PERSPECTIVE www.mrc-journal.de Merging 𝝈-Bond Metathesis with Polymerization Catalysis: Insights into Rare-Earth Metal Complexes, End-Group Functionalization, and Application Prospects Friederike Adams Polymers with well-defined structures, synthesized through metal-catalyzed processes, and having end groups exhibiting different polarity and reactivity than the backbone, are gaining considerable attention in both scientific and industrial communities. These polymers show potential applications as fundamental building blocks and additives in the creation of innovative functional materials. Investigations are directed toward identifying the most optimal and uncomplicated synthetic approach by employing a combination of living coordination polymerization mediated by rare-earth metal complexes and C–H bond activation reaction by 𝝈-bond metathesis. This combination directly yields catalysts with diverse functional groups from a single precursor, enabling the production of terminal-functionalized polymers without the need for sequential reactions, such as termination reactions. The utilization of this innovative methodology allows for precise control over end-group functionalities, providing a versatile approach to tailor the properties and applications of the resulting polymers. This perspective discusses the principles, challenges, and potential advancements associated with this synthetic strategy, highlighting its significance in advancing the interface of metalorganic chemistry, polymer chemistry, and materials science. 1. Introduction The category of rare-earth metals represent the elements of the 3rd subgroup, namely, scandium, yttrium, and the lanthanides (lanthanum and cerium to lutetium), totaling 17 elements, all exhibiting similar chemical properties.[1] Despite the name im- plying rarity, these elements are not scarce; in fact, some are F. Adams Institute of Polymer Chemistry University of Stuttgart Pfaffenwaldring 55, 70569 Stuttgart, Germany E-mail: friederike.adams@ipoc.uni-stuttgart.de F. Adams University Eye Hospital Tübingen Elfriede-Aulhorn-Strasse 7, 72076 Tübingen, Germany The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/marc.202400122 © 2024 The Author(s). Macromolecular Rapid Communications published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. DOI: 10.1002/marc.202400122 more abundant in the lithosphere than, for instance, cobalt, tin, lead, silver, cop- per, or lithium (highest amount in the up- per continental crust: 66 ppm (cerium); low- est amount: ≈0.27 ppm (lutetium)).[2] The nomenclature originated during the dis- covery phase when these elements were found in genuinely scarce ores. Presently, the name remains justified, as rare-earth metals predominantly occur in minerals de- posited in widely dispersed locations and necessitate intricate separation from other elements. From a chemical viewpoint, all elements within this class exhibit a simi- lar behavior. The most prevalent oxidation state is +III (d0). While a variety of com- plexes with other oxidation states have been identified, these are typically less stable.[3] For these d0 rare-earth metal complexes, the catalytic interplay of late transition met- als that commonly involves oxidative addi- tion and reductive elimination cannot oc- cur. Furthermore, the empty d orbitals in the +III oxidation state are responsible for the electronic properties, as the additional electrons in the 4f orbitals are close to the nucleus and are shielded by the 5s and p orbitals. The 4f orbitals are highly diffuse, providing minimal shielding of the increased nuclear charge. This leads to a strong attraction of the outer orbitals and a reduction in atomic radii within the period known as lanthanide contraction. Additionally, relativistic effects intensify this obser- vation. The cumulative contraction is so pronounced that, for instance, the dysprosium cation (atomic number 66) possesses a comparable ion radius to that of the yttrium cation (atomic number 39).[4] Rare-earth metal ions exhibit a strong Lewis acid- ity and affinity for oxygen and demonstrate notable reactivity to- ward unsaturated C–C bonds making them very interesting com- plexes for diverse catalytic applications like polymerization of po- lar and nonpolar monomers, as well as activation of C(sp2)–H and C(sp3)–H bonds.[5–7] Rare-earth metal catalysts often operate in well-defined coor- dination environments, offering enhanced selectivity and con- trol over polymerization reactions. The 18-electron rule, com- monly applicable to late transition metal complexes, does not apply to rare-earth complexes. Instead, the arrangement of lig- ands is predominantly influenced by steric factors rather than electron count. Rare-earth metal alkyl and hydride species ex- hibit noteworthy reactivity, attributed to their dual characteristics Macromol. Rapid Commun. 2024, 45, 2400122 2400122 (1 of 24) © 2024 The Author(s). Macromolecular Rapid Communications published by Wiley-VCH GmbH http://www.mrc-journal.de mailto:friederike.adams@ipoc.uni-stuttgart.de https://doi.org/10.1002/marc.202400122 http://creativecommons.org/licenses/by/4.0/ www.advancedsciencenews.com www.mrc-journal.de of nucleophilicity and basicity. This combination, in combina- tion with their Lewis acidity and affinity for unsaturated C–C bonds of the metal ions, positions rare-earth metals as excep- tional candidates for the development of catalysts.[8] The highly specific coordination geometry enables tailored interactions with monomers, facilitating the synthesis of polymers with desired structures and properties, including (co)polymerization of ethy- lene and polar monomers such as alkyl (meth)acrylates and lac- tones through nucleophilic addition of alkyl or hydride species of the rare-earth complex to a metal-coordinated monomer.[9–12] However, the intricate synthesis requires specialized knowledge. Specially designed ligands are often necessary to stabilize the rare-earth metal centers, adding more complexity to the cata- lyst design and synthesis. Consequently, most complexes are not readily available commercially in a form suitable for catalysis. Furthermore, certain rare-earth metal complexes are sensitive to air and moisture, necessitating special precautions during han- dling and storage to maintain the stability of these catalysts. The increased sensitivity is facilitated by the high reactivity of these complexes, a characteristic that can be effectively utilized for poly- merization catalysis. Reactive catalysts with defined and stabiliz- ing ligands act as initiators and play a key role in the control of the polymerization process. While mechanistic studies on rare-earth complexes with amide- and alkyl-initiators reveal that polymer- izations utilizing these initiators are impeded by potential side reactions. The introduction of novel and more efficient initiators can be achieved through 𝜎-bond metathesis. This perspective ar- ticle examines the first strategies to employ C–H bond activation by 𝜎-bond metathesis for the incorporation of new initiators to rare-earth metal complexes that can act as functional end groups in polymers. This enables applications to extend to diverse prac- tical uses presented herein. 2. Polymerization Catalysis with Rare-Earth Metal Elements Three categories of rare-earth metal complexes exhibit activity in polymerization catalysis: metallocenes, half-metallocenes, and non-metallocenes, the latter also referred to as post-metallocenes. A metallocene is a complex characterized by having two sub- stituted or unsubstituted cyclopentadienyl, indenyl, or fluorenyl units as ancillary ligands, whereas a half-metallocene is a com- plex with only one such ancillary ligand.[11] These complexes show activities in a variety of different polymerization types. The versatility of rare-earth metal complexes allows the polymeriza- tion of both polar and nonpolar monomers in a controlled man- ner, resulting in high molecular weight polymers with high con- version of the respective monomer. The polymerization of olefins holds importance in polymer science, representing a key target for research and development efforts. Metallocenes, the most extensively studied complexes, such as Cp*2LaH or Cp*2LuMe (Cp*= 𝜂5-C5Me5) show activity in ethylene polymerization, while such complexes are unsuccessful in polymerizing propylene. In- stead of polymers, an allyl complex, Cp*2Ln(𝜂3-allyl), was formed through a 𝜎-bond metathesis reaction (vide infra).[13] Typically, hydride complexes exhibit higher activity in ethylene polymer- ization compared to the alkyl analogues. The metallocene hy- dride complexes [Cp*2LnH]2 (Ln = La, Nd, Lu) have been ob- served to serve as extremely active catalysts for ethylene poly- merization to yield high molecular weight polyethylene while their catalytic activity depends on the metal center with the or- der of activity being La > Nd > Lu. The bis(trimethylsilyl)methyl metallocene complexes Cp*2LnCH(SiMe3)2 (Ln = La, Nd, Sm, Lu) show inactivity for ethylene polymerization under identi- cal conditions.[11,13] Although unsubstituted cyclopentadienyl lig- ands (Cp) frequently exhibit low activity, the pentamethylcy- clopentadienyl ligand Cp* typically offers exceptional solubility and stability for a metal ion. As a result, it is the most extensively utilized ancillary ligand in organolanthanide chemistry.[11] Even more stable bulky organolanthanides (e.g., from silylene-linked tetramethylcyclopentadianyl ligands, Scheme 3) developed by Bercaw and others showed enhanced activities in ethylene poly- merization and also catalyzed polymerization of other olefines beside ethylene.[14–19] The activity in polymerization of these neu- tral isoelectronic group 3 metallocene complexes Cp*2LnR (Ln = Sc, Y, lanthanides) that are 14 electron species can be explained by their isolobal analogy to cationic group 4 metallocene species.[14] A prominent example of controlled polymerization of func- tionalized polar olefins with rare-earth catalysts was devel- oped by Yasuda et al.[20] High molecular weight, syndiotactic poly(methyl methacrylate) (PMMA) was synthesized with very low polydispersities (PDI < 1.05) via a living group transfer polymerization employing organolanthanide(III) complexes, such as [Cp*2SmH]2 (Scheme 1). In the initial step, the inser- tion of methyl methacrylate (MMA) into the lanthanide–alkyl bond yielded the enolate complex. Michael addition of MMA to the enolate complex via an eight-membered transition state facilitated stereoselective C–C bond formation, generating a novel chelating enolate complex with two MMA units in which one is functioning as an enolate and the other is coordinated to the metal center in its original form via its carbonyl group. Sequential insertion of MMA led to the formation of polymer. In these polymerizations of polar monomers, the polymerization activity increased with the ionic radius of the metal (Sm > Y > Yb > Lu).