Dongren Wang

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.

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

Alternating ring-opening metathesis copolymerization by Grubbs-type initiators with unsymmetrical N-heterocyclic carbenes.

A series of Ru(IV)-alkylidenes based on unsymmetrical imidazolin-2-ylidenes, that is, [RuCl(2){1-(2,4,6-trimethylphenyl)-3-R-4,5-dihydro-(3H)-imidazol-1-ylidene}(CHPh)(pyridin)] (R = CH(2)Ph (5), Ph (6), ethyl (7), methyl (8)), have been synthesized. These and the parent initiators [RuCl(2)(PCy(3)){1-(2,4,6-trimethylphenyl)-3-R-4,5-dihydro-(3H)-imidazol-1-ylidene}(CHC(6)H(5))] (R = CH(2)C(6)H(5) (1), C(6)H(5) (2), ethyl (3)) were used for the alternating copolymerization of norborn-2-ene (NBE) with cis-cyclooctene (COE) and cyclopentene (CPE), respectively. Alternating copolymers, that is, poly(NBE-alt-COE)(n) and poly(NBE-alt-CPE)(n) containing up to 97 and 91% alternating diads, respectively, were obtained. The copolymerization parameters of the alternating copolymerization of NBE with CPE under the action of initiators 1-3 and 5-8 were determined by using both a zero- and first-order Markov model. Finally, kinetic investigations using initiators 1-3, 6, and 7 were carried out. These revealed that in contrast to the 2nd-generation Grubbs-type initiators 1-3 the corresponding pyridine derivatives 6 and 7 represent fast and quantitative initiating systems. Hydrogenation of poly(NBE-alt-COE)(n) yielded a fully saturated, hydrocarbon-based polymer. Its backbone can formally be derived by 1-olefin polymerization of CPE (1,3-insertion) followed by five ethylene units and thus serves as an excellent model compound for 1-olefin polymerization-derived copolymers.

Cationic versus Neutral RuIIN‐Heterocyclic Carbene Complexes as Latent Precatalysts for the UV‐Induced Ring‐Opening Metathesis Polymerization

A series of cationic and neutral Ru(II) complexes of the general formula [Ru(L)(X) (tBuCN)(4)](+)X(-) and [Ru(L)(X)(2)(tBuCN)(3))], that is, [Ru(CF(3)SO(3)){NCC(CH(3))(3)}(4)(IMesH(2))](+)[CF(3)SO(3)](-) (1), [Ru(CF(3)SO(3)){NCC(CH(3))(3)}(4)(IMes)](+)[CF(3)SO(3)](-) (2), [RuCl{NCC(CH(3))(3)}(4)(IMes)](+)Cl(-) (3), [RuCl{NCC(CH(3))(3)}(4)(IMesH(2))(+)Cl(-)]/[RuCl(2){NCC(CH(3))(3)}(3)(IMesH(2))] (4), and [Ru(NCO)(2){NCC(CH(3))(3)}(3)(IMesH(2))] (5) (IMes=1,3-dimesitylimidazol-2-ylidene, IMesH(2)=1,3-dimesityl-imidazolin-2-ylidene) have been synthesized and used as UV-triggered precatalysts for the ring-opening metathesis polymerization (ROMP) of different norborn-2-ene- and cis-cyclooctene-based monomers. The absorption maxima of complexes 1-5 were in the range of 245-255 nm and thus perfectly fit the emission band of the 254 nm UV source that was used for activation. Only the cationic Ru(II)-complexes based on ligands capable of forming μ(2)-complexes such as 1 and 2 were found to be truly photolatent in ROMP. In contrast, complexes 3-5 could be activated by UV light; however, they also showed a low but significant ROMP activity in the absence of UV light. As evidenced by (1)H and (13)C NMR spectroscopy, the structure of the polymers obtained with either 1 or 2 are similar to those found in the corresponding polymers prepared by the action of [Ru(CF(3)SO(3))(2)(IMesH(2))(CH-2-(2-PrO)-C(6)H(4))], which strongly suggest the formation of Ru-based Grubbs-type initiators in the course of the UV-based activation process. Precatalysts that have the IMesH(2) ligand showed significantly enhanced reactivity as compared with those based on the IMes ligand, which is in accordance with reports on the superior reactivity of IMesH(2)-based Grubbs-type catalysts compared with IMes-based systems.

