthermore, I do not believe Department of Polymer Science, that the word control should be used to define any University of Akron, Akron, Ohio 44325 particular set of characteristics of a polymeriza- tion process. However, I find that controlled poly- Both the article by Darling et al.1 and the mac- merization is helpful when used to emphasize a romolecular nomenclature note2 to which it re- process, living or nonliving, where the choice of sponds represent attempts to standardize nomen- the amounts and types of reactants enables one to clature for polymerization processes that exhibit prepare polymers with predictable structural fea- living polymerization character as initially de- tures. scribed by Szwarc3 and that meet, to varying de- Terms such as quasi-living, pseudo-living, con- grees, the criteria defined for living polymeriza- 4 trolled living, apparently living, and “living” have tion by Quirk and Lee. been used by investigators to refer to polymeriza- If an accepted nomenclature can be adopted, tion systems that exhibit the aforementioned bibliographic indexing and searching in this area pragmatic features. These qualifying terms are will be simplified, and authors will be able to used to indicate that the mechanisms of many of communicate with improved clarity. There may these polymerizations involve equilibria between even be legal consequences (e.g., patent validity) active and dormant chain ends as opposed to associated with the nomenclature adopted. mechanisms of many more classical living poly- In my opinion, no polymerization can be re- merization systems where the chain ends are garded as being perfectly living, and the relaxed thought to be permanently active (probably only 5 definition of Szwarc discussed by Darling et al. true for anionic and cationic processes that in- seems very reasonable. A problem arises concern- volve anions or cations of relatively low nucleo- ing what is a desired maximum size and what can philic or electrophilic character, as in the ring- be considered a negligible degree of termination opening polymerizations of cyclic ethers, esters, or (irreversible) chain transfer. It seems that it and amides). will be necessary to be flexible and pragmatic in Not to muddy the water, but if it is important these matters. to distinguish between polymerizations that do or The pragmatic features that most people will do not involve equilibria between active and dor- associate with living polymerization behavior are mant polymer chain ends, the former might be (1) that the growth of individual polymer chains termed resurrecting, resurrectional, restorable,or occurs throughout a polymerization process and revivable polymerizations to emphasize their ter- (2) that a large proportion of the polymer chain mination–reinitiation features; I do not think it ends are active or capable of being activated at desirable to do so. Instead, I would like to refer to the end of the process and can be involved in all polymerizations that exhibit the pragmatic LIVING OR CONTROLLED? 1721 features discussed previously as living polymer- R. R., Jr.; Moad, G.; Rizzardo, E. J Polym Sci Part izations and to include, where necessary, modify- A: Polym Chem 2000, 38, 1706. ing names or acronyms that indicate what types 2. Matyjaszewski, K.; Muller, A. H. E. Polym Prep of processes are involved. Most readers encoun- (Am Chem Soc Div Polym Chem) 1997, 38, 6. tering terms such as living anionic polymeriza- 3. Szwarc, M. Nature 1956, 178, 1168. tion, living cationic polymerization, living ATRP 4. Quirk, R. P.; Lee, B. Polym Int 1992, 27, 359. 5. Szwarc, M. J Polym Sci Part A: Polym Chem 1998, (atom transfer radical polymerization) polymer- 36, ix. ization, living RAFT (reversible addition frag- 6. Harwood, H. J.; Christov, L.; Guo, M.; Holland, mentation chain transfer) polymerization, living T. V.; Huckstep, A. Y.; Jones, D. H.; Medsker, R. E.; GTP (group transfer polymerization) polymeriza- Rinaldi, P. L.; Saito, T.; Tung, D. S. Macromol tion, and living ROMP (ring opening metathesis Symp 1996, 111, 26–28. polymerization) polymerization will have some knowledge about the particular side reactions as- sociated with these processes and their impor- Comments by tance. Terms need to be agreed on for living po- Professor Koichi Hatada lymerizations that involve reversible interactions Department of Chemistry, Faculty of of propagating radicals with stable (persistent) Engineering Science, Osaka University, radicals (FRP, SFRP, ?) or organometallic com- Toyonaka, Osaka 560, Japan pounds such as cobaloxime and for other ways of obtaining living polymerization behavior in radi- I read the article “Living Polymerization: Ra- cal systems (reversible additions of radicals to tionale for Uniform Terminology” by Darling et al. compounds that form stable radicals is a possibil- with great interest. Certainly, many polymeriza- ity that may eventually be demonstrated).6 By tions have been reported to be living since Dr. using such modifying names or acronyms, writers Szwarc published his article on the anionic poly- will automatically provide subclassifications for merization of styrene in the absence of termina- the systems they are discussing. Agreement need tion and chain-transfer reactions and named the only be reached on what modifying terms should reaction living polymerization, and various be used. When one wishes to refer generically to names such as living cationic, living ring-open- living systems that involve active–dormant ing, and living radical polymerization have been chain-end equilibria, the use of living seems ac- proposed. In addition, new names such as quasi- ceptable. I do not believe it is desirable to incorporate living, pseudo-living, and apparently living have information about initiation or irreversible termi- also been proposed. Now it is time to think deeply nation processes into any definitions of living po- about the terms relating to living polymerization lymerization because they would have to be arbi- to aid communication and avoid confusion. trarily quantitative. This would then bring in the The IUPAC definition of living polymerization subject of control, which would also require arbi- is “a chain polymerization from which chain 1 trary quantitative definitions and would lead to transfer and chain termination are absent.” Note unnecessary complication. I believe that different that in many cases, the rate of chain initiation is aspects of control will be associated with each fast compared to the rate of chain propagation, so living or “living” polymerization system and that that the number of kinetic chain carriers is essen- these will become apparent as workers read about tially constant throughout the polymerization. or use a particular system. We do not need to This is the term defined on the basis of the develop definitions for controlled polymerizations. mechanism of the polymerization reaction. Con- I do not think that the nomenclature for living sequently, a certain polymerization can be re- polymerization systems needs to be made more ferred to as a living polymerization when it occurs complicated than what has been proposed. without termination and chain-transfer reac- tions. If desired, the term living polymerization may be qualified by a self-explanatory adjective or an adjective based on the well-established defini- REFERENCES AND NOTES tion: anionic living polymerization, cationic living polymerization, radical living polymerization, ste- 1. Darling, T. R.; Davis, T. P.; Fryd, M.; Gridnev, reospecific living polymerization, and so forth. A. A.; Haddleton, D. M.; Ittel, S. D.; Matheson, The IUPAC definition of radical polymerization is 1722 LIVING OR CONTROLLED? a chain polymerization in which the kinetic chain Another important problem is what we do carriers are radicals. Therefore, radical living po- when our polymerization is not completely living. lymerization is a chain polymerization in which The IUPAC recommendation1 advises us to re- the kinetic chain carriers are radical and chain solve such a problem in the following way. To transfer and termination are absent. Living rad- present a clear concept, idealized definitions are ical polymerization should mean the same, but necessarily adopted. Deviations from ideality the preference of living radical polymerization usually arise with real polymers and reactions. and radical living polymerization is to be dis- Although such deviations are not explicitly taken cussed. into account in the definitions, the recommended Now the definition of living radical polymeriza- nomenclature can usefully be applied to the pre- tion or radical living polymerization is very clear. dominant features of real polymers or reactions. It is important that the polymer chemist who If necessary, self-explanatory adjectives such as wants to refer to his or her polymerization as essentially, almost completely, or highly can be living polymerization clarify the mechanism of used for qualification. reaction and carefully decide whether or not the So if the polymerization includes chain trans- mechanism fits the definition. fer and termination but in small proportions com- These are very important procedures. The evi- pared to propagation, it can be called essentially, dence of no termination and no chain transfer can almost completely,orpredominantly living.Of be usually obtained by a detailed analysis of the course, the deviation from ideality should be products, for example, an NMR analysis of a poly- small enough, and I cannot agree with the pro- mer with the aid of synthetic means or contriv- posal that the process that yields a living polymer ance. Narrowness of the molecular weight distri- is living polymerization even if chain transfer and bution (MWD) is neither a necessary condition termination occur considerably. In such a case, nor a sufficient condition for living polymeriza- you might mention that a part of the propagating tion; living polymerization sometimes gives a species polymerizes monomers in a living manner polymer with a broad MWD, and nonliving poly- but the reaction as a whole is not living. Chain merization sometimes gives a polymer with a nar- transfer and termination are not simple side re- row MWD. actions. The absence of these side reactions is an Some polymer chemists disagree that polymer- absolute necessity for living polymerization. ization with reversible termination is living, prob- I agree with Darling et al. not to use the term ably because active species are in a dynamic equi- librium with inactive or dormant species.2 I agree controlled for apparently living or “living”. Most that such a polymerization is living as long as all of the chemists carry out the reactions to be con- the chains can add monomer molecules in a suf- trolled in every meaning. So the adjective con- ficiently long time. I think that the IUPAC defi- trolled is too broad and not self-explanatory, and nition of living polymerization does not require we cannot imagine anything specified from the the chain carriers to be activated all the time; term controlled polymerization. Without the ad- they can be reversibly deactivated for some time. jective controlled, we can describe such a living- I myself sleep every day, but no one opposes that like polymerization with appropriate modifiers, I am living. However, I do not like to define new for instance, “predominantly living polymeriza- terms such as quasi-living and pseudo-living po- tion with a small extent of irreversible termina- lymerizations for the polymerization. tion.” The existence of the equilibrium between reac- Finally, I must tell you that “the use of the tive species and dormant species does not affect term controlled radical polymerization instead of the main course of the chain reaction as long as living radical polymerization” is not an IUPAC the reactive sites are regenerated from the dor- recommendation. This is a misunderstanding by mant species. Darling et al. The term controlled may be a pro- The modification of basic terms by prefixes or posal by Matyjaszewski and Muller.3 adjectives that are not self-explanatory some- times causes confusion or misunderstanding. Liv- The author would like to thank Dr. Teiji Tsuruta, ing polymerization with reversible deactivation is Professor Emeritus of the University of Tokyo, and enough if they want to qualify it, as Matyjasze- Professor Tatsuki Kitayama for their helpful discus- wski and Muller mentioned in their article.3 sions. LIVING OR CONTROLLED? 1723

