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Latent and Delayed Action Polymerization Systems Stefan Naumann, Michael R. Buchmeiser* Various approaches to latent polymerization processes are described. In order to highlight recent advances in this field, the discussion is subdivided into chapters dedicated to diverse classes of polymers, namely polyurethanes, polyamides, polyesters, polyacrylates, epoxy resins, and metathesis-derived polymers. The described latent initiating systems encompass metal-containing as well as purely organic compounds that are activated by external triggers such as light, heat, or mechanical force. Special emphasis is put on the different chemical venues that can be taken to achieve true latency, which include masked N-heterocyclic carbenes, latent metathesis catalysts, and photolatent radical initiators, among others. Scientific challenges and the advantageous application of latent polymerization processes are discussed.

1. Introduction A perfect latent catalyst (or initiator) would usually be defined as a compound that is completely inert in the presence of monomer under storage conditions, in handling prior to processing or more generally in the absence of activating triggers. It should be tolerant towards all components of the polymerization mixture, easy and cheap to prepare, but highly reactive after activation. The activation step itself is desired to be quantitative and fast, but at the same time it is mandatory to avoid premature activation. These requirements provide for a very challenging and interesting field of chemistry; indeed, a great deal of effort has been put into research regarding compounds that can be controlled in such a way.[1] As will be outlined below, progress in this field is heterogeneous, with types of reactions that are in commercial use since decades, while others are only just emerging. The methods that were chosen to satisfy these requirements are similarly diverse and selected examples will be considered in the following discussion. There is probably no other area where the application of latent reaction techniques is as advantageous as in S. Naumann, Prof. M. R. Buchmeiser Institute of Polymer Chemistry, University of Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany E-mail: [email protected]

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polymer chemistry. To be able to exactly control the onset of polymerization does not only remove repercussions that can arise from incomplete mixing of highly reactive components, but can also allow for the preparation of storable, ready-to-use formulations. This is desirable because it simplifies the experimental setup (machinery, safety) and extends pot times and processing windows. Furthermore, single-component polymerizations can be realized where before several components or in situ addition of chemicals was necessary. Equally important, modern technologies like reaction injection molding (RIM), resin transfer molding (RTM), or the preparation of photoresists depends on the development of suitable, latent catalytic systems. Indeed, there is hardly any processing method that would not experience significant broadening of application possibilities if the polymerizations where to be triggered exactly where and when it is wanted. At the same time, the preparation of polymers imposes challenging requirements on latent polymerization systems. The “degree of latency” has to be very high; otherwise, the premature formation of polymer would hamper any process that is constructed for the handling of low-viscosity, monomeric components and corrupt control over molecular weight. On the other hand, sluggish activation and liberation of the active species over too long a time would result in slow polymerizations that were of little commercial use and equally

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DOI: 10.1002/marc.201300898

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endangered of ill control over the molecular weights. Thermosets, finally, could not tolerate unwanted polymerization at all, which would result in materials that cannot be processed anymore. Furthermore, it has to be considered that today’s polymers contain numerous additives, fillers, and stabilizers. This means that the latent catalysts or initiators are part of sophisticated mixtures. For example, UV stabilizers can compete with photoinitiators for light, or excessive heat required for activation of thermally latent compounds can induce unwanted side reactions. Residual traces of these compounds in the final material, however, can entail quite adverse effects and contribute to the degradation of the polymers and thus complete consumption of the “catalysts” should be targeted. In spite of this, very intriguing and successful latent polymerization systems have been developed. By recent advances, the polymerization mechanisms that can be triggered in a latent fashion include now not only radical polymerization and polyadditions, but also ionic polymerizations (anionic, cationic, zwitterionic) and metathesis-based mechanisms. The majority of the examples described below utilize heat or (UV) light radiation as trigger to convert the latent catalyst or initiator into the active state. However, other activation methods have the potential to gain importance, among them mechanochemical activation (ultrasound) or electrochemically triggered transformations.[2] Of course, latent processes have also been used outside the context of polymeric materials and many of the underlying principles and mechanisms that are now frequently used to induce latent properties have been elucidated from an organic background, resulting in observations that are now exploited for polymerizations. Examples can be found in the application of isothiocyanate-protected N-heterocyclic carbenes (NHCs) for the thermally latent cyclotrimerization of isocyanates,[3] latent ring-closing metathesis (see below), or pioneering studies on the UV switching of (NHC-containing) structures.[4,5] However, the following discussion is limited to latent catalysis applied to polymer chemistry or very close fields and focuses on the more recent investigations. The presented examples are thought to demonstrate the manifold approaches that can be taken to successfully realize a latent polymerization system and should be understood as inspiration rather than exhaustive discussion of the field. Cases where latent systems have to be activated by in situ addition of chemicals are not considered here. For the sake of simplicity, the active form of latent compounds is referred to as “catalyst” in most cases, though initiator would frequently be more exact. Similarly, the latent compounds are referred to as “precatalysts.”

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Stefan Naumann received his diploma degree in chemistry in 2010 from the University of Stuttgart (Germany), working on metathesisderived polymers. He then started his Ph.D. studies under the supervision of Prof. Michael R. Buchmeiser (Stuttgart) and investigates latent polymerization processes involving protected N-heterocyclic carbenes. His research interests also include organocatalysis and organometallic polymerization catalysis.

Michael R. Buchmeiser received his Ph.D. in 1993 from the University of Innsbruck (Austria). He spent one postdoctoral year at the MIT, USA within the group of Prof. Richard R. Schrock. After his Habilitation in Macromolecular Chemistry in 1998, he held a Faculty Position as Associate Professor at the University of Innsbruck, before accepting a Full Professorship in Leipzig (Germany) together with the position as Vice Director of the Leibniz Institute of Surface Modification in 2004. Since 2009 he is Full Professor at the University of Stuttgart (Germany) and Director of the Institute of Textile Chemistry and Chemical Fibers (ITCF Denkendorf).

2. Polyurethanes Polyurethanes (PU) represent undoubtedly one of the most versatile classes of polymers and this broad range of applications is matched by an equally large number of processing variations.[6] The chemistry that is required not only to catalyze the formation of the polymers but also of the educt monomers and oligomers is still evolving.[7] However, PU formation has long since suffered from that fact that its most effective catalysts were based on toxic or potentially toxic metals (like dibutyltin dilaurate, DBTDL, 1, Figure 1). This situation is even worse where latent catalytic systems are concerned. Here, the dominating compound for decades has been phenylmercury neodecanoate (PMND, 2). This organic mercury(II) derivative had originally been described to be of low activity, but soon after its potential for latent PU synthesis had been recognized.[8,9] Not surprisingly, considerable effort has been put into finding suitable and environmentally more benign alternatives. The main motivation for latent PU preparation is found in the need to retain the performance of classical two-component (2K) polyurethane in situations where 1K PU would be more appropriate and cheaper (e.g., where a certain pot-life is required).

