Quo Vadis Carbanionic Polymerization?

Abstract

Living anionic polymerization will soon celebrate 70 years of existence. This living polymerization is considered the mother of all living and controlled/living polymerizations since it paved the way for their discovery. It provides methodologies for synthesizing polymers with absolute control of the essential parameters that affect polymer properties, including molecular weight, molecular weight distribution, composition and microstructure, chain-end/in-chain functionality, and architecture. This precise control of living anionic polymerization generated tremendous fundamental and industrial research activities, developing numerous important commodity and specialty polymers. In this Perspective, we present the high importance of living anionic polymerization of vinyl monomers by providing some examples of its significant achievements, presenting its current status, giving several insights into where it is going (Quo Vadis) and what the future holds for this powerful synthetic method. Furthermore, we attempt to explore its advantages and disadvantages compared to controlled/living radical polymerizations, the main competitors of living carbanionic polymerization.

Introduction

Polymer science recently celebrated 100 years of existence since Staudinger published the first article on polymerization¹ (1920), indicating that polymers consist of long chains of covalently linked monomeric units. The term “macromolecule” was introduced in a subsequent article by Staudinger in 1922,² referring to polyisoprene.³ These groundbreaking articles initiated the field of macromolecular science, leading to significant developments and many commercial products.^3,4

The first synthetic polymers were prepared by free-radical polymerization of vinyl monomers or by condensation of difunctional molecules.⁵ These polymers showed a wide range of beneficial physical properties such as elasticity, hardness, toughness, and strength. However, these materials were not comparable to natural polymers in terms of properties.⁵ The natural polymers are generally prepared by adding monomeric units one at a time to the growing chain-ends (condensation polymers), resulting in polymer chains with identical molecular weights. Conventional chain-growth polymerizations, such as free-radical polymerization, led to polymers with high dispersity due to chain transfer and termination reactions. Soon, it was realized that new synthetic methods should be established to approach nature’s perfect macromolecules.

First, Ziegler⁶ reported the anionic polymerization of 1,3-butadiene by organometallic initiators in hydrocarbon solvents without exhibiting any chain transfer or termination reactions. At the same time, Medvedev⁷ investigated the kinetics of the anionic polymerization of 1,3-butadiene. In 1940, Flory⁸ described living polymerization without using the term “living”, referring to the case of ethylene oxide. He reported that narrow molecular weight distributions are expected for polymerizations where the initiation rate is comparable to the propagation rate, and the total number of propagating chains does not change throughout the polymerization. However, a significant discovery in macromolecular science was reported in 1956, where Szwarc and co-workers^9,10 coined the term “living polymerization” by studying the mechanism of the anionic polymerization of styrene using sodium naphthalene as the initiator in tetrahydrofuran (THF). This polymerization proceeds without termination or chain transfer reactions when impurities such as oxygen, moisture, or carbon dioxide are excluded. Figure 1 illustrates the “primitive” glass apparatus that Szwarc and co-workers¹¹ used in their experiments to prove the concept of living anionic polymerization. After the formation of living polystyrene (PS), a new quantity of styrene and the proper amount of THF was added into the reactor so that the solution’s concentration remained unaltered. The increase in the solution’s viscosity and the fact that both portions of styrene were converted quantitatively into a polymer proved the living character of anionic polymerization.⁹ The groundbreaking discovery of Szwarc, followed by subsequent developments, inspired many excellent polymer chemists (M. Morton, S. Bywater, L. Fetters, J. Worsfold, J. Roovers, P. Rempp, among many others) to apply the concept of livingness to a plethora of monomers, leading to the synthesis of well-defined polymers with predetermined properties and a wide variety of applications.¹²

Figure 1.

Illustration of the “primitive” custom-made glass apparatus used in Szwarc and co-workers’ first carbanionic polymerization experiments.

In general, carbanionic polymerization consists of two steps: initiation and propagation. It proceeds via organometallic sites (carbanions) with metallic cations as counterions. The main characteristic of using an appropriate organometallic compound as the initiator is the rapid reaction with the monomer at the initiation step of the polymerization, with a higher reaction rate than in the propagation step. The most widely used initiators in anionic polymerization are organolithium compounds since they are soluble in organic solvents.¹³ The propagation proceeds via a nucleophilic attack of a carbanionic site on the monomer, followed by a reformation of the first anionic active center. A similar mechanism occurs in the case of the ring-opening polymerization (ROP) of cyclic monomers containing heteroatoms (lactones, lactides, siloxanes, etc.).¹⁴

Due to the high reactivity of the propagating sites, anionic polymerization necessitates the absence of contaminants or chemical compounds that are sensitive to nucleophilic reagents. Carefully designed apparatuses, reactors, and appropriate techniques are necessary to avoid premature chain termination.^13,15 Inert atmosphere (Schlenk techniques) and high-vacuum techniques have been successfully employed for anionic polymerization. Schlenk techniques are more straightforward to handle, leading to the preparation of relatively large quantities of polymers.¹⁶ However, high-vacuum techniques are the most prominent and reliable experimental tools to synthesize well-defined polymers with complex macromolecular architectures.¹³ They demand specialized skills in glassblowing, which are time-consuming and usually lead to the preparation of small quantities (a few grams) of the desired materials. Nevertheless, these restrictions are a small price to pay, given the ability of anionic polymerization to generate model polymers.¹⁷

The key feature of living anionic polymerization is the absence of chain transfer and termination reactions enabling the synthesis of well-defined polymers with unprecedented precision. This feature gives rise to more advantages of anionic polymerization. First, it produces polymers with the lowest dispersity among all known synthetic methods, better described by a Poisson distribution. Moreover, it provides excellent control over the molecular characteristics through the ratio of monomer and initiator, leading even to extremely high molecular weight block copolymers (about 2 × 10⁶ g mol^–1) by sequential monomer addition (challenging).^18,19 Finally, complete chain-end functionalization is feasible due to the highly reactive but stable (under appropriate conditions) terminal anions. These essential characteristics led to the synthesis of various linear and branched macromolecular architectures and precisely end-functionalized polymers.^20,21 A few of them are given in Figure 2.

Figure 2.

A few examples of linear and nonlinear macromolecular architectures synthesized by carbanionic polymerization.

Several other “living” polymerizations coexist with anionic polymerization. These polymerizations differentiate from living carbanionic polymerization in terms of the nature of initiation and propagation, monomers, catalysts, and reaction conditions. For instance, anionic polymerization of (meth)acrylates proceeds through enolate anions as the active propagating centers. Such anions have intrinsic reactivity lower than that of pure carbanionic species. Baskaran and Kitayama^22,23 have reported comprehensive reviews regarding the stereospecific living anionic polymerization of (meth)acrylates. Group transfer polymerization of (meth)acrylates, acrylonitrile, and acrylamide is different because the propagation occurs via the repetitive Mukaiyama–Michael reaction.²⁴ Cationic polymerization, a living polymerization via carbocation active chain-end, is essential for producing synthetic butyl rubber via the copolymerization of isobutylene and isoprene, followed by cross-linking of isoprene units.^25,26 Moreover, cationic polymerization is used to polymerize vinyl ether, N-vinyl carbazole, and styrene derivatives.^27,28 Stereospecific living (co)polymerizations of conjugated dienes such as 1,3-butadiene, isoprene, allene, etc., are achieved using organometallic complexes (mostly with transition metal).^29,30 The resulting polymers can be converted to highly stereoregular polyolefins via hydrogenation and are essential for industries.²⁹ All polymerizations above are excluded from this Perspective since our discussion’s scope refers to vinyl monomers’ polymerization via carbanionic propagation and the comparison with controlled/living radical polymerizations.