REFERENCES AND NOTES can be argued that no polymerization can ever be truly living; for example, the total exclusion of 1. IUPAC. Pure Appl Chem 1996, 68, 2287–2311. contaminants, terminating moieties, and so forth 2. Living polymerization is defined for chain polymer- from an anionic procedure is a goal that can never ization. Chain polymerization is defined in the IU- be achieved. A good analogy for not using these 1 PAC document as “a chain reaction in which the strict and unobtainable definitions is the field of growth of a polymer chain proceeds exclusively by . Until the recent advent of advanced reaction(s) between monomer(s) and reactive site(s) mass spectrometry techniques, it was difficult to on the polymer chain with regeneration of the re- detect subtle defects in the overall branched active site(s) at the end of each growth step” structure of dendrimers; as a consequence, a per- reactive site 3 Ͻchain growthϾ 3 fect structure was generally assumed or expected. 21 However, MALDI-TOF experiments17 have clearly dormant species shown that divergently grown dendrimers18 are regeneration of 3 Ͻchain growthϾ 3 prone to defects, and even the most carefully reactive site grown convergent structures18 may also contain 21 defects. Should we refrain from using the term dormant species dendrimer17–21 for divergently grown structures 3. See ref. 1 in Darling et al. and only use it for well-defined convergent struc- tures,22 or should we not use the term at all because it is extremely difficult, if not im- Comments by Dr. Craig Hawker possible, to obtain a strictly monodisperse sample IBM Almaden Research Center, 650 with perfect branching? I think the answer is Harry Road, San Jose, California 95120- definitely no; both sets of compounds approach 6099 very closely the goal of an idealized dendritic structure and experimentally have been shown to As with many scientific disciplines, the advent have polydispersities of less than 1.01 and in of new techniques and approaches almost invari- many cases less than 1.001. Although they are not ably results in a reevaluation of existing termi- perfect dendrimers, they are dendrimers. nology and beliefs. Polymer science is not im- As Szwarc23 and Darling et al.1 pointed out, mune, with this discussion concerning living living polymers are distinguished from dead poly- polymerizations and the newer approaches to mers by being “able to grow whenever additional free-radical polymerization in particular being a monomer is supplied.” From this perspective, prime candidate. As pointed out in previous arti- polymer chains prepared under living free-radical cle by Darling et al.1 and recently by Matyjas- methods are far from being dead; they can be zewski and Muller,2 considerable confusion isolated, characterized, stored, and so forth and reigns in the area of living polymerizations; can a then simply reactivated by the addition of mono- free-radical polymerization ever be a living proce- mer under the appropriate conditions. Although dure or is it something else? If it is something it can be argued that some condensation proce- else, then what is it and what do you call it? dures can also be reactivated, the combination of As both sets of authors correctly pointed out, this feature and the ability to prepare narrow living free-radical polymerizations, whether me- polydispersity materials and control the molecu- diated by metal complexes [atom transfer radical lar weight, macromolecular architecture, end polymerization (ATRP)],3–6 nitroxides,7–12 or groups, and block purity for living free-radical such invariably suffer from termination reac- procedures does justify the use of the term living tions. Therefore, in the strictest sense, they are polymerization. As has been shown by a number not true living systems. Numerous authors have of authors,24–26 the ability to prepare linear and recognized this point as the field has grown, and a star polymers with controlled molecular weights greater understanding of the underlying pro- and polydispersities of 1.03–1.08 make these ma- cesses has developed.13–16 To accommodate this, terials similar in numerous respects to those pre- various terms such as pseudo living, living, and pared by more acknowledged living processes controlled have been put forth in the literature, such as anionic procedures. In addition, the field and this profusion of terms has caused consider- of living polymerization is still in its infancy, and able confusion and debate. Unfortunately, a strict as improvements and new concepts emerge, the adherence to guidelines is problematic because it characteristics associated with livingness will 1724 LIVING OR CONTROLLED? certainly improve. As Oscar Wilde emphasized in Comments by The Importance of Being Earnest, “The truth is Professor Allan S. Hay rarely pure, and never simple.” Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec, Canada H3A 2K6 REFERENCES AND NOTES One of the requirements of an article is a title 1. Darling, T. R.; Davis, T. P.; Fryd, M.; Gridnev, and abstract that contain enough information for A. A.; Haddleton, D. M.; Ittel, S. D.; Matheson, a potential reader to identify it as an article of R. R.; Moad, G.; Rizzardo, E. J Polym Sci Part A: interest that he would like to learn more about by Polym Chem 2000, 38, 1706. retrieving and reading. In organic chemistry, as 2. Matyjaszewski, K.; Muller, A. Polym Prep (Am the authors note, terms such as the Diels–Alder Chem Soc Div Polym Chem) 1997, 38, 6. 3. Wang, J. S.; Matyjaszewski, K. J Am Chem Soc reaction are invaluable as a communication tool. 1995, 117, 5614. An even better example is the Friedel–Crafts re- 4. Kato, M.; Kamigaito, M.; Sawamoto, M.; Higash- action, with the many catalysts and reactants imura, T. Macromolecules 1995, 28, 1721. that have been used. 5. Percec, V.; Barboiu, B. Macromolecules 1995, 28, The plethora of terminologies that have arisen 7970. to describe various forms of living polymerization 6. Granel, C.; DuBois, P.; Jerome, R.; Teyssie, P. Mac- reactions and the concomitant duplication pre- romolecules 1996, 29, 8576. sents a problem in identifying articles of interest, 7. Georges, M. K.; Veregin, R. P. N.; Kazmaier, P. M.; especially if mechanical searching is being used, Hamer, G. K. Macromolecules 1993, 26, 2987. 8. Hawker, C. J. J Am Chem Soc 1994, 116, 11314. because these methods may rely principally on 9. Li, I. Q.; Howell, B. A.; Koster, R. A.; Priddy, D. B. keywords. Macromolecules 1996, 29, 8554. I propose that the writers imagine they are 10. Fukuda, T.; Terauchi, T.; Goto, A.; Ohno, K.; Tsujii, writing a comprehensive article on living poly- Y.; Yamada, B. Macromolecules 1996, 29, 6393. merization reactions and prepare titles for an in- 11. Moad, G.; Rizzardo, E. Macromolecules 1995, 28, dex in which the headings and subheadings 8722. would then necessarily provide a recommended 12. Puts, R. D.; Sogah, D. Y. Macromolecules 1996, 29, terminology for the specific area under the um- 3323. brella of living polymerization reactions. 13. Fischer, H. Macromolecules 1997, 30, 5666. 14. Hawker, C. J. Acc Chem Res 1997, 30, 373. 15. Colombani, D. Prog Polym Sci 1997, 22, 1649. 16. Matyjaszewski, K. Controlled Radical Polymeriza- Comments by tion; ACS Symposium Series 685; Oxford Univer- Professor Toshinobu Higashimura sity Press: New York, 1998. Department of , Faculty 17. Leon, J. W.; Fre´chet, J. M. J. Polym Bull 1995, 35, of Engineering, University of Shiga 449. Prefecture, 2500 Hassaka-cho, Hikone, 18. Hummelen, J. C.; van Dongen, J. L. J.; Meijer, Shiga 522-8533, Japan E. W. Chem Eur J 1997, 3, 1489. 19. Gooden, J. K.; Gross, M. L.; Mueller, A.; Stefa- 1 nescu, A. D.; Wooley, K. L. J Am Chem Soc 1998, Darling et al. submitted a constructive com- 120, 10180. ment on living polymerization terminology. In my 20. Durst, H. D.; Tomalia, D. A. Top Curr Chem 1993, view, the confusion about the terminology stems 165, 193. from the following two points. 21. Newkome, G. R.; Moorefield, C. N.; Vogtle, F. Den- Living Polymerization Versus Controlled dritic Molecules; VCH: Weinheim, 1996. Polymerization: As Szwarc, the pioneer of living 22. Hawker, C. J.; Fre´chet, J. M. J. J Am Chem Soc polymerization, stated distinctively in one of his 1990, 112, 7638. books, the reaction is originally defined as a 23. Szwarc, M. J Polym Sci Part A: Polym Chem 1998, polymerization where “propagation proceeds with 36, ix. exclusion of termination and chain-transfer.”2 Be- 24. Patten, T. E.; Xia, J.; Abernathy, T.; Matyjasze- wski, K. Science 1996, 272, 866. cause such an ideal polymerization is, practically 25. Benoit, D.; Chaplinski, V.; Braslau, R.; Hawker, speaking, least likely to exist, he later proposed a C. J. J Am Chem Soc 1999, 121, 40. “relaxed” and pragmatic definition that “we shall 26. Hawker, C. J.; Zech, C. Unpublished results. refer to polymers as living if their end-groups LIVING OR CONTROLLED? 1725 retain the propensity of growth for at least as long confusion in terminology, to make literature a period as needed for the completion of an in- searches flawless, and in turn to promote creative tended synthesis, or any other desired task.”2 research in polymer chemistry. However, Matyjaszewski,3 taking a stand for the earlier, strict definition, claimed that one should not call a polymerization living that is recognized to involve chain-breaking reactions, for example, REFERENCES AND NOTES those that fail to give high molecular weight poly- mers. He accordingly proposed two terms, con- 1. Darling, T. R.; Davis, T. P.; Fryd, M.; Gridnev, trolled polymerization for a process that at least A. A.; Haddleton, D. M.; Ittel, S. D.; Matheson, permits polymers of controlled structures and con- R. R., Jr.; Moad, G.; Rizzardo, E. J Polym Sci Part trolled/living polymerization for a reaction where A: Polym Chem 2000, 38, 1706. controlled polymers can form with an unknown con- 2. Szwarc, M.; van Beylen, M. Ionic Polymerization tribution of chain-breaking reactions. As Darling et and Living Polymers; Chapman & Hall: New York, al.1 pointed out, however, the term controlled poly- 1993; p 12. merization has been used for different meanings 3. Cationic Polymerization: Mechanisms, Synthesis, and is, therefore, unsuitable for defining another and Applications; Matyjaszewski, K., Ed.; Marcel polymerization of particular characteristics such as Dekker: New York, 1996; p 266. those that we are now discussing. From these arguments, I think Szwarc’s pragmatic definition is more appropriate; namely, Comments by the term living polymerization should refer to a Professor Hartwig Ho¨ cker process that produces polymers with the charac- Textile and Macromolecular Chemistry, teristics of the strictly defined living polymers, University of Technology, RWTH Aachen, rather than be confined to chain polymerizations Veltmanplatz, W-5100 Aachen, Germany for which the absence of termination and chain transfer is actually and unambiguously proved Living polymerization has become a fashion- (practically, however, this proof seems almost im- able expression. It seems that a new polymeriza- possible to obtain). tion reaction is generally considered to have a Terminology Based on Polymerization higher value if it is called living, although poly- Techniques—A Proposal: Another confusion mers obtained by polymerization reactions that regarding the living polymerization terminology, cannot be considered living may be better than in my view, is the use of a variety of terms that the living ones. Therefore, a uniform terminology bear particular polymerization techniques, such is very much required. as atom transfer radical polymerization and A living polymerization should yield living iniferter method. If these terms are employed to polymers (or their dormant precursors). It must refer to polymerizations that are thought to pro- be recognized, however, that pseudo first-order duce living polymers (as defined previously), the kinetics (the constant concentration of living particular polymerization technique of interest ends) are realized, along with a degree of poly- should be attached to the term living polymeriza- merization that is proportional to the conversion, * tion, for example, living polymerization by the the proportionality constant being [M0]/c , where * atom transfer method. This terminology is exactly [M0] is the initial monomer concentration and c the same in principle as, for example, “substitu- is the concentration of active species. This implies tion reaction, SN1,” by which a particular substi- that the molecular weight distribution is narrow, Ϸ tution process is distinguished by reference to its that is, Pw Pn. This condition certainly is not reaction mechanism. This technique-based termi- fulfilled when Pw/Pn is equal to 1.5 because this nology may also circumvent the problem in com- value is reached with radical polymerization and putational literature searching with which Dar- termination via combination reactions. It defi- ling et al. expressed some concern. nitely should be lower than 1.2. For referring to polymerization reactions that In conclusion, there always has been controversy lead to polymers with the characteristics of living over the term living because, for example, a living polymers, I would like to recommend attaching a cell is susceptible to apoptosis, except for a cancer phrase for a relevant polymerization technique to cell. If the term living is accepted, however, the the term living polymerization, so as to avoid the living system should be defined as it is herein. 1726 LIVING OR CONTROLLED?

Comments by tional and appropriate flexibility. The reasons for Professor Thieo E. Hogen-Esch this proposed modification are detailed later. Loker Hydrocarbon Research Institute, The nature of the original definition of living University of Southern California, polymerization is a mechanistic and thus theo- Los Angeles, California 90089 retical concept. It seems to make sense that the definition should remain mechanistic and As polymer chemists who are speaking the lan- should not include experimental criteria as well. guage of polymer chemistry, it would seem to However, the degree to which a living polymer- follow that we should agree on its vocabulary. I ization conforms to the ideal model can be exper- imentally tested through a comprehensive list of too am troubled by the current lack of consensus 1,2 on the meaning of living, quasi-living, and pseu- criteria. do-living polymerizations and believe that such Thus, I believe that it would be most practical to apply the term living only to the polymeriza- terms may be contributing to some of the confu- tion and the inherent and unavoidable termina- sion in this area. As I understand it, the authors tion, transfer, or other side reactions of the prop- propose that (1) the term living polymerization be agating chain end. For instance, the ability to redefined as “those processes that yield living carry out subsequent reactions such as end func- polymers (or their dormant precursors)” and (2) tionalization, block copolymerization, formation the terms controlled radical and controlled/living of stars, and other reactions of this type is an radical polymerizations, proposed in their ref. 1, excellent criterion for the degree of conformity should be avoided. I disagree with their first point with the ideal model. However, the success in and agree with their second, but for somewhat carrying out these subsequent reactions, at least different reasons. in principle, depends on their rates relative to the The original definition of an ideal living poly- rate of spontaneous degradation of the active merization as proceeding without termination or chain end. Thus, we may find ourselves in a po- transfer was useful and appropriate.1 However, sition of stating that a polymerization is living for let us not forget that the word living, however one type of subsequent chain-end reaction but not colorful and apt, is metaphorical. Like the word for another. The shelf life of a living polymer pregnant, the term is also absolute, and it is rea- should not be used for its definition but should be sonable to infer that a polymerization is either used as a criterion only. living or not. As originally pointed out by A similar argument can be made with respect Szwarc,1a polymerizations proceeding without to initiation as initiation and polymerization are termination or transfer regardless of timescale different reactions and have different rate con- are probably experimentally unattainable. Fur- stants. For instance, the alkyllithium-initiated thermore, this definition carries specific mecha- polymerization of styrene or isoprene in a hydro- carbon solvent closely approximates an ideal liv- nistic/kinetic implications, that is, the absence of 1,2 irreversible termination or other side reactions ing polymerization. The initiation by the less that deactivate or otherwise compromise the nucleophilic 1,1-diphenylhexyllithium or an even slower initiator would give a wider molecular structure of the chain. weight distribution polymer, especially if the in- I agree with Darling et al. that it is desirable to tended number-average molecular weight is low.2 retain the term living. However, I do not believe Could it then be said that the former polymeriza- that their definition, “those processes that yield tion is living and the latter is not? Depolymeriza- living polymers or their dormant precursors,” is tion reactions and equilibria between active and appropriate, as it uses the very term living that it dormant chains occurring on the polymerization aims to define. A better definition and one that timescale also are known to lead to wider molec- closely follows the original and subsequent defi- ular weight distributions. Do all of these polymer- nitions would be “polymerizations proceeding izations then cease to be living, even though the without termination or transfer on the polymer- chains do not terminate or give chain transfer? ization timescale.” This definition leaves out the For this and other reasons, I do not believe that phrase “or any other desired task” (discussed the terms quasi- and pseudo-living polymeriza- later).1b The clause “on the polymerization time- tions, describing polymerizations proceeding scale” appears to considerably relax the strin- through interconverting reactive and unreactive gency of the original definition and give it addi- (or less reactive) species, are useful because most LIVING OR CONTROLLED? 1727 living polymerizations are covered by that defini- REFERENCES AND NOTES tion. For instance, the living anionic polymer- ization of dienes or styrene initiated by alkyl- 1. (a) Szwarc, M. Nature 1956 178, 1168; (b) Szwarc, M. Ionic Polymerizations Fundamentals; Hanser: lithiums in hydrocarbons involves unreactive or New York, 1996; (c) Szwarc, M.; Van Beylen, M. dormant alkyllithium aggregates in rapid equilib- Ionic Polymerization and Living Polymers; Chap- rium with reactive polydienyllithium chains. In man & Hall: New York, 1993; p 12–15. tetrahydrofuran, some anionic polymerizations of 2. Hsieh, H. L.; Quirk, R. P. Anionic Polymerization: styrene are dominated by tiny amounts of highly Principles and Practical Applications; Marcel Dek- reactive free ions in equilibrium with relatively ker: New York, 1996; p 2. unreactive ion pairs.1,2 3. Matyjaszewski, K.; Wang, J.-S. J Am Chem Soc Indeed, it could be said that the presence of un- 1995, 117, 5614. 4. (a) Webster, O.; Hertler, W.; Sogah, D.; Farnham, reactive intermediates that rapidly interconvert W.; Rajan Babu, T. J Am Chem Soc 1983, 105, with the active chain end greatly contributes to the 5706; (b) Quirk, R. P.; Bidinger, G. P. J Polym Bull living character of many polymerizations. Thus, the 1989, 22, 63. low concentration of the active chain ends in the 5. (a) Zagala, A. P.; Hogen-Esch, T. E. Macromole- living radical polymerizations also greatly reduces cules 1996, 29, 3038; (b) Baskaran, D.; Mueller, intermolecular deactivation reactions and is an es- A. H. E.; Kolshorn, H.; Zagala, A. P.; Hogen-Esch, sential element in conferring living character to this T. E. Macromolecules 1997, 30, 6695. 6. Ishizone, T.; Yoshimura, K.; Yanase, E.; Naka- polymerization.3 There are several other recent re- hama, S. Macromolecules 1999, 32, 955. ports on the living anionic polymerization of 7. Miyamoto, M.; Sawamoto, M.; Higashimura, T. (meth)acrylates in which dormant species are pos- Macromolecules 1984, 17, 265. tulated or demonstrated.4–6 The situation is quite 8. Webster’s New Universal Unabridged Dictionary; similar in some cationic polymerizations in which Barnes & Noble: New York, 1994. several species with widely different reactivities have been demonstrated.7 I agree with Darling et al. that controlled-liv- Comments by ing and similar terms are not desirable and Professor Bob A. Howell should be avoided. The words controlled and con- Center for Applications in Polymer trol, meaning to exercise restraint or direction Science, Department of Chemistry, 8 over, are neither metaphorical nor absolute, and Central Michigan University, Mt. the expression degree of control, as opposed to Pleasant, Michigan 48859 degree of livingness, is internally consistent. In this case, the word control appears to be linked to Over the past several years, attempts to moder- the process leading to an intended outcome of the ate the reactivity of the propagating species in rad- polymerization. This control may include, but is ical polymerization have been multifold. The focus not limited to, molecular structure, molecular of these efforts has been the elimination of un- wanted side reactions (radical coupling, dispropor- weight, molecular weight distribution, stereo- tionation, radical transfer). In the absence of such chemistry, end functionalization, and other prop- reactions that lead to polydispersity broadening and erties but does not have to conform to all, or even the formation of polymer defect structures, the po- most, of these aspects or to the criteria for living lymerization should be much more well-behaved polymerizations. For instance, a polymerization than traditional radical polymerization. It has been may be controlled by the formation of a polymer noted that for “systems that proceed in the absence with a particular number-average molecular of termination and chain transfer, polymers can be weight without the polymerization process neces- prepared with control of molecular weight, molecu- 1,2 sarily being living. lar weight distribution, copolymer composition and In conclusion, definitions should not be changed microstructure, tacticity, chain-end and in-chain without compelling reasons. “If it ain’t broke, don’t functional groups, architecture, and morphology.”1 fix it” would seem to be good advice in this area also. Success has been achieved to a greater or lesser Keeping the definition of living polymerization sim- degree depending on the exact approach used. How- ple but flexible and restricting it to polymerization, ever, none have been completely successful; that is, chain transfer, or termination would avoid much none has provided an ideal polymerization. All of confusion and proliferation of terms. these processes involve the presence of a capping 1728 LIVING OR CONTROLLED? agent (nitroxyl radical, halogen atom, organometal- polymerization, a great variety of terminology lic moiety, or other molecular fragment) to react has sprung up to describe these processes, in- reversibly with the propagating radical to generate cluding pseudo-living and quasi-living. Such a nonradical species that is unreactive. In this descriptions often reflect a desire to include liv- bound state, the propagating radical is stable or ing in the descriptive terminology rather than a dormant and unable to participate in termination or regard for accuracy in description or precision chain-transfer processes. Chain extension is con- of language. More recently, specific descriptors trolled by the position of equilibrium between the such as nitroxyl-mediated radical polymeriza- dormant and active or free propagating radical. Un- tion (NMRP) and atom transfer radical poly- der appropriate conditions, very good mediation of merization (ATRP) have been used in an at- the radical polymerization may be achieved, and tempt to introduce a greater degree of precision some of the characteristics of living polymerization in the description of the polymerization reac- may be observed, such as ready chain extension to tion. This has largely been successful. Neither form block polymers, narrow polydispersity, molec- of these processes is living, but both possess ular weight control (reflecting the monomer/initia- certain characteristics of living polymerization tor molar ratio for systems in which preformed ini- that vary with the exact nature of the polymer- tiators/mediators are used), and end-group purity. ization and the reaction conditions. Because However, none of these reactions meet rigorous cri- these processes have been rather widely ex- teria for living polymerization. plored and their scope and limitations largely These have been defined as follows:2 established, the terms NMRP and ATRP are functionally descriptive; that is, to what they 1. The polymerization proceeds until all the refer is well understood. Neither contains any monomer has been consumed; the further direct reference to the extent of the livingness of addition of monomer results in continued the process that it describes; in either case, this polymerization. is variable depending on the reaction condi-

