Michael R. Buchmeiser

Carbon Fibers: Precursor Systems, Processing, Structure, and Properties

This Review gives an overview of precursor systems, their processing, and the final precursor-dependent structure of carbon fibers (CFs) including new developments in precursor systems for low-cost CFs. The following CF precursor systems are discussed: poly(acrylonitrile)-based copolymers, pitch, cellulose, lignin, poly(ethylene), and new synthetic polymeric precursors for high-end CFs. In addition, structure-property relationships and the different models for describing both the structure and morphology of CFs will be presented.

New synthetic ways for the preparation of high-performance liquid chromatography supports

The latest developments and in particular important synthetic aspects for the preparation of modern HPLC supports are reviewed. In this context, the chemistry of inorganic supports based on silica, zirconia, titania or aluminum oxide as well as of organic supports based on poly(styrene-divinylbenzene), acrylates, methacrylates and other, more specialized polymers is covered. Special consideration is given to modern approaches such as sol-gel technology, molecular imprinting, perfusion chromatography, the preparation of monolithic separation media as well as to organic HPLC supports prepared by new polymer technologies such as ring-opening metathesis polymerization. Synthetic particularities relevant for the corresponding applications are outlined.

Bis(pyrimidine)-based palladium catalysts: synthesis, X-ray structure and applications in Heck–, Suzuki–, Sonogashira–Hagihara couplings and amination reactions

Abstract The synthesis of N,N-bis(pyrimid-2-yl)amine (1), N-acetyl-N,N-bis(pyrimid-2-yl)amide (2), N-norborn-2-ene-5-ylbis(pyrimid-2-yl)carbamide (4) is described. The Pd-complex N-acetyl-N,N-bis(pyrimid-2-yl)amine palladium dichloride (3) has been prepared from compound 2 and its X-ray structure has been determined. A polymer supported catalytic system (6) was prepared via ring-opening metathesis copolymerization of 4 with 1,4,4a,5,8,8a-hexahydro-1,4,5,8-exo-endo-dimethanonaphthalene (HDMN-6) and subsequent loading with PdCl2. Both the homogeneous and heterogeneous catalysts 3 and 6 were active in Heck–, Suzuki–, Sonogashira–Hagihara-type couplings and amination reactions using aryl iodides, bromides and chlorides.

Monolithic Materials: New High‐Performance Supports for Permanently Immobilized Metathesis Catalysts

Polymerchemistry and materials sciences have seen the introductionof new trends by metathesis-based techniques, such as ring-opening metathesis polymerization (ROMP) or acyclic dienepolymerization (ADMET). Complementary, cross-metathesisand ring-closing metathesis find ample application in organicchemistry.

Recent advances in the synthesis of supported metathesis catalysts

The state of the art and recent developments in the synthesis of supported, well-defined metathesis catalysts are reviewed. In this context, their synthesis, selected properties and application to both polymer and organic chemistry are summarised. Special consideration will be given to aspects of activity [discussed in terms of turnover numbers (TONs) and turn over frequencies (TOFs)], regioselectivity and enantioselectivity as well as stability. The general applicability of these supported catalysts to either high-throughput technology or to the manufacture of continuous flow systems is discussed.

Alternating copolymerizations using a Grubbs-type initiator with an unsymmetrical, chiral N-heterocyclic carbene ligand.