Living polymerizations of cyclic monomers are also beyond this article’s scope due to the monomers’ different natures and their polymerization process. Ring-opening polymerization (ROP) is a chain polymerization widely employed for cyclic monomers containing heteroatoms and produces polymers primarily used in (bio)-related medical applications.³¹⁻³³ The living ROP mechanism depends on the type of the cyclic monomer, the catalytic/initiating system, and the nature of the resulting active species.³¹ Living ring-opening metathesis polymerization (ROMP) is a transition metal-catalyzed polymerization of cyclic olefins in which the cyclic unsaturated olefin is opened, and two new carbon–carbon double bonds are created.³⁴ ROMP is a versatile pathway for the production of narrowly distributed, low molecular weight, and functional PE mimics under relatively mild conditions.³⁵

The scope of this Perspective is not to investigate the mechanism, kinetics, and thermodynamics of living carbanionic polymerization and review what has been reported over the last seven decades of its existence. Numerous excellent review articles and books have documented these aspects over the previous years.³⁶⁻⁴³ Furthermore, advances in anionic polymerization and macromolecular architectures are also well-reported in the literature.⁴⁴⁻⁴⁹ We aim to present the high importance of anionic polymerization of vinyl monomers by providing some examples of its major achievements, investigating its current status, exploring its advantages and disadvantages compared to the most popular controlled/living radical polymerizations (ATRP, RAFT), and providing insight into where it is going (Quo Vadis) and what the future holds for this powerful synthetic method.

Importance of Living Carbanionic Polymerization

The significance of living anionic polymerization in today’s research and commercial activity is undisputed.⁵⁰ Nowadays, almost 70 years after its discovery, the field has reached a certain degree of maturity. The wide range of anionically synthesized model polymers generated numerous studies in polymer physics, physical chemistry, theory, and applications of polymers.⁴⁷ In general, polymer physicists prefer to work with well-defined polymeric materials providing the highest possible molecular, compositional and structural homogeneity. These model materials offer the opportunity to explore the exact structure–property relationships, test theoretical concepts, or develop new ones.¹⁷ Phase diagrams of block copolymers and terpolymers,⁵¹⁻⁵⁵ discovery of new morphological phases of linear and nonlinear block copolymers,⁵⁶⁻⁵⁹ as well as theoretical concepts on the rheology of linear, branched and cyclic polymers, were verified experimentally based on anionically synthesized materials.⁶⁰⁻⁶⁴ Furthermore, most complex macromolecular architectures (star, graft, cyclic, branched, dendritic, tadpole, 8-shaped, etc.) were initially synthesized by combining anionic polymerization with the appropriate linking chemistry. Additionally, organic nanoobjects (e.g., Janus particles) and hybrid materials composed of polymers and inorganic fillers were synthesized through carbanionic polymerization, leading to materials with unprecedented chemical or physical properties.^65,66

Concerning industrial applications, thermoplastic elastomers (TPEs) based on polystyrene (PS), poly(butadiene) (PB), and poly(isoprene) (PI) (e.g., SBS, SIS,) have been commercially available since the mid-1960s (Shell), via sequential anionic polymerization. TPEs are an increasingly important class of polymers that combines the elasticity of chemically cross-linked elastomers with the melt processability of thermoplastics.⁶⁷ Nowadays, TPEs represent one of the most significant sectors of the polymer industry, with a wide range of applications from adhesives and sealants to footwear and asphalt modifiers.⁵¹ Specifically, styrenic block copolymers (SBCs) synthesized by anionic polymerization are considered the largest in volume and lowest-priced category of TPEs.⁶⁸ The global demand for SBCs was estimated to be USD 7.3 billion in 2020 (over 2.3 million tons) and is projected to reach USD 10.4 billion by 2027 with a compound annual growth rate (CAGR) of 3.8%.⁶⁹ Anionic polymerization has offered the potential of synthesizing many well-defined TPEs in short production times, with quantitative conversion, and in large volume, including linear and nonlinear architectures (e.g., star-block and graft copolymers).⁷⁰ The high importance of carbanionic polymerization in the TPEs’ industry also relies on the absolute control of molecular weight and composition, leading to the ultimate control of the self-assembling morphologies. Moreover, the hydrogenated products of SBCs (hydrogenated SBS and SIS) are used in many high-performance applications due to their outstanding stability (UV and thermal stability) and superior mechanical performance (higher modulus).⁷¹ In addition to linear and star-block structures, other star-shaped polymers have been commercialized. For example, BASF produces a variety of star-block copolymers of PS and PB.⁷² The materials with low PB content (Styrolux) are used as impact-modified thermoplastics, while the ones with high PB content (Styroflex) are highly flexible and transparent wrapping materials.³⁹

A few noteworthy and significant examples of anionic polymerization’s major achievements are followed. Based on the work of Roovers,^73,74 and Fetters⁷⁵ during the 1970s and 1980s, Hadjichristidis’⁷⁶ and Mays’⁷⁷ groups were the first to synthesize miktoarm star polymers (derived from the Greek word μικτός, meaning mixed) via the combination of anionic polymerization and appropriate chlorosilane chemistry. One of the authors initially coined the term “miktoarm” for stars having arms with chemical asymmetry, which later included stars with molecular weight asymmetry and stars with similar chemical nature but different end-functional groups.⁷⁸ Many reports and studies were followed, revealing the tremendous effect of miktoarm stars in polymer science.⁷⁹⁻⁸⁸ The star structures synthesized by anionic polymerization guided scientists working with other types of polymerization techniques, such as controlled/living radical, ring-opening, catalytic, and ring-opening metathesis polymerizations.⁸⁹⁻⁹¹

Interdisciplinary research between polymer chemists and physicists led to many discoveries and developments in polymer science, where anionic polymerization played a key role. For example, the discovery of the ordered bicontinuous double diamond (OBDD) structure.⁹² This morphology was not expected from theoretical predictions in neat block copolymers. It was reported on anionically synthesized inverse star-block copolymers [(PS-b-PI)₂(PI-b-PS)₂]. OBDD, along with other ordered bicontinuous morphologies, are very important in the area of nanotechnology, providing opportunities for potential applications such as high-temperature membranes, photonic band gap materials, etc.⁹³ Another example is the microphase separation of the four different blocks in a tetrablock quarterpolymer [consisting of PS, PI, poly(dimethylsiloxane) (PDMS), and poly(2-vinylpyridine) (P2VP)] synthesized by anionic polymerization and a heterobifunctional linking agent.⁹⁴ Four phases of the microphase-separated structure were observed for the first time by transmission electron microscopy (TEM) after selectively staining the different blocks. The PI, PDMS, and P2VP blocks formed triple coaxial cylinders with a hexagonal shape packed in a hexagonal array in the PS honeycomb-shaped matrix (Figure 3).

Figure 3.

Schematic illustration of the hexagonal triple coaxial cylinder structure observed in a PS-b-PI-b-PDMS-b-P2VP quarterpolymer and the corresponding TEM image. Reproduced from ref (94). Copyright 2002 American Chemical Society.

