Macromolecular Nomenclature Note No. 19

[The Nomenclature Committee of the ACS Division of Polymer Chemistry (E. S. Wilks, chairman) presents a guest contribution.]

Béla Iván*

In this Macromolecular Nomenclature Note an attempt will be made to clarify the currently existing controversy, diversity, and uncertainties related to the terminology for addition polymerizations in the absence of irreversible chain breaking events, and terminology for these polymerizations will be suggested on the basis of the elementary polymerization reactions taking place. Conventional chain addition polymerizations involve four elementary reactions that occur simultaneously in most cases: initiation, propagation, chain transfer, and termination. The last two, i.e. chain transfer and termination, are also called chain breaking processes in line with the nomenclature of chain reactions. Anionic polymerizations in the absence of chain breaking reactions were first described by Ziegler¹ and Abkin and Medvedev² in the 1930s. In 1956, Szwarc³ named the macromolecules formed in these processes as ‘living’ polymers (within quotation marks). Later, the quotation marks were abandoned, and anionic polymerizations in the absence of elementary chain breaking reactions, such as termination and chain transfer, were called living polymerizations. The definition and description of ‘living’ polymers emphasized that the "polymerization process does not involve a termination step", the "polymeric molecules ‘live’ for an indefinite period of time", and "the living ends are potentially able to grow" if monomer is available.³ For later considerations in this note it should be mentioned that this definition of ‘living’ polymers excludes macromolecules unable to propagate (grow) in a polymerization system. These are obviously nonliving (nonpropagating) polymers.

Although Morton⁴ found the living name a "somewhat euphemistic term" it has been widely used in the literature in the last four decades. In his book,⁴ Morton used the term "nonterminating chain addition polymerization" for anionic polymerization in the absence of chain-breaking reactions.

In spite of significant efforts, polymerizations of vinyl monomers with other mechanisms, e.g. carbocationic, radical, Ziegler-Natta polymerizations, that would afford the same synthetic advantages as terminationless and transferless anionic polymerizations were not discovered until about a quarter of a century after the Szwarc article on ‘living’ polymers. The first breakthroughs came with two discoveries in the early 1980s. Faust, Fehérvári, and Kennedy⁵ found that carbocationic polymerization of a-methylstyrene with reversible termination and chain transfer, i.e. equilibrium between propagating chains and nonpropagating (nonliving) macromolecules, provides linear relationship between M_n and conversion to a certain extent as expected for anionic polymerizations in the absence of termination and chain transfer. According to Szwarc’s definition, propagating chains can be called living polymers although these macromolecules do not ‘live’ for an indefinite period of time, but these chains are able to react with monomer, and the polymer molecules grow as a consequence of this process. Polymerizations with equilibrium between propagating (living) and nonpropagating (nonliving) polymer chains were defined as "quasiliving polymerizations".^5,6 Kennedy, Kelen, and Tüdős⁶ analyzed and classified these polymerizations in detail with respect to the theoretical possibilities for reversible chain breaking reactions. Later, their definition and classification were revised and modified by Kennedy and Iván.⁷ The second important event was the discovery of group transfer polymerization of methyl methacrylate by Webster and coworkers.⁸ Although the stoichiometry clearly indicated the occurrence of equilibrium between propagating and nonpropagating chains in these polymerizations, this was not considered in this study.⁸ These findings were followed by discovery of other polymerizations without irreversible chain breaking reactions but with equilibrium between propagating (living) and nonpropagating (nonliving) polymer chains via either reversible termination, for instance, carbocationic,^7,9-12 ring-opening metathesis,^13-15 and radical^16-22 polymerizations, or by reversible chain transfer.²³