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Figure 1. Structures of DBTDL (1), PMND (2), cyclic guanidine, both free (3) and blocked by isocyanates (4) and selected examples for protected NHCs (5–11).[14,21]

One of the principle approaches to induce latent behavior in PU chemistry is the blocking of the isocyanate functionality.[10,11] By reacting it reversibly with nucleophiles that act as blocking group its inherent high reactivity is attenuated or blocked and not accessible for reaction with polymerization components anymore (polyols, amines). The crucial step is the thermal deblocking (Scheme 1), which is heavily dependent on i) the blocking group (usually N H containing molecules like amines, pyrazoles or lactams), ii) the present co-catalyst (which is commonly the same as the urethane-forming one), and iii) various other factors like concentration, polymerization medium, or isocyanate structure. Overall, in spite of its simple appearance, the application of blocked isocyanates for latent polymerizations entails a complex chemistry.[12] However, since the deblocking temperatures (see Scheme 1 for examples) can nowadays be selected from a broad range to suit the intended application, this technique finds widespread industrial use.[13] Quite inverse to this method, isocyanates were very recently described to form thermally labile precatalysts based on cyclic guanidines (4, Figure 1).[14] While the free guanidines (3) have been shown to be efficient catalysts for PU synthesis from alcohols and isocyanates, the heterocyclic, isocyanate-masked compounds of the type 4

displayed low activity at 20 °C but a jump to considerably higher activity at 60 °C.[14,15] The overall performance of the latent catalysts was somewhat weaker than that of the free guanidine, probably due to incomplete thermal activation. Though there is so far no conclusive mechanistic evidence, the authors suspect the regeneration of the free organocatalysts to take place by attack of alcoholic functionalities on the isocyanurate group in the heterocycle of the type 4. Molecular weight distribution and molecular weight of PU prepared by this new delayed action catalysts was shown to be relatively well controlled (PDI about 1.6 in all samples, Mn up to 77 000 g mol−1). A very potent group of versatile (organo)catalysts has emerged in the so-called protected NHCs. After NHCs were first successfully isolated in 1991, they were quickly discovered to be powerful ligands for transition metal complexes.[16,17] Additionally, NHCs are ever more recognized as interesting on their own, because they can act as nucleophiles and Brønstedt bases. Combined with the structural diversity that is accessible for NHCs, this makes them attractive organocatalysts.[18] In the recent years, they also have found growing entrance to polymer chemistry.[19] Harnessing (“protecting”) NHCs in a way that allows for controlled release of these compounds from less reactive, but more favorable to handle progenitors

Scheme 1. Schematic representation of blocked-isocyanate technique for thermally latent PU synthesis. Bottom: Exemplaric deblocking temperatures using different blocking agents.[10]

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has many advantages, and one of them is the potential to realize latent polymerization systems. Indeed, as will be shown in the following discussion, protected NHCs have been successfully implemented as thermally latent catalysts or initiators in very different polymerization reactions. PU formation was one of the first examples where protected NHCs revealed their potential as latent catalysts.[20,21] A number of NHCs was reacted with CO2 or diverse metal salts to form NHC–carboxylates or NHC– metal complexes, respectively (see Figure 1 for examples). The performance of these protected NHCs was then compared with the activity of the commercial catalysts DBTDL (1) and PMND (2) in the synthesis of cross-linked PU from a commercial polyol and cycloctrimeric hexamethylene diisocyanate. As expected, at room temperature Hg(II)based PMND showed no activity, while DBTDL produced polymer under these conditions. More interestingly, compounds 5–11 also showed pronounced latent behavior. Only after thermal activation polymerization occurred; polymerization rates were in some cases even better than that of PMND, which also induced polymerization after heating. Not surprisingly, it was found that the protecting group of the latent catalysts influenced both the degree of latency and the polymerization rates. Generally, the best activity was observed using CO2, Zn(II), or Sn(II) salts as masking agents, while Mg(II) salts delivered somewhat slower polymerizations and Al(III)-based protected NHCs displayed rather low activity. Most important, pseudosecond-order kinetics were observed in most cases, which means that the concentration of the catalysts remained constant during the polymerization. This, in turn, can be related to a fast and quantitative deprotection of the NHCs relative to the polymerization rates, rendering this system well controlled and avoiding residues of non-reacted precatalyst in the final product. Mechanistically, the application of heat leads to a decarboxylation of NHC CO2 adducts and subsequent liberation of the free NHC. In the case of NHC–metal complexes, increased temperatures will entail dissociation of the complex, thus increasing the concentration of free NHCs in solution (Scheme 2). NHC CO2 adducts provide access to entirely organocatalytic polymerizations, which obliterates concerns regarding toxic metal residues or polymer purification. Metal-protected NHCs, on the other hand, offer the opportunity of joint action of NHC and Lewis acidic metal ion after dissociation. Such a mechanism is proposed for the formation of polyurethanes and will also be met again when polyester are discussed. For PU synthesis, it is reasonable to assume that the generated free NHC will activate (or deprotonate) part of the hydroxylic functionalities of the polyol, rendering the functional group more nucleophilic in every case. Metal salts like Sn(II) chloride are prone to coordinate to the isocyanates, removing electron

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Scheme 2. Thermal liberation of NHCs from protected precursors. Bottom: Dual activation of both alcohol and isocyanate functionality by dissociated NHC–metal complex.[20,21]

density this way and thus increase the propensity of the functional group for nucleophilic attack (Scheme 2). Both effects supplement each other and combined they entail a swift and complete polymerization.

3. Polyamides The overall very favorable properties of polyamides (PA) are related to the strong hydrogen bonding interactions that occur between the amide functionalities of the polymer chains, which generally enables the material to display considerable degrees of crystallinity and high melting points.[22] Since the amide density can be well controlled by selection of suitable monomers or by appropriate copolymerization, a broad variety of properties can be targeted (melting points, water uptake, and mechanical characteristics). Apart from polycondensation, the other main strategy to synthesize polyamides is ring-opening polymerization (ROP) of cyclic amides (lactams).[23] The latter process can be induced by anionic polymerization and is comparably fast, especially when initiated in the presence of preactivated monomer (12, Scheme 3).[23] This rapid polymerization process enabled the use of PA in techniques like resin transfer molding (RTM) and reaction injection molding (RIM), where the polymer is formed in situ from monomer and catalyst (plus activators and other additives).[24] These methods profit from the use of lowviscosity monomeric melts that have a number of advantages where the production of complex polymer parts or composite materials is desired. The pressure required for RIM is lower by orders of magnitude than that for traditional injection molding.[25] The reduced pressure safes energy, but more important no special machinery for

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Scheme 3. Mechanism for anionic ROP of lactams for the synthesis of polyamides. Bottom right: Structure of preactivated monomer commonly added to eliminate induction times (12) and typical initiator (13).[28]

high-pressure application is required. Large or complex mold shapes can be used for RIM, where classical polymer melts cannot be injected. The low viscosity is also beneficial for the preparation of composite materials; the wetting of and diffusion into woven mats of carbon- or glass fibers is much enhanced. In situ polymerization then yields the desired fiber reinforced material. To compete with other polymer processing technologies, the cycle times for RIM and related processes need to be as short as possible. Altogether this makes for a situation where a latent polymerization system would be tremendously useful; current technology uses strong bases (commonly sodium amides, 13) in combination with activator (12) to start the polymerization. However, both have to be added directly prior to polymerization, which then immediately starts. It is not possible to store all components together for even short times. In view of the importance of PA-based polymeric materials and the success that has met with the implementation of thermally labile NHC-precursors, efforts were made in our group to realize a latent polymerization system for polyamides with these very compounds. Initially, investigations were hampered by a complete lack of literature describing the interactions of NHCs and lactams. One single patent had been filed by Du Pont, where free NHCs were used for the polymerization of lactams.[26] The results based on five-membered imidazol-2-ylidenes and imidazolin-2-ylidenes indicated