The tremendous abilities of anionic polymerization high-vacuum techniques enabled polymer chemists to synthesize a plethora of well-defined complex macromolecular architectures (such as dendrimers and other branched architectures).⁹⁵⁻¹⁰⁰ In one of these reports, a series of well-defined second (G-2) and third (G-3) generation dendritic PBs (Cayley-tree polymers) were synthesized by the coupling reaction of G-2 and G-3 living dendrons with methyltrichlorosilane.¹⁰¹ Rheological studies of these branched polymers revealed the signature of each layer relaxation in the plateau modulus, where each layer contributed distinctly to the terminal relaxation of the Cayley-tree polymer, following the principle of hierarchical motion of branched structures (Figure 4).¹⁰² Furthermore, dynamic studies (dielectric and viscoelastic measurements) were performed in similar architectures of PI, suggesting the importance of consistent coarse-graining in the molecular description of the dynamics of branched chains in general.¹⁰³

Figure 4.

General reaction scheme for the synthesis of (a) second-generation (G-2) Cayley-tree PBs with the corresponding experimental data for G′ and G″ and (b) third-generation (G-3) Cayley-tree PBs with the corresponding experimental data for G′ and G″. Reproduced from ref (102). Copyright 2007 American Chemical Society.

Hirao’s group¹⁰⁴ introduced a novel methodology for synthesizing well-defined multiarmed and multicomponent miktoarm stars based on a conceptual iterative strategy. This strategy utilizes functionalized diphenylethylene (DPE) derivatives based on a stepwise approach by introducing a definite number of functional groups at the chain-end or along the chain.¹⁰⁵ The proposed method is designed so that the same reaction site is always reintroduced after the insertion of arms in the reaction sequence, which is a repeatable process (Figure 5).¹⁰⁴ The iterative strategy was further developed, generating a series of well-defined exact graft polymers^106−108 with increased branching points and high-generation (up to G-7) dendritic polymers incorporating different polar and nonpolar segments.^{96,97,109,110}

Figure 5.

General scheme for the synthesis of miktoarm star polymers by the iterative methodology.

An example directly relevant to the industry is examining the role of long-chain branching in polymer processing. A wide range of model-branched polyethylenes (PE) was synthesized by anionic polymerization of 1,3-butadiene and chlorosilane chemistry, followed by hydrogenation.¹¹¹ Shear and extensional rheology of the pure materials and blends with linear PE were studied, and theoretical models were developed.¹¹² A better understanding of the structure–property relationships of branched materials led to the design of new PE structures with improved processability and final properties.

As mentioned in the Introduction, one of the advantages of anionic polymerization is that it provides excellent control over the molecular characteristics, leading to the synthesis of very high molecular weight polymers. The careful design and synthesis of extremely high molecular weight PS, PI, and PB (having different microstructures) in different architectures (1–1.16 × 10⁶ g mol^–1 for the linear homopolymers and 4.2 × 10⁶ g mol^–1 for the 18-arm star PB), led to an unexpected discovery of the formation of stable “written” patterns in solution induced by continuous-wave visible laser light at low power level.¹⁹

Tremendous advances have occurred in the last decades in membrane science^113−117 and nanotechnology,¹¹⁸ particularly in block copolymer lithography.^119−123 A variety of well-defined nanostructured materials (block co/terpolymers, star copolymers, etc.) were synthesized via anionic polymerization,^124−131 leading to outstanding developments in the field and guiding the electronics industry to the next level of modern applications. Polymer physicists took advantage of the ability of anionically prepared block copolymers to self-assemble to a wide variety of periodic nanoscale patterns and succeeded in finding the proper conditions that lead to a very long-range order of the domains.¹³²

Current Status of Living Carbanionic Polymerization

Although living carbanionic polymerization was discovered almost 70 years ago, it has evolved significantly during the past decade. Undoubtedly, the advent of controlled radical polymerizations [IUPAC has recommended the term “reversible deactivation radical polymerization” (RDRP)]¹³³ has decreased the interest of polymer chemists in anionic polymerization due to RDRP’s experimental simplicity. Nowadays, atom transfer radical polymerization (ATRP),^134,135 reversible addition–fragmentation chain transfer (RAFT) polymerization,^136−138 and nitroxide-mediated polymerization (NMP)^139,140 have rapidly taken the lead in polymer synthesis. However, anionic polymerization remains active and significant contributions have been documented in the past decade. In the following section, we briefly review the most important recent advances (past decade) in the anionic polymerization of vinyl monomers.

Many synthetic groups were focused on discovering and studying new vinyl monomers compatible with anionic polymerization, leading to the synthesis of novel materials with exciting properties.^141−146 Ishizone’s^147−150 and Ma’s^151,152 groups, reported the synthesis of several novel styrene and 1,3-diene derivatives bearing various substituents. For example, the anionic polymerization of styrene derivatives with bulky adamantyl groups generated well-defined polymers exhibiting higher T_g values (>200 °C) than PS.¹⁴⁹ Except for vinyl monomers, anionic polymerization was applied to inorganic monomers.^153,154 Phosphaalkenes (P=C analogues of olefins), were anionically polymerized to afford homopolymers and block copolymers with controlled architectures and unique chemical functionality.^155,156

Recently, new initiators for the anionic polymerization of vinyl monomers,^157,158 and isocyanates,^159−161 were investigated, and some “forgotten” monomers were revisited and studied through modern anionic polymerization techniques.^162,163 Lee and co-workers¹⁶¹ reported a novel dimeric self-associated sodium diphenylamide (NaDPA) as a robust chain-end-protective initiator for the initiator-transfer anionic polymerization (ITAP) of n-hexyl isocyanate (HIC) in the absence or presence of sodium additives. The ITAP offers precise control over a wide range of molecular weights due to the improved livingness caused by the chain-end protection of NaDPA.

It is well-known that in alkyl lithium-initiated polymerization of vinyl monomers in a hydrocarbon solvent, additives such as ethers or amines enhance the reactivity of the organolithiums, even if added in small quantities.³⁸ Other additives that affect the rate of anionic polymerization are phosphazene bases. These non-nucleophilic strong bases generate highly reactive anionic species, which have been successfully used in anionic ring-opening polymerization (AROP) of epoxides, cyclosiloxanes, and cyclic esters and the anionic polymerization of vinyl monomers such as methacrylates, acrylates, and isocyanates.^164,165 Recent studies have shown that the presence of phosphazene bases affects the PB microstructure similarly to N,N,N,N-tetramethylethylenediamine (TMEDA).¹⁶⁶ Phosphazene bases were also used for the controlled ultrafast anionic polymerization of styrenic monomers by an organolithium initiator (sec-BuLi). Specifically, by employing the “seeding” technique, the monomers were consumed in only 5 min leading to homopolymers with the desired molecular weight and narrow dispersity.¹⁶⁷

In the past decade, significant advances occurred in the field of TPEs, synthesized by anionic polymerization. Several research groups reported synthetic strategies to increase the upper service temperature of traditional TPEs (SIS or SBS) and generate polymers for high-temperature applications by replacing PS with polymers exhibiting higher glass transition temperature (T_g).⁷⁰ Toward this direction, conjugated monomers (rigid dienes) that can be anionically polymerized in hydrocarbon solvents at mild temperatures were employed.^168,169 Ishizone and co-workers reported the anionic polymerization of benzofulvene and the copolymerization with isoprene in benzene¹⁶⁹ and THF.¹⁷⁰ A study of the mechanical properties of the synthesized triblock copolymers of polybenzofulvene-b-poly(isoprene)-b-polybenzofulvene (PBF-b-PI-b-PBF) revealed that at 14% vol. of PBF shows values of ultimate stress (14.3 MPa) and strain at break (1390%) similar to commercially available Kraton materials (Kraton D1112P).¹⁷¹