[20] With such rare-earth metal catalytic systems it was possible for the first time to synthesize poly(alkyl acrylates) (alkyl = methyl (Me), ethyl (Et), n-butyl (nBu), tert-butyl (tBu)) with very high conversions to give high molecular weight polymers with very narrow molecular weight distributions by inhibiting termination and side reactions due to the acidic 𝛼-proton in these systems.[21] Beyond methacrylates, other polar monomers (e.g., (meth)acrylamides, vinyl phosphonates, 2-vinylpyridine (2VP), 2-isopropenyl-2-oxazoline (IPOx)) were successfully polymerized using various trivalent lanthanide complexes.[12,22,23] In contrast to the findings reported by Yasuda et al. for the polymerization of MMA, the polymerization rate of dialkyl vinylphosphonates (DAVPs) increases as the ionic radius of the metal center decreases. Late lanthanide initiators demonstrate higher ini- tiator efficiencies and low polydispersities (Lu > Yb > Tm > Er > Ho > Dy) when using tris(cyclopentadienyl)lanthanide complexes (Cp3Ln, Ln = Gd to Lu, Cp = 𝜂5-C5H5).[24] Par- ticularly noteworthy is that these DAVPs present challenges for polymerization with conventional techniques such as radical or classical anionic polymerizations.[23] These poly- mers exhibit notable biocompatibility and water solubil- ity, positioning them as promising candidates for diverse applications (fire retardation,[25] drug delivery,[26] battery systems,[27] photocatalysis[28]). However, frequently used alkyl Macromol. Rapid Commun. 2024, 45, 2400122 2400122 (2 of 24) © 2024 The Author(s). Macromolecular Rapid Communications published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.mrc-journal.de www.advancedsciencenews.com www.mrc-journal.de Scheme 1. Polymerization of MMA as described by Yasuda with the different organolanthanide catalysts.[10,30] initiators were identified as suboptimal initiators for DAVP, initiating through the abstraction of the 𝛼-acidic proton.[29] While the term coordination polymerization (or coordi- native anionic, or coordination addition polymerization) fre- quently denotes zirconocene-mediated polymerizations of polar monomers, for rare-earth metal-mediated polymerizations, the nomenclature rare-earth metal-mediated group transfer polymer- ization (REM-GTP) has become prominent in recent years, par- ticularly in the context of polymerizing Michael-type monomers. This term originates from the mechanism of a polymeriza- tion process initiated with silyl ketene acetals (SKA).[12,31–34] It refers to the initiator-assisted polymerization of Michael-type monomers, i.e., 𝛼,𝛽-unsaturated carbonyl compounds or other types of monomers in which carbon–carbon double bonds are connected with a nucleophilic site that can be stabilized by keto– enol tautomerism.[32,35] All polymerization subtypes (SKA, rare- earth metal-mediated, Lewis-pair polymerization, etc.) share a similar mechanism: propagation occurs through repetitive 1,4- additions, wherein the enolate form is coordinated and stabilized in each step by a metal cation or a positively polarized atom. Hence, this constitutes a distinct form of anionic polymeriza- tion. However, in contrast to conventional anionic polymeriza- tions, the growing chain ends in these polymerization methods are stabilized. This stabilization effectively prevents nearly all side reactions. Consequently, polymerizations can be conducted at room temperature or even elevated temperatures in contrast to classic anionic polymerization that are typically conducted at −78 °C. This positions group-transfer polymerization as superior to conventional anionic polymerization in terms of practicality and controllability.[31,36,37] While also numerous methods exist for polymerizing lac- tones, such as cationic or anionic pathways, they often face challenges associated with uncontrolled side reactions, resulting in increased polydispersity and impeding efficient and con- trollable polyester synthesis. Consequently, various catalysts have been developed for the coordinative ring-opening poly- merization of lactones. They provide distinct advantages such as narrow polydispersity, controlled molecular mass, stereoreg- ularity introduction, side reaction suppression, rapid reaction kinetics, targeted synthesis of diverse polymer architectures, and the ability to synthesize block copolymers enabled by a living-type polymerization mechanism.[38–44] Rare-earth metal complexes have enabled the controlled rare-earth metal-mediated ring-opening polymerization (REM-ROP) of lactones (e.g., 𝛽- propiolactone (PL), 𝛽-methylpropiolactone/𝛽-butyrolactone (BBL), 𝛿-valerolactone (𝛿VL), 𝜖-caprolactone (𝜖CL)), lactide, and other cyclic esters in ring-opening polymerizations.[40,41,45–47] New yttrium and scandium complexes with diketiminato-, bis(phenolate)ether, salen- or guanidinato-ligands were investigated.[48,49] A significant category of REM-ROP cata- lysts comprises the bis(phenolate)metal complexes introduced by Carpentier and co-workers in 2003.[48,50–53] The first complexes in this class were bis(dimethylsilyl)amide (bdsa = N(HSiMe2)2) yttrium-complexes with a tert-butyl substituted aminomethoxy- bis(phenolate) ligand ([ONOO]tBu) (Scheme 2a). Over time, a multitude of other initiators,[54–57] ligand substituents,[58,59] rare-earth metals,[55–57,59–63] chain transfer agents,[64] and side arms/backbones[55–57,59,60,62,63,65,66] were introduced to the bis(phenolate) catalysts. By using alcohols (e.g., isopropyl alco- hol) as chain-transfer agents the polymerization is transferred to Macromol. Rapid Commun. 2024, 45, 2400122 2400122 (3 of 24) © 2024 The Author(s). Macromolecular Rapid Communications published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.mrc-journal.de www.advancedsciencenews.com www.mrc-journal.de Scheme 2. Synthesis of [ONOO]tBuY(Nuc)(THF) complexes (Nuc = CH2TMS or bdsa) (TMS = trimethylsilyl/Me3Si) and the use of these bis(phenolate) yttrium catalysts in syndiospecific ring-opening polymerization of racemic 𝛽-butyrolactone (rac-BBL). an immortal polymerization leading to higher catalyst activities, and improved initiator efficiency, and the systems are capable of polymerizing monomers like lactide or BBL (Scheme 2b).[50,51] Reaction kinetics of various bis(phenolate) lanthanide catalysts (Ln = Sm, Tb, Y, Lu) were examined for BBL polymerization revealing an increasing catalytic activity with decreasing ionic radius (Sm < Tb < Y < Lu).[61] Biobased lactones held particular significance, as considera- tions regarding degradability and the source of monomer feed- stocks have gained prominence in recent years. Biobased poly- mers can be derived from diverse sources, encompassing natural lactones, natural acids, sugars, terpenes, terpenoids, lignin-based structures, and other bioderived materials.[38,40,67–77] With more than 30 000 terpenes/terpenoids known and some even being byproducts from large-scale industrial pro- cesses, they represent a predestined class of building blocks for polymers.[77–81] The terpenoid feedstocks of interest for ring- opening polymerization are limonene, with menthol and carvone as derived terpenoids, and turpentine oil, composed of 𝛼-pinene, 𝛽-pinene, and 3-carene.[40,82,83] Due to the unique dual catalytic function of rare-earth metal complexes for both polar and nonpolar monomers, these com- plexes (Cp*2LnR with Ln = Sm, Yb, Lu and R = H, Me) and silyl-bridged complexes were active in block copolymeriza- tions of ethylene and higher 1-olefins with polar monomers such as MMA and lactones (Scheme 3).[19,84–87] Adding 𝛿VL or 𝜖CL to a growing polyethylene end proceeded successfully yielding AB-type copolymers in high yield. However, when the respective monomers (MMA or lactones) are added first, followed by ethylene, block copolymerization is not initiated. Instead, only homopolymerizations of the polar monomers occurred. This was attributed to relatively weaker coordina- tion or donating properties of ethylene compared to MMA or lactones.[84] 3. 𝝈-Bond Metathesis with Rare-Earth Metal Complexes In organometallic chemistry, 𝜎-bond metathesis ranks among the most effective for cleaving nonconjugated C–H bonds, which, characterized by bond dissociation energy between 350 and 550 kJ mol−1 (sp3 ˂ sp2 ˂ sp) and low acidity as well as basicity, are considered among the least reactive and therefore most chal- lenging to cleave.[88–91] It is only within the last four decades that numerous catalysts have been developed, enabling the selective activation of C–H bonds under mild conditions. Despite the progress in this re- search field, it remains primarily oriented toward fundamental studies, with practical applications of this type of metal catalysis being infrequent to date. The term 𝜎-bond metathesis refers to a C–H bond activation strategy involving trivalent lanthanides and d0 transition metals. In this process, oxidative addition is precluded due to the absence of electrons, resulting in a reaction proceeding without altering the oxidation state.[92,93] Fundamentally, 𝜎-bond metathesis in- volves the coordinated substitution of a metal–ligand 𝜎-bond with that of an incoming substrate. Alkyl or hydride complexes of “early” transition and lanthanide metals with d0 (or d°fn) elec- tronic configurations exhibit this class of reactions.[94–98] These metals predominantly belong to group 3 of the periodic table (Sc, lanthanides, and actinides). Consequently, these metal com- pounds frequently serve as well-examined model complexes for studying 𝜎-bond metathesis reactions. However, studies involv- ing metals from groups 4 and 5 have also been documented.[99] This reaction demonstrates a first-order kinetic dependent on concentrations of the electrophilic metal complex and the sub- strate and progresses through a [2𝜎 + 2𝜎 ] cycloaddition, incor- porating a metal–ligand bond and the C–H or heteroatom-H bond of the substrate (Scheme 4). Given the concerted nature Macromol. Rapid Commun. 2024, 45, 2400122 2400122 (4 of 24) © 2024 The Author(s). Macromolecular Rapid Communications published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.mrc-journal.de www.advancedsciencenews.com www.mrc-journal.de Scheme 3. Block copolymerization of 1-olefins with polar monomers (MMA and different lactones) catalyzed by organolanthanides(III).[84–87] Scheme 4. C–H bond activation by d0 transition metal complexes and trivalent lanthanides through 𝜎-bond metathesis.