Reports on alternating copolymers prepared by metathesis copolymerization of two different monomers are relatively rare. In 2005, Chen et al. reported on the mechanismbased design of a ROMP (ring-opening metathesis polymerization) catalyst for sequence-selective copolymerization. Here, we report on an alternative system for the synthesis of alternating copolymers based on Grubbs-type initiators containing an unsymmetrical, chiral N-heterocyclic carbene (NHC) ligand. As part of our on ongoing research on ruthenium metathesis catalysts with saturated unsymmetrical NHC ligands initiator 1 was prepared from [RuCl2(PCy3)2(CHPh)] and 1-mesityl-3-((1’R)-1-phenylethyl)-4,5-dihydroimidazolium tetrafluoroborate in 90% yield. This initiator was used for the copolymerization of norbornene (NBE) and cyclooctene (COE) using a stoichiometry of 1/NBE/COE of 1:2000:100000. Reactions kinetics recorded for this copolymerization in CH2Cl2 revealed that NBE was consumed in less than 15 min. Terminating the copolymerization after 60 min in fact provided an almost perfectly alternating copolymer of NBE and COE. Unfortunately, the polymer was only partially soluble in standard solvents such as CHCl3. However, the soluble fraction showed a unimodal molecular weight distribution (PDI= 2.19), andMn = 40400 gmol 1 was found. Longer reaction times led to the formation of substantial amounts of a poly(COE) homopolymer block attached to the alternating copolymer. This homopolymer block is easily identified by NMR spectroscopy. The C NMR spectrum of the alternating copolymer (97% alternating units, d = 128–136 ppm) is shown in Figure 1. Hardly any signals for poly(NBE) or poly(COE) can be detected. The signals for the alternating copolymer are observed around d = 135.0 and 128.5 ppm. The first set of signals around d = 135.0 ppm is assigned to the C=C CH2 carbon atoms and consists of eight different signals at d = 135.28, 135.26, 135.14, 135.00, 134.98, 134.95, 134.83, and 134.80 ppm, which we attribute, without specific assignment, to the ccc, tcc, ctc, cct, ttc, tct, ctt, and ttt triads. The fact that all eight signals display roughly the same intensity is indicative for a cis content of about 50% and is in accordance with the H NMR spectrum, which shows signals of the cis and trans double bonds in a 1:1 ratio at d = 5.36 and 5.28 ppm, respectively. Such a high cis content is rather unusual for polymers prepared by Grubbs-type initiators; most display a cis/trans ratio of roughly 25:75. The less well-resolved signals around 128.5 ppm are assigned to the C=C CH2 carbon atoms and correspond to the same triads. DSC measurements of the alternating copolymer revealed a single glass transition at Tg = 50.9 8C (Dcp = 0.32 Jg )), which is in between the glass transition temperatures of poly(COE) ( 78 8C, 50% trans) and various poly(NBE)s (40–64 8C). We next turned our attention to the initiation efficiency of 1. Second-generation Grubbs-type initiators having one NHC and one phosphine ligand are known to display rather poor initiation efficiencies as a result of unfavorable ratios of the rate constants of initiation and polymerization (ki and kp, respectively). H NMR experiments carried out with 1 and either of the two monomers, that is, NBE or COE, in a 1:5 ratio in fact revealed poor initiation efficiencies of less than 1 and 3.8%, respectively. We therefore prepared the monopyridine adduct of 1, [RuCl2(Py)(1-mesityl-3-((1’R)-1-phenylethyl)-4,5-dihydroimidazolin-2-ylidene)(CHPh)] (2), in analogy to the known complex [RuCl2(Py)2(IMesH2)(CHPh)] (IMesH2 = 1,3-dimesitylimidazolidin-2-ylidene), by reaction of 1 with excess pyridine (Scheme 1). Owing to the high solubility of 2 in conventional organic solvents, purification by recrystallization was not possible. Instead, a Cu-loaded polymer-bound triphenylphosphine was used to remove the released PCy3 and isolate 2 in a pure form. The fact that only one pyridine ligand is capable of coordination to the catalyst is indicative for the significant steric demand of the NHC ligand [17] and is relevant to mechanistic considerations (vide infra). To our great delight, catalyst 2 displayed significantly improved initiation efficiencies for NBE and COE—values of Figure 1. C NMR spectrum of poly(NBE-alt-COE)n prepared with initiator 1.