2. The number-average molecular weight, Mn tions, is reflected experimentally, and is im- (or Xn, the number-average degree of poly- plicit in the descriptor. For both these cases and merization), is a linear function of conver- for others in which a mediating agent is sion. present, the propagating radical is neither free 3. The number of polymer molecules (and ac- nor living. However, because of the presence of tive centers) is a constant, which is sensi- the mediating species, unwanted side reactions bly independent of conversion are suppressed (to a greater or lesser degree 4. The molecular weight of the polymer can depending on reaction conditions), and the re- be controlled by the stoichiometry of the sulting polymer is more nearly ideal than that reaction. which can be obtained in the absence of the 5. Narrow molecular weight distribution mediating agent. Therefore, it is perfectly ap- polymers are produced. propriate to refer to these processes as medi- 6. Block copolymers can be prepared by se- ated radical polymerization (MRP) rather than quential monomer addition. as controlled radical polymerization (the con- 7. Chain-end functionalized polymers can be trol is not complete in any case and is variable prepared in quantitative yields. from case to case) or living radical polymeriza- 8. Linearity of a kinetic plot of the rate of tion (all mediated radical polymerizations ex- propagation as a function of time. hibit some features of living polymerization, but 9. Linear dependence of the degree of poly- none display all such characteristics). The term merization as a function of time. MRP is not only descriptive of a wide range of radical polymerizations carried out in the pres- To fairly be termed living, a polymerization ence of agents to modify the reactivity of the must meet all these criteria because some are propagating species but avoids the confusion sensitive to certain characteristics of living po- associated with the terms living free-radical, lymerization whereas others are sensitive to living radical,andcontrolled radical polymer- other features. Because none of the reported ization. mediated radical polymerizations may be legit- The goal of any nomenclature/terminology imately described as living but, at the same should be clarity. To broaden the definition of time, have some of the characteristics of living living to include a variety of mediated radical LIVING OR CONTROLLED? 1729 processes serves only to destroy the utility of the Comments by term living. This term, as it has been, should be Professor Shohei Inoue reserved for processes that truly meet the criteria Department of Industrial Chemistry, for livingness. It was argued by Darling et al. that Faculty of Engineering, Science such a broadening of the term living is consistent University of Tokyo, Kagurazaka, with Occam’s razor. Nothing could be further Shinjuku, Tokyo 162-8601, Japan from the case. Rather than providing a simplifi- cation, it would only introduce confusion into an The authors’ proposal is quite reasonable. The area currently well-defined, that is, living poly- meaning of the term living polymerization should merization. be relaxed and be defined as those processes that Darling et al. suggested that reactions should give living polymers. Living polymers are defined be classified on the basis of mechanism and then as those being able to grow whenever additional proceeded to use the Diels–Alder reaction (a re- monomer is supplied, in contrast to dead poly- action, as they correctly noted, that may occur by mers. The word controlled is too broad to define multiple mechanisms) as an example. The term polymerization. There can be even a mechanical Diels–Alder simply refers to the conversion of a way to control the polymerization process. A def- diene and an appropriate monoene to an unsat- inition should not be dependent on a detection urated cyclic adduct, as first observed by Diels limit that can change with the progress of meth- and Alder, without regard to mechanism. The ods. more specific way to refer to these processes However, no one can restrict the proposal of would be to do so by reference to mechanism, that new terminology because a scientist has a right to is, as a concerted 4 ϩ 2 cycloaddition reaction, a emphasize his or her identity. It is, of course, also cycloaddition involving a diradical intermediate, possible for other scientists to either agree with or or a cycloaddition as a consequence of ligand re- oppose any proposal. There is a long history of organization at a metal center. The term Diels– similar discussions about language. Opinions Alder simply refers to the nature of the product have been often expressed by those worried about formed and not to the process by which it was the confusion of a language they would correct. formed. As a consequence, it is a far less useful Even so, languages have gradually changed and term than concerted 4 ϩ 2 cycloaddition.Noone will continue to change in the future. Terminology would argue that radical polymerization con- in chemistry cannot be an exception. A rigorous ducted in the presence of a mediating agent definition does not make much sense. should be referred to simply as polymerization (somewhat akin to the use of Diels–Alder to de- scribe cycloaddition). However, it can be de- Comments by scribed in a much more definitive way based on Professor Aubrey D. Jenkins how it occurs, that is, based on mechanism. It is School of Chemistry, Physics, and not living polymerization or controlled polymer- Environmental Science, University of ization. It may exhibit some characteristics of ei- Sussex, Brighton, BN1 9QJ, ther. To avoid confusion, to introduce clarity, and United Kingdom to maintain mechanisms as the basis for descrip- tive terminology, this process may best be de- Although there is an erroneous reference in scribed as MRP. this article to “a recent IUPAC nomenclature note,” it is astonishing to find no further allusion to IUPAC, in particular to the work of the IUPAC Commission on Macromolecular Nomenclature, REFERENCES AND NOTES the body charged with the responsibility of rec- ommending the nomenclature and terminology to be employed internationally in polymer science. 1. Hsieh, H. L; Quirk R. P. Anionic Polymerization: Principles and Practical Applications; Marcel Dek- The fact that some of the authors participate in ker: New York, 1996; p 2. IUPAC work, within the Macromolecular Divi- 2. Hsieh, H. L; Quirk R. P. Anionic Polymerization: sion of IUPAC, adds to the surprise. Principles and Practical Applications; Marcel Dek- In view of the importance, stressed by Darling ker: New York, 1996; p 76–77. et al.,1 of using terms uniformly throughout the 1730 LIVING OR CONTROLLED? polymer science community, the first thing to do Comments by in any discussion of this kind is to refer back to Professor Robert Jerome the nomenclature already established in, for ex- Director, Center for Education & ample, the Glossary of Basic Terms in Polymer Research on Macromolecules, Science2 and the Compendium of Chemical Ter- University of Lie`ge, Sart-Tilman, minology.3 The only way to avoid chaos in nomen- B6 4000 Lie`ge, Belgium clature is to build on the existing body of termi- nology, rather than to invent new terms ad hoc. Although the article entitled “Living polymer- In the Glossary,2 Definition 3.21, we find that ization: Rationale for Uniform Terminology” by living polymerization is “a chain polymerization Darling et al. aims at clarifying the concept of from which chain transfer and chain termination livingness, I do have the feeling that at the end of are absent.” Turning to the Compendium,3 a discussion that is basically not convincing, the (chain) termination is defined as “the steps in a reader is left unsatisfied by the final proposal. chain reaction in which reactive intermediates I do not understand at all the authors who par- are destroyed or rendered inactive, thus ending allel the problem of defining a living polymerization the chain.” and the fact that “one of the basic principles of The very last phrase in the latter definition is chemistry is to name reactions on the basis of their the one most pertinent to this discussion. If it is mechanisms, not their yields.” Polymerizations are held that the intention was to convey a sense of indeed clearly named on the basis of their mecha- ending the chain permanently, then there is no nisms (anionic, cationic, etc.), whereas livingness problem in using the term living radical poly- refers to characteristic features shared by different merization to describe the kinds of system in mechanisms. question, where the reactive intermediates re- When the authors propose to define living po- lymerizations as those processes that yield living vive, perhaps many times, after temporary qui- polymers (or their dormant precursors), where is escence. This is the view that I would person- the clarification, with no definition being pro- ally advocate. posed for living polymers ? Having said that, it could be argued that it The authors also argue that “a definition would be useful to add an adjective that distin- should not be dependent on the limitations of guishes such a system from the type, well-known current technology (a detection limit).” Never- in anionic polymerization, in which the chains are theless, anyone who is conducting polymeriza- not interrupted at all. The simplest suggestion I tion experiments needs criteria to which he can make for this purpose is to use either inter- (she) can refer to conclude to the livingness of rupted living radical polymerization or intermit- the polymerization or not. This need for ac- tent living radical polymerization (in either case, cepted criteria and for experimental measure- ILRP). I believe that these expressions accurately ments (techniques) to quantify them is by far and succinctly convey the desired impression, in the most important issue. When the authors contrast to controlled, which tells us nothing admit that various characteristics of the final about what is controlled or how. polymers are associated with livingness (poly- dispersity, molecular weight, end functionality, etc.), they indirectly address this basic ques- tion, although they make no proposal to answer REFERENCES AND NOTES it (which is a gap to be filled). A definition is completely useless as long as there is no agreement on how to handle it and draw firm 1. Darling, T. R.; Davis, T. P.; Fryd, M.; Gridnev, conclusions from experimental observations. A. A.; Haddleton, D. M.; Ittel, S. D.; Matheson, R. R., Jr.; Moad, G.; Rizzardo, E. J Polym Sci Part Finally, I never considered narrow polydispersity A: Polym Chem 2000, 38, 1706. an attribute of living polymerization. The relative 2. IUPAC Commission on Macromolecular Nomencla- rate constant of the initiation compared to the prop- ture. Pure Appl Chem 1996, 68, 2287. agation does not prevent chains from growing 3. McNaught, A. D.; Wilkinson, A. Compendium of whenever additional monomer is supplied, if I refer Chemical Terminology; Blackwell: Oxford, En- to the distinction of living polymers from dead poly- gland, 1997; p 415. mers, according to ref. 4 of the article. LIVING OR CONTROLLED? 1731