[96] of this process, the formal cycloaddition step represents a four- membered transition state rather than an intermediate.[88] How- ever, the “real” transition state is distorted from a square four- membered construct and is rather a kite-like transition state.[100] In light of the simplicity inherent in the square shape, most il- lustrations continue to conform to this form. The proposed mechanism was primarily proposed for the hy- drogenation of metal–alkyl bonds and the H/D scrambling of metal–hydride bonds but 𝜎-bond metathesis can involve H–H, C–H, and also other E–H bonds (E = As, P, Si, Sn, etc.) of dif- ferent substrates. For d0 metals, the tendency to form the tran- sition state depends on both electronic and steric effects.[88,100] Especially computational calculations can help to determine the product outcome of these reactions in terms of regio- and chemoselectivities.[93] Generally, the 𝜎-bond metathesis of C–H bonds follows the trends: 1) C–H bonds with a higher s-orbital character (sp) react faster than C–H bonds with a higher p-orbital character (sp > sp2 > sp3), 2) sp3 bond activation shows a selec- tivity toward less-hindered primary groups, and 3) C–C bond ac- tivation is not favored for these metals.[88,101,102] The C–H bond activation by 𝜎-bond metathesis (not yet re- ferred to by this name at this point in time) was initially Macromol. Rapid Commun. 2024, 45, 2400122 2400122 (5 of 24) © 2024 The Author(s). Macromolecular Rapid Communications published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.mrc-journal.de www.advancedsciencenews.com www.mrc-journal.de Scheme 5. Methane exchange reaction with isotopically labeled methane and cyclopentadienyl-lanthanide methyl complexes Cp*2LnMe (Cp* = 𝜂5- C5Me5, Ln = Lu, Y). demonstrated by Watson in 1983, utilizing isotopically labeled methane and also involving the activation of benzene, pyridine, tetramethylsilane, and phosphorylides with cyclopentadienyl- lanthanide methyl complexes Cp*2LnMe (Ln = Lu, Y) and lutetium hydride complexes Cp*2LuH to give stable products through metalation at a carbon of the substrate (Scheme 5).[94,95] This marked the first instance of activating the sp3-hybridized C–H bond in methane, which is typically unfavorable for 𝜎-bond metathesis, using homogeneous catalysis. Bercaw and co-workers focused on the 𝜎-bond metathe- sis using Cp*2 metallocenes, e.g., using Cp*2ScR (R = CH3, CH2C6H5, C6H5, C6H4CH3 (ortho, meta, para)) for the study of hydrogen activation to form n−1[Cp*2ScH]n.[96,103] It seems that the strongly bound and sterically hindered Cp* ligands and smaller rare-earth metals are more favorable for such a re- action and were also predominantly tested for C–H bond ac- tivations afterward. Further examples encompass the 𝜎-bond metathesis reactions involving hydrocarbons,[104–106] heteroaro- matic substrates,[107–110] and internal alkynes[110,111] with rare- earth metallocenes. In this context, Teuben and co-workers used alkyl organolanthanides Cp*2LnCH(SiMe3)2 (Ln = La, Ce) and 2-alkynes CH3C≡CR (R = Me, Et, nPr) for a propargylic met- alation of the 𝛼-methyl group of the substrate, resulting in Cp*2LnCH2≡CR complexes. Similar products were obtained with [Cp*2YH]2, but not when using Cp*2YCH(SiMe3)2.[111] Af- terward, alkyl complexes Cp*2LnCH(SiMe3)2 (Ln = Y, Ce, La) or Me2Si(𝜂5-C5Me4)2CeCH(SiMe3)2 were tested toward reaction with a broader scope of 1-methylalk-2-ynes CH3C≡CR (R = Me, Et, nPr, tBu, SiMe3, Ph, C6H4Me-2, C6H3Me2-2,6, C6H3 iPr2- 2,6, C6F5) to obtain the corresponding 𝜂3-propargyl/allenyl complexes Cp*2LnCH2CCR or Me2Si(𝜂5-C5Me4)2CeCH2CCR via propargylic metalation.[112] The Cp* complexes manifest a spec- trum of “tuck-in” (one methyl group of Cp* coordinates to the metal center) and “tuck-over” (methyl group of Cp* ligand coordinates to another metal center bridging two metal cen- ters) conformations in the context of bond metathesis reactions (Figure 1).[113–115] This underscores a significant potential obsta- cle in the design of supporting ligands for these reactive met- als. Therefore, not only metallocene lanthanide alkyls and hy- drides were examined, but also investigations were carried out on non-metallocene rare-earth metal complexes. The advancement of polydentate and anionic ligands, possessing adequate size, sol- ubility, and robustness to effectively stabilize the complex during 𝜎-bond metathesis reactions, has predominantly concentrated on ligands with nitrogen and oxygen donor functionalities. In 1993, Teuben and co-workers initiated investigations into the reactivity of non-metallocene yttrium catalysts Figure 1. a) “Tuck-in” and b) “tuck-over” complexes derived from Cp*– lutetium complexes by 𝜎-bond metathesis with a methyl group of the Cp* ligand.[113–115] (PhC(NSiMe3)2)2YCH(SiMe3)2 and [(PhC(NSiMe3)2)2Y(μ- H)]2 (PhC(NSiMe3)2)2 = N,N′-bis(trimethylsilyl)benzamidato ligands) toward the C–H bond activation of ethyne and various terminal alkynes.[116,117] In contrast to previously considered systems, the newly formed Y-μ-C bond in the formed bis(N,N ′-bis(trimethylsilyl)benzamidinate)yttrium complexes ([(PhC(NSiMe3)2)2Y(μ-C≡CR)]2 (R = H, Me, nPr, SiMe3, Ph, CMe3) is stabilized through two chelating N,N′- bis(trimethylsilyl)benzamidato ligands rather than cyclopen- tadienyl systems. The reaction leads to the formation of μ-acetylide dimers that can be cleaved by the addition of tetrahydrofuran (THF). To evaluate such non-metallocene yt- trium complexes for the activation of other compounds and the formation of new metal–carbon bonds, the same group explored the C–H bond activation of heteroaromatic sub- stances such as pyridine or 𝛼-picolyl derivatives. A bis(N,O- bis(tert-butyl)alkoxy(dimethylsilyl)amido)yttrium complex [Me2Si(NCMe3)(OCMe3)]2YCH(SiMe3)2 reacts with pyridine and the corresponding methyl, ethyl, and dimethyl-substituted pyridine (ortho-substituted) via 𝜎-bond metathesis, leading to the respective bis(alkoxysilylamido)yttrium-pyridyl and -picolyl complexes (Scheme 6). This reaction necessitated elevated temperatures but resulted in low to moderate yields due to decomposition reactions of the reactant at these temperatures. In reactions with 2-picolin and ethylpyridine, only C–H bond activation of the sp3-hybridized 𝛼-alkyl group occurred, not involving the aromatic ring. Crystallographic and NMR analy- ses illustrated that the picolyl–ligand coordinates through an 𝜂3-(C,C,N)-aza-allylic bond.[118] While polymerization studies were not included in these investigations, they constituted the foundational groundwork for subsequent research endeavors. In these subsequent studies, structurally derived pyridines were employed as initiators for polymerization through C–H bond activation, aiming at the synthesis of functional polymers using various monomers. Macromol. Rapid Commun. 2024, 45, 2400122 2400122 (6 of 24) © 2024 The Author(s). Macromolecular Rapid Communications published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.mrc-journal.de www.advancedsciencenews.com www.mrc-journal.de Scheme 6. Synthesis of bis(alkoxysilylamido)yttrium-pyridyl and - picolyl complexes through 𝜎-bond metathesis reaction of pyridine and ortho-substituted methyl, ethyl, and dimethyl-substituted pyridine with [Me2Si(NCMe3)(OCMe3)]2YCH(TMS)2.[118] 4. Chain-End Functionalized Polyolefins Metallocene alkyls and hydrides of group 3 metals and trivalent lanthanides can be regarded as neutral analogs of the cationic group 4 metallocene 14-electron species, recognized for their efficacy in catalytic olefin polymerization.[5,13,108] The first tests in ethylene polymerization with organolanthanides were per- formed with bis(pentamethylcyclopentadienyl) lanthanide hy- drides [Cp*2LnH]2 and alkyl complexes exhibiting high activity in the polymerization of ethylene, leading to the formation of high molecular weight poly(ethylene) without including any function- ality to these inert polymers.[5,9,13] Additionally, these complexes can also undergo C–H bond activation (vide supra), special at- tention was also given to activation with Cp*2LnCH(SiMe3)2 (Ln = Y, La, Ce).[94–98] This C–H activation potential hinders an effi- cient propene polymerization due to the formation of an inactive 𝜂3-allylic species but can be utilized for C–H activation of small molecules with metallocene alkyls and hydrides.[108] Polyolefins with incorporated functionalities offer superior physical properties, including dyeability, adhesive properties, and compatibility with diverse materials (especially more polar mate- rials), compared to nonfunctionalized polyolefins. Nevertheless, achieving selective and catalytic integration of polar functionali- ties into these chemically inert polymers is challenging.[119] 4.1. (Co)polymerizations Using Electron-Deficient Chain Transfer Agents The first attempts of produced end-functionalized polyolefins (e.g., poly(ethylene), poly(1-octene), poly(styrene)) using metal catalysts have been studying electron-deficient or electron- neutral reagents (e.g., group 13 and 14: boranes,[120–122] alanes,[123,124] silanes[125,126]) in which the E–H bond was activated (E=B, Al, Si) (Scheme 7). However, most of these studies focused on zirconocene or titanocene catalysts. Some of them could gen- erate hydroxy-terminated polyolefins after transformation of the terminal group or other functional groups that could react fur- ther with other monomers to form copolymers (e.g., MMA, lac- tones, etc.).[120,121,123] Using boranes as chain-transfer agents dur- ing metallocene-catalyzed olefin polymerization not only offers a convenient route to introduce a reactive group at the polymer chain end but also allows for the synthesis of polyolefin diblock copolymers.[120–122] The selective autoxidation of the borane-end group by oxygen forms stable polymeric radicals at the end-group that can be used for chain extension free radical polymerization of polar monomers such as MMA. Using these electron-deficient or electron-neutral chain trans- fer agents, the heteroatom undergoes chain transfer to the poly- mer chain at the end of a hydride-based catalytic propagation cy- cle by C-heteroatom bond formation in the polymer chain. In this proposed mechanism, this final C-heteroatom bond-forming step at the end of the propagation cycle occurs through a four- centered 𝜎-bond metathesis transition state.[119,126,127] The efficient and selective use of silanes (PhSiH3, nBuSiH3, PhSiH3) as chain transfer agents to produce silyl-terminated polyethylene was enabled by using [Cp*2LnH]2 (Ln = Sm, Lu, Y, La) or Me2SiCp′2SmCH2TMS (Cp′ = (CH3)4C5).