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that application of these NHCs was rather problematic; yields were generally very low, long reaction times and high loadings of catalyst were required. Presumably, these observations were related to the basicity of the NHC in action. It is well known that the so-called ring expanded NHCs (six- and seven-membered cycles, i.e., tetrahyropyrimidin-2-ylidenes and tetrahydro-1,3-diazepin2-ylidenes) are much stronger Brønstedt bases than their five-membered counterparts.[27] And indeed, the application of compounds like 1,3-dicyclohexyltetrahydropyrimidnium-2-carboxylate (14, Scheme 4) yielded polyamide in high (PA 6) or quantitative (PA 12) yields when used in bulk polymerization of commercial-grade monomer at 180– 220 °C.[28,29] It was shown that pKa value and activity of the parent NHC were directly related (compare Scheme 4). The anionic character of the polymerization was also evident in the occurrence of induction times (observed via in situ rheology) and certain molecular weight-limiting side reactions.[29] The CO2-protected NHCs thus act as thermally latent Brønstedt bases. Furthermore, the NHC CO2 adducts could be stored together with the monomer (ε-caprolactam or laurolactam) over weeks, without losing activity or initiating premature polymerization, exclusion of water provided. The mixtures of the powdery NHC–carboxylates and the lactams were stable at room temperature, but polymerization was effortlessly started by simple heating of the mixture and progressed in few minutes to high conversions. It should be underlined that above discussion is a good example for the potential that lies with protected NHCs. By manipulation of the NHC structure (ring expansion) the possibility to efficiently polymerize lactams was opened. By protection of these NHCs with CO2, they became of practical use (harnessed reactivity, powders instead of oily compounds) and allowed for preparing the first stable, latent polymerization system that can be used as a single-component, completely metal free composition.

4. Polyesters Recent developments have focused on the latent ROP of cyclic esters to yield the desired polyesters, the majority of the examples again based on NHCs. Bielawski and Neilson presented a UV-switchable NHC, where a change of light source induced a cyclization or opening of the NHC backbone.[30] This results in a considerable change of electron density at the carbene center, rendering imidazolium derivative 15 active for the ROP of δ-valeroclactone and ε-caprolactone in the presence of benzylic alcohol as co-initiator, while derivative 16 is strongly deactivated (k15/k16 = 59, Scheme 5). Though it is required to keep the sample under UV light to retain delayed reactivity (while

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Scheme 4. Application of CO2-protected NHCs as latent bases for the preparation of PA 12 via anionic ROP of laurolactam (LL). Right: Comparison of polymer yields and dependence on pKa values of the parent NHCs.[29]

the opposite would perhaps be more convenient), this case elegantly shows an alternative venue to manipulate the properties of NHCs by a UV trigger. Notably, the NHC system itself has to be generated by addition of base to the parent imidazolium salts prior to polymerization. In this context, one should see recent efforts to synthesize photoswitchable guanidine catalysts bearing azobenzene moieties for the ROP of lactides, though catalyst activity seems to be still a problem.[31]

Scheme 5. Preparation of a polyester by a photoswitchable NHC. UV irradiation entails formation of a large conjugated system, rendering 16 almost inactive.[30]

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The ability of NHCs to reversibly insert into O H and strongly polarized C H bonds has been one of the first characteristics of NHC chemistry that was exploited for controlled release of the active species.[32,33] Insertion products like 17–20 (Scheme 6) were used for the polymerization of lactide. It was found that the chloroform- and especially the pentafluorobenzene-adducts were useful precatalysts at elevated temperatures.[32] The alcohol adducts, which after dissociation liberate both the free NHC and the co-initiating alcohol and thus present the opportunity for single-component polymerization,

Scheme 6. Thermally labile insertion products of an NHC into O H and C H bonds.[32,33,35,36]

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Scheme 7. Influence of steric demand on the polymerization activity of imidazolium-CO2 precatalysts[38] and polymerization mechanism according to reference[39] (left). Right: Alternative initiation mechanism for strongly basic NHCs via deprotonation of alcohol.

are easily prepared. However, the thermal stability of these compounds is strongly dependent on the structure of both the NHC and alcohol and on reaction conditions. Furthermore, it should be mentioned that the aforementioned adducts were found to display no latency at all in PU formation.[20,34] Interestingly, the MeOH adduct 17 allowed for the stepwise polymerization of lactide by simply switching the temperature between 25 and 90 °C; adduct 18 on the other hand initiates polymerization at room temperature.[33,35] Reactions employing 18 were finished within minutes, while 17 required hours to achieve quantitative monomer consumption (50 h or more).[36] This certainly illuminates that the desired combination of latency at lower temperatures and high activity after thermal triggering is a challenge and difficult to achieve. Efforts in our group were hence directed at improving this situation. Poly(ε-caprolactone) (PCL) as a polyester with favorable properties and applications (tissue engineering, drug delivery) was chosen as a target material. A typical catalyst for the preparation of this polymer from ε-caprolactone (ε-CL) is Sn(II) octanoate but also various organocatalysts have been employed to synthesize PCL, all in a non-latent manner.[37] We first applied a large range of CO2-protected NHCs to investigate whether they would be latent in the presence of alcoholic co-initiator and which structural features render the NHC most active after decarboxylation.[38] Interestingly, it was found that all NHC CO2 adducts did not react at room temperature over night, yet after heating (70 °C) polymerization occurred. Imidazolium-derivatives behaved according to a mechanism proposed by Waymouth and Hedrick,[39] which rendered sterically less congested, nucleophilic

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NHCs the most active (Scheme 7). The more basic sixmembered compounds (see above) seemed to act as latent bases, a situation reminiscent to the case of polyamide preparation, thus initiating polymerization by deprotonating the co-initiating alcohol (Scheme 7). While it is interesting to be able to control the polymerization mechanism by the appropriate choice of NHC (both pathways yield the same polymer with the same end groups), the reaction itself was found to be comparably slow, requiring more than 20 h to reach completion when the most active NHC CO2 precatalysts was used. Implementation of metal salts as protection groups solved this problem remarkably well.[38] A homologous series of metal complexes was synthesized, employing SnCl2, ZnCl2, and MgCl2 as blocking groups. Polymerization experiments revealed a strong accelerating effect of the metal groups, where the activity of the precatalysts under identical conditions was found to be MgCl2>ZnCl2>SnCl2. It should be noted that the parent NHC is sterically too much encumbered to initiate polymerization via nucleophilic ringopening; at the same time, it is the weakest base of all the NHCs that were investigated in our study and not able to deprotonate the co-initiating alcohol efficiently. Accordingly, the CO2-protected homologue did not show any activity. The emerging high activity when introducing metal salts as protecting groups must hence result from an activating effect of the Lewis acidic metal ions or from a cooperative effect of the Lewis acid (metal salt) and the Lewis basic NHC. After stirring with monomer and alcoholic co-initiator at room temperature for a prolonged time (20 h), ZnCl2-protected compound 21 displayed full latency and no monomer consumption at all. Heating the solution subsequently to 130 °C resulted in quantitative