Spontak and co-workers¹⁷² documented a highly effective and fast-acting approach for self-disinfecting materials based on anionically synthesized TPEs. The multiblock polymer studied was poly[tert-butylstyrene-b-(ethylene-alt-propylene)-b-(styrenesulfonate)-b-(ethylene-alt-propylene)-b-tert-butylstyrene] (TESET). Due to the presence of sulfonic acid groups along the polymer backbone, the TESET materials possess a unique antimicrobial ability. These materials fully inactivate many bacteria and viruses (including methicillin-resistant Staphylococcus aureus, MRSA, and SARS-CoV-2) that are typically responsible for several infections.¹⁷³ The inactivation mechanism derives from a low pH (<1) environment at the polymer/pathogen interface and depends on the number of sulfonic acid moieties on each polymer molecule.¹⁷²

A new class of styrenic TPEs exhibiting higher modulus, mechanical strength, toughness, and elastic recovery was investigated through a collaboration of Fredrickson’s and Avgeropoulos’ groups.¹⁷⁴ Initially, Fredrickson’s group¹⁷⁵ designed and studied A(BA′)_n miktoarm star copolymers through self-consistent field theory (SCFT) calculations, leading to the prediction of a stable cylindrical morphology of A block inside the matrix of B block, with the volume fraction of A block up to 70%. This concept was proven experimentally by synthesizing a series of S(IS′)₃ miktoarm star copolymers via anionic polymerization and appropriate chlorosilane chemistry, leading to fascinating results. Furthermore, by blending with PS homopolymers, stiffer TPE materials with aperiodic “bricks and mortar” mesophase morphologies were observed even at higher PS compositions.^176−178

Living anionic polymerization has evolved as a powerful method to control comonomer sequences and tailor the properties of the resulting materials.⁷¹ The “living” nature of anionic species leads to the preparation of block sequences by the subsequent addition of different monomers or monomer mixtures, resulting in narrow molecular weight distributions. Since the 1960s, the statistical copolymerization of styrene and 1,3-dienes were performed in hydrocarbon solvents initiated by alkyllithium, presenting great industrial and academic interest.^71,179−181 Recently, many contributions have been documented in sequence control copolymerization strategies utilizing carbanionic polymerization.^182−187 For example, Frey’s and Müller’s groups introduced online monitoring of the one-step synthesis of tapered block copolymers consisting of PI and PS or poly(4-methylstyrene) (P4MS) in cyclohexane by statistical living anionic copolymerization.^186,188,189 A detailed study of the monomer gradient was performed via real-time NMR kinetics, in situ near-infrared (NIR) spectroscopy, DFT calculations, and kinetic Monte Carlo simulation. The potential of this one-pot strategy for block copolymer synthesis has been demonstrated through a series of tapered block copolymers from low to ultrahigh molecular weight materials (>10⁶ g/mol).¹⁹⁰

Numerous reports were documented on the copolymerization of styrene and dienes with diphenylethylene (DPE) and DPE derivatives resulting in several sequence-controlled (functional) polymers with well-defined structures.^191−198 Hutchings and co-workers¹⁹³ reported the anionic copolymerization of DPE with styrene and 1,3-butadiene under various reaction conditions and solvents. Specifically, the copolymerization of DPE with styrene generated nearly perfectly alternating copolymers in a polar solvent (THF). Additionally, the authors studied the copolymerization of styrene and 1,3-butadiene with a less reactive DPE derivative [1,1-bis(4-tert-butyldimethylsiloxyphenyl)-ethylene (DPE-OSi)] in benzene, leading to the formation of copolymers with controlled comonomer sequences, either telechelic or alternating copolymers.¹⁹³ The same group expanded these studies by reporting the statistical terpolymerization of 1,3-butadiene, styrene, and DPE via a one-pot, one-shot process (“fire and forget” method).¹⁹⁴ This methodology generates copolymers with a block-like structure [poly(styrene-co-DPE)-b-polybutadiene], which undergo microphase separation, containing a high T_g glassy “block”. This approach proceeds in a faster and more scalable way compared to the traditional sequential addition of monomers method (Figure 6).

Figure 6.

Schematic illustration of the (a) one-shot simultaneous “fire and forget” terpolymerization of 1,3-butadiene, styrene, and DPE leading to statistical block terpolymer and (b) two-step sequential addition of monomer approach to generate poly(styrene-co-DPE)-b-polybutadiene copolymer.

Ma and co-workers^{195−197,199,200} documented a series of studies on the anionic copolymerization of styrene with several DPE derivatives bearing different functional groups by controlling the monomer sequence leading to a variety of statistical in-chain functionalized polymers. Recently, they reported a “locked-unlocked” mechanism in living anionic polymerization, where the anionic species can be quantitatively locked by end-capping with 1-(tri-isopropoxymethylsilylphenyl)-1-phenylethylene (DPE-Si(O-iPr)₃) and subsequently unlocked by adding an alkali metal alkoxide (Figure 7).¹⁹⁸ The authors investigated the features of this mechanism by sequential feeding strategies, which revealed a new method for better controlling the composition, the molecular weight distributions, and the sequence of polymers.^201,202

Figure 7.

Schematic illustration of the locked-unlocked mechanism in living anionic polymerization.

The controlled functionalization of polymers via living anionic polymerization, either at the chain-end or in-chain, has drawn great attention because it provides a route to various cross-linking reactions, branched architectures, and supramolecular noncovalent bonds, among others.³⁸ Toward this direction, many reports were documented by employing anionic polymerization and postpolymerization reactions for synthezing noncovalent bonded block polymers (hydrogen bonding, stereocomplexation, etc.).^203−212 Our group²⁰⁹ reported the synthesis of supramolecular block copolymers (PS-DAT-sb-PI-Thy) via anionic polymerization and hydrogen bonding. Well-defined thymine end-functionalized PI (PI-Thy) and diaminotriazine (DAT) end-functionalized PS were synthesized by employing the EO end-capping method of the corresponding carbanionic living chains followed by suitable transformation reactions. Microphase separation studies of the obtained supramolecular block copolymers were also performed and compared to the corresponding covalently linked analogues. This strategy was further developed for the synthesis of asymmetric supramolecular triblock copolymers (PS-b-PI-sb-PS′).²¹⁰

Recently, significant attention and many contributions were reported on the synthesis of amphiphilic diblock copolymer nanoparticles via polymerization-induced self-assembly (PISA).^213,214 This strategy offers many advantages over traditional self-assembly procedures, which typically involve postpolymerization techniques that are performed in dilute solution (<1% w/w).^215,216 Although most research groups have studied controlled radical polymerization methods (especially RAFT polymerization) for PISA,²¹⁷ a few reports were documented using anionic polymerization. Initially, Wang and co-workers²¹⁸ explored the anionic polymerization in combination with the PISA process for synthesizing all-styrenic diblock copolymers (PtBS-b-PS) exhibiting controlled molecular characteristics. The authors used heptane as the solvent of the polymerization, n-BuLi as the initiator, t-butylstyrene as the first monomer, and styrene as the second one. Heptane serves as a good solvent for the PtBS block and a poor solvent for the PS block. Moreover, the same group extended this strategy to the synthesis of PI-b-PS copolymers. The generated nano-objects included the spherical, wormlike, and vesicular micelles, as well as their mixtures.²¹⁹ Additionally, the formed nano-objects can be efficiently stabilized using divinylbenzene (DVB) as the cross-linking agent.