[126] The mechanism was assumed to be analogous to that of a hydrosilylation process while first forming Ln–C (poly- mer) bonds followed by Si–H/Ln–C transposition transfer- ring the silicon group to the terminal end of the polymer chain. When using Me2SiCp′2SmCH2TMS as a precata- lyst, the reaction with PHSiH3 led to the formation of the active catalyst [Me2SiCp′2SmH]2. Copolymerizations were enabled using the more efficient Me2SiCp′2SmCH2TMS and Me2SiCp′2NdCH2TMS precatalysts. As copolymers, poly(ethylene-co-1-hexene) and poly(ethylene-co-styrene) were obtained. 4.2. Polymerization Using Electron-Rich Chain Transfer Agents Electron-rich substances like phosphines or thiophenes show a different mechanism compared to electron-deficient substrates, which is in analogy to hydrophosphination or hydroamination reactions.[128,129] In these cases, initiation and therefore the first step of the propagation cycle takes place by ethylene insertion into the Ln–E (E = P, C(sp2), C(sp3), N, etc.) bond and not into the Ln–H bond leading to a C–E or C–C bond formation (Scheme 8).[127] In these cases, the C–H bond activated lanthanide complex Cp*2LnERm serves as the catalytic species.[11] Kawaoka and Marks investigated the synthesis of phosphine- terminated poly(ethylene)s catalyzed by lanthanide complexes by the use of primary and secondary phosphines as chain trans- fer agents.[119,127] The proposed catalytic process for generating phosphine-terminated poly(ethylene) chains involves C═C in- sertion into a lanthanide–phosphide bond, in which the lan- thanide Cp*2LnPR2 (Ln = Lu, Y, Sm, La) was generated by reaction of the respective phosphine (diphenyl-, dicyclohexyl-, Macromol. Rapid Commun. 2024, 45, 2400122 2400122 (7 of 24) © 2024 The Author(s). Macromolecular Rapid Communications published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.mrc-journal.de www.advancedsciencenews.com www.mrc-journal.de Scheme 7. A proposed catalytic mechanism for the synthesis of functional-group terminated poly(ethylene) using [Cp*2LnH]2 catalysts with electron- deficient or electron-neutral chain-transfer agents.[127] di-iso-butyl-, diethyl-, diphenyl-, cyclohexyl-, and phenyl phos- phine) with [Cp*2LnH]2 which was obtained from hydrogena- tion of Cp*2LnCH(SiMe3)2. Successive ethylene insertions into the resulting Ln–C bond and protonolysis of the growing poly- olefin chain by incoming phosphine complete the cycle and re- generate the lanthanide–phosphide species. Due to the Brønsted acidity of HPPh2 protonolytic chain transfer takes via a four- centered 𝜎-bond transition state place instead of transferring the phosphine to the polymer chain end (Scheme 8(ii)).[119,127] The absence of vinylic resonances and a 1:1 ratio of PPh2 to CH3 end groups in NMR spectra underlined the absence of 𝛽-H elimination as a chain transfer or termination reaction and a phosphine moiety on one end-group per chain when diphenylphosphine is used as a chain-transfer agent. For some other chain-transfer agents and metal catalyst combinations (e.g., Cp*2YPiBu2, Cp*2YPCy2), 𝛽-H elimination is a competing ter- mination process. High polymerization activities and narrow polydispersities are generally observed when secondary phos- phines are present. However, the use of dicyclohexylphosphine did not yield satisfactory outcomes. Regardless of the lanthanide metal utilized, diphenylphosphine proves to be the most effec- tive secondary phosphine in ethylene polymerization resulting in polymers with low polydispersities. Polymerizations mediated with the respective lanthanum catalyst in comparison to samar- ium, yttrium, and lutetium yielded only low molecular weight poly(ethylene) indicating that ionic radius and molecular weight Macromol. Rapid Commun. 2024, 45, 2400122 2400122 (8 of 24) © 2024 The Author(s). Macromolecular Rapid Communications published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.mrc-journal.de www.advancedsciencenews.com www.mrc-journal.de Scheme 8. A proposed catalytic mechanism for the synthesis of functional-group terminated poly(ethylene) with electron-rich chain-transfer agents. i) Insertion of C═C into the Ln–E bond, ii) successive incorporation of multiple C═C units into the developing polymer chain, and iii) Ln–polymer protonolysis, accompanied by the simultaneous regeneration of the active lanthanide species Cp*2LnERm with the Ln–E bond, completing the catalytic cycle.[130] are reversely related. In the case of ethylene polymerizations me- diated by Cp*2YPPh2, an inversely proportional correlation is ob- served between the number-average molecular weight and the diphenylphosphine concentration, consistent with a phosphine protonolytic chain-transfer mechanism. Polymerizations carried out in the presence of primary phosphines yielded products with low molecular weights showing that protonolysis of the Ln–C bond might be favored over ethylene insertion.[127] Additionally, phosphine-coordination of less bulky phosphines to the metal center could further hinder ethylene coordination and insertion and favor protonolysis. To further understand the polymeriza- tion mechanism, the reactivities of these phosphines and the de- pendence on the metal center on protonolysis experiments were performed with Cp*2LnCH(SiMe3)2 and six different phosphines at 60 °C. The rate of protonolysis in dependence of the phos- phine was HPCy2 < HPiBu2 ≈ HPEt2 < HPPh2 ≈ H2PCy ˂˂ H2PPh, which correlates to the steric bulk of the substitutes and Brønsted acidity and proved the high efficiency of diphenylphos- phine amongst the secondary phosphines.[127] Another study from Hessen et al. was successful in synthe- sizing thienyl-capped poly(ethylene), thus including another heteroatom functionality to inert polymers.[108] The Cp*2La complex enabled both the polymerization of ethylene and the activation of C–H bonds in thiophene, ultimately yielding poly(ethylene) with 2-thienyl end groups through catalysis (Scheme 9). [Cp*2LaH]2 was reacted with thiophene resulting in Macromol. Rapid Commun. 2024, 45, 2400122 2400122 (9 of 24) © 2024 The Author(s). Macromolecular Rapid Communications published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.mrc-journal.de www.advancedsciencenews.com www.mrc-journal.de Scheme 9. A proposed catalytic mechanism for the synthesis of thienyl-capped poly(ethylene). Initiation involves the insertion of ethylene into the La–C(thienyl) bond. Chain growth proceeds with successive insertions of ethylene into the La-alkyl species formed. Chain transfer occurs through C–H activation of thiophene by the La-alkyl species, resulting in the liberation of the polymer as an alkane (protonolysis) and the regeneration of the La-thienyl species.[108] the 2-thienyl complex [Cp*2La(2-C4H3S]2, a product that was not formed when the respective yttrium precursor was used, proba- bly due to the larger ionic radius of lanthanum. The lanthanum complex was able to polymerize ethylene resulting in 2-(n-alkyl)- thiophenes. It was proven that C–H activation of thiophene was the only chain-transfer reaction happening leading to defined thiophene-end groups of poly(ethylene). Due to stabilization Cp*2La(CH2CH2)n(C4H3S) for n = 1,2 larger amounts of 2-ethyl thiophene and 2-butyl thiophene and no formation of high molecular weight poly(ethylene) was observed. The reversible coordination observed in the La/thiophene system, along with the relatively slow initial insertion into the La–C(sp2) bond of the thienyl species required for initiation, contributes to the modest activity of the catalytic system.[108] Nonetheless, the catalyst demonstrates robustness under the applied conditions. Despite the apolar nature of the thienyl functionality, due to the delocalized 𝜋-electron system, it remains open to further functionalization including but not limited to oxidative coupling, cross-coupling, Friedel–Crafts alkylation/acylation, desulfuriza- tion, halogenation, metalation, and metal insertion.[131–133] Macromol. Rapid Commun. 2024, 45, 2400122 2400122 (10 of 24) © 2024 The Author(s). Macromolecular Rapid Communications published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.mrc-journal.de www.advancedsciencenews.com www.mrc-journal.de A combination of ethylene conversion and C–H bond activa- tion was reported before with the heteroaromatic pyridine and [Cp*2YH]2.[134] The high stability of the initial insertion prod- uct of ethylene into the M-pyridyl bond of Cp*2Y(2-pyridyl), fa- cilitated by intramolecular N-coordination of the pyridyl group, leads to the formation of 2-ethyl-pyridine with only minimal traces of higher alkyl products. The weaker and softer Lewis basic characteristics of thiophene makes it more amenable to the generation of higher molecular weight products.[108] In con- trast to these observations, Marks and co-workers reported on the first amine chain transfer mechanism, producing amine- capped poly(ethylene) by organolanthanide catalysis.[130,135] As catalysts, Cp*2LnCH(SiMe3)2 (Ln = Lu, Y, Sm, La) was re- acted with amines. In comparison to the analog phosphine- capped polymers, Ln−C protonolyses were ≈104 times faster. Tuning of the steric and electronic characteristics of the amine chain transfer agent is essential for chain propagation. The efficiency of amine chain transfer follows the sequence: C6H5NH2 ≈ nC3H7NH2 << [Si(CH3)3]2NH ≈ secBu2NH < N- tBu[Si(CH3)3]NH ≈ iPr2NH < Cy2NH.[135] This sequence re- sults in the formation of poly(ethylene) with the structural mo- tif H(CH2CH2)nNRR′, where an effective chain-transfer agent is characterized as a substance that terminates polymer chain growth and concurrently enables the reinitiation of polymer chain growth. The molecular weights of poly(ethylene) prod- ucts capped with dicyclohexyl amine were observed to exhibit an inverse relationship with the concentration of dicyclohexyl amine. This observation aligns with the assumption of an amine chain transfer mechanism. NMR spectroscopy proved that chain termination via 𝛽-hydride elimination is insignificant for these mechanisms due to the absence of vinylic resonances in these spectra.[135] Rheingold and co-workers describe the synthesis and characterization of neutral lanthanide silyl complexes (Cp*2LnSiH(SiMe3)2 (Ln = Sm, Nd, Y) and Cp″2LnSiH(SiMe3)2 (Cp″ = 𝜂5-C5Me4Et; Ln = Sm, Nd)) via a 𝜎-bond metathe- sis reaction of H2Si(SiMe3) with Cp*2LnCH(SiMe3)2 and Cp″2LnCH(SiMe3)2 at 85 °C, respectively.[136,137] The structure of Cp*2SmSiH(SiMe3)2 shows that this compound forms dimers in the solid state via intermolecular Sm–CH3–Si interactions but is monomeric in pentane solution. The novel silyl complexes are extremely air- and moisture-sensitive, but are thermally stable in solution and the solid state. Until then only a few silyl derivatives of lanthanides have been reported and studied, synthesized by salt metathesis reaction of the corresponding chloride complex.