Shortly, I would propose that living polymer- of the propagation rate. Usually, we determined ization is any process that is free from irreversible the initiation rate by using benzoquinone, DPPH, chain-breaking reactions, whatever the relative verdazyl, and TEMPOL (1-hydroxy-1,1,6,6-tetra- rate constants of the constitutive processes. In methyl piperinin-1-oyl). TEMPOL quantitatively practice, this definition will be relaxed (to use the catches up the initiating radical via a coupling same term as the authors in their proposal) de- reaction below 60 °C, whereas the other inhibitors pending on the experimental criteria that each of have extra reaction sites such as nitro and car- us accept to use for drawing conclusions. Once bonyl groups.1 Accordingly, we used TEMPOL as these criteria are clearly defined, it will appear the best inhibitor for the determination of the that the notion of yield (I suppose that the au- initiation rate. From my experience, I think that thors mean monomer conversion) is important the polymerization with nitroxyl radical is tem- because it might happen that the criteria are perature-controlled radical polymerization; poly- fulfilled up to a certain monomer conversion and merization with reversible activation and deacti- that increasingly more important deviations ap- vation takes place at temperatures greater than pear at longer reaction times. Finally, that the 100 °C. Moreover, I think the concentration of polydispersity is narrow or broad (depending on nitroxyl radical is also an important factor in the the relative rate constant for initiation and prop- achievement of well-controlled radical polymer- agation) merely makes livingness very well suited ization with reversible activation and deactiva- or not to the macromolecular engineering of the tion. Living polymerization with an anionic initi- polymers under consideration. The problem of the ator, which was used by Professor Szwarc, usu- initiator efficiency (number of chains formed per ally does not contain irreversible termination initiator) should be treated in the same way, par- processes. Accordingly, I think controlled radical ticularly if this efficiency increases with monomer polymerization is more realistic than other termi- nology, such as living polymerization, for union conversion. So, my proposal is (1) to use a defini- terminology. tion that is very close to an early definition, (2) to emphasize the need for experimental criteria on the basis of which this (or any other) definition REFERENCES AND NOTES will be considered fulfilled or not, and (3) to con- sider that in some instances livingness might be 1. Kamachi, M.; Honda, Y. Department of Macromo- not suited to macromolecular engineering. lecular Science, Osaka University. Unpublished Although at this point I do not embark myself paper, 1983. in establishing a list of criteria for livingness (I do believe that those ones that are most often used by many authors, such as the dependence of Mn Comments by on conversion, the dependence of ln([M]o/[M]) on Professor Konstantin S. Kazanskii time, and the dependence of (Mn)exp on (Mn)theor Laboratory of Hydrophilic Polymer are a valuable basis), I hope that these consider- Materials, Polymer and Composite ations will be of some help in the rationale for Department, N. N. Semenov Institute of uniform terminology, something most of us con- Chemical Physics, Russian Academy of sider highly desirable. Sciences, ul. Kosygina 4, Moscow 117977, Russia Comments by Professor Mikiharu Kamachi THE BEST IS THE ENEMY TO THE GOOD Department of Applied Chemistry and Physics, Fukui University of Technology, It is well-known that the notion of living polymer- 6-3-1 Gakuen, Fukui 910-0028, Japan ization relates not to any particular type of chem- ical reaction but rather to a specific regime of Twenty years ago, we determined the propaga- chain polymerization processes. The determining tion rate constants for several monomers by using feature of this regime is that within the time the rotating sector method. In the course of this sufficient to attain a definite synthetic aim, chain- study, we needed to determine the initiation rate termination or chain-transfer reactions do not under the same conditions as the determination seem to take place. As a rule, it does not strictly 1732 LIVING OR CONTROLLED? mean that the mentioned reactions are really ab- ously one of the most pure examples of living sent in the given polymerizing system but that polymerization. In fact, poly(ethylene oxide) of they involve only an insignificant portion of the practically any predetermined molecular weight propagating macromolecules. Thus, being gener- can be easily prepared with the appropriate ated simultaneously, these species are living; that choice of initiator and solvent.4,5 Despite some is, they retain the activity of their terminal review publications,6 there are not even signs of groups, which in turn allows the polymer chain to termination or transfer reactions in the ethylene be extended with the same or another monomer oxide anionic polymerization in bulk or tetrahy- and also allows any reactions of these groups to be drofuran. Thus, if some kinetic aspects are not carried out (e.g., functionalization, branching, taken into account, it is an ideal living polymer- and crosslinking). ization with all the consequences. Naturally, it The number of nines needed to characterize the seems to be completely controlled as well. extent of livingness of a growing polymer on the Nevertheless, if anybody intends to obtain whole (0.99, 0.999, 0.9999, or even more) depends poly(ethylene oxide) of a molecular weight as high on the aim of the synthesis or the pedantry of any as 107, he obviously will meet serious problems or particular chemist. From this pragmatic point of completely fail, because in this case the purity of view, the absolute livingness without any limita- the system must be assured on the 10Ϫ4 M level. tions in time becomes an unattainable ideal, so The same problem occurs if quantitative function- nobody gives even a finger for its reality. This is alization of the polymer is needed. The analysis quite reasonably noted by Darling et al.1 and experiments show that it cannot be done In regard to the experimental foundations of without the expected molecular weight of the living polymerization, the first appeared very polymer being taken into account, and for certain long ago. It is worth remembering the work of cases, the value of 105 seems to be an essential Perry and Hibbert2 on the anionic polymerization limit.7 However, in both these examples, purely of ethylene oxide, where the stepwise increasing living polymerization simply gets out of control. of the molecular weight at the successive portion- So, the livingness by itself as a determining fea- ing of the monomer was clearly shown for the first ture of this polymerization gives no guarantee time. These data subsequently inspired Flory to that it can be controlled in all conditions. Is this give his classical treatment of the statistics of the reason to consider this reaction as living terminationless polymerization.3 within some range of conditions and nonliving The explosion of the 1950s and 1960s, mainly outside this range because of purely technical in the study of the anionic polymerization of sty- problems? The answer is obviously no. rene, completed the formulation of the term living The circumstances responsible for the con- polymerization and the understanding of all its trolled state of any living polymerization can be preparative advantages in designing a variety of arbitrarily subdivided into internal and external polymeric systems. This mighty wave overflowed ones. The first group of factors is related exclu- first cationic and, quite recently, radical polymer- sively to the nature of the active species, mono- ization, with a vast literature on this problem mer, and polymer units. The existence of any resulting. reactions between these three main components At the same time, the limitations of the precise of the polymerizing system (irreversible termina- use of this term and the synthetic approach itself tion, chain transfer, and depropagation), except are becoming more and more numerous, giving for chain growth, usually excludes the possibility rise to the effort of improving the terminology. of living polymerization. Some of these unfavor- The most useful alternative terms are living and able internal factors can be more or less avoided controlled, as is quite clear from Darling et al.1 by the appropriate choice of temperature and Which of them is the broader and more reason- timescale. The majority of us know that living able? Which reflects to a greater extent the fun- polymerization is quite real even in the case of the damentals of the process and which relates to thermodynamic reversibility of the process. some particulars? In this respect, my vote is def- Therefore, we have the elaborated theory and initely for the first of these terms, and I am going practice of polymerization control, the character- to argue for this choice. istic times of the said reactions being decisive. My own experience in the field of living poly- As for so-called external factors (solvents, ad- merization is restricted entirely to the anionic ditives, purity of the whole system, and all ma- polymerization of ethylene oxide, which is obvi- nipulations with living polymer), the tactics to LIVING OR CONTROLLED? 1733 keep them under control are quite different but 2. Hibbert, H.; Perry, S. J Am Chem Soc 1940, 62, are often quite successful. 2599. The problem of applying the habitual terminol- 3. Flory, P. J. J Am Chem Soc 1940, 62, 1561. ogy of living polymers to radical polymerization 4. Kazanskii, K. S.; Solovyanov, A. A.; Entelis, S. G. appears more complicated and gives rise to a Eur Polym J 1971, 7, 1421. great variety of terms.1 Nevertheless, the situa- 5. Dubrovskii, S. A.; Kumpanenko, I. V.; Goldberg, V. M.; Kazanskii, K. S. Vysokomol Soedin A 1975, tion here is quite similar to many other living 17, 2733. polymerizations, although the inexorable ten- 6. Powell, G. M.; Bailey, F. E. In Encyclopedia of dency of living radicals to bimolecular termina- Chemical Technology; Standen, A., Ed.; Inter- tion makes it necessary to control more strongly science: New York, 1960; 2nd Suppl. Vol., p 597. the lifetime of the radicals in a living state. There 7. Kazanskii, K. S.; Ptitsyna, N. V. Makromol Chem is also the seriously elaborated methodology to 1989, 190, 255. avoid this internal factor of instability. In many cases, this tactic is quite successful. It seems clear that there are enough reasons to Comments by consider the term living as more fundamental, Professor Joseph P. Kennedy whereas the term controlled is more particular in Maurice Morton Institute of Polymer its meaning. Moreover, the term controlled looks Science, University of Akron, 302 East like a question, expecting an explanation, of how Buchtel Avenue, Akron, Ohio 44325-3909 this process is controlled. Therefore, I believe the first of these terms must retain its customary I do not agree with the proposed relaxation “of place in the nomenclature of polymer science, re- the term living polymerization [to] be defined as gardless of the term controlled. those processes that yield living polymers.” I pre- fer to use the classical definition of living poly- merization, that is, polymerizations in which TWO ADDITIONAL NOTES IN R ϭ R ϭ 0. The requirement that R be CONCLUSION tr,M t i greater than Rp is of secondary significance; if Ri is greater than R , molecular weight distribution The term living polymerization should be kept for p control is possible (and a narrow molecular one more reason. This helpful and lasting term weight distribution is useful for the diagnosis of carries a memory of the romantic period, having livingness), but this inequality is not part of the freed polymer chemists from many earlier restric- rigorous definition. After all, polymerizations tions of their fantasy. In any event, a host of with a relatively low R can still be living. distinguishing achievements of polymer chemis- i try is the result of this freedom. Finally, we can draw a purely illustrative anal- ogy to human life. Really, why should we deprive Comments by Dr. Young H. Kim a man of the attribute living, even being aware of DuPont Central Research & his mortality? There are also a lot of internal and Development, Experimental Station external factors of livingness that result in a life- E328/261, Wilmington, Delaware 19880- time distribution that is analogous to chain- 0328 length distribution. However, within a mean life- time, every man has a right to plan all his affairs, What criteria need to be met for a polymeriza- and his relatives and colleagues may expect of tion to be worthy of being classified as living po- him all the activity inherent in the notion of liv- lymerization and what we can conveniently call a ing. Is this not true? type of polymerization reaction that may render living polymerization are two different issues. Living Polymerization: Like Szwarc’s origi- REFERENCES AND NOTES nal definition, the only criteria for living polymer- ization should be that the polymer chain ends are 1. Darling, T. R.; Davis, T. P.; Fryd, M.; Gridnev, stable enough at the complete consumption or A. A.; Haddleton, D. M.; Ittel, S. D.; Matheson, near complete consumption of monomers so that a R. R., Jr.; Moad, G.; Rizzardo, E. J Polym Sci Part new supply of monomers can continue the poly- A: Polym Chem 2000, 38, 1706. merization on all the resting ends of the polymer 1734 LIVING OR CONTROLLED? chains. To make all the polymer chains to be group transfer polymerization (GTP) is a living involved in the growth of the chain, there should polymerization, even though it is not spelled out be no initiator left at the point of incremental as living GTP. If naming a particular reaction monomer addition. These requirements are com- helps communication between scientists, it monly confirmed by the first reaction order depen- should be employed. It has been common practice dence of monomers for living polymerization. If to use such a name for many years. Hundreds of the reaction rate of the initiation reaction is faster so-called named reactions have been a marvel of than that of the polymer propagation, the result- organic chemistry for centuries. Certain polymer ing polymer will have a narrow molecular weight reactions involving a common technique or mech- distribution, which is another common require- anism can be named for convenience for scientific ment for living polymerization. communication. It is up to a scientist to learn and Now, let us think about why living polymeriza- understand what the scope and limitation of a tion has attracted so much fascination. There are named polymerization are, if it is worthwhile. two important advantages that living polymeriza- Leave it to the free-market economy; it works. tion can offer better than any other polymeriza- tion. One is rendering polymers with a narrow molecular weight distribution, and the other is Comments by complete chain-end functionalization or forma- Professor Bert Klumperman tion of block copolymers. In comparing these two Laboratory for Polymer Chemistry, virtues that living polymerization renders, the Eindhoven University, P.O. Box 513, latter attribute is far more important than the 5600 MB Eindhoven, The Netherlands first attribute because the advantage of narrow molecular weight distribution in the material It is beyond doubt that the term living poly- properties, process, or both has not been unam- merization is loaded with controversy. In my opin- biguously demonstrated yet. Nevertheless, in re- ion, it is a good idea to strive toward uniform ality the quantity of polymers required in com- terminology. The situation with living radical po- merce needing these attributes is very small. All lymerization is probably the most complex. The the research and hype on living polymerization technique is derived from free-radical polymeriza- seem to be out of proportion compared to other tion, where attempts are undertaken to suppress polymeric materials of far more importance. Inas- chain-breaking reactions. This background could much as the objective of the polymerization reac- call for a name like living free-radical polymeriza- tion is to obtain complete end-group functional- ization, many polymerization reactions that may tion, but this would needlessly further complicate not fit the rigorous definition of living polymeriza- the terminology. In some cases [e.g., in atom tion can provide excellent means to achieve this transfer radical polymerization (ATRP)], it is un- goal. TEMPO-mediated vinyl free-radical poly- clear whether free diffusion of the radicals takes merization and some Lewis-base-mediated vinyl place. In trying to use names that are as detailed ether cationic polymerizations are examples. as possible with respect to the mechanism, there The definition of living polymerization does not will be no uniformity, and searching for examples need to be refined from what Szwarc’s defined, in Chemical Abstracts will be difficult. and we do not need to use new terminology to The suggestion of Darling et al. to simplify the describe the degree of the perfection of livingness. terminology is the only way to prevent an unlim- An ideal living polymer should have a moiety ited growth of names for comparable techniques. from the initiator, and the other end has to have The use of living radical polymerization seems to reactive ends for the polymerization at the time of be the best approach. The definition of living po- the complete consumption of monomer. There lymerization would then be a chain polymeriza- could be many analytical methods to confirm this tion in which chain-breaking reactions are mini- requirement, but an analytical method should not mized. The degree to which the living character of be used to define the livingness of the polymer- the polymerization is achieved can be reported to ization reaction. show the level of success in performing a living Nomenclature: Whether a particular poly- polymerization. merization is living or not, the naming of the To recognize the underlying mechanism, it reaction is a quite different issue. Most people in would be advantageous to mention the living rad- the field of polymer science would recognize that ical polymerization method in the keywords of a LIVING OR CONTROLLED? 1735 publication [e.g., ATRP, reversible addition frag- s–Alder reactions, Friedel–Crafts reactions, and mentation chain transfer (RAFT), and nitroxide- so forth are specifically confirmed by their reac- mediated]. This would greatly facilitate searches tion pattern, mechanism, product structures, or in databases such as Chemical Abstracts. some combination of these. Other examples are Wittig reactions and Mannich reactions, which are defined simply by the starting reagents and Comments by product structures. Professor Shiro Kobayashi In contrast, living polymerization is a general Department of Materials Chemistry, concept to explain a phenomenon of polymerization Graduate School of Engineering, Kyoto reactions, involving living propagating species, that University, Kyoto, 606-8501, Japan brings about molecular weight control, narrow poly- dispersity, end-group control, and the ability to This article describes the authors’ proposal for chain-extend. This concept concerns the phenome- the definition of living polymerization, which has non observed in various polymerization reactions, recently been used in various ways depending on including not only different reaction mechanisms of the results obtained by the many research groups. anionic, cationic, radical, transition-metal-cata- It is quite understandable that when researchers lyzed, and enzyme-catalyzed processes but also dif- find something new in chemistry, they often pro- ferent reaction types of vinyl, olefin, acetylene, pose a new terminology to stress their findings. diene, and ring-opening polymerizations. In these This has resulted in confusion in the terminology complex circumstances, it is very difficult to define typically associated with living polymerization. It living polymerization as a general uniform termi- is also recognized that these authors tried to rem- nology. This situation is like “there is no general rule without some exceptions.” edy the confused usage concerning living poly- I think we should return to the original concept merization. They proposed the following as a uni- of Szwarc. To practically characterize the gener- form terminology, that “the meaning of the term ally accepted living polymerization for more living polymerization be relaxed and defined as quantitative consideration, we need two factors, those processes that yield living polymers (or the yield of living polymers and the polydisper- their dormant precursors). The term living poly- sity, as minimum requirements. In the perfect merization should be used regardless of the yield case, initiation is much faster than propagation, of the process.” without termination and chain transfer. The yield According to the definition of the IUPAC No- of living polymers is 100%. Then, polydispersity menclature Committee, living polymerization is must be 1.0. However, nothing is actually perfect. “a chain polymerization from which chain trans- 1 The confusion in terminology stems from the fact fer and chain termination is absent.” Note that that researchers have different rigidities for us- in some living polymerizations, reversible deacti- ing a polydispersity value. Sometimes, living po- vation of chain carriers may occur. However, in lymerization is claimed even when the value is what case should we use living polymerization for greater than 1.5, where livingness is too broadly practical publication in academic articles? understood. Therefore, we require criteria. I Now, I partially agree with this proposal for would like to propose that the process be called the following reasons. I think that the authors’ living polymerization when the yield of living terminology is too relaxed. If the proposed termi- polymers, including their dormant species, is nology is applied, almost all processes can be greater than 95% and when the polydispersity is called living polymerization because almost all less than 1.2. These values of 95% and 1.2 need polymerization reactions yield living polymers to further arguments, but I think these may be prac- some extent, even in a very low yield. In many tically meaningful and reasonable values. A pro- cases, polydispersity (Mw/Mn) should become cess showing the respective values 95% Ͼ yield Ն Ͻ Յ wider. This does not reflect actually expected liv- 90% and 1.2 Mw/Mn 1.5 is then to be called ing polymerization. living-like polymerization. Again, the values of The authors referred, in regard to the yield of 90% and 1.5 need to be argued. the reaction, to the Diels–Alder reaction as a named reaction. I do not think this is an appro- REFERENCES AND NOTES priate analogy. In the case of name reactions, naming the reaction is quite clear. Typically, Diel- 1. IUPAC. Pure Appl Chem 1996, 68, 2287–2311. 1736 LIVING OR CONTROLLED?