The endless effort of the scientific community to mimic nature’s complexity gave birth to polymeric materials with complex structures like stars, grafts, cyclic, dendrimers, and multiblock polymers.⁴⁷ Living anionic polymerization is the most representative method for synthesizing well-defined polymers with complex macromolecular architecture with absolutely predictable structure and composition. In this area, many contributions were reported in the past, summarized in excellent reviews and books;^{12,20,45,47,48,220} however, significant advances in the synthesis of complex macromolecular architectures were developed during the past decade. A few recent examples are followed.

Avgeropoulos’ group reported the synthesis and self-assembly of various types of well-defined miktoarm star polymers and dendrimers consisting of PS and immiscible polydiene blocks or PDMS via anionic polymerization high-vacuum techniques and appropriate chlorosilane chemistry.^221−224 On the other hand, other groups developed new multifunctional initiators for the anionic polymerization of styrene and dienes generating star homo/copolymers.^225,226 These DPE-based new initiators were soluble in hydrocarbon solvents and did not require polar additives during the polymerization.²²⁵

Recently, many other fascinating, complex architectures such as miktoarm stars, H-shaped, cyclic, tadpole, 8-shaped, graft copolymers, and dendrimers were synthesized by the anionic polymerization method (or through the combination with other polymerization strategies) and efficient coupling reactions.^{191,227−241} Hirao and co-workers^242−245 expanded their iterative methodology by introducing new strategies leading to the synthesis of a series of well-defined exact graft and multigraft copolymers containing polar and nonpolar blocks. A significant advantage of this methodology is that complicated multistep selective reactions, generally required for synthesizing macromolecular architectures, are avoided because the polymer blocks are introduced one by one in each reaction step.²⁴⁶

Hadjichristidis’ group²⁴⁷ reported the synthesis of cyclic triblock terpolymers of PI, PS, and P2VP by employing anionic polymerization and Glaser coupling reaction. The produced α,ω-dialkyne triblock terpolymer was submitted to Glaser coupling for the intramolecular ring closure at room temperature to afford the cyclic terpolymer. The authors extended this methodology by synthesizing well-defined tadpole homo/co/terpolymers derived from the appropriate chemical modification reactions of the corresponding 3-miktoarm star homo/co/terpolymers.²⁴⁸ Morphological characterization was also carried out to explore how architecture can influence self-assembly since tadpoles combine two topologies in the same molecule (ring polymer as the head and linear polymer as the tail).

Our group reported an interesting approach regarding the synthesis and self-assembly of well-defined PE-based miktoarm star architectures by combining anionic polymerization high-vacuum techniques and an efficient coupling reaction followed by the in situ polyhomologation of dimethylsulfoxonium methylide (C1 polymerization).^249−251 This strategy led to the formation of star-block copolymers of (PS-b-PE)₃ and (PI-b-PE)₃,²⁵⁰ as well as more complex miktoarm star polymers of PI₂(PI′-b-PE)–OH and PI₂(PS-b-PE)–OH co/terpolymers, respectively (Figure 8).²⁵¹

Figure 8.

General scheme illustrating the synthesis of well-defined PE-based miktoarm star co/terpolymers through anionic polymerization, appropriate coupling reaction, and polyhomologation of dimethylsulfoxonium methylide. Reproduced from ref (251). Copyright 2020 American Chemical Society.

Multiblock copolymers have gained increased attention due to their relatively straightforward reaction methodologies. Modern synthetic methods provide access to a broad portfolio of multiblock macromolecular architectures,²⁵² and specifically, anionic polymerization plays a key role in this field. The power of living carbanionic polymerization has been demonstrated in several examples by synthesizing multiblock copolymers with rubbery, glassy, and semicrystalline segments.^253−260 In most cases, sequential monomer addition methodology and monomers with similar reactivity were employed. The resulting multiblock copolymers have been extensively investigated for their mechanical properties, phase behavior, and morphology. Moreover, a continuous process for the production of multiblock copolymers through anionic polymerization in a loop reactor was reported by He and co-workers.²⁵⁷ The authors presented the living anionic copolymerization of styrene and 1,3-butadiene; moreover, this strategy can be extended to any other living copolymerization system of suitable monomer pairs.^261,262

Recently, the significant difference in the reactivity ratios of isoprene and 4-methylstyrene or styrene led to the synthesis of tapered alternating multiblock copolymers by a one-pot sequential addition of monomer mixtures.^263,264 The resulting polymers contained up to 10 blocks, with molecular weights ranging from 80–400 kg/mol, and exhibited relatively low dispersity values (1.06–1.28). Additionally, the thermomechanical properties and phase separation behavior of the tapered multiblock polymers were extensively studied.²⁶⁵ The authors extended their studies by reporting the synthesis of well-defined multiblock 4-arm tapered star copolymers [PS-b-(PI-grad-PS)]₄ on a multigram scale under short reaction times.²⁶⁶ Subsequently, catalytic hydrogenation of the PI blocks was achieved selectively, leading to polystyrene-b-poly((ethylene-alt-propylene)-grad-polystyrene)₄ (SEP/S)₄ (Figure 9)_. The mechanical properties (strain hardening, modulus, toughness, and ultimate strength) of the tapered star copolymers and their linear precursors were investigated thoroughly.

Figure 9.

One-pot synthesis route for well-defined multiblock tapered star copolymers. Reproduced from ref (266). Copyright 2020 American Chemical Society.

The future success of the synthetic polymer industry will rely on the development of sustainable polymers derived from renewable feedstocks that can be recycled or disposed of in environmentally friendly ways.²⁶⁷ Nowadays, substituting fossil resources with renewable ones has received increased attention toward the plastic circular economy.^268,269 As an example, terpenes are promising biobased monomers to replace 1,3-dienes since they can be polymerized through anionic polymerization.²⁷⁰ Both β-myrcene and β-farnesene, showing high structural similarity to isoprene, have been reported as monomers for anionic polymerization.²⁶⁸ Myrcene is a 2-substituted-1,3-butadiene that can be incorporated [as poly(myrcene)] as the rubbery midblock in a styrenic TPE. Hillmyer and co-workers²⁷¹ developed new, high-performing, and sustainable TPEs using terpenes as a naturally occurring feedstock. The authors synthesized, through living anionic polymerization and appropriate chlorosilane chemistry, ABA-type triblock copolymers consisting of poly(α-methyl p-methylstyrene) as the hard segment and poly(myrcene) as the soft block. This renewable TPE exhibits improved performance at high temperatures compared to current styrenic TPEs, making it attractive for applications requiring higher service temperatures. Recently, many other research groups have shown tremendous interest in the anionic polymerization of terpenes and the postpolymerization reactions of these polymers (hydrogenation, epoxidation, etc.).^272−277 Furthermore, major contributions on the synthesis and study of terpene-based linear and branched macromolecular architectures have been reported by Frey’s group.^{268,278−282} They have synthesized via anionic polymerization, several combinations of linear and complex macromolecular architectures of terpene-based copolymers (linear copolymers with PS, graft copolymers, polymerization of functionalized terpenes, etc.) (Figure 10). In another interesting report, Schlaad and co-workers²⁸³ demonstrated the anionic polymerization of β-myrcene and isoprene in “green” ether solvents (cyclopentylmethyl ether and 2-methyltetrahydrofuran) with sec-BuLi as initiator. This synthetic route generated well-defined polydienes with predictable molecular weights, suggesting that conventionally used solvents can be replaced by environmentally more friendly alternatives.