[138–142] The group also investigated the reac- tivity of these complexes toward several molecules, including ethylene, however without giving mechanistic details or polymer analysis. The studies confirmed that the Ln–Si bonds of the silyl complexes are highly reactive. 5. End-Functionalized Polar Polymers The electron-deficient nature of rare-earth metal d° complexes imparts Lewis acidic character to the metal center. This Lewis acidity enhances the affinity of the metal for coordination with Lewis basic sites, especially present in oxygen and nitrogen- containing monomers. Early investigations into metal-catalyzed group-transfer polymerization centered around metallocene complexes. In 1992, Yasuda et al. employed a neutral samarocene [Cp*2SmH]2 for the polymerization of MMA.[20] Demonstrating activity across a broad temperature range (−95 °C to 40 °C), the catalyst yielded PMMA with low molecular weight distributions (Ð < 1.05) and high syndiotacticity reaching up to 95%. 5.1. Boosting Initiator Efficiencies Depending on the nature of the initiating ligand, initiation with rare-earth metallocene catalysts can occur through the abstrac- tion of the acidic 𝛼-CH of the monomer, the transfer of a nucle- ophilic ligand to a coordinated monomer, or a monomer-induced ligand-exchange reaction.[29] The initiators employed, particu- larly the traditionally used strongly basic methyl, CH2TMS or hy- dride initiators, were found to have distinct limitations, thereby constraining their efficient utilization to specific monomers only. To introduce more efficient initiators, Rieger and co- workers successfully demonstrated the C–H bond activation of N-heteroaromatic compounds using rare-earth cyclopentadienyl systems.[143] The reaction of Cp2Y(CH2TMS)(THF) with one equivalent of 2,4,6-trimethylpyridine (sym-collidine/sym-coll) in toluene at room temperature resulted in the formation of Cp2Y(CH2(sym-coll) after 30 min through 𝜎-bond metathesis. It was not directly stated that the initiator was bound through aza-allylic coordination, but a partial double bond character was discussed using the X-ray structure of Cp2Y(CH2(C5H2Me2N)) implying it is the same metal coordination environment. The analogous C–H activation with the lutetium catalyst exhibited lower activity and required an overnight reaction for quantitative completion. Rieger and co-workers primarily focused on the polymeriza- tion of phosphorus-containing polymers, i.e., DAVPs, via REM- GTP to obtain poly(dialkyl vinylphosphonates). Alkyl initiators were identified as suboptimal initiators, initiating through the abstraction of the 𝛼-acidic proton. The resulting allenyl phospho- nate anion serves as the real initiator. However, despite the chem- ical similarity, the nucleophilic attack of the phosphonate anion at the double bond of a second activated phosphonate monomer is impeded, resulting in low initiator efficiencies and prolonged initiation times.[23] The polymerization of diethyl vinylphospho- nate (DEVP) conducted with Cp2Y(CH2(sym-coll), either isolated or generated in situ, demonstrated a living character, character- ized by linear growth of molecular weight with conversion and high initiator efficiencies.[143] The isolated poly(diethyl vinylphos- phonate) (PDEVP) exhibited narrow molecular weight distribu- tions (Đ = 1.05). Using this catalyst, initiation can occur through an eight-membered transition state analogous to propagation (Scheme 10). This process is significantly favored over initiation via deprotonation. End-group analysis of oligomers using electro- spray ionization mass spectrometry (ESI-MS) demonstrated that each polymer chain was covalently bound to the initiator. In a different approach, yttrium-based constrained geom- etry complexes (CGCs) were developed to induce isotacticity to PDEVP.[144] To enhance initiator efficiencies CH2TMS was replaced with 2,6-lutidine by C–H bond activation. Regulat- ing stereoregularity remains a crucial aspect of a precision polymerization technique and is fundamental for the advance- ment of functional materials. The authors conducted detailed Macromol. Rapid Commun. 2024, 45, 2400122 2400122 (11 of 24) © 2024 The Author(s). Macromolecular Rapid Communications published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.mrc-journal.de www.advancedsciencenews.com www.mrc-journal.de Scheme 10. The heteroaromatic initiator coordinates to the metal in the form of a carbanion or enamide as proposed by the authors, and an eight- membered transition state is proposed for initiation of DEVP. Reproduced with permission.[143] Copyright 2015, American Chemical Society. structural analyses using multidimensional NMR (1H−1H and 1H−13C(−31P)) correlation experiments to investigate the mi- crostructure of the resulting polymers. The proposed chain-end control mechanism elucidates the isospecific yttrium-mediated catalysis, providing the missing link in understanding the preci- sion in group-transfer polymerization of DEVP. Such high and enhanced initiator efficiencies as observed for DEVP polymerization with pyridyl-cyclopentadienyl-yttrium complexes in comparison to the corresponding CH2TMS complex were not observed for all types of monomers and rare-earth metal catalyst.[145] Furthermore, some metallocene lanthanides exhibit inadequate activity in the polymerization of 2VP, IPOx, or N,N-dimethylacrylamide (DMAA).[23] Mashima and co-workers demonstrated the polymerization of 2VP after C(sp3)–H or C(sp2)–H bond activation of N-heteroaromatic substances (including but not limited to 4-methylpyridine, 2-phenylpyridine, sym-collidine or 2,3,5,6-tetramethylpyrazine) using yttrium en-diamido complexes.[145–147] Exploration of the controlled coordination polymerization of 2VP has been limited in previous research endeavors. In Mashima’s study, the catalysts with novel pyridyl initiators were prepared in situ through 𝜎-bond metathesis and subsequently investigated in the living polymerization of 2VP.[146] Based on these findings, a sys- tematic study with isolated and fully characterized isostructural 2-aminoalkoxybis-(phenolate)yttrium and lutetium complexes was performed by Rieger and co-workers.[145] In contrast to findings in DEVP and DMAA polymerization, 2-aminoalkoxy- bis(phenolate)yttrium complexes [(ONOO)tBuY(initiator)(THF)] with heteroaromatic initiators (sym-collidine was introduced as an initiator via C–H bond activation) exhibited reduced efficiency in polymerization with electron-donating monomers (2VP and IPOx). Computational studies emphasized that the coordination of nitrogen-donating monomers was hindered when utilizing yttrium catalysts with electron-donating ini- tiators. It was calculated that a high electron density/charge localization on the nucleophilic carbon atom of alkyl initiators is crucial for effective nucleophilic attack on the loosely coor- dinated nitrogen-coordinating monomers. The combination of an electron-donating nitrogen atom and reduced electron density on the activated CH2 group of collidine (due to charge delocalization) was insufficient for efficient nucleophilic trans- fer to these electron-donating monomers. Additionally, using both, electron-donating monomers and N-coordinating initiator, weakens the binding energy of the collidine moiety leading to detachment after reacting with the first monomer due to an overload of electron density to the metal center.[145] Addi- tionally, switching the metal center to Lutetium, these catalysts [(ONOO)tBuLu(initiator)(THF)] demonstrated the highest activity for REM-GTP of 2VP at that time. Attributable to the increased Lewis acidity of lutetium, a more pronounced polarization of the coordinating monomer in the propagation mechanism may be the reason for its higher activity. However, a stronger binding of the CH2TMS initiator to the lutetium center was reflected by lower initiator efficiency compared to the corresponding yt- trium compounds. Unlike the yttrium complexes, transitioning from CH2TMS to sym-collidine as the initiator proved to be an effective strategy for enhancing initiator efficiency, resulting in an ≈45% increase.[145] While these studies primarily constituted fundamental research, their findings played a crucial role in understanding initiation mechanisms and significantly im- proved the polymerizability of various monomers. Furthermore, a newly acquired ability for diverse end-group functionalization was achieved through C–H activation of pyridine-analogous structures. The 2-methylpyridine framework offers a myriad of modification possibilities that can substantially influence the properties and potential applications of the polymer. 5.2. Copolymer Architectures In a study on metallocene-mediated group-transfer polymer- ization, the precursor complex Cp2Y(CH2TMS)(THF) under- went threefold C–H bond activation of 1,3,5-tris(3,5-dimethyl- 4-pyridinyl)benzene, resulting in a trinuclear catalyst.[148] This catalyst facilitated the rapid catalytic synthesis of star-shaped polymeric structures. Incomplete initiation led to the forma- tion of short linear, long linear, and star-shaped structures with mol fractions being 32:46:22 (star-shaped:long linear:short linear) calculated using probabilities of chain growth occur- ring in three, two, and one directions and initiator efficien- cies. Visualization was accomplished by converting poly(2-iso- propenyl-2-oxazoline) (PIPOx) into P(IPOx-g-EtOx) (EtOx = 2- Macromol. Rapid Commun. 2024, 45, 2400122 2400122 (12 of 24) © 2024 The Author(s). Macromolecular Rapid Communications published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.mrc-journal.de www.advancedsciencenews.com www.mrc-journal.de Figure 2. Top: Scheme for transformation of star-shaped PIPOx into a macroinitiator for grafting of EtOx by using living cationic ring-opening polymer- ization and termination using piperidine. Bottom: a) Overall view of an AFM scan of P(IPOx-g-EtOx). b–d) Sections of this AFM scan: (b) star polymer, (c) long linear polymer, and (d) short linear polymer. Adapted with permission.[148] Copyright 2017, American Chemical Society. ethyl-2-oxazoline) and subsequent analysis using atomic force microscopy (AFM) (Figure 2). Star-shaped polymers, charac- terized by their compact structure, display remarkable physi- cal properties that set them apart from their linear counter- parts. Consequently, these polymers are well-suited for spe- cific applications, for example, as viscosity index improvers and thermoplastic elastomers, in cosmetics or the paper industry.