Comments by As Szwarc aptly stated, living polymers are Professor Mikhail Lachinov polymers that are able to grow whenever addi- Department of Chemistry, Moscow MV, tional monomer is supplied, and living polymer- Lomonosov State University, Vorobevy izations are polymerizations that involve living Gory, Moscow 119899, Russia polymers. As there are no perfect stereospecific polymerizations, perfect living polymerizations, I agree with the authors of this article on the where no dead polymer is formed at all, do not existing problem, concerning the terminology of a exist either. Hence, if the aforementioned defini- common name for several radical polymerization tion is applicable and a polymerization exhibits reactions, that recently has attracted wide atten- all or most of the characteristics of livingness tion. This problem exists not only from an author’s such as low polydispersity, end functionalization point of view but also from the point of view of by terminating agents, and formation of block polymer chemistry teaching. After many authors copolymers, then the polymerization can be re- investigated several radical polymerization reac- garded as a living polymerization. If such charac- tions, each with its own special name, including teristics are not very evident, the polymerization iniferter polymerization, nitroxide-mediated poly- should be called a polymerization involving long- merization, atom transfer radical polymerization lived species or a quasi-living polymerization.I (ATRP), and reversible addition fragmentation also think that the name controlled polymeriza- chain transfer (RAFT), we no longer had questions tion does not make sense in itself and is not di- about the names of the reactions (they all are quite rectly related to living polymerization. Here I touch on our studies. We have been work- correct). The question is what common name to give ing on the living polymerization of substituted acet- them if they have common properties. In fact, they ylenes by using MoOCl -based multicomponent cat- do have properties that permit us to control the 4 alysts (for a recent review, see ref. 1). For instance, kinetic parameters of polymerization to a higher a catalyst composed of MoOCl , n-Bu Sn, and eth- extent than in the case of classic radical polymer- 4 4 anol polymerizes a variety of substituted acetylenes ization and to obtain well-defined products. There- (e.g., 1-chloro-1-octyne, o-CF -phenylacetylene, and fore, we can say that the IUPAC working party 3 tert-butylacetylene) in a living fashion to provide recommendation is correct enough. polymers with narrow molecular weight distribu- The second question is whether we can name all tions (polydispersity ratio ϭ 1.02–1.10). Further- these radical reactions as living polymerizations? more, block copolymers can be easily obtained from All of these reactions have the same features, such these monomers. These findings demonstrate that as the growth of molecular weight with conversion these polymerizations are living polymerizations. and a low polydispersity of produced polymers, that One problem in our systems is that the initiation are common for the case of living polymerization. reaction is not quantitative. However, this does not However, in most cases these reactions have more keep it from being a living polymerization and is or less dead chains that result from inevitable bi- rather convenient for obtaining high molecular weight molecular termination, chain transfer to monomer, living polymers from a practical point of view. or both. Therefore, I suppose that not all of these reactions satisfy Szwarc’s definition of living chains. Nevertheless, we should find ways to have real liv- REFERENCES AND NOTES ing radical polymerization. 1. Masuda, T.; Hayano, S.; Iwawaki, E.; Nomura, R. J Mol Catal 1998, 133, 213–220.

Comments by Professor Toshio Masuda Comments by Department of Polymer Chemistry, Professor Lon J. Mathias Graduate School of Engineering, Kyoto Department of Polymer Science, University, Kyoto 606-8501, Japan University of Southern Mississippi, P.O. Box 10076, Hattiesburg, Mississippi I read your letter and the article entitled “Liv- 39406-0076 ing Polymerization: Rationale for Uniform Termi- nology” by Darling et al. I basically agree with the Words are fascinating creatures: they beguile idea of the authors. and delight, sadden and disgust, define and ob- LIVING OR CONTROLLED? 1737 scure, reveal and hide. Words are the mechanism with other knowledge, we extend and develop and of conscious thought and the basis for action and discover, and soon we have a new reality to rep- creative endeavor. Words can define and explain, resent with the words we have. The words must generalize and differentiate. Sometimes, the change to reflect this new reality, to generalize same word means very different things to differ- our understanding to other systems, and to allow ent people. better communication of what we are seeing and To discuss Darling et al.’s article correctly, we thinking to others. must understand the purpose and function of Let us deal with specifics. If we standardize the words: they help us to think, act, and communi- use of living now, will that change what is already cate. As scientists, we seek objective facts, con- written? Should we change the historical legacy cepts, relationships, and definitions. We would in light of our current knowledge? Of course not, like our words to be precise and controlled, saying for this would hide the human aspect of science: for us exactly what we mean and no more. How- we learn and grow. Should we mandate a more ever, language is a dynamic, changing part of precise (or more general) use of the term now and human experience, as much in science as in any in the future? Should we ban or limit the use of other human activity. Because the words of sci- the other terms discussed in the article so that we ence are of human origin and subject to human all “speak the same language?” These beg the real manipulation, they undergo continuous modifica- questions that need to be asked: what are the uses tion through use. The word living itself is an these terms enjoy? Why are the multiple terms example of such change: it once applied only to with variations of specific details used? The ques- biological systems, to plants and animals, but tion is not of how to restrict and limit, or expand now has been adapted to chemical reactions pos- and generalize; the question is what terms and sessing certain characteristics of biological life. definitions do we need to effectively think and act This article, on the uses and misuses of the and communicate. term living in describing the behavior of polymer- Words change how we think in both subtle and izations, raises issues regarding generality and broad ways. Take the term phase transfer catalysis. specificity. Does the term refer to the ideal of an Before this term was coined, interfacial chemistry indefinite lifetime for a propagating chain end, as was done on a routine basis: the Schotten–Bau- mentioned by the authors of the article? This menn synthesis of amides from amines and acid would be a characteristic that did not pertain to chlorides was known to be catalyzed by tertiary its original use concerning plants and animals, for amines. However, it was not until someone actually which termination by death was inherent in the coined the term, gave a broad definition to the con- word’s meaning. Should it now include reversible cept of a catalyst for the phase transport process, termination, controlled addition, assisted inser- that we were able to communicate the idea freely tion, and systems that look as if they are living and apply the method more generally. That is, we but are not according to some specific detail of a had to have a new way of thinking about the process specific definition? Does it matter? If it does, how to use it better. So is this the problem with which and why does it matter? the article deals, that we need a new term to facil- Words, especially scientific words, should help itate our thinking about the general concept? In a us to do our work better. So what does the word word, no. The concept is there, it is broadly used, living communicate to each of us? How does it but the problem is that of the evolution of the defi- help us think more clearly or creatively and do nition and the reality to which it applies. As the our work more productively? Well, depending on reality increased in breadth and complexity, adjec- the context and background, it can mean different tives were added and new terms were developed to things. More to the point, its meaning can change increase the communication of the unique specifics and, in fact, must change as our knowledge and of the individual system being described. Is this understanding of what we use the word for bad? In fact, it is both good and necessary. Within changes. A word such as living represents a part the broad concept and the general words used to of chemical reality in some objective way. How- describe it are specifics that are unique, that encom- ever, our understanding of that reality evolves pass a smaller range of characteristics. There is over time, both in what and how we see it, and in richness in these variations, an ability to help us its creative extension: we learn more because we think and act and talk about the unique within the can see more, and think more clearly, about that general. It is not an either/or situation but a both. objective reality. As we combine this knowledge We need the specific and the general definitions. 1738 LIVING OR CONTROLLED?

The latter helps us to extrapolate beyond what we process should fulfill to become living. It is not know now, the former helps us to understand and enough to say that atom transfer radical polymer- apply the unique aspects to real situations. Al- ization (ATRP), nitroxide-mediated polymeriza- though I cherish being human and honor my family tion (NMP), and reversible addition fragmenta- name for what it gives me of my ancestry, my given tion chain transfer (RAFT) are living radical po- names make me unique as a man and an individual. lymerizations. As an example, the NMP of methyl So should we broaden or limit the definition of methacrylate is a typical nonliving process, but living? Yes and no. Both will inevitably happen as the NMP of styrene may, under some conditions, the needs of people who use the terms evolve. In fulfill criteria of CLRP. discussing the definitions and the details, we in- We disagree with the authors’ statement that crease our understanding of what the terms mean living polymerization is mechanistically different and how they can be used as words in our thought from conventional radical polymerization and do processes to extend knowledge and create or dis- not understand the analogy to Diels–Alder chem- cover new systems and applications. “The only istry. The step of the conversion of monomer into constant is change,” and this applies especially to polymeric unit is mechanistically exactly the words and how we use them. Sure, we can define same in both CLRP and conventional radical po- and use the word living more carefully, but that lymerizations, as confirmed by the same stereo-, definition will in turn have to be modified, lim- regio-, and chemoselectivities. Several years ago, ited, or expanded, as our knowledge increases and we stressed the identical mechanism of conven- tional and living carbocationic polymerization, our understanding expands. By all means, dis- 2 cuss and experiment with the word and its defi- contrary to some other proposals at that time. nitions, but it is the usage of the word that will We think that an identical situation occurs in control its change. Embrace that change, enjoy it, CLRP. Because living polymerization does not de- fine a mechanism of the polymerization in the discuss it as needed; just do not argue about it. organic chemist’s sense but only the ratio of rates and rate constants of the involved reactions, it is Comments by very important to quantify the rate constants of all chain-breaking reactions. Professor K. Matyjaszewski It has to be recognized that if the rate of con- Department of Chemistry, Carnegie ventional polymerization and CLRP is the same, Mellon University, 4400 Fifth Avenue, the concentration of chains irreversibly termi- Pittsburgh, Pennsylvania 15213-3890 nated is also approximately the same because the rate of propagation is first order and the rate of When a field becomes mature, such as con- termination is second order in respect to radical trolled (or living) radical polymerization (CLRP), concentrations. Thus, from the point of view of the the appropriate definitions should be discussed by rates and rate constants of termination, there is the IUPAC Nomenclature Committee rather than currently no improvement in chemoselectivities in a polymer journal. for polymerizations proceeding with the same Living polymerization is more difficult to de- rates. Only when CLRP is slower than conven- fine than living polymers, as originally proposed tional radical polymerization is the termination by Szwarc, because it is easier to determine the rate reduced. However, although the absolute boundaries of a material than those of a process concentrations of dead chains are similar in con- 1 that has a kinetic meaning. ventional and CLRP systems occurring with the The IUPAC Nomenclature Committee pro- same rate, nevertheless, in conventional systems poses to define living polymerization as a chain- practically all chains are dead, whereas in CLRP growth process without chain-breaking reactions. systems their proportion is small in comparison to My understanding is that this refers to the ab- dominating dormant chains. sence of irreversible chain-breaking reactions; However, it is possible to imagine that true thus, living polymerization is compatible with re- living radical polymerizations can be developed in versible deactivation (also known as reversible the future. In these systems, the rate and rate termination) and with reversible transfer (such as constant of termination could become signifi- degenerative transfer). cantly lower than in conventional processes. This The authors of the submitted article do not can happen when growing radicals are complexed provide any definition or criteria that a radical by a specific reagent, as previously reported for LIVING OR CONTROLLED? 1739 phosphoric acid,3 or propagation may occur in a cerning living polymerization and its terminol- confined space (e.g., clathrates or zeolites).4 In ogy. I am not an expert in that particular field, both cases, the rate constants of termination be- nor I am very fond of my grammar, especially in a tween two growing radicals could be significantly foreign language, but I am intrigued by the pro- reduced because of steric or electronic effects. cess of rationalization for uniform terminology. I To summarize, the extension of the definition would like to express my thoughts here, by which of living polymerization to currently developed it should become clear that using different termi- radical systems is not appropriate. Also, the re- nologies is one of the most prominent exposures of laxation of the term living polymerization may living science and we are very well equipped to lead to a situation in which any chain-growth live with that. One can even argue that as soon as system will become defined as living, especially we all agree on a generally accepted nomencla- when no definition and criteria are provided. Fi- ture for events such as reaction types, mecha- nally, we would suggest that all researchers in nisms, phenomena, effects, and fields of chemis- the CLRP field make continuous efforts in mea- try, most aspects of that topic are known and suring the contribution of chain-breaking reac- hardly of interest for further investigations. Ob- tions and providing data on end functionalities viously, the opposite is true for topics such the that, in addition to the ratios of expected and IUPAC nomenclature for compounds or the val- measured molecular weights and polydispersi- ues for fundamental constants; without a uniform ties, are very important, especially for block copo- terminology, we are desperate. lymerization and in the synthesis of well-defined polymers. TOTAL SYNTHESIS OF NATURAL PRODUCTS REFERENCES AND NOTES The total synthesis of natural products is a field in chemistry that probably everyone knows of and 1. It has to be remembered that polymers with de- grees of polymerization predetermined by the ratio relates to a uniform terminology: you synthesize a of the concentrations of the reacted monomer to the molecule that is present in nature from scratch. introduced initiator and with low polydispersities However, what is allowed as your starting com- can be obtained in living polymerizations only pound? Something that is not natural? Or can one when the rate of initiation becomes comparable or final reaction on another natural product also larger than that of propagation. result in a total synthesis? The debates on the 2. Matyjaszewski, K.; Sigwalt, P. Polym Int 1994, terminology of total synthesis are nicely described 35, 1. by the Johnson in his recently published book, A 3. Kabanov, V. A. J Polym Sci Polym Symp 1975, 50, Fifty-Year Love Affair with Organic Chemistry.1 71. Following the terminology of Cornforth and Rob- 4. Farina, M.; Silvestro, G. D. Chem Commun 1976, inson, we use the term formal total synthesis for a 842. synthesis that depends on relays through inter- mediates that were supplied (for further steps) by Comments by the degradation of natural products. Woodward, however, never used the word formal; he refused Professor E. W. (Bert) Meijer to recognize that there was any difference be- Laboratory of Macromolecular and tween a synthesis that was carried all the way Organic Chemistry, Eindhoven through from simple chemicals and that relied on University of Technology, P.O. Box 513, relays, citing Johnson from his book. We probably 5600 MB Eindhoven, The Netherlands all agree with Johnson, that when a significant number of the steps of a synthesis are performed with material derived from the natural product LIVING SCIENCES DO NOT USE A (by a process that is called partial synthesis), it is UNIFORM TERMINOLOGY FOR EVENTS not right to call your final product totally syn- thetic material. Obviously, the interpretation of “a It was with pleasure that I accepted the invitation significant number of the steps” is very personal, by the editors to write a short personal comment but it does not influence the use of the term total on the contribution of a group of scientists con- synthesis. The moral of this example of total syn- 1740 LIVING OR CONTROLLED? thesis stresses the importance of a personal inter- fact that authors like to call it living or controlled pretation of a terminology that has become uni- shows that many aspects of the different radical form with time. polymerizations still need to be worked out, or that some investigators expect to make signifi- STEREOCHEMISTRY cant progress on the livingness of this type of polymerization. I would like to give everyone the Stereochemistry is probably the area of chemistry freedom to add those words to their polymeriza- in which debates on chemical terminology are the tions, to use what they think expresses their ideas most hectic. Some people like to use terms such as most accurately. At this time, counting the num- homochiral, chiral center, and optical yield, ber of articles with the different names can be whereas others will never use these words and compared to a popularity poll before a champion- prefer enantiomerically pure, stereocenter, and en- ship. However, for knowing the name of the antiomeric excess. You should use the R and S champion, we have to wait until all the games are nomenclature in the case of molecules with ste- played. Therefore, I am interested in seeing the reocenters and P and M nomenclature for helices, outcome in the debates on the terminology for whereas D and L configurations are useful for living polymerization, especially because this plea carbohydrates. However, it is obvious to many of for uniformity is enriched with the discrimination us that it is very easy to make mistakes in this between living and ideal living in ref. 1 of Darling area, and we like to consult specific articles to use et al. the most adequate terms. Again, you will see that the nomenclature for molecules is uniform, whereas the nomenclature of events or phenom- ena is under debate, with different schools sup- REFERENCES AND NOTES porting different terminologies. I personally encountered one of those prob- lems at the famous Bu¨ rgenstock Conference on 1. Johnson, W. C. A Fifty-Year Love Affair with Or- Stereochemistry in 1996.2 In my lecture, I used ganic Chemistry; American Chemical Society: the term cryptochirality for a series of chiral Washington, DC, 1998; p 54. dendritic macromolecules that did not show any 2. Hunziker, J. Chimia 1996, 50, 282. optical activity, despite the fact that the mole- 3. Peerlings, H. W. I.; Strijik, M. P.; Meijer, E. W. cules were enantiomerically pure. It was Dunitz 1998, 10, 46. 4. Mislow, K.; Bickart, P. Isr J Chem 1977, 15, 1. of the ETH Zu¨ rich who, in the discussion, cor- rected me: “Young fellow, I like to remind you that an object is either chiral or a geometrical property.” I used that word according to the Comments by definitions of Mislow,3 another eminent stereo- Professor Martin Moeller chemist, after an extremely fruitful correspon- Organic Chemistry Division III, dence on the question of whether a certain den- University of Ulm, Albert-Einstein Allee drimer is more chiral than another. It was only 11, D-89081 Ulm, Germany after the meeting that I found out that cryp- tochirality is only one of the many examples of a Terminology questions are certainly an impor- difference in preferred terminology between the tant issue that has to be discussed broadly and in Zu¨ rich and Princeton schools of stereochemis- detail. In most cases, the result will be some com- try. Because the chirality in our molecules is promise, and I think the discussion should be very really hidden, I liked and still like the term objective and dispassionate. cryptochirality, which probably is wrong, but it Personally, I agree with the authors. The ma- nicely makes use of the wealth of a language to jority of the people in this field use the term. They express something that is otherwise very diffi- understand that it means sufficient control of cult to describe. chain breakage to control molecular weight and end-group functionality and to prepare block co- LIVING POLYMERIZATION polymers. To me, living implies mortality, and I think the major question is addressed very well The question of whether a living polymerization by the statement of Szwarc, provided we do not is living, quasi-living,orpseudo-living and the talk about oligomers only. LIVING OR CONTROLLED? 1741