Figure 10.

General scheme that represents the ability to synthesize linear and complex macromolecular architectures of terpene-based polymers through anionic polymerization. Reproduced from ref (268). Copyright 2021 American Chemical Society.

Living Carbanionic Polymerization vs Controlled/Living Radical Polymerizations

Conventional free-radical polymerization of vinyl monomers has been extensively used over the years on an industrial scale. Some advantages of radical polymerization over living polymerizations are the relative insensitivity to impurities, and the available multiple polymerization processes (bulk, solution, and emulsion polymerization). However, this polymerization method has some significant limitations, such as limited control over the molecular weight, composition, molecular weight distribution, site-specific functionality, and chain architecture.

Anionic polymerization paved the way for discovering controlled/living radical polymerizations, which have been developed rapidly over the last two decades. RDRP methods overcome the significant issues of conventional free-radical polymerization providing better control of the polymerization and the molecular characteristics of the synthesized polymers. However, some degree of termination or chain transfer occurs in these methods compared to the absence of termination reactions in living anionic polymerization (Figure 11a).²⁸⁴ RDRP techniques rely upon either a reversible termination of active chain-ends (Figure 11b) or a rapid degenerate exchange between dormant and active chain-ends (Figure 11c).^285,286 Undoubtedly, RDRP methods have influenced many fields in materials science, ranging from biomedical²⁸⁷ and optoelectronic²⁸⁸ applications to nanocomposites, etc.¹³⁴ These polymerizations are often described as “living” or “controlled” because they preserve the essential characteristics of living polymerizations (low dispersity and efficient chain-end functionalization).^133,284 In the following section, we will attempt to compare some general features of living anionic polymerization with the most popular and widely used RDRP methods (ATRP and RAFT). We have excluded the NMP method as well as other “living” metal-catalyzed (i.e., sulfonyl chloride) radical polymerizations.²⁸⁹

Figure 11.

Illustration of living and RDRP methods. (a) Living polymerization methods with no termination or chain transfer. The propagating active site (*: either anion, cation, radical, or catalyst) remains active after the conversion of the monomer. (b) Reversible deactivation polymerization in which equilibrium between dormant and active chain-ends can lead to polymerization with living characteristics. (c) Reversible deactivation polymerization in which chain transfer with a rapid exchange between chain-ends can lead to polymerization with living characteristics.

Concerning ATRP, the control over the molecular weight distribution occurs through the reversible deactivation of the polymeric radicals with the help of a complex. The activator and deactivator complexes switch the polymers between active and dormant states.²⁸⁶ Various vinyl monomers have been successfully polymerized with this method, such as styrenes, (meth)acrylates, (meth)acrylamides, and acrylonitrile, which contain substituents that can stabilize the propagating radicals.²⁹⁰ However, the ATRP of dienes (1,3-butadiene, isoprene, etc.) has been a longstanding polymer chemistry challenge, as cross-linking is unavoidable with these monomers in the ATRP process. Nevertheless, very recently, Asandei’s group reported the synthesis of well-defined PBs with high Br-chain-end functionality (>90%) and relatively high dispersity values (1.3–1.5).²⁹¹ Another limitation of ATRP, and generally of RDRP techniques, is that it cannot produce high molecular weight polymers and is most efficiently applied to synthesizing polymers with low and moderate molecular weights (except the case of acrylates which have high propagation rate). The proportion of terminated chains and loss of functionalities increases with the chain length and concentration of growing radicals. Thus, generating long chains will require long reaction times, where several chain-breaking reactions may become more dominant.²⁹² On the other hand, a well-controlled ATRP method can afford many functional groups to synthesize polymers with predetermined molecular weight, dispersity, and composition. The main advantage of ATRP is that various functional polymers can be prepared at the expense of simple experimental conditions (solvents, temperature, etc.) with a high yield. In contrast, during anionic polymerization, functional groups should be excluded from the monomers or suitably protected before the polymerization.

Another widely used RDRP method is RAFT polymerization, capable of polymerizing many vinyl monomers having vulnerable functionalities with controlled molecular weight, compositions, and dispersity.^136,293 In this case, the control over the molecular weight distribution occurs by a degenerative transfer mechanism that allows efficient switching between active and dormant species.²⁸⁶ In an ideal living RAFT process, all chains should initiate at the beginning of the polymerization and grow at a similar rate without termination. A very narrow dispersity is achieved if the initiation is faster than propagation. However, similar to other controlled radical polymerizations, unavoidable termination reactions, even after establishing a rapid equilibrium between the active and dormant species, are evident. In contrast to ATRP, high molecular weight polymers have been synthesized using the RAFT emulsion process.²⁹⁴ In addition to the acrylates that have high propagation rates (acrylates), other monomers such as styrene have been used to generate very high molecular weight polymers (up to 10⁶ g mol^–1) with relatively high dispersity values (Đ = 1.39).²⁹⁴

As mentioned above, for the ATRP process, radical polymerization of the industrially significant conjugated diene monomers is still a considerable challenge for polymer chemists due to gelation issues. Nevertheless, some reports documented the RAFT polymerization of diene monomers (e.g., isoprene, 1,3-butadiene, chloroprene) with a much higher conversion before the gelation.^295−298 For example, Perrier and co-workers polymerized isoprene using a trithiocarbonate compound and/or 2-(2-cyanopropyl) dithiobenzoate as a RAFT agent, which shows a broad molecular weight distribution (Đ = 1.4).²⁹⁹ Ameduri’s group also reported the RAFT polymerization of 1,3-butadiene using a dithiobenzoate derivative as a RAFT agent above 110 °C.²⁹⁷ However, all the diene polymers synthesized via RAFT had some gel content (cross-linking) and did not achieve a very high control in their molecular weight and dispersity. Moreover, in all cases, the 1,4-content of the resulting polydienes did not exceed the value of 80 wt %, in contrast with anionic polymerization that produces polydienes with high 1,4-content (up to 93 wt %). Another advantage of living anionic polymerization of dienes is that it provides absolute control over the microstructure. The choice of the appropriate solvent or additives controls the resulting microstructure depending on the desired properties.

During the past decade, RDRP methods contributed significantly to the synthesis of sequence-controlled multiblock copolymers generating new functional materials.²⁵³ Although anionic polymerization plays a key role in the synthesis of well-defined and controlled multiblock copolymers, it is only suitable for a specific range of monomers. Moreover, it must be performed under particular polymerization conditions, where several linking reactions or more steps are usually necessary.³⁰⁰ On the other hand, RDRP techniques allow multiple chain extension procedures while retaining a high-end-group fidelity of the obtained polymers.²⁵³ As an example, Perrier and co-workers³⁰¹ reported for the first time (in 2013) the one-pot synthesis of a 20-block acrylamide polymer via RAFT polymerization; however, the degree of polymerization (DP) for each block was kept very low (DP = 3). In another report, Anastasaki and co-workers³⁰² reported the synthesis of several multiblock polymers using photoinduced ATRP of four acrylates with alternating structures in various combinations. The same group recently established a rapid one-pot synthetic procedure of sequence-controlled multiblock copolymers with on-demand control over dispersity and molecular weight, leading to the generation of highly ordered pentablock, octablock and decablock copolymers.³⁰³ However, for multiblock copolymers synthesized by RDRP methods, reports on the properties of these materials (e.g., phase separation studies) have not been documented due to the short length of the blocks.