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Figure 2. Comparison of the latent behavior of 21 and its MgCl2-protected homologue 22 in the bulk polymerization of ε-caprolactone in the presence of alcoholic co-initiator. Note the perfect latency (left), that is somewhat compromised if the weakly coordinating Mg(II) is used as protecting group (right). Both compounds deliver the polyester quantitatively after thermal activation at 130 °C in only 5 min polymerization time.[38]

polymer yield after only 5 min (Figure 2). This sharp step in monomer consumption comes close to the description of a perfect latent catalyst, the more so since 21 is prepared in a single step. Quite fitting to this theory, the MgCl2-protected homologue (22) displays a somewhat shorter pot-time, but even higher activity. Most probably, this is due to the weaker coordinating power of Mg(II), rendering this compound more sensitive to thermal activation. Accordingly, the NHC is released very fast when heating the polymerization mixture, while also at room temperature, a small concentration of free NHC seems to exist in solution. In summary, the often problematic balance between high latency on one side and high activity on the other side is circumvented in this case by the dual use of the metal ions: once activation by heat occurs, the passive role as protecting group changes to an active acceleration of the polymerization, and the latent behavior is not induced at the expense of attenuated reactivity. Comparable to the PU-forming metal-protected NHCs (see above), this can be understood as a case of “dual catalysis,” a concept that gains increasing interest in polymerization catalysis.[40]

standard applications require well-behaved polymerization systems. The suppression of discoloration, high resolution, working systems despite competing UV stabilizers, and avoidance of residual non-reacted catalysts are some of the main challenges that inspire constant development of suitable latent catalytic processes. Though it is possible to polymerize acrylic monomers by direct UV activation, the presence of a photolabile catalyst usually entails much more effective curing under milder conditions.[43] The following discussion is limited to catalyst-containing polymerizations. Pioneered by Ciba, α-hydroxy and α-amino ketones like 23 or 24 (Scheme 8) have found widespread use.[44] A newer development for white coatings was found in bisacylphosphine oxides (BAPO, 25). These compounds

5. Polyacrylates Acrylic polymers find some of their most important applications in coating industry.[23] Coatings allow for using (UV) light-triggered polymerizations since the required penetration depth is manageable; at the same time, a certain pot life of the polymerization compositions is desired and the coating itself is more easily applied when using lowviscosity educts. Thus, photolatent catalysts can be used at great advantage, usually involving radical mechanisms.[41] The technology is well established, yet growing customer demand for special properties still inspires a lively development in this sector.[42] Outdoor applications or microelectronics demand sophisticated compositions, but even

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Scheme 8. Typical structures of commercial photolatent catalysts including BAPO (25). α-Cleavage can occur twice, resulting in a total of four generated radicals and complete degradation of the π-system absorbing in the visible. MA = methyl acrylate.[44,45]

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Scheme 9. Ru-based compound 26 for photo-induced free radical polymerization of acrylate monomers and fac-[Ir(ppy)3] (27) for the photocontrolled living radical polymerization of MMA.[47,48]

effectively liberate highly reactive phosphinoyl and benzoyl radicals upon irradiation.[45] As this cleavage can occur twice, a total of four radicals can be generated by BAPO, while importantly the residual fragments contain no chromophore anymore that is absorbing light in the visible range. This way, the optical density is decreasing during polymerization and yellowing is avoided (“photo bleaching”) (Scheme 8). BAPO is also very conveniently modified for special applications, for example, when dental acidic aqueous primer formulations are targeted.[46] There, the strongly polar medium can prove problematic for preparation of storable mixtures containing monomer and photocatalyst due to the low solubility of BAPO. Extending the aromatic groups in BAPO with suitable side chains successfully induced better solubility under these conditions. The light-induced generation of radicals for polymerization of acrylic monomers can also be achieved by metal-based compounds, both for free radical or controlled radical polymerization (CRP). While ruthenium (Ru)-based 26 (Scheme 9) has been applied as photolatent initiator for a free radical process,[47] Ir compound 27 in combination with an alkyl bromide initiator allowed for the controlled radical polymerization of methyl methacrylate (MMA).[48] Importantly, in the absence of visible light the polymerization stops, thereby rendering this process not only latent with regard to initiation but also to the polymerization process as a whole (Scheme 9). Numerous repetitions of these “on–off” cycles were possible. Molecular weights were in good agreement with the expected values, especially when very low catalyst loadings were applied (0.005 mol%). Polymerization time in order to arrive at high conversions was in the range of several hours, however. Well-advanced photocontrol including “on–off” cycles has also been reported in the case of free radical polymerization of MMA, again mediated by a Ru-based photo-redox catalyst.[49] The synthesized PMMA displayed PDI values around 2.0, which is acceptable for a free radical process. Re-starting of the polymerization by visible light radiation after stopping the process several times by removing the light source was successfully demonstrated, though after each step

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the polymerization rates showed a certain deceleration. Overall, light-induced and light-controlled radical polymerizations seem a very powerful tool in the constantly growing array of latent polymerization processes.[50] Matyjaszewski and co-workers realized an appealing setup that made use of an external electrochemical trigger.[51] Methyl acrylate was polymerized using the well-established atom transfer radical polymerization technique. The mediating copper complex was sensitive to the applied electrochemical potential, which allowed not only switching between active and dormant species (“on–off”), but also for a real time tuning of the polymerization rates, since the equilibrium of both species is dependent on the potential. Activation or deactivation time to reach full or zero polymerization rates after applying the necessary potential was in the range of 15–20 min. This achievement is certainly impressive, as it takes the concept of latency one step farther by being in control of the onset of polymerization and of the polymerization rate that is desired at the same time. Contrary to all above examples, protected NHCs offer latent access to ionic polymerization mechanisms, using heat as trigger. While UV irradiation is an elegant method to cure coatings or other thin layers of material, thermally latent polymerization systems can find application where larger polymer bodies are concerned or where no light source can be implemented. Furthermore, it is obvious that the ionic polymerization mechanisms (see below) that follow from the interaction of NHCs and acrylic monomers will allow other polymer architectures, other copolymers and other end groups compared to the radical processes. Additionally, they have the potential to be very fast and controlled, though they are more sensitive to impurities and less tolerant to some functional groups, which overall makes them a complementary choice to the more established technologies. The fundamental challenge in polymerizing monomers like MMA directly by the action of NHCs is the initiation step. A conjugate addition is required, resulting in a zwitterionic species, which can lead to further polymerization (Scheme 10) or other side reactions (dimerization, formation of stable enamines).[52] Conjugate addition itself is a relatively