[149] Another article presents the synthesis of copolymers using a hydrophobic poly(dimethylsiloxane) (PDMS) block as a macroini- tiator for the formation of PDEVP and through REM-GTP.[150] Known for its high hydrophobicity and unique applications Macromol. Rapid Commun. 2024, 45, 2400122 2400122 (13 of 24) © 2024 The Author(s). Macromolecular Rapid Communications published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.mrc-journal.de www.advancedsciencenews.com www.mrc-journal.de Scheme 11. PDMS macroinitiators containing 2,6-dimethyl pyridine motifs for C–H bond activation with Cp2Y(CH2TMS)(THF) and subsequent poly- merization of DEVP catalyzed by these in situ generated complexes. Reproduced with permission.[150] Copyright 2020, American Chemical Society. spanning various fields, including antifoaming agents, medicinal uses, and soft lithography.[151,152] Additionally, PDMS is generally considered nontoxic. Copolymers from PDMS can have attrac- tive features for more advanced applications in, e.g., the biomed- ical sector. Macroinitiators were synthesized via hydrosilylation of several Si–H-containing PDMS substrates with 4-(allyloxy)-2,6- dimethylpyridine (Scheme 11). These macroinitiators were uti- lized in the C–H bond activation with Cp2Y(CH2TMS)(THF) and used in the polymerization of DEVP, resulting in the correspond- ing block copolymers.[150] This strategy was supplemented by in- corporating 2VP as an alternative Michael-type monomer. More- over, the controlled ring-opening polymerization of 𝜖CL to yield P𝜖CL-b-PDMS-b-P𝜖CL was successfully conducted. This marks the first publication of a C–H bond-activated metallocene yttrium system effectively applied in the REM-ROP of lactones. The synthesis of (multi-)block copolymers is facilitated through living-type polymerization characteristics of rare-earth metal-mediated polymerization.[22,38,153] Sequential addition al- lows for copolymerization of various monomers into block copolymers, but a specific addition order must be taken into ac- count. During propagation, the next monomer unit must dis- place a coordination site of the polymer chain to the catalyst, requiring its coordination to be stronger than that of the poly- mer unit. This holds within one monomer class, but not across different monomer classes. Copolymerization screenings have determined the following relative coordination strength series (for the same metal center) for REM-GTP: DEVP > DMAA > MMA > IPOx > 2VP.[154] Hence, monomers with distinct sub- stitutions from the same class can be statistically copolymer- ized, whereas combinations involving different monomer classes are constrained to form block structures exclusively. Therefore, achieving a BAB structure is not feasible with conventional ini- tiators. This limitation can be overcome by using bifunctional ini- tiators enabling chain growth in two directions.[26,155] Specifically in the synthesis of polymers tailored for drug delivery, the inves- tigations carried out have proficiently showcased the significant potential of this methodology in creating multi responsive drug carrier systems. Sequential copolymerization of 2VP with DEVP, as well as its methyl and propyl derivatives, was conducted.[26,156] This pro- cess utilized the monometallic yttrium bis(phenolate) catalyst with an alkyl (CH2TMS) (Scheme 2), a methylpyridine initiator, and the bimetallic yttrium bis(phenolate) catalyst with a 2,3,5,6- tetramethylpyrazine (TMPy) initiator (Scheme 12). The latter catalyst was synthesized through 𝜎-bond metathesis of the N- heteroaromatic compound with (ONOO)tBuY(CH2TMS)(THF). X-ray crystallographic structures showed that the observed dual 𝜂3-(C,C,N)-aza-allylic coordination takes place on adjacent rather than opposing methyl groups in TMPy.[26] This approach success- fully achieved the synthesis of amphiphilic AB-, ABB′-, and BAB- type block copolymers. The C–H bond activation of TMPy with (ONOO)tBuY(CH2TMS)(THF) enabled the synthesis of so far unattained well-defined PDEVP-b-P2VP-b-PDEVP (BAB) structures (Ð ≤ 1.20) (Scheme 12). These copolymers consist of a hydrophobic P2VP block covalently linked to hydrophilic PDEVP blocks showing amphiphilic characteristics and a dual responsive behavior (pH and temperature).[26,156] These BAB- type block copolymers were further investigated regarding their fluorescein release behavior under varying pH and temperature Macromol. Rapid Commun. 2024, 45, 2400122 2400122 (14 of 24) © 2024 The Author(s). Macromolecular Rapid Communications published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.mrc-journal.de www.advancedsciencenews.com www.mrc-journal.de Scheme 12. Using the bimetallic catalyst [(ONOO)tBuY(THF)]2 (TMPy) obtained by 𝜎-bond metathesis of (ONOO)tBuY(CH2TMS)(THF) with TMPy to synthesize PDEVP-b-P2VP-b-PDEVP via sequential addition of 2VP and DEVP.[26] conditions. Additionally, critical micellization concentration and particle size were examined using transmission electron microscopy and dynamic light scattering. Loading with the anti- cancer drug doxorubicin (Dox) and its release into HeLa cells has been investigated. A decreased reducing power of HeLa cells was observed via AlarmaBlue assay with Dox-loaded BAB micelles while empty micelles did not impact the reducing power in the same assay, making the presented polymers attractive candidates for targeted drug delivery.[26] However, more studies on toxicity and drug delivery behavior are necessary. The same group also investigated the REM-ROP of lactones with these complexes to test other initiators for ring-opening polymerization apart from often used bis(di- or trimethylsi- lyl)amide (bdsa, btsa), iso-propanol or other alcohols as ini- tiators/chain transfer agents.[153] Until that point, heteroaro- matic initiators, binding to the yttrium center via 𝜂3-(C,C,N)- aza-allylic coordination, were not recognized for initiating ring- opening polymerization. They were subsequently identified as effective initiators for the first organolanthanide-mediated polymerization of (−)-menthide, a lactone derived from (−)- menthone via Baeyer–Villiger oxidation, and syndioselective BBL polymerization.[153] Using mild conditions, amorphous poly((−)- menthide) (PM) with a very low dispersity was successfully ob- tained indicating a controlled polymerization and the absence of side reactions such as transesterification. The sym-collidine and TMPy-based catalysts demonstrated high activity, enabling the polymerization of BBL with enhanced conversions and obtain- ing poly(hydroxybutyrate) (PHB) with lower dispersities even at room temperature as compared to the catalyst reported in the literature, which featured a bdsa-initiator. Due to the low strain at which the isolated semicrystalline syndiotactic PHB fractures, the stress–strain properties can be modified by incorporating PHB into block copolymers. The bimetallic catalyst system was employed to produce BAB block copolymers from PM (block A) and PHB (block B). The expected outcome for the BAB system in- volved the occurrence of microphase separation, as the semicrys- talline PHB segments are embedded in an amorphous PM ma- trix. The inclusion of PM in the structure led to a reduction in the elastic modulus and an increase in the elongation at break.[153] However, further studies on these biobased BAB polymers as thermoplastic elastomers were not conducted. The copolymerization of Michael-type monomers and lactones via group transfer, followed by subsequent ring-opening poly- merization, has been a relatively unexplored area. This method holds the potential for creating block copolymers that combine the high functionality introduced by Michael monomers with the degradability inherent in polyesters. The utilization of cat- alytic methods and a sequential addition approach could offer a highly versatile and controllable polymerization technique, en- abling the facile synthesis of copolymers with diverse function- alities. Despite some known catalytic approaches for copolymers derived from methacrylates and lactones catalyzed by transition and main group elements,[157–159] the initial proof of concept for copolymerization with organolanthanides was conducted by Ya- suda et al. in 1995. Copolymerization of MMA and MA with BBL, 𝛿VL, and 𝜖CL was examined.[19,21] Despite this pioneering work, no further investigations were undertaken. Characterization of the material properties and mechanistic elucidation have been largely overlooked. Gaining insights into these aspects would be crucial for a comprehensive understanding of the copolymeriza- tion process and the development of tailored copolymers with de- sired properties. In the initial studies on the REM-GTP of Michael-type monomers and REM-ROP with lactones, there was a distinct sep- aration concerning the initiators when utilizing 2-aminoalkoxy- bis(phenolate)lanthanide complexes. Typically, alcoholates and amides served as initiators for lactone polymerization, whereas alkyl initiators were explored mostly for REM-GTP.[41,147] The distinct separation of initiators seemed to pose a challenge for the combination of both polymerization methods. However, Macromol. Rapid Commun. 2024, 45, 2400122 2400122 (15 of 24) © 2024 The Author(s). Macromolecular Rapid Communications published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.mrc-journal.de www.advancedsciencenews.com www.mrc-journal.de Scheme 13. AB and BAB block copolymer synthesis of 2VP with 𝜖CL (R1 = R2 = H) or (−)-menthide (R1 = Me, R2 = iPr) with (ONOO)tBuY(sym- collidine)(THF) or [(ONOO)tBuY(THF)]2(TMPy). Reproduced with permission.[160] Copyright 2021, American Chemical Society. it is noteworthy that both polymerization types exhibit a monometallic initiation and propagation mechanism with first- order kinetics in catalyst and monomer.[147] Initial attempts to merge these approaches were made by employing alkyl ini- tiators for BBL polymerization, resulting in PHB with mod- erate polydispersity (<1.6). Unfortunately, block copolymeriza- tion involving Michael-type monomers and BBL in a sequen- tial addition manner (first block: 2VP, DEVP or IPOx; second block: BBL) proved unsuccessful in this study.[147] After sym- collidine and TMPy were reported to undergo 𝜎-bond metathesis with [ONOO]tBuY(CH2TMS)(THF) to enable AB- and BAB-block copolymers either via group-transfer or via ring-opening poly- merization, another attempt was made to produce block copoly- mers from Michael-type monomers and lactones.