Overall, the discovery of RDRP methods revolutionized polymer science during the last two decades providing accessible tools for synthesizing various polymers derived from vinyl monomers. Especially in the previous decade, these polymerization methods have advanced to the next level due to the use of external stimuli, which yielded well-defined polymers with increased end-group fidelity in a facile manner.³⁰⁴ Some limitations of applying RDRP techniques in the industry include the higher cost of controlling agents, additional cost to eliminate metal or thio-containing end groups, and macroinitiator purification. For example, the industrial price of the RDRP agents is at least 1 order of magnitude higher than the average cost of the monomers.³⁰⁵ Moreover, for ATRP, removing metal catalysts is a major concern for material applications in food packing or medical devices.⁷⁰ Similarly, for RAFT, removing thio-containing end groups is vital for applications where traces of odor and color are undesirable. Table 1 summarizes the general features and differences between carbanionic polymerization and RDRP methods.

Table 1. Comparison of Carbanionic Polymerization with RDRP Methods.

eneral features	carbanionic polymerization	RDRP methods (ATRP, RAFT)
termination or transfer reactions	absence of termination or transfer reactions, especially for styrenes and dienes (truly “living” polymerization)	some degree of termination and transfer reactions
synthesis of high MW polymers with low Đ (<1.1)	extremely high MW polymers (>2 × 106 g mol–1) exhibiting Đ < 1.1	moderate to high MW polymers (<106 g mol–1) exhibiting Đ < 1.1 (except acrylates)– Emulsion RAFT produces very high MW polymers (up to 106 g mol–1, Đ ∼ 1.3–1.4)
experimental conditions (purification of reagents, polymerization conditions)	demanding	simple
	extensive purification procedures for monomers, solvents, initiators, etc. Polymerization is performed under high vacuum or Ar conditions at low or moderate temperatures	mild experimental conditions, no requirements on extensive purification of the reagents
type of vinyl monomers	styrenes, dienes, meth(acrylates), vinylpyridines; functional groups must be excluded or protected	styrenes, meth(acrylates), (meth)acrylamides, vinylpyridines, and a broad choice of functional monomers
type of polymerization (in bulk, solution etc.)	solution, bulk	bulk, solution, and emulsion polymerizations
purity of polymers	complete elimination of Li compound, depending on the termination agent, by reprecipitation	multipurification steps for the removal of controlling agents (metal residuals or CTA agents)
macromolecular architectures	access to a wide library of well-defined (model) macromolecular architectures (linear block polymers, multiblocks, stars, grafts, cyclics, branched, dendrimers) through multifunctional initiators or appropriate coupling reactions (e.g., chlorosilanes)	access to a wide library of well-defined macromolecular architectures (linear block polymers, multiblocks, stars, grafts, cyclics, brush, branched, hybrid materials) through multifunctional initiators
functionalized polymers	direct method for chain-end and in-chain functionalization	postpolymerization modifications, limited access to in-chain functionalization
large-scale industrial applications	thermoplastic elastomers (TPEs), rubber industry (SBR)	currently limited large-scale industrial use due to the high cost of controlling agents, difficulty in elimination of metal or thio-containing end groups

Undoubtedly, the invention and advances of living anionic polymerization inspired and have driven the development of RDRP techniques. Along with them, many other living polymerizations, such as cationic polymerization, coordination, ring-opening polymerization, etc., are also known for their controlled character in molecular weight, compositions, and dispersity. On the other hand, a significant disadvantage of RDRP methods is the control of stereochemistry, tacticity, and regio-/stereoselectivity. However, only the anionic polymerization shows the true living nature due to the absence of termination or chain transfer reactions among all living polymerization techniques. It is the most reliable technique when a certain application demands very high molecular weights, extremely low dispersity, precise structure and block copolymer composition. Furthermore, compared with carbanionic polymerization, RDRP techniques have much more limited access to polymers with functional groups in the backbone.³⁰⁴

What Does the Future Hold for Living Carbanionic Polymerization?

Over the past 70 years, polymer synthesis has made remarkable progress. We believe that the “initiator/catalyst” of this progress was the pioneering discovery of living anionic polymerization by Szwarc in 1956.⁹ Nowadays, an increasing number of monomers are available and used in anionic polymerization after polymer chemists explored the basic mechanisms of the polymerization. The endless effort of the scientific community to mimic nature’s complexity gave birth to polymeric materials with complex structures, such as star, cyclic, graft, dendrimer, and multiblock polymers, to mention a few of them.⁴⁷ Through living carbanionic polymerization and appropriate linking chemistries, it was feasible to synthesize all these complex macromolecular structures allowing polymer physicists to develop new theoretical concepts and provide practical solutions to the polymer industry.

Anionic polymerization remains a field of intense interest for academia and industry since it represents the most powerful and reliable technique for synthesizing polymers with the highest degree of control over molecular weight, dispersity, composition, architecture, and microstructure. All these factors controlled by anionic polymerization helped the industry (Kraton, INEOS, BASF, among others) to commercialize an extensive library of SBCs having various applications on a large scale. The range of TPEs continues to expand, and physical or chemical modification of existing copolymers provides novel routes to stimuli-responsive and functional polymers. As mentioned above, recent studies of chemically modified SBCs have shown fascinating antimicrobial properties. Many developments are expected from functionalized TPEs in biomedical applications. Particularly, SBCs synthesized by anionic polymerization are expected to lead the market of TPEs over the next decades. However, these materials suffer from limited thermal resistance due to PS’s relatively low glass transition temperature. New monomers that can be copolymerized anionically with dienes in hydrocarbon solvents need to be developed to overcome these limitations, in addition to the ones already cited in the literature.^168−171

Styrene butadiene rubber (SBR) is another important category of copolymers (production rate ∼8 million tons in 2021) with industrial applications, mainly for tires.^306−309 SBR are generally prepared by two main methods, (a) solution (S-SBR) and (b) emulsion (E-SBR) processes.^310−313 The solution process mainly utilizes carbanionic polymerization, leading to better SBR in dispersity, microstructure, and controlled monomer distribution with high end-group functionality. Nowadays, many SBR manufacturers are changing their technology from E-SBR to S-SBR due to the high demand for high-performance tires (fuel efficiency, high abrasion, reduced rolling resistance, and low carbon footprint).³¹¹ Additionally, functionalized S-SBRs are compatible with polar fillers like silica, making them a suitable candidate for high-performance tire applications. In this regard, anionic polymerization can provide several polymers with various functional groups in the S-SBR family. Therefore, it is expected that the functionalized S-SBR synthesized via anionic polymerization will expand the scope of its industrial applications toward high-performance tires. For example, ARLANXEO has marketed a new family of high-performance S-SBR (Buna FX) functionalized with polar end-groups to increase the interaction with silica fillers, which reduces the in-tread hysteresis and rolling resistance of the tire.³¹⁴

As it is known, improvements are always necessary and usually derive from the applications side.³¹⁵ When the applications require polymers with very high molecular weights, extremely low dispersity, and exact design of the blocks in terms of composition (nanotechnology applications), anionic polymerization is still in the lead. It has also opened the way for the advent of controlled radical polymerization methods (ATRP, NMP, RAFT), which provided a resurgence in polymer synthesis over the last two decades. RDRP techniques overcome a severe disadvantage of anionic polymerization, such as the polymerization of functional monomers, and deliver easier experimental conditions for polymer chemists. Nevertheless, in contrast with living anionic polymerization, these controlled radical polymerizations cannot generate high molecular weight polymers (>100 kg mol^–1) with extremely narrow Đ (<1.1).