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Scheme 10. Proposed mechanisms for NHC-initiated polymerization of MMA from thermally latent precursors. Top: Anionic pathway accessible for the stronger basic NHCs.[55] Bottom: Zwitterionic polymerization as was observed for the thermally liberated 28.[54,55]

new concept in NHC chemistry,[53] and not surprisingly the first direct polymerization of MMA was only recently reported in 2012.[54] A screening of a large array of differently constituted CO2-protected NHCs indeed underlined the massive influence that is exerted by the NHC structure, and demonstrated that these compounds can be held in solution at room temperature in bulk MMA, without undesired polymerization occurring.[55] Thermal activation (50–85 °C) then effected polymerization in some cases, though heavily dependent on the NHC in action. Interestingly, there is some evidence that also classical anionic polymerizations can be induced by NHCs. It was found that the strongly basic six-membered NHCs (see above paragraphs) are unable or extremely ineffective for conjugate addition; in the presence of DMSO, however, they gained activity.[55] Preliminary, a mechanism is proposed, where the NHC–carboxylate acts as a latent base, thus forming deprotonated DMSO after decarboxylation, which, in turn, initiates polymerization (Scheme 10). Mechanistically, the formation of cyclics and the addition chemistry of NHCs to Michael acceptor systems (as is MMA) in general are still under debate, but it seems that the development for these particular case of organopolymerization is just gaining momentum.[52] The NHC-catalyzed cyclotetramerization of acrylates was only recently reported.[56] Metallocenes have been reported to be efficient photoinitiators for the anionic polymerization of cyanoacrylate after being exposed to UV radiation, while in the absence of light, no monomer consumption was observed.[57] An electron transfer to form a radical anion of the monomer is responsible for initiation of the polymerization. The robust and well-available nature of the Fe and Ru metallocenes is certainly advantageous for application of this method. Finally, there are also efforts to introduce mechanical force as external stimulus in acrylate chemistry. Incorporation of a substituted cyclobutane moiety that is sensitive to mechanical force (“mechanophore”) in polymethacrylate (PMA, Scheme 11) was shown to entail increased

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and selective bond scission after exposure to ultrasound.[58] The cyclobutane moiety degrades to generate cyanoacrylate-terminated polymer fragments. The chain ends were indirectly observed by trapping experiments. Generally, the highly reactive, newly formed cyanoacrylates allow for further manipulation of the polymers after bond cleavage, which is thought to find application in self-healing materials. It should be noted that the above few examples span a range of fundamentally different triggers (light, heat, electrochemical potential, mechanical force), polymerization mechanisms (free radical, controlled radical, zwitterionic, anionic) and types of control (initiation, polymerization progress), that were all successfully applied for latent polymerization of acrylic monomers. Their inherently complementary properties underline that it is principally possible to match the various processing and application types of polyacrylates with an equally versatile set of

Scheme 11. A substituted cyclobutane group is incorporated into PMA to serve as a predestinated point of scission when mechanic force is applied via ultrasound. Degradation results in the formation of cyanoacrylate-terminated polymeric fragments that can rapidly undergo subsequent reactions.[58]

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Scheme 12. Structures of bisphenol A diglycidyl ether (BADGE, 29) and phthalic anhydride (30) as well as various methods to generate tertiary amines from thermally or photo-labile progenitors.[63–70]

latent polymerization systems that can be used to advantage in each individual case.

6. Epoxy Resins Epoxy thermosets are very versatile materials, which originates from the multiple structure variations that can be incorporated into the polymer.[23] Apart from a broad range of epoxide-functionalized moieties (monomeric, oligomeric), perhaps most typically represented by diglycidylethers (29, Scheme 12), there is an equally rich array of hardeners for curing. Widely applied hardeners are amines, anhydrides, phenols, thiols, and carboxylic acids, each imposing very individual properties on the resulting materials, which is further diversified by factors like curing temperatures, curing times and ratio of epoxy functionality and hardener.[59,60] In short, it is much beyond the scope of this feature article to discuss exhaustively the (potential) applications of latent polymerization systems. Some interesting developments, however, will be presented, again focused on the manifold approaches that

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can be chosen to realize a beneficial implementation of latency. In principle, latent catalysis or initiation can only be applied where the educts, e.g., epoxy compound and hardener, do not react immediately by themselves. A suitable combination is found in anhydride-based compositions. Acid anhydrides (like phthalic anhydride, 30), sometimes referred to as “latent hardeners,” allow a certain pot time of 1 K formulations and require elevated temperatures for curing. This latency (in contrast to room temperature hardening amines), however, comes at the price of slower curing, which is why accelerators are commonly added.[61,62] It is very important to ensure that the addition of accelerators does not compromise the latency of the composition, otherwise premature setting of the duroplast will ruin its processability. A proper harnessing, preferentially in a chemical way, must thus allow controlling the activity of the accelerator. Typical accelerators are tertiary amines, and since elevated temperatures are needed to ensure sufficient crosslinking in epoxy/anhydride systems, it is sensible to introduce the amine from a thermally labile progenitor that is chemically inert towards the educt materials. An example for this strategy can probably be found in a recently filed patent family by BASF, employing N,N′-dialkylated imidazolium salts.[63] When heated, the imidazolium salts (31) dealkylate, which liberates N-alkyl imidazole, a reaction that is well known.[64] Being a tertiary amine, this compound then catalyzes the cross-linking reactions. It was found that at 25 °C, the compositions remained castable after more than 60 d, while at 80 °C, the pot time shrank to about 300 min, which is still substantial.[63] As counter ion and N-substituents can be chosen from a broad variety, this method is certainly versatile. Curing temperatures, however, have to be rather high (160–200 °C), which is obviously due to the fact that a relatively stable N C bond has to be broken. In the same context, investigations into photobase generators have to be considered, though it should be understood that generation of bases is not only interesting in epoxy applications.[65] While it is interesting to be able to generate primary or secondary amines from photolabile precursors,[66] it is especially the development of compounds that liberate tertiary amines that is expected to find promising applications.[67] Reduced amidines of the type 32 were developed by Ciba to release DBN (1,5-diazabicyclo[4.3.0]non-5-ene, Scheme 12) after irradiation with light. The photogeneration most probably involves a radical step. DBN is basic enough to act as a catalyst in the formation of different resins.[67] Quaternary ammonium salts have also been demonstrated to be photosensitive and prone to generate tertiary amines (33).[68] Interestingly, the released amine was used for curing of epoxides, while the involved radical step was exploited for the