Comments by Comments by Professor Klaus Mu¨ llen and Professor R. J. M. Nolte Dr. Markus Klapper Department of Organic Chemistry, Max-Planck Institute for Polymer NSR Center, University of Nijmegen, Science, Ackermannweg 10, D-55128 Toernooiveld 1, 6525 ED Nijmegen, Mainz, Germany The Netherlands

We fully concur with the authors that, in re- The naming problem, raised by Darling et al., spect to a common language in research, an of when a polymerization reaction should be agreement for definitions and naming should be called living, is reminiscent of similar nomencla- reached in this area. However, these definitions ture problems in organic chemistry. For many have to be clear and unambiguous. years, synthetic organic chemists have been de- Recently, Matyaszewski and Mu¨ ller1 published scribing chemical transformations in complex re- a similar article, also cited by the authors, about action sequences without much proof of mecha- the naming of controlled or living polymeriza- nisms. Natural product synthesis has not really tions. We believe this article clearly mentions all suffered from this lack of mechanistic informa- the necessary criteria concerning the designation tion, and the same can be said for materials sci- of a controlled or living process. The kinetic as- ence, although this field is much younger and still pects are especially well discussed and explained. in an emerging state. Synthetic results are often Thus, one should clearly distinguish between liv- published because authors would like to inform ing or controlled radical polymerization; clear def- the scientific community that they have prepared initions are already given. new compounds or have improved existing syn- A further relaxation of the word living is un- thetic methodologies. In many cases, they are not necessary and even deleterious. Language gives concerned with the precise reaction mechanisms us the chance to express things in an exact way. by which the products have been synthesized, Clear definitions are especially desirable and nec- unless they have encountered serious problems essary in science. Living suggests the perfection that have hampered further progress. Because of the reaction. When side reactions occur, such as the situation in the field of polymer synthesis is chain-breaking reactions, one should use the not much different from that in organic synthesis word controlled. Why not express a different be- [scribitur ad narrandum non ad probandum (one havior with a different word? From the kinetic writes to narrate, not to prove)], we should not point of view, the controlled radical polymeriza- worry too much about whether a radical polymer- tion corresponds to a free-radical polymerization ization reaction is strictly living or not. The prod- at low free-radical concentrations, but nobody ucts and the procedures by which they are pre- would name the normal free-radical polymeriza- pared, as efficiently as possible, are far more im- tion living. portant. I am in favor of the terminology proposed In conclusion, a clear definition is desirable, by the authors. We should take living as easily as and in our opinion one should focus on living possible; life is complicated enough. polymerization and controlled (radical) polymer- ization as defined by Matyjaszewski and A.H. E. Mu¨ ller.1 Expressions such as quasi, pseudo, and controlled living should no longer be used, as they Comments by are imprecise and ambiguous. Controlled is the Professor Oskar Nuyken best descriptive word because of the principal ef- Lehrstuhl fu¨ r Makromolekulare Stoffe, fect, that the polymerization mechanism is not Technische Universita¨t Mu¨ nchen really affected even when the polymerization be- Lichtenbergstrasse 4 havior is controlled by concentrations and reac- D-85747 Garching, Germany tivity. Although I fully agree that there are too many misleading terms in the literature concerning liv- REFERENCES AND NOTES ingness, I do not completely agree with the au- thors view in which they argue with Diels–Alder 1. Matyjaszewski, K.; Mu¨ ller, A. H. E. Polymer Prepr reactions. Nobody would call a polymerization (Am Chem Soc Div Polym Chem) 1997, 38, 6. controlled if he had found 5% polymer only !! 1742 LIVING OR CONTROLLED?

Personally, I share M. Swarc’s view. If a poly- ingness or its reasoning should be explained in a mer is able to grow when more monomer is added, proper manner, for instance, by polydispersity. its molar mass is controlled by [M] : [I], it has a narrow molar mass distribution, its molar mass increases with conversion, and we find head and Comments by end group control, I would call it living. It would Professor Takayuki Otsu not bother me whether the active species is stable and Professor Akikazu Matsumoto over the whole reaction or whether we have re- Department of Applied Chemistry, versible termination (indeed, we included the re- Faculty of Engineering, Osaka City versible termination into the inifer mechanism University, Sugimoto, Sumiyoshi-ku, 1 2,3 already in 1982; shortly later we found the Osaka 558-8585, Japan experimental evidence for that ). I am pleased to see this reversible termination now included in The proposal for the use of uniform terminology almost every mechanism. for living polymerization is important, but its exe- cution is not easy. In a nomenclature note in 1997,1 it was proposed that several polymerization sys- REFERENCES AND NOTES tems, including radical polymerization, be called controlled or controlled/“living”, but not living po- 1. Nuyken, O.; Pask, S. D.; Vischer, A. Makromol lymerization. However, the term controlled has Chem 1983, 184, 553. been used everywhere in the field of polymer chem- 2. Pask, S. D.; Nuyken, O.; Vicher, A.; Walter, M. In istry, and the use of a quotation mark is nothing Cationic Polymerization and Related Processes; more than a compromise. The other terms based on Goethals, E. J., Ed.; Academic: London, 1984; p. 25. polymerization techniques, such as atom transfer 3. Nuyken, O.; Pask, S. D.; Vischer, A.; Walter, M. In radical polymerization and reversible addition frag- Cationic Polymerization and Related Processes; mentation chain transfer, can represent the feature Goethals, E. J., Ed.; Academic: London, 1984; p. 35. of each polymerization, but they are not all-inclu- sive names. Radical polymerization should be dis- tinguished from any other living polymerizations Comments by that proceed via an ionic or coordination mecha- Professor Yoshio Okamoto nism, even if the radical polymerization is fully con- Department of Applied Chemistry, trolled without any detectable chain-breaking reac- Graduate School of Engineering, Nagoya tions because of the features of the polymerization University, Furo-Cho, Chikusa-Ku, that are different from the other living polymeriza- Nagoya 464-8603, Japan tions. That is, the propagating species is neutral and forms dormant (covalent) species in competi- It is a good time to define the meaning of the tion with the bimolecular termination. In 1982,2 one term living polymerization in a form that we can of the authors proposed the concept of iniferter for accept and properly use in the field of polymer the design of polymer chain-end structures, and it science without confusion. Generally, nomencla- simultaneously was applied to a new model for liv- ture (naming) for a reaction (polymer) can be ing radical polymerization. Fortunately, many peo- given for an ideal example. Therefore, living po- ple favor the term living radical polymerization in lymerization may be defined as a polymerization many scenes. Therefore, we think that the use of that yields an ideal living polymer as described in living radical polymerization, which includes all ref. 4 in the article by Darling et al. However, as (and unavoidable) characteristics of radical poly- the authors mentioned, actual reactions far from merization, is the best choice for escaping the con- the ideal case are often still called Diels–Alder fusion of the terminology at this time. reactions for convenience. This situation appears in most naming. For example, although isotactic polymer is defined for an ideal (100%) isotactic REFERENCES AND NOTES polymer, we often use the term isotactic polymer for a polymer with a low isotacticity. Analogously, 1. Matyjaszewski, K.; Mu¨ ller, A. H. E. Am Chem Soc the term living polymerization may be conve- Dive Polym Chem Polym Preps 1997, 38(1), 6. niently used for a polymerization with a lower 2. Otsu, T.; Yoshida, M.; Tazaki, T. Makromol Chem, degree of livingness. However, the degree of liv- Rapid Commun 1982, 3, 133. LIVING OR CONTROLLED? 1743

Comments by tory role during the initial stages of the polymeriza- Professor Timothy E. Patten tion. It is also important to note that a true steady Department of Chemistry, University of state is not reached in these polymerizations and California, Davis, One Shields Avenue, that the concentrations of propagating and persis- Davis, California 95616-5295 tent radicals are not constant throughout the poly- merization. The concentration of propagating The authors ask whether we should broaden radicals will decrease with time, whereas the con- and apply the term living to nitroxyl-radical-me- centration of persistent radicals will increase diated polymerizations, atom transfer radical po- with time. Thus, irreversible termination occurs lymerization (ATRP), reversible addition frag- throughout the course of the polymerization. Al- mentation chain transfer, and other such classes though the extent of termination is small for a typ- of radical polymerizations in lieu of controlled (or ical free-radical polymerization technique exhibit- controlled/“living”). I think that the key question ing molecular weight control, it can have measur- underlying the discussion of this matter is, “Can a able impacts on the polymerizations. For example, polymerization be termed living if irreversible in ATRP, in which multiple chains are grown from termination is a fundamental component of the a single core,2 the small but finite probability that mechanism of polymerization?” two chain ends can terminate via coupling is mag- A living polymerization in the strictest sense of nified by the fact that a single macromolecule is the definition is an impossible ideal to achieve. The composed of many of these growing chains linked to intermediates used in such polymerizations are suf- the same core. In these polymerizations, coupling ficiently reactive or unstable that, given enough products can comprise a significant fraction of the time, irreversible termination or transfer will occur sample if the conversion of the polymerization is to some fraction of the active chain ends. Thus, sufficiently high. Thus, irreversible termination is although irreversible chain-breaking reactions are an integral part of the mechanism of these polymer- not part of the polymerization mechanism, their izations and can have a measurable impact on the occurrence is imminent. Nevertheless, experimen- resulting macromolecule. tal conditions usually can be found, such that dur- The term living polymerization is a construct ing the timescale of the polymerization, these chain- created and applied to the subset of polymeriza- breaking reactions are negligible. Such is the case tion methods that meet the abstract criterion of for living anionic, cationic, and transition-metal co- an absence of chain-breaking reactions via a set of ordination polymerizations. The situation with po- experimentally verifiable criteria. Expanding this lymerizations proceeding via free-radical interme- term to encompass experimental methods that diates that exhibit molecular weight control, how- are living during the timescale of the polymeriza- ever, is quite different. In these polymerizations, tion but may exhibit the inevitable irreversible irreversible termination is an integral part of the termination transfer at longer times because the mechanism of polymerization. end groups are not indefinitely stable, as pro- As discussed by Fischer,1 molecular weight con- posed by Szwarc,3 is a practical adjustment. For trol in these radical polymerizations is derived from polymerizations that exhibit molecular weight the persistent radical effect. In the initial stages of control but have irreversible termination as an the polymerization, the concentrations of transient integral part of the mechanism of polymerization, (propagating chains) and persistent radicals are another term really needs to be defined, such as formed at equal rates. Because the propagating rad- controlled.4 An advantage to using this new term icals can undergo fast termination via coupling, dis- is that it encompasses polymerizations that are proportionation, or both, their concentration de- demonstrably not living yet display molecular creases, and the concentration of the persistent rad- weight control under certain experimental condi- ical builds up. Eventually, the concentration of tions and can be useful for preparing block copol- persistent radicals will be sufficiently large that the ymers and other such macromolecules. rate at which the propagating radicals react with To conclude, is this discussion over living and the persistent radicals in a deactivation (or revers- controlled terminologies for polymerization meth- ible termination) step is much faster than the rate ods a matter of semantics, or does this distinction at which the propagating radicals react with one serve a useful purpose? I think that it makes a another in an irreversible termination step. Thus, useful distinction in terms of the polymerization irreversible termination plays an important regula- mechanism. Others may disagree. Either way, 1744 LIVING OR CONTROLLED? this debate should continue until a workable so- mers should be called living or controlled. The lution is agreed on for the benefit of everyone. authors strongly advocate the first term. I do not think this is correct. These novel radical polymer- izations involve two kinds of terminations, but REFERENCES AND NOTES only one is reversible; the other is irreversible. 1. Fischer, H. Macromolecules 1997, 30, 5666–5672. Thus, the process is inherently nonliving: active 2. Angot, S.; Murthy, S.; Taton, D.; Gnanou, Y. Mac- species disappear during the polymerization pro- ͗ ͘ romolecules 1998, 31, 7218–7225. cess. The higher the target Mn is, the higher the 3. Szwarc, M. J Polym Sci Part A: Polym Chem 1998, proportion of the irreversible processes is. There- 36, ix–xv. fore, until someone comes along with a real dis- 4. Matyjaszewski, K.; Muller, A. H. E. Polym Prepr covery that will allow sufficiently increasing the (Am Chem Soc Div Polym Chem) 1997, 38, 6–9. kp/kt/ktr ratio (no transfer and termination could be detected), these polymerizations should just be Comments by called controlled (if control is indeed achieved). Thus, for example, “Controlled ATRP of …”, Professor Stan Penczek “Persistent Radical Controlled Polymerization of Center of Molecular and Macromolecular …”, and “Nitroxyl-Radical Controlled …” are cor- Studies, Polish Academy of Sciences, rect titles. If control is attempted but not Sienkiewicza 112, 90-363 Lodz, achieved, the title should specify the method The terms living polymerization and controlled used. In this way, we may have “Controlled ATRP polymerization are being defined by the Nomen- of …” and “ATRP of …” clature Commission of the Macromolecular Divi- sion of IUPAC. At the time when the document Kinetic and Thermodynamics of Polymerization Comments by was mostly completed, this commission was Professor Nicolai A. Plate and chaired by Stepto (Hess is now its Chairman); Professor Arkady D. Litmanovich Marechal and I have been assigned to coordinate Russian Academy of Sciences, AV the work of the commission on this document TOPCHIEV, Petrochemical Synthesis (Baron, Hatada, Hess, Jenkins, Jones, Kahovec, Institute, Moscow 117912, Russia Kubisa, Wilks, and Vohlidal are involved). The entire document (94 entries), before becoming an First of all, we would like to note that the official IUPAC document, will be published as a authors of the article put forth the actual termi- proposition open for discussion. In this document, nological problem and considered it comprehen- the two terms discussed by Darling et al. have sively. Indeed, 4 decades ago, a sense of the term been defined as follows: Living polymerization: A living invoked by Szwarc1 was absolutely clear: chain polymerization from which chain transfer an active anionic end of the growing chain re- and chain termination are absent. Note that in mained alive any time provided the reaction me- some living polymerizations, reversible deactiva- dium was pure enough. As a consequence, it was tion of chain carriers may occur. Controlled poly- possible to carry out postpolymerization of new merization: A polymerization process allowing the portions of the monomer or to prepare a block preparation of macromolecules with a predeter- copolymer, both of a narrow molecular weight mined M , polydispersity index, and end groups. n distribution. Eventually, many types of polymer- Note the following: (1) Controlled polymerization ization processes were discovered for which no may but does not have to be living. (2) Controlled canonical termination step was observed and polymerization with a known transfer, termina- which were used to design a variety of macromo- tion step, or both should not be called living (e.g., a controlled radical polymerization involving ter- lecular structures. In particular, with free-radical mination: a stable radical modified or atom trans- living polymerization, it is possible to obtain not fer polymerization, a controlled cationic polymer- only linear block polymers and random copoly- ization involving transfer, etc.). mers but also structures such as hyperbranched polymers and hybrid dendritic–linear polymers.2 This article discusses the terminology problem, This situation raised a multiplicity of terms, as namely, whether recently developed radical poly- pointed out in the article. LIVING OR CONTROLLED? 1745