Much confusion has arisen with respect to the terminology by the appearance of addition polymerization reactions with equilibria between propagating (living) and nonpropagating (nonliving) polymer chains, and by the limits of these new polymerizations in the scientific literature worldwide. Many scientists have tried to present their results as polymerization systems that match the characteristics of perfect (or idealized) ‘living’ anionic polymerizations. Although the general classification of polymerizations with reversible chain breaking reactions (termination and chain transfer) as quasiliving polymerizations was published in the early 1980s,⁶ and revised a decade later,⁷ numerous authors described their specific polymerizations with different terms, such as truly living,^24-26 true living system,²⁷ pseudoliving,^28-30 immortal,^23,31 apparently living,³² livingness enhancement,³³ enhanced livingness,^34,35 living polymerization with reversible termination (or chain transfer),^36,37 and recently controlled/"living" ("living" with quotation marks) (see e.g. Refs. 21,38) or "living" (e.g. Refs. 38-40). This diversity in terminology might indicate that a variety of fundamentally different ‘living’ polymerizations exist, and therefore a unified general description, terminology, and classification cannot be provided for these polymerization systems. The broad selection of terms also reflects the dilemma of the scientific community on how to express the differences between the idealized ‘living’ anionic polymerization and the recently discovered polymerization systems with equilibria between propagating (living) and nonpropagating (nonliving) macromolecules. Another problem is how to treat the apparent irreversible decomposition of the propagating chain ends after a certain time (mainly at complete monomer conversion) in several cases. In two comprehensive studies^41,42 by Iván, the general role and the importance of the equilibria between propagating (living) and nonpropagating (nonliving) polymer chains and the kinetic consequences were analyzed, and distinction between ideal living polymerizations and quasiliving polymerizations was proposed. Another major problem arises by the exactly same overall kinetic characteristics for both the idealized ‘living’ polymerization and polymerizations with equilibrium reactions between living (propagating) and nonliving (nonpropagating) macromolecules when the rate of initiation is equal to or faster than the rate of propagation^41,42 (linear M_n versus monomer conversion plot and first order monomer consumption). Recent attempts by forcing the original ‘living’ terminology under the controlled/"living" term (see e.g. Refs. 38-40) have not solved these fundamental problems. It is quite obvious and logical that the rational correct scientific terminology of chemical processes should be based on the underlying chemical reactions and mechanisms, not on the kinetic consequences and characteristics (even worse, on certain subjective expectations and/or marketing goals). Therefore, distinguishing between ideal living polymerizations and quasiliving polymerizations is necessary. As will be shown in the next section, this allows us to express and describe precisely all the major characteristics of these polymerization systems discovered earlier and in recent years.

As already indicated in the previous section, the large majority of polymerization systems that currently offer synthetic routes for obtaining structurally well-defined polymers with predetermined molecular weight and relatively narrow molecular weight distribution in most cases involve the following elementary reactions: initiation, propagation, and equilibrium reactions (reversible termination and/or chain transfer) between propagating (living) and nonpropagating (nonliving) polymer chains. In ‘living’ anionic polymerizations in the absence of nonpropagating ionic aggregates, only initiation and propagation occur. In other words, from a mechanistic point of view, two kinds of polymerizations exist in the absence of irreversible chain breaking reactions: polymerizations with or without equilibrium reactions between propagating and nonpropagating polymer chains. Let us briefly summarize the major polymerization systems belonging to these types of general mechanisms.

In ‘living’ anionic polymerization only two elementary reactions take place: initiation and subsequent propagation. In other words, the rates of the other elementary processes of this chain reaction, i.e. termination and chain transfer, are zero. Although this polymerization process is quite rare, it can be accomplished for certain monomers, e.g. styrene, dienes and (meth)acrylates, in the presence of polar solvents or certain cation complexing or other additives.

In many cases, equilibrium exists between propagating polymeric anions and nonpropagating (or relatively unreactive) ionic aggregates (see e.g. Ref. 37 and references therein) in a large variety of terminationless and transferless anionic polymerizations. Thus the Coulombic interaction and the solvating power of the reaction media play important roles in these polymerizations, but new chemical bonds are not formed in these processes. The aggregation-dissociation equilibrium and the propagation in this polymerization are shown in Scheme 1, in which P, M, and Mt represent a polymer chain, monomer and metal, respectively.