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Figure 3. Phosphorous-based compounds for base-catalyzed preparation of epoxy polymers.[71–73]

polymerization of acrylates. Borate salts chelated by benzoyl ligands in combination with ammonium counter ions were found to be thermally latent catalysts for the formation of epoxy-phenolic resins.[69] Ammonium benzoate was proposed to be the true initiating compound, formed from the precatalyst by reaction with phenolic alcohol and activated at elevated temperatures. Below 100 °C, only a slow build-up of product was observed. Aminimide 34 was more recently reported as novel thermally labile pre-catalyst for anionic applications.[70] It was used for the polymerization of epoxides and thermal decomposition seems to liberate the tertiary amine, which is simultaneously formed with an isocyanate (Scheme 12). Both compounds are in equilibrium with the corresponding urethane, which can be understood as a blocked isocyanate (see paragraph on PU), though inversely in this case the isocyanate serves as a blocking agent for the desired tertiary amine. Their action as thermally latent Brønstedt bases has rendered phosphorous-based compounds 35 and 36 (Figure 3) suitable precatalysts for the polymerization of epoxy monomers as reported by Endo and co-workers. Phosphonic ester amide 35 was found to release the secondary amine piperidine after heating to 110 °C, which entailed the polymerization of glycidyl phenyl ether.[71] Below this temperature, no conversion was observed. At catalyst loadings of 3 mol% and 12 h polymerization time at 190 °C, conversions >90% were achieved. 35 was also used to cure epoxy resins.[72] Phosphonium ylides of the type 36 catalyzed the polyaddition of BADGE (29) and bisphenol A.[73] The acidic phenolic protons can be abstracted by the ylide, which results in a poshponium compound that activates the epoxy functionality that is in turn opened by the phenolic alcoholate. Interestingly, by appropriate choice of substituents R, the limiting starting temperature could be modulated, ranging roughly between 90 and 130 °C. Below this temperature, no formation of polymer was detected. Cationic polymerization of epoxides and curing of epoxy resins is obviously fundamentally different from the anionic process.[23] Accordingly, the latent catalysts are very different from the ones discussed above. Both

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Scheme 13. Photo-labile epoxides.[71–76]

precatalysts

for

UV

curing

of

thermally labile and photolatent catalytic systems have been investigated in detail, and some of them are used in large-scale applications.[74,75] Ionic compounds, onium salts, dominate this chemistry; probably most iconic are Crivello’s diaryliodonium- and triarylsulfonium salts (37 and 38, Scheme 13).[75] In rather complex fashion, UV irradiation of these onium salts results in the formation of acids, which then are able to initiate cationic polymerizations, among them the ROP of epoxides. Principally, the organic part (cation) controls the photochemistry and thermal stability, while the anion determines the pKa value of the resulting acid and the rate and nature of the propagating species (ion pairing). Modification of these systems is readily done by tuning the conjugated systems, for example, by introducing side chains on the aromatic moieties. In general, compounds of the type 37 and 38 are sufficiently photolatent and thermally stable to allow homogeneous dispersion in thermosetting compositions prior to UV curing. Frontiers for improvement remain solubility issues (non-ionic compounds are desired), tuning of the conjugated system to optimize or shift the absorption range and curing efficiency. A manifold of different structures for these photoacid generators have been realized, for example, the phenacyl anilinium compound 39 as reported by Yagci and co-workers.[76] Ciba has revealed the use of ferrocenium salts (40) for photo-induced cationic ROP of epoxides, exploiting a distinctly different mechanism based on monomer activation by the in situ-formed Lewis acid (Scheme 14).[77]N-Benzylpyrazinium hexafluoroantimonates (41) have been shown to induce cationic polymerization of a bio-based epoxy monomer, thermally triggered in this case; below 60 °C, monomer conversion practically ceased.[78] A Zn–salen complex was reported to catalyze the polyaddition of multifunctional hemiacetal esters with diepoxides in a thermally latent fashion, where manipulation of the electronic properties of the ligand influenced the required polymerization temperature.[79]

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Scheme 14. Left: Ferrocenium salt as photolatent initiator via loss of arene ligand and activation of monomer by coordination to the Lewis acidic metal center. Right: N-Benzylpyrazinium salt as thermally latent curing agent.[77,78]

Above-mentioned examples illustrate some of the various approaches to solving the basic problem that still exists in the curing chemistry of one-component thermoset compositions, which is the need to induce effective and fast curing in spite of the use of “latent hardeners” (i.e., hardeners with a relatively low reactivity towards the epoxide functionality, like anhydrides or dicyanodiamide).[80] This requires powerful catalysts that at the same time can be controlled enough to prevent premature reaction or cross-linking. Progress has been made, though very traditional latent catalysts like boron trifluoride-amine adducts[81] are still in use, perhaps documenting that there is still more than enough room for improvement. It seems like especially the chemistry of latent base generators contains as yet unexplored potential. One of the aims will be to increase the Brønstedtbase power of the generated bases; substitution of the commonly employed tertiary amines by more effective catalysts could prove very beneficial. From what was discussed in the paragraphs above, protected NHCs seem an obvious choice for this task. Indeed, preliminary results indicate a very promising behavior of these compounds for the curing of epoxy resins.[82]

7. Metathesis-Derived Polymers As ring-opening metathesis polymerization (ROMP) grants access to important polymers like polynorbornenes, polymers derived from cyclooctene or cyclooctadiene and dicyclopentadiene (DCPD)-based thermosets, latent metathesis catalysis has been mainly driven again by polymer chemistry.[83] In order to implement latency in a metathesis catalyst, a number of strategies exist, not surprisingly focused on the robust Ru-type compounds.[84] A very fundamental difference is the absence or presence of the alkylidene moiety in the precatalysts (Figure 4). The former type (A) requires the triggered generation of the alkylidene by either light or heat; this can be problematic with regard to initiation efficiency and molecular weight control, but offers the possibility to achieve good latency, usually coupled with a convenient preparation of the precatalysts. The compounds in which the alkylidene already exists (B) can employ a variety of blocking or delaying methods

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to suppress premature metathesis. This can be realized by the use of deactivating ligands (B1), electron-rich carbene moieties (B2) and, perhaps most intensely researched, by chelating ligands (B3 and B4). Hafner and co-workers (Ciba) presented p-cymene complexes of Ru and osmium of the general structure 42 (Figure 4) as one-component catalysts for thermally or light-triggered polymerization applications.[85] In the case of M = Os and L = PCy3 or PiPr3, the compounds proved to be highly active after UV activation, however, the compounds are not fully inert even at room temperature and can thus be thermally activated too. Interestingly, more bulky substituents suppressed polymerization activity. Comparable Ru-derived complexes did not show any thermal latency, i.e., were fully active at room temperature. Furthermore, it was observed that for 42 with M = Ru and L = PCy3 a stable mixture of the precatalyst and dicyclopentadiene could be prepared, which was stable for weeks and could be activated by heating above 80 °C. Not surprisingly, the versatility of the general complex structure 42 inspired several working groups to investigate the consequences of modifications.[86] Here again, NHCs play an important part. After their first incorporation in transition metal complexes,[17] they soon found their way to metathesis chemistry.[87] Delaude et al.[88] reported on the UV-induced ROMP applying 42 (L = NHC). Buchmeiser et al.[89] similarly employed NHCs and exchanged additionally the chlorides for trifluoro acetate ligands. Norbornenes (both enatiomerically pure exo- and endo-monomers) were thus polymerized after thermal activation. It was found that exo-norbornene was consumed faster, and calculations were presented that backed the assumption of an 1,2-H-shift being responsible for initial formation of the alkylidene moiety. In 2008, Buchmeiser and co-workers[90] published the cationic Ru(II) complex 43a to effect UV-induced ROMP. Mixing of 43a with various norbornenes, cyclooctene, and dicyclopentadiene at room temperature did not entail polymerization after 24 h, thereby demonstrating full thermal stability. In contrast, UV radiation at 308 nm wavelength and better at 254 nm for 60 min resulted in up to quantitative yields of polymer depending on the structure of the monomer. A dicyclopentadiene-based thermoset was successfully created using a mask and UV