Synthesis of a Silica‐Based Heterogeneous Second Generation Grubbs Catalyst

The synthesis of a second generation Grubbs catalyst immobilized onto non-porous silica is described. For this purpose, a polymerizable cationic NHC precursor, 1,3-bis(1-mesityl)-4-{[(bicyclo[2.2.1]hept-5-en-2-ylcarbonyl)oxy]methyl}-4,5-dihydro-1H-imidazol-3-ium tetrafluoroborate (5) was prepared and characterized by X-ray analysis. Oligomers were prepared therefrom using both the well-defined Schrock initiator Mo(N-2,6-i-Pr2C6H3)(CHCMe2Ph)[OCMe(CF3)2]2 and the first generation Grubbs catalyst Cl2Ru(CHPh)(PCy3)2. Ru-initiated oligomerizations were terminated with ethyl vinyl ether, Mo-initiated oligomerizations were terminated by addition of (EtO)3SiCH2CH2CH2NCO. (EtO)3Si-terminated oligomers obtained by the Wittig-like reaction between the Mo-containing oligomer and the isocyanate were used for the immobilization of the NHC-precursor containing oligomers on non-porous silica. Both oligomerizations were characterized by quantitative consumption of the corresponding initiator. This allowed the controlled synthesis of oligomers via stoichiometry. Using both non-porous and porous silica, degrees of derivatization of 0.04 and 0.02 mmol, respectively, of cationic NHC precursor/g silica were obtained. These precursors were converted into the corresponding NHCs by standard procedures and used for the generation of a heterogeneous second-generation Grubbs catalyst. Ruthenium loadings of 5.3 and 1.3 μmol/g, corresponding to 0.5 and 0.1 weight-% of catalyst were realized. Additionally, coating techniques were applied, where C18-derivatized silica-60 was loaded with oligo-5. Conversion into the corresponding heterogeneous catalyst revealed 4.1 μmol/g, corresponding to 0.4 weight-% of catalyst. All supported catalysts prepared by this approach were successfully used in RCM in slurry type reactions.

N-Acyl-N,N-dipyridyl and N-acyl-N-pyridyl-N-quinoyl amine based palladium complexes. Synthesis, X-ray structures, heterogenization and use in Heck couplings

A series of homogeneous and heterogeneous palladium(II)-catalysts and their use in Heck-couplings is described. Starting from four different amines, N -pyrid-2-yl- N -(3-methylpyrid-2-yl)amine ( 1 ), N -pyrid-2-yl- N -(6-methylpyrid-2-yl)amine ( 2 ), N -(6-methylpyrid-2-yl)- N -(4-methylquinolin-2-yl)amine ( 3 ) and N -bis(6-methylpyrid-2-yl)amine ( 4 ), the corresponding acetyl- and norbornene derivatives, N -pyrid-2-yl- N -(3-methylpyrid-2-yl) acetamide ( 5 ), N -pyrid-2-yl- N -(6-methylpyrid-2-yl) acetamide ( 6 ), N -(6-methylpyrid-2-yl)- N -(4-methylquinolin-2-yl) acetamide ( 7 ), N -bis(6-methylpyrid-2-yl)acetamide ( 8 ) and N -pyrid-2-yl- N -(3-methylpyrid-2-yl)- endo -norborn-2-ene-5-carbamide ( 9 ), N -pyrid-2-yl- N -(6-methylpyrid-2-yl)- endo -norborn-2-ene-5-carbamide ( 10 ) and N -(6-methylpyrid-2-yl)- N -(4-methylquinolin-2-yl)- endo -norborn-2-ene-5-carbamide ( 11 ), respectively, have been prepared. The acetyl derivatives 5 – 8 have been used for the preparation of the homogeneous Heck catalysts N -acetyl- N -pyrid-2-yl- N -(3-methylpyrid-2-yl)amine palladium dichloride ( 13 ), N -acetyl- N -pyrid-2-yl- N -(6-methylpyrid-2-yl)amine palladium dichloride ( 14 ), N -acetyl- N -(6-methylpyrid-2-yl)- N -(4-methylquinolin-2-yl)amine palladium dichloride ( 15 ) and N -acetyl- N -bis(6-methylpyrid-2-yl)amine palladium dichloride ( 16 ). X-ray data were obtained for compounds 9 , 11 , 13 and 14 . Polymer supports 17 – 19 were prepared via ring-opening metathesis copolymerization of the norbornene derivatives 9 – 11 with 1,4,4a,5,8,8a-hexahydro-1,4,5,8- exo - endo -dimethano-naphthalene (HDMN-6) and subsequently loaded with palladium(II) chloride. Both the homogeneous catalysts 13 – 16 and the heterogeneous catalysts are active in the vinylation of aryl iodides and aryl bromides with turn-over numbers (TONs) of up to 220 000. Comparably low yields (=34%) and TONs (=2100) are achieved in the tetrabutylammonium bromide- (TBAB-) assisted vinylation of aryl chlorides as well as in the amination of aryl bromides. Structural data of compounds 9 , 11 , 13 and 14 were compared with those of the parent systems, N -acetyldipyridylamine palladiumdichloride ( 20 ) and the poly( N , N -dipyrid-2-yl- endo -norborn-2-ene-5-carbamide)based resin ( 21 ), respectively. Thus, methyl-substitution leads to a significant perturbation of the almost perfect square planar coordination of Pd found in 20 resulting in lowered stabilities of the corresponding Pd-complexes 13 – 16 and consequently lowered coupling yields compared to 20 and 21 .