[160] Another modification implemented was the transition from the four- membered lactone BBL to larger-sized lactones, i.e., the seven- membered ring lactones 𝜖CL and (−)-menthide. It was shown that the active chain end of P2VP itself can successfully initiate the polymerization of these seven-membered lactones leading to AB or BAB polymers (Scheme 13). All synthesized polymers ex- hibited acceptable polydispersities, and the confirmation of block formation was validated through analyses such as SEC, ESI-MS, and DOSY-NMR. The copolymerization kinetics of 2VP with lac- tones were determined by using the sterically more demand- ing (−)-menthide, revealing a living-type mechanism. Differently composed block copolymers were prepared via simple monomer feed variation and characterized regarding their solution prop- erties, showing a pH-dependent micellization. Additionally, mi- crophase separation was observed for both the semicrystalline P2VP-b-PCL block copolymer and the fully amorphous P2VP-b- PM, which was shown using DSC and pXRD analyses.[160] Mate- rial properties or drug delivery of these polymers were not further investigated. Recent studies investigated the potential combination of aliphatic polycarbonates with polar polymers.[161] Zinc 𝛽- diiminate (BDI) complexes are well-studied complexes that can serve as highly active catalysts for the ring-opening copolymer- ization (ROCOP) of CO2 and epoxides such as cyclohexene oxide (CHO).[162–166] Figure 3. Left: ROCOP of CHO and CO2 with the Lewis acidic zinc moiety to obtain PCHC. Right: REM-GTP of 2VP or IPOx with the yttrium center of the bimetallic complex.[161] Recognizing that BDI complexes are inactive in group-transfer polymerization, a heterobifunctional complex was synthesized based on spatially separated but connected zinc and yttrium centers.[161] The zinc unit facilitates the ROCOP of CHO and CO2, while the yttrium moiety, introduced through C–H bond activation of the spacer (3-((2,6-dimethylpyridin-4-yl)oxy)propan- 1-ol) already connected to the zinc, is active in group-transfer polymerization. Terpolymerizations involving CHO, CO2, and 2VP or IPOx were conducted (Figure 3). NMR studies revealed Macromol. Rapid Commun. 2024, 45, 2400122 2400122 (16 of 24) © 2024 The Author(s). Macromolecular Rapid Communications published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.mrc-journal.de www.advancedsciencenews.com www.mrc-journal.de Scheme 14. 𝜎-Bond metathesis of (ONOO)tBuY(CH2TMS)(THF) with 6,6′-dimethyl-2,2′-bipyridine (6-Me2bpy) and 6-methyl-2,2′-bipyridine (6-Mebpy) to obtain bipyridine-functionalized yttrium bis(phenolate) complexes. Reproduced with permission.[28] Copyright 2018, Wiley-VCH GmbH. catalyst decomposition upon exposure to CO2. Consequently, REM-GTP was consistently performed initially before the ad- dition of CHO and CO2. Copolymer formation was assessed through SEC analysis, MALDI measurements, and solubility be- havior tests, revealing the presence of a terpolymer structure.[161] In conclusion, a novel class of polymers with promising prop- erties has been synthesized, however, demanding further investigation in subsequent studies. 5.3. Postpolymerization Modifications Postpolymerization functionalization is a robust approach in polymer chemistry where specific functionalities are modified or introduced onto preformed polymers following their synthesis. This technique enables the customization of polymer properties, including reactivity, solubility, or functionality, without requir- ing alterations to the polymerization process. Well-established postpolymerization methods are thiol–ene reactions,[167] active ester chemistry,[168] Michael-type addition reactions,[168] Huis- gen [3 + 2] cycloaddition,[168] and Diels–Alder reactions.[169] C– H bond activation with 𝛼-methylated bipyridines was performed with (ONOO)tBuY(CH2TMS)(THF) to obtain novel bpy-yttrium catalysts (Scheme 14).[28] These catalysts were tested in the homopolymerization of 2VP, as well as for copolymerization. The bpy-capped block copolymers enabled metal complexation shown by the reaction of bpy-2VP-DEVP-block copolymers with [Re(CO)5Cl] providing a [Re(CO)3(bpy)Cl] motif as the end group. It was revealed that these Re-2VP-DEVP-block copolymers still form unimodal micelles in water. As such block copolymers were proven to enter cells and bipyridines are widely used ligands in metalorganic compounds,[170] these bpy-block copolymer mi- celles are promising candidates as carriers for hydrophobic metal complexes. To prove that the polymer part of this system does not influence the properties of the metalorganic complex, the Re-2VP-DEVP-block copolymers were tested in the photocatalytic reduction of CO2 to CO. Comparing the catalytic performance of the rhenium-functionalized block copolymers with its analog polymer-free complex, the turnover numbers were not only re- tained with respect to the polymer-free system, but they even increased.[28] In alignment with the studied initiator sym-collidine in non-metallocene mediated group-transfer polymerization, a substrate for C–H activation with protected functional groups was developed.[171] C–H bond activation was per- formed by reacting [(ONOO)tBuY(CH2TMS)(THF)] with tert-butyl-dimethyl-silyl-functionalized 𝛼-methylpyridine to obtain the complex [(ONOOtBuY(X)(THF)] (X = 4-(4′-(((tert- butyldimethylsilyl)oxy)methyl)phenyl)-2,6-dimethylpyridine). Homopolymers of 2VP and DEVP showed low to moderate polydispersities (1.01 ≤ Ð ≤ 1.33), with P2VP being synthe- sized under more controlled conditions. Various deprotection routes were examined for the production of hydroxyl-terminated P2VP and PDEVP in which the most promising route in- volved the treatment of the polymers with glacial acetic acid in a tetrahydrofuran–water mixture under ambient conditions. The resulting hydroxyl groups have the potential to serve as anchoring sites for small bioactive molecules, enabling postpoly- merization functionalization, or functioning as macroinitiators for subsequent polymerizations. Mashima and co-workers also successfully introduced alkyne functional groups to poly(2-vinylpyridine). Propargyl yttrium ini- tiators, investigated for their efficacy in the polymerization of 2VP, were synthesized through C(sp3)–H bond activation of the propargylic position of internal alkyl acetylenes (1-trimethylsilyl- 1-propyne, 1-phenyl-1-propyne, and 2-hexyne) employing an alkyl yttrium complex with an ene-diamido ligand. Based on the ESI-MS measurements, it was observed that all polymers pos- sessed a terminal group corresponding to the alkyne molecule added (Figure 4). In the case of 1-phenyl-1-propyne and 2-hexyne, it was proven that alkynyl- and allenyl-terminated P2VP was observed.[146] Metal and nonmetal catalyzed functionalization of alkynes are well-established techniques in organic chemistry and transform alkynes to functional multisubstituted alkenes and alkanes.[172] The optimization of polymerization for the utilization of func- tional end groups in biological applications also occupies a sig- nificant role in this context. In alignment with the extensively studied sym-collidine initiator, substrates for C–H activation with different functional groups were developed by the Rieger group.[173] Three initiator structures featuring protected func- tional groups (–O-tert-butyldimethylsilyl, -2,5-dimethylpyrrole, and –S-triphenylmethyl (= S-Trityl)) and the 2,6-dimethylpyridyl motif were synthesized and used for C–H activation with Cp2Y(CH2TMS)(THF).[173] Afterward, the in situ generated com- plexes were used to polymerize DEVP. Initiator efficiencies of the catalysts were between 40% and 80% generating PDEVP with Macromol. Rapid Commun. 2024, 45, 2400122 2400122 (17 of 24) © 2024 The Author(s). Macromolecular Rapid Communications published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.mrc-journal.de www.advancedsciencenews.com www.mrc-journal.de Figure 4. a) Structure of alkyne-capped P2VP. b) ESI-MS spectrum of alkyne-capped P2VP. c) Plot of m/z values versus the number of 2VP repeating units. d) 1H NMR spectrum of alkyne-capped P2VP. Reproduced with permission.[146] Copyright 2011, American Chemical Society. low to moderate polydispersities (Ð ≤ 1.20). Subsequent depro- tection experiments led to the removal of the respective protec- tion groups giving access to the –OH, –NH2, and –SH termi- nated polymers, respectively (Scheme 15). Model substances to couple with these end groups were cholesteryl chloroformate (hy- droxy and amino-end groups) and N-phenyl maleimide (thio-end group). Such end-groups are susceptible toward reactions like ac- tive ester chemistry and thiol–ene reactions. Additional applications involving polymer conjugates, incor- porating targeting ligands, fluorophores, surfaces, or pharmaco- logically active substances, were not the focus of this study. How- ever, they have paved the way for potential future applications in these areas. Terminal double bonds containing heteroaromatic sub- stances, i.e., (2,6-dimethyl-4-(4-vinylphenyl)pyridine and 2-(4- vinylphenyl)pyridine were additionally developed.[174] The de- rived catalyst was employed in the polymerization of DEVP to produce end groups featuring double bonds that are suscep- tible to thiol–ene reactions. Additionally, some of the DEVPs in the polymer chain were converted into mixed pyrene ethyl vinylphosphonates through transesterification, enabling detec- tion through fluorescence in subsequent biological experiments. Finally, through thiol–ene click reactions, folic acid or choles- terol was attached to the double bond of the initiator/end group (Scheme 16). The biocompatibility of the polymers, assessed through MTT assay (MTT = 3-(4,5-dimethyl-2-thiazolyl)–2,5- diphenyl-2H-tetrazolium bromide) on immortalized cell lines, i.e., kidney (HEK-293) and endothelial (HMEC-1) cells, appears to be low. In most instances, the presence of the two anchor units seems to confer an advantage in terms of toxicity compared to polymers lacking functionalization.[174] However, the statement of the authors that the polymers “seem to be beneficial for the cells” due to values above 100% in MTT is rather vague. The MTT assay examines only a limited aspect of many processes that can influence cell viability. Nevertheless, the MTT assay, and also its adapted MTS assay (MTS = 3-(4,5-dimethylthiazol-2-yl)–5-(3- carboxymethoxyphenyl)–2-(4-sulfophenyl)–2H-tetrazolium), are widely used attributed to their simplicity, reliability, and versatil- ity, establishing them as a valuable tool for researchers in diverse fields such as pharmacology, toxicology, and cell biology. Subsequent confocal microscopy investigations with HMEC- 1 cells revealed that the poly(vinylphosphonate)–cholesterol con- structs adhere to the cellular membrane, whereas the polymers anchored with folic acid are transported into the cells.