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Figure 4. Several strategies to induce latent behavior in metathesis catalysts categorized.[108a]

radiation to prove the convenient application of 43a. Furthermore, its use in the photochemically triggered, spatially resolved functionalization of polymeric monolithic capillary columns was demonstrated.[91] Mechanistically, a reaction sequence for the formation of the metathesisactive catalysts was proposed, which was supported by quantum chemical calculations and laser flash spectroscopy. 43a is thought to lose two tBu-CN ligands, which is facilitated by radiation and subsequent excitation to the triplet state. The free coordination site is occupied by the olefinic functional group of the monomer (norbornene) and the π-complex is converted to the alkylidene by an 1,2-H-shift, forming essentially a Ru(IV)-based Grubbstype catalyst. The experimental results confirm this by displaying typical Grubbs-type-like polymerization results. Notably, the synthesis of the related cationic compound 43b (Figure 4), bearing a triflate ligand, and comparison of its performance with analogous precatalysts bearing only monodentate ligands, demonstrated the truly latent behavior of 43-type compounds to stem from both their cationic character and the use of potentially chelating anionic ligands.[92] The latter are able to stabilize the transition state after loss of the first tBu-CN ligand by coordinating in a bidentate fashion. More recently, Buchmeiser and co-workers[93] continued this research into alkylidene-free latent precatalysts,

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presenting 44 (Figure 4) as a new addition to the growing array of UV-active ROMP catalysts. At room temperature, norbornene was converted extremely slowly if L3 = PPh3 and X = chloride or trifluoro acetate, while UV-induced polymerization (254 nm) resulted in yields >90% in only 30 min. The initiation efficiency was calculated to be about 25%, as evident from the high experimental molecular weights (three to four times higher than calculated). The strong influence of ligand L was demonstrated by the fact that the bismethallyl complex bearing PCy3 was active at room temperature. In contrast to A-Type precatalysts, there is a great wealth of latent olefin metathesis catalysts based on types B1–B4. Since it is not the scope of this article to present all reports exhaustively and since the situation has been reviewed in 2009,[83] the following discussion is limited to selected examples under newer developments. Catalysts of the type B1 keep all the characteristics of Grubbs-like compounds; latent behavior must hence originate from a blocking/deactivating property of ligands X, L3, and L4. This strategy is straightforward as the catalysts can be easily accessed, yet it is inherently problematic because the active state of the catalysts may be much attenuated in reactivity. Sufficient activation is indeed only achieved by external addition of acids; P’Pool and Schanz reported an example where reversible

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Scheme 15. Activation of a Ru biscarbene complex by application of mechanical force to generate a metathesis-active site for ROMP.[96,97]

deactivation–activation cycles of ROMP were controlled by the addition of pyridine (deactivator) and phosphoric acid (activator),[94] Grubbs and co-workers published a complex bearing large alkoxide ligands.[95] Reactivity in ring-closing metathesis (RCM) in the latter case was low, both at room temperature and at 60 °C; however, generally the usefulness of latent systems in RCM is not clear at all. Activator in this case was hydrochloric acid, which entailed a substitution of the alkoxides by chloride ligands, thus generating a situation much like in typical Grubbs-type catalysts. Unfortunately, in situ addition of activating chemicals cancels out some of the benefits that are desired to arise from the application of latent catalysts. This problem can be solved elegantly when photoacid generators are used (see below). Radically different, Sijbesma and co-workers have recently demonstrated ROMP induced by mechanical force, both in solution via ultrasound[96] and in the solid state by application of pressure.[97] The precatalyst in these cases was a polymer-bound Ru biscarbene complex (45, Scheme 15) that was activated by dissociation of one NHC ligand. In the ROMP of cyclooctene in toluene, a considerable influence of the molecular weight of the polymers was observed, documenting the importance of the transfer of force via the polymer chains. When sonication was stopped, monomer consumption also ceased. However, this was due to decomposition of the catalyst; the activation proved to be irreversible. Kinetic experiments established that most scission events lead to active catalysts, provided the pendant polymer chains possess high enough molecular weight. Furthermore, it was found that scission by ultrasound followed first-order kinetics. Life times of the active sites were generally longer for ROMP than for RCM, amounting to several hours.[96] The same precatalyst 45 was also used in the solid state, namely in a semicrystalline matrix consisting of polyTHF together with norbornene or bifunctional monomer 46. When pressure was applied (0.8 GPa) in cycles of 5 min each, successively growing monomer consumption was observed.[97] Future applications are seen in smart selfhealing materials, where this method could effect crosslinking exactly where the mechanical stress (and risk of material failure) occurs. In 2000, van der Schaaf reported Ru-based complexes bearing arylthiocarbene moieties (47, Scheme 16).[98] Similarly, Grubbs and Louie extended on this work by

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preparation of complexes of the type 48.[99] With respect to polymerization, these B2-type compounds displayed delayed-action but no truly latent behavior. It was thus possible to handle mixtures of dicyclopentadiene and 47, with gel times of at least 10 min. Nonmodified Grubbs first-generation catalysts, in contrast, would entail immediate polymerization and thus drastically limit the processing window for this thermoset. Comparably, catalysts of the type 48 were attenuated in their reactivity towards norbornene, thus offering enough time to prepare proper mixtures of the precatalyst and the monomer, resulting in polymerization in the range of minutes instead of seconds. Very recently, Grubbs and co-workers published a procedure for the creation of photoresists based on related complex 49.[100] 1,5-Cyclooctadiene (COD) was thereby polymerized by a third-generation catalyst, the volatiles then removed after quenching with ethyl vinyl ether and the polymer dissolved in difunctional ethylidene-2-norbornene (50, Scheme 16). Interestingly, it was found that the quenched Ru species (49), which is sensitive to heat, light, and oxygen, survived for weeks under ambient conditions in the viscous and olefin-rich matrix. The authors attributed this to stabilizing, coordinating effects of the double bonds provided by the norbornene matrix. The mixtures proved to be useful UV photoresists and it was possible to generate patterns with relatively high resolution this way. Though the mixtures were processable after weeks of storing under ambient conditions,

Scheme 16. B2-type delayed-action catalysts and preparation of photoresists employing in situ formed 49.[98–100]