Although the detailed mechanism of group transfer polymerization^8,26 is still not clarified (see e.g. Ref. 26 and references therein), there is no doubt that these polymerizations involve equilibrium between propagating and nonpropagating chains. Group transfer polymerization of methyl methacrylate and other monomers are usually initiated by silyl ketene acetals in the presence of nucleophilic anions or Lewis acid catalysts.^8,26 The existence of equilibrium between propagating (living) and nonpropagating (nonliving) polymer chains is obvious on the basis of the initiator/catalyst concentration ratio (it is usually much higher than one). The "group transfer" terminology reflects the propagation mechanism. This mechanism is shown in Scheme 2.

Scheme 3 shows the fundamental processes of carbocationic polymerizations with reversible termination in the absence of chain transfer.⁷ As shown in this Scheme, active polymer chains with carbocationic propagating species (PÅ ) can either participate in propagation with the monomer (M) or in ion collapse with the gegenion to yield inactive, terminated (halogenated) macromolecules and the corresponding Lewis acid. The reverse process, reinitiation between the inactive (P-X) chains and the Lewis acid (MtX_n), leads to the active carbocationic species.

It has been experimentally proved that ionic and covalent species coexist in ring-opening cationic polymerization of cyclic monomers, such as tetrahydrofuran (THF) (see Ref. 25 and references therein). In such equilibria, the onium ions in conjunction with counterions are the propagating species while chains with terminated (covalent) structures (P-X-Y) are nonpropagating (nonliving) polymers, as shown in Scheme 4.

In free radical polymerizations, there are two major processes based on equilibria between propagating (living) and nonpropagating (nonliving) polymer chains. Polymerizations based on so-called iniferters¹⁶ and stable radicals^17-19 involve reversible dissociation leading to propagating carbon-centered radicals and to terminated chain ends in the reversible recombination (termination). This is shown in Scheme 5. In the other method, propagating (living) radicals are

20-22

Ring-opening metathesis polymerization (ROMP) of cyclic olefins occurs by a unique cyclic process involving terminated, nonpropagating (nonliving), polymerization-inactive metallocyclic and propagating (living) metallocarbene species.^13-15 The ring-opening process and the propagation by living metallocarbenes are shown in Scheme 7.

Polymerization of a variety of epoxides and lactones induced by aluminum porphyrins in the presence of protic chain transfer agents (alcohols, acids) can occur in the absence of irreversible chain breaking reactions.²³ In these polymerization systems, reversible chain transfer exists between propagating porphinatoaluminum alkoxide chain ends and nonpropagating polymers with hydroxyl termini, as shown in Scheme 8. These polymerizations with equilibrium between propagating (living) and nonpropagating (nonliving) macromolecules via chain transfer reactions were named "immortal" polymerizations.²³

These examples clearly show that a large majority of polymerizations in the absence of irreversible chain breaking processes involve in general the following elementary reactions: initiation, followed by propagation by living (propagating) polymers; and equilibrium reactions between propagating (living) and nonpropagating (nonliving) polymer chains. Accepting the definition of a living polymer as a polymer chain that is able to grow if monomer is available,³ living (propagating) and nonliving (nonpropagating) chains coexist in the polymerizations shown in Schemes 1-8. Consequently, these polymerizations cannot be called simply living polymerizations. This does not allow for that fact that in addition to propagation, substantially important equilibrium reactions occur, and that polymerization systems simultaneously contain living (propagating) polymers and nonliving (nonpropagating) chains that are unable to react with the monomer. (Usually the concentration of nonliving chains is higher by orders of magnitude than that of the living (propagating) chains in these polymerizations.)

Readers may notice that until now only elementary polymerization reactions have been mentioned in this section. However, at this point the question arises of what to call polymerizations proceeding in the presence of propagating (living) and nonpropagating (nonliving) polymer molecules in equilibrium with each other. On

historical grounds, these polymerizations can be called quasiliving polymerizations.⁶ The pseudoliving term might be also considered. The quasiliving nomenclature has been adopted and used in the latest edition of a textbook by Odian.⁴³ In their excellent book on radical polymerizations, Moad and Solomon⁴⁴ refer to these type of polymerizations as "quasi- or pseudo-living radical polymerizations", and several other authors also use the term quasiliving in recent publications (see e.g. Refs 45,46).