CO2 and SnII Adducts of N-Heterocyclic Carbenes as Delayed-Action Catalysts for Polyurethane Synthesis

Catalytic rivals: Both CO(2)-protected tetrahydropyrimidin-2-ylidene-based N-heterocyclic carbenes (NHCs) and Sn(II)-1,3-dimesitylimidazol-2-ylidene, as well as Sn(II)-1,3-dimesitylimidazolin-2-ylidene complexes (example displayed), have been identified as truly latent catalysts for polyurethane (PUR) synthesis rivaling all existing systems both in activity and latency.A series of CO(2)-protected pyrimidin-2-ylidenes as well as 1,3-dimesitylimidazol-2-ylidene and dimesitylimidazolin-2-ylidene complexes of Sn(II) have been prepared. Selected single-crystal X-ray structures are reported. The new compounds were investigated for their catalytic behavior in polyurethane (PUR) synthesis. All compounds investigated showed excellent catalytic activity, rivaling the industrially most relevant catalyst dibutyltin dilaurate. Even more important, all compounds displayed pronounced latent behavior, in selected cases rivaling and exceeding the industrially relevant latent catalyst phenylmercury neodecanoate both in terms of latency and catalytic activity. This allows for creating one-component PUR systems with improved pot lifetimes. Pseudo-second-order kinetics were found for both CO(2)-protected tetrahyropyrimidin-2-ylidenes and for [SnCl(2)(1,3-dimesityldihydroimidazol-2-ylidene)], indicating a fast pre-catalyst decomposition prior to polyurethane formation. 1,3-Di(2-propyl)tetrahydropyrimidin-2-ylidene was additionally found to be active in the cyclotrimerization of various isocyanates, offering access to a broad variability in polymer structure, that is, creating both urethane and isocyanurate moieties within the same polymer.

Cationic RuII Complexes with N‐Heterocyclic Carbene Ligands for UV‐Induced Ring‐Opening Metathesis Polymerization