Problems of terminology are solved via agree- merizations have been refined and expanded to ment. We agree with the authors of the article include (1) the prediction and control of the num- that the term controlled is too broad. Also, we ber-average molecular weight, (2) the control of agree with the principle of naming reactions “on the molecular weight distribution when the rate the basis of their mechanism, not their yields.” of initiation is competitive with or faster than the The essence of the mechanism of the processes rate of propagation,5 (3) the control of branching under consideration is the absence of irreversible architecture by linking reactions, and (4) the con- termination of the growing chains. For such pro- trol of functional end groups by controlled termi- cesses, living appears the most relevant term. nation with functionalizing agents.3 Certainly, to describe any concrete reaction, Thus, the definition of living polymerization as one needs some additional definitions, say, living enunciated by Szwarc et al.1 is not an esoteric, ring-opening isomerization polymerization.3 It is unrealizable ideal. On the contrary, it is a funda- in the order of things. mental concept in polymer science that has spe- So let the term living polymerization remain cific, useful, general synthetic and mechanistic alive. consequences. For a living polymerization, it is possible to prepare polymers with predictable, well-defined structures and with low degrees of REFERENCES AND NOTES compositional heterogeneity. When termination and transfer occur, these reactions lead to compo- 1. Szwarc, M. Nature 1956, 178, 1168. sitional heterogeneity and a loss of the ability to 2. Malmstrom, E. E.; Hawker, C. Macromol Chem prepare polymers with well-defined structures. Phys 1998, 199, 923. A further attribute of the SLM definition of a 3. Miyamoto, M.; Watanabe, T.; Kimura, Y. Macromol living polymerization is that it can be applied, Chem Phys 1998, 199, 2237. and synthetic consequences can be realized, for chain polymerizations regardless of mechanistic type. Thus, in addition to the classic living anionic Comments by polymerization described by Szwarc and cowork- Professor Roderic P. Quirk ers,1–3 living polymerizations have been reported Maurice Morton Institute of Polymer for cationic,6 Ziegler–Natta,7 ring-opening met- Science, University of Akron, athesis,8 and radical9 chain-reaction polymeriza- Akron, Ohio 44325-3909 tions. For each of these mechanistic types, to the extent that the designation of living is correct, the The enunciation of the concept of living poly- synthetic consequences of a living polymerization merization by Szwarc, Levy, and Milkovich1 for the precision synthesis of well-defined poly- (SLM) in 1956 has had a profound influence on mers has been realized. These developments fur- polymer science in general and on synthetic poly- ther validate the correctness and usefulness of mer science specifically. They simply stated that a the original definition of living polymerization. living polymer is produced from a chain-reaction For example, for any living polymerization (re- polymerization that does not have a termination gardless of the nature of the chain-carrying spe- (or transfer) step.2 The importance of living poly- cies), it is possible to prepare polymers with con- merization is that it provides methodologies for trolled, well-defined structures and with low de- the synthesis of polymers with well-defined struc- grees of compositional heterogeneity. tures and low degrees of compositional heteroge- Darling et al. have proposed that “the meaning neity.3,4 of the term living polymerization be further re- In their pioneering article, Szwarc et al.1 rec- laxed and be defined as those processes that yield ognized and described some of the most important living polymers (or their dormant precursors) … synthetic consequences of a living polymerization; and that ”the term living polymerization should that is, chains grow until all of the monomer is be used regardless of the yield of the process.“ In consumed (except when near the ceiling temper- other words, these authors propose that if any ature), the subsequent addition of monomer in- living polymer chains are formed, this process creases the molecular weight (all chains continue should be defined as living. The authors go on to to grow),5 and block copolymers can be formed by state that ”it should, however, also be recognized sequential monomer addition. In the intervening that the various characteristics associated with years, the synthetic consequences of living poly- livingness … will be maximized when irreversible 1746 LIVING OR CONTROLLED? termination processes such as chain transfer or proposed trivializing of the term living as pro- self reaction … are negligible.“ Thus, these au- posed by Darling et al. thors propose to generalize and trivialize the def- 3. The authors incorrectly state that “the at- inition of living polymerization to include nonliv- tributes of living polymerization are well-known ing systems because it will now include systems and include molecular weight control, narrow in which transfer and termination occur (”regard- polydispersity, end-group control, and the ability less of the yield“) and for which the characteris- to chain extend.” tics associated with livingness will not necessar- Rebuttal: As delineated in numerous learned ily be a consequence of the process. discussions on the subject of living polymerization Polymer science, with its underlying theories and reviewed in depth by Henderson and and definitions, must be capable of change to ac- Szwarc10 in 1968, a narrow molecular weight dis- commodate new data and new ideas. However, tribution should not be used as a criterion for these changes should be based on careful, skepti- living polymerizations because it is neither a nec- cal analysis and the quest for clarity, correctness, essary nor sufficient condition to indicate whether and rationality. In this respect, it is important to a given polymerization is living.5 evaluate the rationale proposed by Darling et al. 4. The authors state that “a definition should to justify changing the definition of living poly- not be dependent on the limitations of current merization to include nonliving systems. technology (a detection limit) or be based on sub- 1. The authors state, “The terms controlled po- jective argument (an expectation).” They go on to lymerization, living, living/controlled, pseudo-liv- say that “one criterion that has been suggested is ing, living polymerization with reversible deacti- that the polydispersity of living polymerization vation, and others are scattered throughout the should be less than 1.1. The 1.1 limit was intro- literature. Lack of a common language creates duced when the determination of polydispersities confusion, wastes time and journal space, and has less than 1.1 was not possible by the analytical the potential to inhibit computational literature tools then available.” searching. Rebuttal: Once again it should be noted that Rebuttal: The fact that the term living is mis- narrow molecular weight distribution is not a used and that people propose ambiguous terms to generally useful criterion for a living polymeriza- describe systems with less than living character- tion (see the rebuttal to 3).5 Furthermore, the istics is not a compelling argument for abandon- description of a polydispersity less than 1.1 was ing the precise definition of living. Efficient and not proposed as a criterion of livingness by Fet- rapid searching of the chemical literature has ters11 but as a useful guide to distinguish narrow always been a challenge, but the proposal to in- distribution polymers; to wit, if Mw/Mn is greater clude nonliving systems under the umbrella of than 1.1, it should not be classified as narrow. living for expediency of literature searching is a This had nothing to do with the state of the art for questionable goal. the determination of narrow molecular weight 2. With respect to a recent proposal to use the distributions. term controlled radical polymerization, the au- 5.The authors state that “one of the basic prin- thors state that “it seeks to reserve the term liv- ciples of chemistry is to name reactions on the ing for an impossible ideal (immortality), thereby basis of their mechanisms, not their yields” and removing a valuable term from the vocabulary of “there is little precedent for assigning reaction the polymer scientist.” names on the basis of a perceived absence … of Rebuttal: The term living is not an impossible side reactions.” ideal. For example, in the alkyllithium-initiated Rebuttal: The term living polymerization has polymerization of styrenes and dienes in a hydro- very specific implications with respect to the carbon solution under high-vacuum conditions, it mechanism of polymerization. A mechanism is is possible to prepare polymers with predictable defined as “a detailed description of the path-

Mn’s, narrow Mw/Mn’s, block copolymers without way leading from the reactants to the products contamination for the first block segment, star- of a reaction, including as complete a character- branched polymers with well-defined arms, and ization as possible of the composition, struc- narrow molecular weight distributions and with ture, and other properties of the reaction inter- quantitative end-group functionality. Whether mediates and transition states.”12 The authors the term controlled is a useful term for certain are confusing named reactions (which usually systems is neither critical nor relevant to the have historical rather than mechanistic signif- LIVING OR CONTROLLED? 1747 icance) with mechanisms. The Diels–Alder re- 2. Szwarc, M. Nature 1956, 178, 1168. action is not a mechanism; the designation as a 3. Hsieh, H.; Quirk, R. P. Anionic Polymerization: 4ϩ2 cycloaddition reaction describes a mecha- Principles and Practical Applications; Marcel Dek- nism. Living polymerization has mechanistic ker: New York, 1996. significance because it describes the pathway 4. Webster, O. W. Science 1991, 251, 887. 5. Quirk, R. P.; Lee, B. Polym Int 1992, 27, 359. from reactants (initiator, monomer) to products 6. Cationic Polymerizations: Mechanisms, Synthesis, (living polymers that retain their active chain and Applications; Matyjaszewski, K., Ed.; Marcel ends) without the intervention of termination Dekker: New York, 1996. and transfer steps. As such, it does not describe 7. Doi, Y.; Keii, T. Adv Polym Sci 1986, 73/74, 201. the mechanism of the formation of terminated 8. Grubbs, R.; Risse, W.; Novak, B. M. Adv Polym Sci or transfer products. Thus, this distinction be- 1992, 102, 47. tween named reactions and mechanisms is con- 9. Controlled Radical Polymerization; Matyjaszewski, fusing and does not serve to enlighten the is- K., Ed.; ACS Symposium Series 685; American sues at hand. Chemical Society: Washington, DC, 1998. 6. The authors propose that “the meaning of 10. Henderson, J. F.; Szwarc, M. J Polym Sci Macromol the term living polymerization be relaxed and be Rev 1968, 3, 317. defined as those processes that yield living poly- 11. Fetters, L. J. In Encyclopedia of Polymer Science and Engineering; Kroschwitz, J. I., Ed.; Wiley In- mers (or their dormant precursors). The term liv- terscience: New York, 1987; Vol. 10; p 19. ing polymerization should be used regardless of 12. Glossary of Terms Used on Physical Organic the yield of the process.” Chemistry. Pure Appl Chem 1979, 51, 1725. Rebuttal: First, one cannot define living po- lymerizations by stating that they yield living polymers. This tautology does not address the Comments by question of what living means. If this relaxation Dr. Joseph C. Salamone of the term living were universally adopted, the Vice President, Chemical Research, term living polymerization would be trivialized Global Scientific Affairs, Baush & Lomb, and meaningless. What would be the value of 1400 North Goodman Street, this designation? It would not have mechanistic Rochester, New York 14609 or synthetic consequences that could be rigor- ously investigated, characterized, and under- Professor Emeritus, University of stood. Massachusetts, Lowell, Lowell, Darling et al. have not presented a convincing Massachusetts 01854 case for changing the meaning of the term living polymerization to include polymerizations in Difficulties in terminology have long been as- which nonliving polymer chains are formed. The sociated with the polymer field, and the topic un- designation as a living polymerization specifically der consideration, namely, how to classify a rad- refers to a chain-reaction polymerization proceed- ical polymerization reaction that can exhibit liv- ing in the absence of chain-termination and ing characteristics, is a continued manifestation chain-transfer reactions. This mechanistic desig- of our attempts to classify various polymerization nation has implicit kinetic and synthetic conse- reactions. Consider, for example, the difficulties quences that can be tested and verified experi- in polymer nomenclature. If IUPAC- or CAS- mentally. This definition has been in place for based nomenclatures were used, particularly for approximately 43 years and has served to codify complicated structures, few scientists would un- widely diverse processes and successfully stimu- derstand the polymer’s structure without the late research to find living processes involving structure being represented schematically. If this different types of chain-carrying reaction inter- is not difficult in itself, other polymer nomencla- mediates and design new well-defined molecular tures may use abbreviations or source-based architectures. names. Thus, the current state of polymer nomen- clature leaves much to be desired in understand- ing the structure of the polymer under consider- REFERENCES AND NOTES ation. Related circumstances are often present in 1. Szwarc, M.; Levy, M; Milkovich, R. J Am Chem Soc many polymerization reactions. In the area of 1956, 78, 2656. coordination polymerization, reactions are often 1748 LIVING OR CONTROLLED? initially categorized as one type of process, only to granted, overall I agree with the proposal by Dar- have this designation altered over time, as more ling et al. that the term living polymerization data become available. should be used without seemingly ambiguous While preparing the Polymeric Materials Ency- variants. For this uniform terminology to be prop- clopedia,1 I encountered the same difficulty dis- erly executed, of course, we must carefully cussed by Darling et al., namely, how to classify present solid and reproducible experimental re- radical polymerization that is considered living. sults that confirm the occurrence of such polymer- In general, the polymer community has a ten- izations, and my agreement is based on the as- dency to use the simplest terminology to describe sumption that, whenever we use the term living, a system, probably because of the complexities the reactions thus called are experimentally dem- involved in using precise designations. The origi- onstrated to clearly fulfill the definition of living nal concept of living polymerization was based on polymerization. no termination and that chain growth could con- Since the beginning of the 1980s,1 an increas- tinue when the initial monomer was exhausted if ing number of new polymerizations have been additional (polymerizable) monomer was added. reported that exhibit phenomenology that con- Other concepts, such as initiation and propaga- form with the definition of living polymerization. tion rates, kinetic and thermodynamic parame- A common characteristics of these, including our ters, and molecular weights and molecular weight cationic2 and metal-mediated radical3 living poly- distributions, were added to clarify the character- merizations, is that they involve dormant species, istics of living polymerization. covalent precursors that generate the true grow- However, times change, and all fields of science ing species via catalysis or a physical stimulus evolve, with the continued generation of new con- such as heat and light. Dormant species are in cepts. As scientists, we must be cognizant of these dynamic equilibria with the true growing end, changes, and, when appropriate, these changes and thus the formation of the latter from a dor- must be accommodated. For a polymerization re- mant end is reversible. The backward reaction, action where fresh monomer is added to a pre- that is, the generation of a dormant species from sumably dead polymer and the polymerization a growing end, corresponds to a termination re- continues, this reaction should be classified as a action in conventional chain-growth polymeriza- living polymerization. Ultimately, what is the dif- tions, and this seems to be the reason for the ference between a living ion-pair chain end and a spreading use of various terms related to living living, covalently bonded chain end? In this re- polymerizations. In my view, however, if the in- viewer’s opinion, reversible termination leading terconversion between dormant and true growing to continued chain growth in a radical polymer- species is reversible, the involvement of dormant ization should be classified as a living radical species does not deny the use of the term living polymerization. polymerization. Herein I would like to emphasize that to claim any living polymerization, we must present solid REFERENCES AND NOTES and reproducible experimental facts to confirm that it fulfills the definition of living polymeriza- tion. For this, the following set of results may be 1. Polymeric Materials Encyclopedia; Salamone, presented: J. C., Ed.; CRC: Boca Raton, FL, 1996.