On the basis of the common mechanistic elementary steps, the following rational definitions can be suggested: ideal living polymerizations involve initiation followed by propagation with no chain-breaking reactions occurring; quasiliving polymerizations proceed by initiation followed by propagation, and the propagating (living) species are in equilibrium with nonpropagating (nonliving) polymer chains in the absence of irreversible chain-breaking reactions. In other words, quasiliving polymerizations take place with the coexistence of propagating (living) and nonpropagating (nonliving) polymers in equilibrium with each other. This means that in addition to initiation and propagation, the equilibrium reactions and at least two kinds of polymeric species, propagating (living) and nonpropagating (nonliving), should be considered in quasiliving polymerization processes. The equilibrium reactions between propagating (living) and nonpropagating (nonliving) polymer chains are also known as quasiliving equilibria.

Simple reaction models for ideal living polymerization and quasiliving polymerizations are shown in Schemes 9 and 10, respectively. The model of ideal living polymerization also represents the model of ideal chain reactions.

If P* represents the propagating species and [N_p] is the total concentrations of polymer molecules in the polymerization systems, then the rates of polymerization for ideal living (R_p,IL) and quasiliving polymerizations (R_p,QLP) are as follows:

R_p,IL = k_p [P*] [M] = k_p [N_p] [M]

R_p,QLP = k_p [P*] [M] = k_p [N_p] [M] / (1 + K)

where k_p, K, and [M] are the rate constant of propagation, the equilibrium constant of the quasiliving equilibrium, and the concentration of monomer, respectively. Comparing Schemes 9 and 10 and these two equations, it can be concluded that the ideal living polymerization can be derived from the more general quasiliving polymerizations when the equilibrium constant (K) of quasiliving polymerizations becomes zero (see also Refs. 41,42).

In quasiliving polymerizations the major mechanistic steps are initiation, propagation, and the equilibrium reactions. Polymerizations are usually distinguished by the chemical nature of the propagating species, such as free radicals, anions, cations, and coordinative complexes. Quasiliving polymerizations with either of the propagation mechanisms can be also classified according to the mechanism of the underlying quasiliving equilibria. Such classification can precisely express the fundamental elementary process involved in the equilibrium between propagating (living) and nonpropagating (nonliving) species. Quasiliving equlibria can occur by reversible aggregation, termination, and chain transfer. Thus, as shown in Scheme 11, the following classes of quasiliving polymerizations (QLP) can be distinguished:

aggregative QLP,

terminative QLP,

transferative QLP,

ideal living polymerization (or equilibriumless QLP).

Ideal living polymerization can be considered as a special subclass of quasiliving polymerizations taking place in the absence of nonpropagating (nonliving) chains (K=0) and any chain breaking events, i.e. only initiation and propagation, are involved.^41,42 In terminative QLPs the active chains undergo termination and the terminated chains are reinitiated in a reversible process. Quasiliving free radical, cationic, group transfer, ring-opening metathesis, and coordinative polymerizations are terminative QLPs. The so-called "immortal" polymerization³¹ of cyclic monomers catalyzed by metal complexes in the presence of chain transfer agents is transferative quasiliving polymerization in which the equilibrium occurs by reversible chain transfer. Direct activity exchange between polymer chains that is assumed to take place in group transfer polymerizations⁴⁷ fits also into the class of transferative QLPs.

It has to be noted that Quirk and Lee³⁶ proposed an alternative possibility for expressing quasiliving polymerizations (QLP) by using the old ‘living’ terminology together with describing the underlying mechanism of the quasiliving equilibria. For example, terminative QLP and transferative QLP according to this terminology would be referred to as "living polymerization with reversible termination" and "living polymerization with reversible chain transfer", respectively.^36,37 Although these expressions might be considered as possibilities for naming these polymerizations, these are quite long terms, and sound more like definitions than names for chemical phenomena. In contrast, the importance of the underlying mechanism is precisely described by the quasiliving term together with the mechanistic processes which are involved in the quasiliving equilibrium, as presented in Scheme 11.