[175] An- other biocompatibility evaluation with fluorescent polymers on HMEC-1 and HEK-293 cells was performed by the same authors. In situ C–H bond activation of a chromophoric initiator 2,5- bis(2-(2-picoline-4-yl)-vinylene)hydroquinone dineopentyl ether with Cp2Y(CH2TMS)(THF) and subsequent polymerization of DEVP led to fluorescent chromophore/PDEVP conjugates. Macromol. Rapid Commun. 2024, 45, 2400122 2400122 (18 of 24) © 2024 The Author(s). Macromolecular Rapid Communications published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.mrc-journal.de www.advancedsciencenews.com www.mrc-journal.de Scheme 15. Top: Polymerization of DEVP with N-heteroaromatic initiators with a 2,6-pyridine motif bearing different protected functional groups. Bot- tom: Deprotections strategies for O-tert-butyldimethylsilyl, 2,5-dimethylpyrrole, and S-Trityl leading to OH, NH2, and SH-end groups. Adapted with permission.[173] Copyright 2020, American Chemical Society. The studied cell lines did not incorporate PDEVP as con- firmed by fluorescence microscopy.[176] As a limitation, poly- mers had a high molecular-weight due to low initiator efficien- cies. Given that molecular weights can influence cell uptake and viability, it is essential to conduct tests with polymers of varying molecular weights to comprehensively evaluate the im- pact of molecular weight. Additionally, the question arises as to whether the elaborate procedure of synthesizing a substan- tial quantity of only partially active catalyst, is reasonable. Al- ternatively, it might be more beneficial to develop a method that minimized the waste and cost of unreacted catalyst. These limitations were partially targeted by a recently published ap- proach. The parallel conjugation of cholesterol and folic acid molecules to poly(vinylphosphonates) was achieved by copoly- merization of different vinyl phosphonates using Cp2Y(sym-coll) as a catalyst with high initiator efficiency.[177] The monomers utilized in this procedure include diallyl vinylphosphonate and di trimethylsilyl propargyl vinylphosphonate alongside the hy- drophilic DEVP, facilitating the integration of diverse function- alities prone to orthogonal postpolymerization functionalization. The monomer diallyl vinylphosphonate was already used before to functionalize polymers via thiol–ene reaction using diverse substrates.[155,178,179] 5.4. Surface Functionalization In terms of surface functionalization, Cp2Lu(CH2TMS)(THF) was reacted in situ with an azide-derivative of sym-collidine (4- azido-2,6-dimethylpyridine=PyN3) generating Cp2Lu(PyN3).[180] Afterward, DEVP was polymerized to obtain an azide-capped PDEVP. All polymers exhibited low polydispersities below 1.24, emphasizing the controlled polymerization of DEVP facilitated by the in situ generated catalyst however only with very low ini- tiator efficiencies of 8–15%. The initiator efficiencies observed in this study are approximately half of the literature-reported values for the structurally similar lutetium catalyst Cp2Lu(sym- coll)(THF) lacking azide functionalities, which reported an effi- ciency of 21%.[181] These reduced initiator efficiencies resulted in the formation of high molecular weight polymers with molar masses exceeding 35 kg mol−1, even at low catalyst:monomer ra- tios of 25:1. To covalently tether the synthesized polymers to mul- tiwalled carbon nanotubes (MWCNTs), a [2 + 1] cycloaddition is employed. This cycloaddition involves the 𝜋-electrons of the car- bon nanotubes and an in situ generated nitrene, formed through thermally induced nitrogen extrusion from the azide moiety of the polymers. To evaluate the loading of the nanotubes with the polymer, thermogravimetric as well as elemental analysis was conducted. The loading was calculated based on the percentage of phosphorus/DEVP found. 10–22 wt% of polymer were found in the PDEVP@MWCNT. The water-soluble PDEVP on the surface on the carbon nanotube was capable of stabilizing suspensions of MWCNTs in water against coagulation (Figure 5).[180] The catalyst used in that study was only partially active. It might be more beneficial to develop a method that minimizes the waste and cost associated with unused catalyst. Just recently, a graft-to strategy was developed in which the activated 2,6- dimethyl-4-((trimethylsilyl)ethynyl)pyridine induced higher ini- tiator efficiencies in yttrium-mediated group-transfer polymer- ization with values between 44% and 60%.[182] After deprotec- tion, the alkyne was coupled to an azide-dopamine via alkyne– azide click chemistry. Since dopamine serves as a nanoparticle coating agent, the hydrophilic polymers could be grafted to gold nanoparticles resulting in stable and uniformly distributed col- loids suitable for various applications. Another study on surface-functionalization was performed by Zhang et al. This group introduced a method for synthesizing bottle-brush brushes by combining self-initiated photografting and photopolymerization (SIPGP) with REM-GTP.[183] Bottle- brush polymers have emerged as a fascinating class of ma- terials with diverse applications, ranging from supersoft elas- tomers and organic optoelectronics to templates for crafting 1D Macromol. Rapid Commun. 2024, 45, 2400122 2400122 (19 of 24) © 2024 The Author(s). Macromolecular Rapid Communications published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.mrc-journal.de www.advancedsciencenews.com www.mrc-journal.de Scheme 16. Conjugation of cysteamine and folate-NHS or thio- cholesterol to double-bond end-capped PDEVP via thiol–ene reaction.[174] Figure 5. Capability of PDEVP to stabilize the nanotubes against co- agulation. Suspensions of 0.05 mg mL−1 of the MWCNT:PDEVP com- pounds in water are prepared and pictures are taken immediately after suspension, 5 min, 3 h, and 24 h. A) MWCNT:PDEVP200 = 1:5, B) MWCNT:PDEVP100 = 1:5, C) MWCNT:PDEVP50 = 1:5, D) MWCNT:PDEVP100 = 1:2, and E) MWCNT:PDEVP100 = 1:1. I) Pure MWC- NTs without polymer functionalization, and II) MWCNTs after functional- ization with a nonazide-functionalized PDEVP as control experiment. Re- produced with permission.[180] Copyright 2022, Wiley-VCH GmbH. nanomaterials, as well as applications in energy storage and biomedical devices.[184] The presented strategy involves the gen- eration of poly(3-((2,6-dimethylpyridin-4-yl)oxy)propyl methacry- late) (PDMPPMA) brushes on a silicon wafer, followed by their conversion to surface-grafted macroinitiator brushes through C–H bond activation with Cp2Y(CH2TMS)(THF). The surface- bound macroinitiator exhibits high activity for the polymer- ization of heteroatom-containing monomers, such as different DAVPs (DMVP, DEVP, DPVP), 2VP, and IPOx, thereby enabling the efficient synthesis of bottle-brush brushes, which cannot be Macromol. Rapid Commun. 2024, 45, 2400122 2400122 (20 of 24) © 2024 The Author(s). Macromolecular Rapid Communications published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.mrc-journal.de www.advancedsciencenews.com www.mrc-journal.de Scheme 17. Monomers, self-assembled monolayer preparation, PDMPPMA brushes, surface-bound macroinitiator, bottle-brush brushes, and the mech- anism of polymerization with the surface-bound dimethylpyridine–yttrium catalyst. Reproduced with permission.[183] Copyright 2017, American Chemical Society. synthesized using conventional polymerization techniques. This leads to the formation of side chains along the PDMPPMA back- bone (Scheme 17). This versatile approach does not apply to poly- merization of conventional MMA.[183] 6. Conclusion The synthesis of polymers with well-defined structures through metal-catalyzed processes, featuring end groups of varying po- larity and reactivity, has garnered significant attention in sci- entific and industrial fields. These polymers serve as crucial components and additives for the development of innovative functional materials. Current investigations focus on advancing the synthetic approach by combining living coordination poly- merization mediated by rare-earth metal complexes with C–H bond activation through 𝜎-bond metathesis. This combined ap- proach produces catalysts with diverse functional groups from a single precursor, facilitating the direct production of terminal- functionalized polymers without the need for sequential termina- tion reactions. This innovative methodology allows precise con- trol over end-group functionalities, offering a versatile approach to tailor the properties of the resulting polymers. This aspect is particularly relevant for applications across diverse fields from biomedicine to material science. However, the current applica- tion of this method primarily lies within fundamental research, where various aspects have been explored, shedding light on the general reactivity of rare-earth metal complexes toward 𝜎- bond metathesis of different substrates and various polymeriza- tion techniques. Mechanisms underlying these processes have also been elucidated. A potential limitation for widespread in- dustrial adoption stems from the fact that most rare-earth metal complexes are not commercially available in a catalytically suit- able form, impeding their broad industrial utilization. Addition- ally, certain rare-earth metal complexes exhibit high sensitivity to air and moisture, posing challenges during handling and stor- age. Despite these hurdles, research in this field demonstrates promising avenues that, through further development and opti- mization, may be overcome in the future. Acknowledgements F.A. is thankful for funding by the Federal Ministry of Education and Re- search (BMBF) and the Baden-Württemberg Ministry of Science as part of the Excellence Strategy of the German Federal and State Governments. The author thanks Dr. Moritz Kränzlein for his valuable assistance in re- viewing and discussing the manuscript. Open access funding enabled and organized by Projekt DEAL. Conflict of Interest The authors declare no conflict of interest. Keywords C–H bond activation, ethylene polymerization, functional end groups, group-transfer polymerization, rare-earth metal catalysis, ring-opening polymerization, 𝜎-bond metathesis Received: February 29, 2024 Revised: May 15, 2024 Published online: June 3, 2024 [1] A. Naumov, Russ. J. Non-Ferrous Met. 2008, 49, 14. [2] K. Hans Wedepohl, Geochim. Cosmochim. Acta 1995, 59, 1217. [3] W. J. Evans, Polyhedron 1987, 6, 803. [4] E. Wiberg, N. Wiberg, Inorganic Chemistry, Academic Press, Cam- bridge, MA 2001. [5] P. L. Watson, J. 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