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Figure 5. Some B3-type latent olefin metathesis catalysts.[101–105]

the spin-casted solutions were found to be much less stable, most probably due to the increased surface-tovolume ratios. Numerous bi- and tridentate ligands have been used to prepare Ru-based alkylidene complexes (type B3). This was done in the hope to exploit the stabilizing effect of chelating ligands, which are less prone to loss of the ligand and thus effectively block the free coordination sites necessary for metathesis polymerization. This would bestow thermal stability onto the compounds and thus make them useful latent catalysts. Furthermore, in view of the abundant structural variety of available chelating ligands, this offers the opportunity to alter the properties of the catalyst in a fast and systematic way. Ozawa and co-workers,[101a] Grubbs and co-workers,[101b] and Slugovc and co-workers[101c] investigated compounds bearing tridentate and anionic hydridotris(pyrazolyl) borato ligands (see for example 51, Figure 5 The anionic character of the ligand combined with its three points of coordination entail thermally very stable compounds; unfortunately this hampers activity, even at elevated temperatures like 80 °C and leads to slow polymerization rates in ROMP and shuts down RCM activity, highlighting the inherent problematics of type B3 complexes. Even if (partial) dissociation occurs, ligand and substrate stand in competition to each other, i.e., latency is achieved at the price of much attenuated activity. Compound 52 in combination with photoacid generators (38, Scheme 13) was used elegantly in high conversions for the ROMP of cyclooctene, several norbornenes, and cross-linking of DCPD.[102] In the absence of UV light or in the absence of 38, no polymerization occurred, thus rendering this system photolatent. Mechanistically, it was assumed that the acid liberated from 38 protonates the acetylacetonate and introduces chloride ligands instead (regenerating a typical Grubbs-type catalyst); accordingly, 38 with chloride as counterion induces polymerization upon irradiation, while non-nucleophilic nonaflate (C4F9SO3−) as counter ion rendered the whole system inactive. Molecular weights were considerably higher than expected, which was related to only partial activation of the catalyst (ca. 10% after 2 h of irradiation).

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N,O-Bidentate ligands were used in further efforts to synthesize latent B3-type compounds. Schiff base ligands were employed for synthesis of complex 53 and derivatives.[103,104] 53 was used for the ROMP of COD at elevated temperatures (90 °C), while at room temperature, very good latent behavior was observed.[104] Similarly, crosslinking of DCPD did only occur after heating to 150 °C, while storage for 1 week at room temperature did not entail undesired polymerization. Structurally related compound 54 was reported to gain activity for ROMP of cyclooctene at elevated temperatures.[105] Chelating or “dangling” alkylidene ligands present the perhaps best method to induce latent behavior in a metathesis catalyst (B4). The chelating ligand stabilizes the catalyst towards degradation, but at the same time, it will be removed from the active site in the course of propagating polymerization. Competitive coordination is prevented this way, which allows for increasing the reaction rates in comparison to B3-type catalysts. If the chelating power of ligand L3 is strong enough to suppress initiation of metathesis at room temperature, it is thus thought possible to obtain a powerful latent catalyst that fulfills the criteria of perfect latency/high activity. The development of “dangling” alkylidene ligands started with efforts to improve the chelating ability of Grubbs–Hoveyda catalysts (55, Figure 6), which cannot be considered as latent.[106] Van der Schaaf reported the use of 2-pyridylethanyl alkylidene ligands (56), which caused the catalytic activity to drop somewhat, but allowed for a better handling of DCPD mixtures due to longer gel times.[98] These catalaysts were later extended on by substitution of the phosphine by an NHC, which revealed an interesting case of cis/trans-isomerization (57).[107] Observation of the cross-linking of DCPD using the isolated isomers strongly demonstrated the influence of the trans-effect by the electron donating NHC ligand, rendering the transisomer much more reactive (less latent, shorter gel times) because the pyridyl ligand decoordinates more easily. Further examples for N-chelating alkylidenes are found in the versatile class of Schiff base derivatives[108] or Grela’s more rigid quinoline systems.[109] With a novel Ru–triazene complex, Buchmeiser and co-workers[110] made a

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Figure 6. B4-type metathesis catalysts.[105–114]

contribution to these N-chelating systems (58). However, in this case, activation of the complex is achieved by irradiation with UV light. Excitation of the N N N-moiety entails a fast π–π* transition, which was expected to lead to an equally fast change in the coordination of this fragment in turn. Indeed, it was found that all complexes of this type were thermally stable. Photoactive behavior was observed where X = Cl in the ROMP of cyclooctene and of DCPD. In the absence of UV light, practically no polymer was formed even after prolonged latency times of 24 h, while irradiation (2 h) resulted in high or very high yields. Strongly electron-withdrawing substituents diminished or shut down the photoactivity of 58. O-Chelating alkylidene ligands have also been investigated to some extent, most intensively by Grela and Slugovc, who demonstrated that manipulation of the benzylidene ring allowed for influencing the latency properties of the catalysts (59).[111] An intriguing case is finally found in S-chelating alkylidene ligands. Lemcoff and co-workers were able to show that the sulphur-containing analogue to the Grubbs-Hoveyda catalyst exists in cis-dichloro configuration (60).[112] The catalyst displayed excellent latent properties in RCM, and even allowed for switching between “on–off” states repeatedly, by simple change of reaction temperature. Applying 60 to ROMP of a norbornene derivative resulted in equally good latent behavior.[113] Again thermoswitchable behavior was observed. The authors attributed this to a very small degree of activation, which resulted in only minute amounts of the catalyst actually performing polymerization. In the absence of heat, no new active sites are generated and polymerization effectively stops when the

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life time of the active catalysts is used up; heating again re-starts the polymerization. This reasoning was supported by GPC analysis. Lemcoff and co-workers extended their investigations also on UV-triggered processes and synthesized more recently several bi- and tridentate S-chelating alkylidenes, which were either very stable or more easily activated, depending on the chelating structure (61).[114]

8. Conclusions Polymer chemistry is the main driving force behind the ongoing development in latent catalytic systems. Apart from offering considerable advantages in handling and processing of polymers, this technology will play a crucial role in high-tech areas like microelectronics, self-healing materials, smart devices, or light weight construction, all of which depend on control over the polymerization process. Latent polymerization catalysis can realize this, and as was discussed above, the “latent” tool box now encompasses diverse polymerization mechanisms, various types of monomers, radically different activation mechanisms and in growing number also “switching” properties. A promising current trend is certainly found in the advent of organocatalysis, and here especially in NHCs, which are in general stronger bases than the competing tertiary amines, while maintaining good nucleophilic properties and unparalleled structural diversity. Progress in the field of latent ionic polymerization is closely linked to the success of latent (organo)catalysis, an area that is still somewhat underdeveloped. Latent metathesis chemistry is

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dominated by Ru-based compounds, a trend that seems unlikely to be broken. Improvements are currently focused on novel chelating ligands and in situ formation of alkylidene ligands. The challenge at the heart of all is the proper balancing between inertness in the absence of activating stimuli and high activity after being triggered; at the same time, all this has to be achieved at reasonable cost in a complex chemical environment. This requires clever solutions, and it can be safely assumed that the future is bright for latent processes as the strategies to address this problem evolve ever more. Acknowledgements: This work was supported by the Ministry of Finance and Economics, State of Baden-Württemberg, Germany. Received: December 9, 2013; Revised: January 8, 2014; Published online: February 11, 2014; DOI: 10.1002/marc.201300898 Keywords: anionic polymerization; curing of polymers; metathesis; photopolymerization; reactive processing

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Latent and delayed action polymerization systems.

Various approaches to latent polymerization processes are described. In order to highlight recent advances in this field, the discussion is subdivided...
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