It may have been realized that until now the "dormant" term has not been used in this discussion. Nonpropagating (nonliving) aggregates and complexes in anionic polymerizations were called "dormant" species (or "dormant" polymer chains).^48,49 This terminology expresses the fact that the chains in the nonpropagating (nonliving) aggregates have the same negative charge as the propagating ones, i.e. the chemical nature (structure) of the chains in the species inactive toward monomer addition (propagation) is similar (ionic) to the propagating ones, electron transfer does not occur, and no new chemical bonds (e.g. covalent bonds) are formed in the equilibrium process between propagating (living) chains and polymers incorporated into the nonpropagating (nonliving) aggregates or complexes. In other words, the nonpropagating (nonliving) polymers were considered as ‘living’ but "dormant" polymer chains. However, new chemical bonds and species are formed in the quasiliving equilibria in the course of quasiliving polymerizations with reversible termination or chain transfer, such as quasiliving carbocationic, free radical, group transfer, ring-opening metathesis, and transition metal complex mediated ring-opening polymerizations. Therefore, the "dormant" term is incorrect, misleading, and obsolete for polymerizations taking place with simultaneous equilibrium reactions between propagating (living) and nonpropagating (nonliving) polymer chains. The chemistry involved in quasiliving equilibria is more accurately expressed by the propagating (or living) and nonpropagating (or nonliving) terms.

The terms "active", "inactive", "activation", and "deactivation" are also used in the literature. Since the reactions involved in quasiliving polymerizations can be precisely expressed by elementary polymerization and general chemistry reactions, use of these expressions is not recommended.

The term "controlled" polymerization has been recently used for a variety of processes. Matyjaszewski and Müller³⁸ tried to define the term "controlled" on the basis of certain kinetic and synthetic aspects of polymerizations. Unfortunately, the term "controlled" is not well-defined at all. This provides opportunities for individual definitions and use of the expression controlled polymerizations. Thus, in the definition of "controlled" polymerizations it is stressed that slow initiation and slow equilibrium reactions in quasiliving polymerizations are not considered to be controlled. However, slow initiation can also lead to well-defined macromolecular architectures. The same also holds for quasiliving polymerizations with relatively slow equilibrium reactions. Another debatable requirement for "controlled" polymerization is narrow molecular weight distribution (MWD). However, it is known that for several processing methods of polymers with well-defined structures, broad rather than narrow Poisson MWD would be preferred. Therefore, control of MWD over a wider range would be a desired requirement from certain practical points of view. This might obviously lead to self-contradiction if the "controlled" term were used only for a narrow range of polymerization/polymer characteristics from a narrow point of view.

The suggested term "controlled" does not also involve control of several important structural aspects, such as stereochemical microstructure and control of branching, which are very important not only from the synthetic point of view, but also in many applications. Recent examples of such controlled polymerizations are summarized by Mülhaupt and coworkers.⁴⁸ This study is also an indication that the terminology of "controlled" polymerization would require further clarification.

Exact definitions of characteristics considered for using the expression "controlled" should be given in scientific publications. Another important matter is analyzing precisely and providing solid data for the degree or extent of control for given factors such as kinetic, structural, property, etc. Otherwise, the meaning of the term "controlled" remains questionable.

Helpful and stimulating discussions on the subject of this note with T. Erdey-Grúz, T. Fónagy, P. Werner Groh, I. Szanka, and T. Szakács are gratefully acknowledged.

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*Address: Department of Polymer Chemistry and Material Science, Institute of Chemistry, Chemical Research Center, Hungarian Academy of Sciences, H-1525 Budapest, Pusztaszeri u. 59-67, P. O. Box 17, Hungary

E-mail: bi@cric.chemres.hu