Metathesis chemistry and, in the context of polymer chemistry, ring-opening metathesis polymerization (ROMP) have gained a strong position in chemistry and materials science. ROMP is strongly associated with two classes of well-defined metal alkylidene based initiators, molybdenumbased Schrock and ruthenium-based Grubbs type initiators. Despite the tremendous achievements in catalyst development, both families of initiators are still experiencing ongoing, vivid development. Most Grubbs type initiators work at room temperature or require only gentle warming to work properly. More recently, an increasing number of reports on latent Ru-based initiators has appeared. Such precatalysts are of particular interest in technical applications of ROMP, since they allow for premixing, that is, the preformulation of a monomer/precatalyst mixture, its storage over a longer period of time even at elevated temperatures (usually less than 45 8C), and, most importantly, the shaping and profiling of such mixtures prior to polymerization (“curing”). Numerous latent Grubbs type initiators have been reported recently; however, all these precatalysts are triggered thermally. By contrast, surface modification and functionalization require UV-triggerable precatalysts. Few such systems have been reported to date. The synthesis of photoactive Schrock type tungsten-based compounds as well as ruthenium and osmium arene compounds of the general formula [Ru(p-cymene)Cl2(PR3)] and [Os(p-cymene)Cl2(PR3)] (R= cyclohexyl, etc.) were first reported by van der Schaaf et al. They also investigated the photoinduced polymerization of different functionalized norbornenes and 7-oxanorbornenes using various [Ru(solvent)n]X2 complexes, (X= tosylate, trifluoromethanesulfonate) as well as Ru half-sandwich and sandwich complexes. Noels and co-workers reported on the visiblelight-induced ROMP of cyclooctene using [RuCl2(IMes)(pcymene)] (IMes= 1,3-dimesitylimidazol-2-ylidene). Some of these systems were also used in ring-closing metathesis reactions. Most of the systems available to date, however, have significant disadvantages. They either show low activity, resulting in low polymer yields (less than 30%) in the photochemically triggered process, or the irradiation wavelength necessary to trigger ROMP is 360 nm or higher. In the latter case, the initiatorsA thermal stability is generally poor, thus discouraging their application in photoinduced ROMP. Thus, none of the systems reported to date was entirely thermally stable above or even at room temperature. Therefore, these systems do not fulfill the requirements of a truly latent photocatalyst. Herein, we report the development of the first thermally stable, truly UV-triggerable precatalysts for ROMP and their application in surface functionalization. We commenced our investigations with [Ru(IMesH2)(CF3CO2)(tBuCN)4)] CF3CO2 (PI-1) and [Ru(IMes)(CF3CO2)(tBuCN)4)] CF3CO2 (PI-2), which were prepared from [Ru(CF3CO2)2(L)(p-cymene)] [31,32] (L= IMes or IMesH2, 1,3-dimesityl-4,5-diyhdroimidazolin-2-ylidene) by reaction with excess tBuCN. Both compounds can be handled in air. H and C NMR spectroscopy data and elemental analysis reveal the presence of one N-heterocyclic carbene (NHC) ligand, two inequivalent trifluoroacetate groups, and four tBuCN ligands, suggesting cationic Ru complexes. The structures of PI-1 and PI-2 were confirmed by X-ray analysis; the structure of PI-1 is shown in Figure 1 (see also the Supporting Information). Upon mixing of either PI-1 or PI-2 with monomers 3–8 (Scheme 1), no reaction was observed at room temperature within 24 h. Even highly reactive (distilled) dicyclopentadiene (4) did not react with PI-1 or PI-2 at room or elevated temperature (RT<T< 45 8C) in the absence of light. Heating a mixture of 8 with PI-1 or PI-2 in 1,2-dichloroethane to 60 8C resulted in the formation of much less than 10% polymer within 24 h. However, exposing mixtures of either PI-1 or PI2 in chloroform with these monomers to 308-nm light at room temperature resulted in the formation of the corresponding polymers. Yields were between less than 5 and 99% (Table 1). Increasing the energy of the light by switching from 308 nm to a 254-nm Hg lamp gave raise to high, in most cases virtually quantitative, yields (Table 1). The molecular weights [*] Dr. D. Wang, Dr. W. Knolle, Dr. U. Decker, Dr. L. Prager, Dr. S. Naumov, Prof. Dr. M. R. Buchmeiser Leibniz-Institut f.r Oberfl/chenmodifizierung e.V. (IOM) Permoserstrasse 15, 04318 Leipzig (Germany) Fax: (+49)341-235-2584 E-mail: michael.buchmeiser@iom-leipzig.de Homepage: http://www.iom-leipzig.de/index_e.cfm