It was mentioned above how vital anionic polymerization is for polymer science via the synthesis of various linear and nonlinear macromolecular architectures in a controlled way utilizing different monomers. Nevertheless, there are still many challenges that anionic polymerization faces. Gaining access to sequence control in synthetic polymers, with the same level as in peptides or nucleic acids, has been a “Holy Grail” for polymer scientists for many decades.³¹⁶ Although this has to be achieved in high molecular weight synthetic polymers, some significant developments in anionic polymerization have initiated a growing interest in this direction, as discussed in a previous section of this article.

Another challenge of anionic polymerization that needs to be overcome is the synthesis of polymers based on highly reactive polar monomers (acrylonitrile, vinylidene cyanide, etc.). Moreover, developing new nonmetallic sophisticated initiators for vinyl and diene monomers soluble in hydrocarbon solvents will solve many issues of anionic polymerization. These new initiators could lead to the synthesis of 100% cis-1,4 microstructure of polydienes, providing many options to the tire industry. In addition, the use of organic catalysts is also intriguing because they offer enormous possibilities for tuning the reactivity of monomers and the microstructure of the resulting polymers. By choosing the appropriate organic catalyst, it is feasible to accelerate the polymerization rate of less reactive monomers such as styrene and dienes^166,167 and to moderate the reactivity of highly reactive polar monomers. Moreover, easier experimental conditions that can enable the large-scale synthesis of anionically prepared polymers are highly desirable. The high number of synthetic steps and highly demanding synthetic techniques lead to small-scale production. In general, anionic polymerization under high-vacuum techniques is carried out via a one-pot/several steps through sequential addition of monomers. However, further studies are needed to perform anionic polymerization in a one-pot/one-step controlled way, similar to the recently reported process in the anionic ROP field.³¹⁷ As mentioned in a previous section, significant efforts have been documented recently using continuous flow reactors. Therefore, more advances are expected, using different monomers under continuous flow-controlled anionic polymerization.

Many polymers synthesized by anionic polymerization have provided numerous applications due to their advantages regarding low cost, wide availability, and the possibility to tailor properties through chemical modifications. Anionic polymerization can produce functional polymers using protected functional initiators or termination with a suitable electrophile.³¹⁸ This feature allows the synthesis of telechelic polymers containing several functional groups, including hydroxyl, amino, sulfonate, etc. Furthermore, mechanistic transformations where two or more different polymerization techniques can be performed (e.g., anionic polymerization to RDRP methods) through postpolymerization reactions would lead to a more comprehensive range of block copolymers with fascinating properties.^318,319 Nowadays, “smart” materials,³²⁰ such as stimuli-responsive³²¹ and self-healing polymers,³²² are developing fast, providing opportunities beyond the traditional ones. Except for conventional covalent bonding in polymers, noncovalent bonding interactions (hydrogen bonding, stereocomplexation, dynamic covalent bonding, etc.) are favored in modern polymer science to provide adaptive/sensing materials.³²³ Anionic polymerization has already contributed to the field of self-healing materials and is expected to be involved in the synthesis of dynamic covalent bonding polymers for the formation of well-defined macromolecular architectures and controlled vitrimers. Vitrimers are an important class of polymers combining thermosets’ properties and thermoplastics’ processing.³²⁴ Carbanionic polymerization and suitable postpolymerization reactions can provide another alternative synthetic method for vitrimers, opening routes to new well-defined materials with promising properties.

Anionic polymerization has also contributed to other applications involving block copolymers, such as solid polymer electrolytes for lithium batteries.^325−327 Epps and co-workers³²⁸ documented that block copolymer nanoscale phase separation introduces sufficient mechanical stability into polymer electrolyte membranes while maintaining ionic conductivity similar to that of the solvent-free homopolymer electrolyte. These unique properties of block copolymers have provided more opportunities to fabricate solid polymer electrolytes in high-performance lithium batteries. The design of novel polymer electrolytes using carbanionic polymerization is beneficial due to its unique control of molecular characteristics and the ability to produce nanostructured polymers with precise macromolecular architectures. This ability generally leads to the fabrication of nanostructures with high precision and well-defined orientations in many materials crucial for advancing nanotechnology. The design of novel nanomaterials for next-generation thin film and membrane applications (gas separation and water purification) is highly important and necessitates the use of carbanionic polymerization.

In addition, the future of the synthetic polymer industry will rely on developing sustainable polymers derived from renewable feedstocks that can be recycled or disposed off in an environmentally friendly way. Substituting fossil resources with renewable ones has received increased attention toward the plastic circular economy. Recently, the reports regarding anionically synthesized polymers derived from sustainable monomers have rapidly increased and are expected to grow more. Anionic polymerization should provide more alternatives to well-defined polymers by enriching the library of commodity materials such as polyolefins from renewable and sustainable sources. These materials can substitute traditional polymers in the TPEs industry, one of the most important categories of the polymer industry related to carbanionic polymerization.

Looking ahead, there are great potentials for carbanionic polymerization in future applications. For example, the wire and cable industry is interested in functionalized polyolefins for the next generation of power cable insulators. This application requires the study of the physical properties of these materials through model polyolefins. The basic physical properties of polyolefins have been advanced using model polymers synthesized via anionic polymerization, followed by hydrogenation. This feature may prove to have even greater uses in the future. Another example is the use of anionically synthesized polymeric materials in additive manufacturing (3D printing). Anionic polymerization can provide high-performance materials (possibly complex architectures) that will help overcome the current challenges in processing (viscosity issues).

In summary, we present in this Perspective the past, the present, and possibly what will follow in anionic polymerization. Even after almost 70 years of its existence, carbanionic polymerization is still strong. The remarkable progress in polymer physics and engineering over the last 50 years would have been unimaginable without the model and well-characterized polymers’ availability. It is fair to mention that state-of-the-art anionic synthesis has challenged other fields, such as improved rheological capabilities, advanced characterization, and simulations. As described in this Perspective, the outstanding achievements of carbanionic polymerization have given a mature character to this method and opened the way to greater future possibilities. We believe that anionic polymerization is still very important for polymer science and industry thanks to the unique model polymers can provide. It will continuously support polymer chemistry to produce even more sophisticated structures, allowing polymer scientists to further explore the exact structure–properties relationship leading to new commercial products and advanced technological applications. Many fields in polymer science are waiting to be explored, where carbanionic polymerization will guide once again and lead the progress. We hope to convince more polymer scientists to be involved in the forthcoming fascinating journey of carbanionic polymerization.

Author Contributions

CRediT: Konstantinos Ntetsikas investigation (equal), methodology (equal), resources (equal), validation (equal), writing-original draft (lead), writing-review & editing (equal); Viko Ladelta investigation (equal), methodology (equal), resources (equal), validation (equal), writing-original draft (equal), writing-review & editing (equal); Saibal Bhaumik investigation (equal), methodology (equal), resources (equal), validation (equal), writing-original draft (equal), writing-review & editing (equal); Nikos Hadjichristidis conceptualization (lead), investigation (equal), methodology (equal), project administration (lead), resources (equal), supervision (lead), validation (lead), visualization (lead), writing-review & editing (lead).

The authors declare no competing financial interest.