1. A linear increase of the number-average mo-

Comments by lecular weights (Mn) in direct proportion to Professor Mitsuo Sawamoto monomer conversion or polymer yield. Department of Polymer Chemistry, 2. A further increase in Mn, again proportional Graduate School of Engineering, Kyoto to conversion or yield, when a fresh feed of University, Kyoto 606-8501, Japan monomer is added to a completely polymer- ized reaction mixture. As most of us understand, living polymeriza- 3. Molecular weight distributions (mostly size tion is defined as chain-growth polymerization exclusion chromatograms) before and after without termination, chain transfer, and other such sequential monomer addition, as in Re- chain-breaking reactions. With this definition sult 2, that shift to higher molecular LIVING OR CONTROLLED? 1749

weights without leaving any fractions indic- transfer, many terms have been proposed to de- ative of growth-inactive polymers. scribe various types of living polymerizations. Living polymerizations are methods of precise po- Results 1 and 2 demonstrate the absence of lymerizations in which some controversies are chain transfer (but not necessarily the absence of also involved, especially in the case of the recent termination), whereas Result 3 visually demon- living radical polymerization, which makes the strates the absence of both chain transfer and meaning of living polymerization vague. There- termination. The absence of termination may also fore, as the authors proposed, it seems that a be shown in other ways, such as linear first-order more uniform terminology is needed among poly- plots for monomer consumption and quantitative mer chemists regarding living polymerization. analysis of end groups by NMR. Therefore, I want to propose that the term liv- In any case, a single result usually cannot dem- ing polymerization be used when the polymeriza- onstrate unequivocally the absence of chain tion is characterized under conditions such as a transfer and termination, and we need these com- Ͻ narrow polydispersity (Mn/Mw 1.01), a molecu- bined sets of results for experimental verification lar weight control of more than three times, a of living polymerization. In particular, when cou- molecular weight of more than 10,000, and the pled with the molecular weight changes against formation of a block polymer together with a more conversion, molecular weight distribution data, if specific description of the relevant polymerization properly obtained, are quite informative in show- [e.g., nitroxide-mediated polymerization (NMP), ing the absence of chain-breaking reactions dur- atom transfer radical polymerization, and revers- ing the course of polymerizations. Unfortunately, ible addition fragmentation chain transfer there seems to be some confusion or misunder- (RAFT)], excluding the words controlled, control, standing in some of the published articles in quasi, and pseudo, regardless of the polymeriza- which, for example, very narrow molecular tion yield. weight distributions alone or molecular weight data without molecular weight distribution evi- dence are often regarded as sufficient to claim Comments by living polymerizations. As often pointed out, abuse of the term living should be avoided to keep Professor Teiji Tsuruta University of Tokyo, 1-1-1-609 Shimoda- the important research activity in this field ac- cho, Kohoku-ku, Yokohama 223-0064, cepted by the majority of scientists, particularly Japan those working outside the field of polymer chem- istry. I basically agree, with some reservations, with Darling et al. that the meaning of the term living REFERENCES AND NOTES polymerization should be further relaxed and de- fined as those processes that yield living polymers 1. Webster, O. W. Science 1991, 251, 887. (or their dormant precursors). 2. Sawamoto, M. Prog Polym Sci 1991, 16, 111. Darling et al.’s definition of living polymeriza- 3. Sawamoto, M.; Kamigaito, M. CHEMTECH 1999, tion has the same logical structure as that of 29, 30. stereospecific polymerization. An IUPAC nomen- clature document states that “stereospecific poly- merization” is “polymerization in which a tactic Comments by polymer formed.1” Here, it is to be noted that the Professor Masayoshi Tabata IUPAC document defines tactic polymer indepen- Department of Molecular Chemistry, dently, and then defines stereospecific polymeriza- Graduate School of Engineering, tion by using the term tactic polymer to avoid Hakkaido University, Sapporo 060-8628, circular logic. Japan Darling et al. stated that living polymers are distinguished from dead polymers, being “able to Since Szwarc discovered the living anionic po- grow whenever additional monomer is supplied.” lymerization of styrene and suggested that living However, this is not enough because by consensus polymers grow to a desired maximum associated a living polymer is formed by chain polymeriza- with a negligible degree of termination or chain tion.2 Darling et al. must strictly define living 1750 LIVING OR CONTROLLED? polymer first, then define living polymerization by recommendation. Collapsing language describing using the term living polymer. fundamentally different processes into a single The comparative discussion of living polymer- term, living polymerization, is an oversimplifica- ization and Diels–Alder reactions cannot be a ra- tion and, worse, a distortion of the concepts cur- tionale for Darling et al.’s proposal. In this dis- rently denoted by the word living. cussion, we have two contrasting reactions: living The concept of living polymerization already and nonliving polymerizations. For example, if has a simple, well-established meaning. It is an 20% of a polymerization process takes place un- ideal and not a mechanism. Like the abstract der a living mechanism, the whole polymerization concepts of high yield, selective, rapid, and simple, cannot be called living polymerization because it is subjective but measurable. Many different 80% of the polymerization was a nonliving poly- polymerizations can be made to function as living, merization. The same logic may be applied to, for with significantly different mechanisms even example, Markovnikov and anti-Markovnikov ad- within broad categories such as anionic, cationic, dition reactions. A reaction which obeys Mark- group transfer, and radical. Livingness denotes ovnikov’s rule 20% cannot be called a Markovni- the fraction of polymer chains that grow, without kov-type addition reaction because an anti-Mark- termination, transfer, or other side reactions, ovnikov-type reaction obviously predominates. from the beginning to the end of a polymerization The applicability of the term living polymeriza- process. A polymerization is practically living tion to the whole polymerization system depends when a sufficient fraction of chains needed to on the living/nonliving ratio in the relevant poly- impart the desired polymer properties survive merization system, but I do not think it wise to while growing to the target molecular weight of consider “threshold numerals for living/nonliving the process, all of which are subjective criteria. ratio” for qualifying the polymerization as living. Also, Darling et al. are inconsistent in arguing Every experienced polymer scientists (see the that “one of the basic principles of chemistry is to preamble of ref. 2) should know the ratio. How- name reactions on the basis of their mechanisms” ever, it is safe to say, for instance, that living while recommending that many mechanisms be polymerization takes place to an extent of 20% in combined under the name living. To call a process this reaction system. living based on mechanism, “whether the yield is 99.99% or 50%, or even 20%” as the authors sug- gest, is like referring to a low-yield high-yield REFERENCES AND NOTES reaction, defined as high-yield by its mechanism. Most scientists already apply Occam’s razor 1. IUPAC. Basic Definitions of Terms Relating to naturally and do not create terms unnecessarily. Polymers. Pure Appl Chem 1974, 40, 477. The existing terminology related to controlled po- 2. IUPAC. Glossary of Basic Terms in Polymer Sci- lymerization processes is evidence of a rich and ence. Pure Appl Chem 1996, 68, 2287. varied scientific landscape. Communication in this field will not be improved by the oversimpli- fication or broadening of the definition of living Comments by Howard W. Turner, until it is effectively meaningless. Ralph B. Nielsen, Adam L. Safir, Gerrit Klaerner, and Christopher Goh Comments by Symyx Technologies Inc., 3100 Central Professor Robert M. Waymouth Expressway, Santa Clara, California Department of Chemistry, Stanford 95051 University, Stanford, California 94305-5080 The article by Darling et al. recommends the uniform application of the term living polymeriza- The article by Darling et al provides a useful tion to a very broad range of radical polymeriza- commentary on the nomenclature of living poly- tion processes and mechanisms, regardless of the merizations, particularly as it applies to free-rad- mechanistic details or the yield of living chains ical polymerizations. There is little substantive from a particular process. Although we appreciate added here beyond what Quirk1 argued persua- the authors’ concerns, we do not agree with their sively in his discussion of the experimental crite- LIVING OR CONTROLLED? 1751 ria for living polymerization. I would find this a the term living polymerization. It is agreed that useful comment to stimulate dialogue to come to the diversity of nomenclature, ranging from pseu- some consensus on a consistent terminology for do-living to controlled polymerization, is less than these classes of polymerization reactions. I found ideal for those examples that do not meet the the arguments reasoned and the authors’ position basic criteria established for living polymeriza- defensible. tion and that uniformity is needed. However, The editors experiment to provide a series of broadening the term living to all conditions that commentaries on various sides of the issues is a lead to some fraction of chains exhibiting living quite reasonable one. character is not an acceptable solution. The statement buried within ref. 7 of the arti- cle, suggesting that the polydispersity criterion be REFERENCES AND NOTES reduced to less than 1.01, is extreme and likely beyond the human error associated with the ex- 1. Quirk, R. P.; Lee, B. Polym Int 1992, 27, 359. perimental preparation of the polymers. This statement is made to demonstrate that the crite- ria for the evaluation of livingness should not be Comments by Dr. Owen Webster dependent on the detection limits of current tech- Central Research & Development, E. I. nology; however, a rationale based on molecular du Pont de Nemours & Company, weight dispersity is overemphasized throughout DuPont Experimental Station, the article. Moreover, a single criterion cannot be Wilmington, Delaware 19898 the basis for the determination of a living poly- merization process, and there is no reason to ne- I agree with the overall suggestion that the term glect or abandon the well-established identifying living with respect to polymerization processes in- properties, including molecular weight control clude all methods in which, after monomer deple- and narrow molecular weight distribution result- tion, the addition of new monomer starts chain ing from initiation rates that far exceed propaga- growth again. These systems allow one to make tion rates, a linear increase in the degree of poly- block and star polymers and to control end function- merization as a function of monomer conversion, ality and molecular weight. By and large, the ma- and the remaining reactivity of the chain end(s) jority of workers in the field use the term living in following monomer consumption because of the this respect when communicating orally. The writ- limitation of irreversible termination reactions, ten language should follow suit. which allows for end-group compositional control Along with the term living, we will still need and the ability to chain-extend for the production other modifiers, such as living free-radical, anionic, of block copolymers or other macromolecular ar- and cationic polymerization. To narrow down the chitectures. The most interesting and useful ap- field further, terms such as stable free radical poly- plications of living polymerizations do not rely merization (SFRP), atom transfer radical polymer- solely on the narrow molecular weight distribu- ization (ATRP), reversible addition fragmentation tion that is the focus of the article (e.g., ref. 7). In chain transfer (RAFT), and group transfer polymer- addition, narrow molecular weight distributions ization (GTP) should be used. are not necessarily limited to true living polymer- In general, languages evolve by usage, and at- izations that yield chains with ends of specific and tempts at control will be short-lived. equal reactivity, as its value primarily relies on the relative kinetics of the initiation, propagation, ӷ and irreversible termination reactions (ki kp ӷ Comments by kt). A situation can be imagined in which side Professor Karen L. Wooley reactions leading to irreversible chain termina- Department of Chemistry, Washington tion occur on a timescale preventing convenient University, One Brookings Drive, St. postpolymerization chain-end manipulation, even Louis, Missouri 63130-4899 though the polymer chain growth appears from molecular weight and molecular weight distribu- The confusion that has arisen from the vari- tion control to result from a living process. Fur- able terminology found for radical polymeriza- thermore, ref. 3 of the article states that slow tions that approach or exhibit living character is initiation does not by itself cause chain death— the basis for the authors’ proposed relaxation of this is obvious. Although slow initiation relative 1752 LIVING OR CONTROLLED? to propagation will lead to an increase in the merization that is actually nonliving. I suggest molecular weight distribution, the yield of living using this word more correctly: polymer chain ends can be quantitative if the rate of propagation is still much faster than the rates 1. Because living polymerization means the for irreversible termination reactions. The discussion regarding named reactions in or- lack of termination and chain-transfer re- ganic chemistry is presented in support of the argu- actions, the conversion or polymer yield ment that “a new name should not be defined when should be more than 98% in short or long it is realized that some side reaction also occurs.” polymerization times. Although this is a valid point, especially with re- 2. The molecular weight distribution (Mw/Mn) spect to technique-specific nomenclature, the term should be less than 1.05 or at least less living indicates that side reactions are limited. than 1.15 when Mn (not Mw) is greater The article presents a problem in terminology, than 10,000. When Mw/Mn is greater than but it does not offer a viable solution. The proposed 1.15, the word pseudo-living or controlled solution is to use the term living polymerization for polymerization should be used. When Mn is all processes that yield living polymers (or their greater than 100,000, Mw/Mn should be dormant precursors) regardless of the yields of the less than 1.25. processes. This will most certainly lead to a broad 3. Because living polymerization means no use of the term living polymerization beyond those termination or no chain transfer, Mn cases that actually exhibit living polymerization be- should increase to more than 100,000. Oli- havior, which will become a nuisance for those re- gomerization exhibiting an Mn of less than searchers who are in search of chemistries that can 10,000 with a rather small polydispersity efficiently generate polymer chains containing should be called controlled or pseudo living chain ends that are accessible for clean and com- polymerization. plete employment in the construction of more elab- 4. Living polymerization indicates that M orate compositions and structures. It is not suffi- n should increase linearly with conversion. cient to recognize that “the various characteristics This should be confirmed experimentally. associated with livingness … will be maximized 5. The actual meaning of living polymeriza- when irreversible termination processes … are neg- tion lies in the possibility of performing AB ligible,” while still assigning the term living poly- merization regardless of the yield of the process. diblock copolymerization or ABA or ABC Perhaps a better solution would be settlement on a triblock copolymerization. In these cases, single term, such as pseudo-living or quasi-living, the catalytic efficiency or initiator effi- for those polymerizations that give either less than ciency should be greater than 75%. We optimal yields of polymer chains with remaining have realized greater than 80% catalytic chain-end reactivity or that give polymers with less efficiency in the rare-earth-metal-initiated than ideal control over molecular weight properties. polymerization of methyl methacrylate (MMA)1 and block copolymerization.2 Comments by Professor Hajime Yasuda Department of Applied Chemistry, REFERENCES AND NOTES Faculty of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, 1. Yasuda, H.; Yamamoto, H.; Yamashita, M.; Yokota, Japan K.; Nakamura, A.; Miyake, S.; Kai, Y.; Kanehisa, N. Macromolecules 1993, 26, 7134. The term living polymerization is used widely. 2. Ihara, E.; Morimoto, M.; Yasuda, H. Macromole- However, the use of this term has spread to poly- cules 1995, 28, 7886.