Functional polyolefins: poly(ethylene)-graft-poly(tert-butyl acrylate) via atom transfer radical polymerization from a polybrominated alkane.

Poly(cis-cyclooctene) is synthesized via ring-opening metathesis polymerization in the presence of a chain-transfer agent and quantitatively hydrobrominated. Subsequent graft polymerization of tert-butyl acrylate (tBA) via Cu-catalyzed atom transfer radical polymerization (ATRP) from the non-activated secondary alkyl bromide moieties finally results in PE-g-PtBA copolymer brushes. By varying the reaction conditions, a series of well-defined graft copolymers with different graft densities and graft lengths are prepared. The maximum extent of grafting in terms of bromoalkyl groups involved is approximately 80 mol%. DSC measurements on the obtained graft copolymers reveal a decrease in T(m) with increasing grafting density.

Regio- and stereospecific cyclopolymerization of 1,6-heptadiynes and 1,5-hexadiynes

The synthesis of poly(ene)s exclusively based on one single repetitive unit, i.e., 1,2- cyclopent-2-enylenvinylenes, is described. Polymers containing >96% 1,2-cyclopent-2-enylenvinylene units were obtained by low-temperature-initiated cyclopolymerization of diethyl dipropargylmalonate (DEDPM) by Mo(N-2,6-i-Pr2-C6H3)(CHCMe2Ph)(OCH(CH3)2)2. In the presence of quinuclidine, >96% 5-membered ring structures were realized at room temperature, e.g., using Mo(N-2,6- Me2-C6H3)(CHCMe2Ph)(OC(CH3)3)2. 4-(Ethoxycarbonyl)-4-(1S,2R,5S)-(+)-menthyl-1,6-heptadiyne was cyclopolymerized to determine the configuration of the double bond (i.e., the cis/trans ratio) and the tacticity of the poly(ene) backbone. Poly(4-(ethoxycarbonyl)-4-(1S,2R,5S)-(+)-menthyl-1,6- heptadiyne) again consisted of >96% 5-membered rings and possessed an all-trans tactic, presumably isotactic structure. A linear plot of number of monomers added (N) vs. molecular weight suggested that cyclopolymerization of DEDPM proceeded in a living manner. At least a class-V living system was confirmed by the stepwise synthesis of poly(DEDPM). Molecular weight distributions (PDIs) of 1.16-1.37 resulted from ratios of the rate constants for polymerization (kp) to initiation (k i), k p/k i >> 1. Influences of temperature, the base and steric and electronic effects of the arylimido and alkoxy ligand are presented. Complementary to the use of well-defined Schrock initiators, poly(DEDPM) containing exclusively 5-membered rings was accessible by the use of the quaternary systems MoCl5-n-Bu4Sn-EtOH-quinuclidine and MoOCl4-n-Bu4Sn-EtOH-quinuclidine. Though virtually identical polymers were obtained, the polymerization systems themselves were controlled, yet not living, as is the case with Schrock initiators. Selected polymer properties such as stability, degradation behavior, conductivity, and thermoresponsive behavior will be discussed. Finally, the polymerization behavior of other monomers such as N,N-dipropargylaniline (1), dipropargyl sulfide (2), 1,8-diethynylnaphthalene (3) and 1,2-diethynyltetramethyldisilane (4) is presented.

Ruthenium(IV)–Bis(methallyl) Complexes as UV-Latent Initiators for Ring-Opening Metathesis Polymerization

The RuIV‐based transition metal catalysts [Ru(η3:η3‐C10H16)Cl2(PPh3)] (1), [Ru(η3:η3‐C10H16)Cl2(PCy3)] (2), and [Ru(η3:η3‐C10H16)(CF3COO)2(PPh3)] (3) have been synthesized and investigated for their use as initiators in the thermally and photo‐initiated ring‐opening metathesis polymerization (photo‐ROMP) of norborn‐2‐ene (NBE). Compounds 1 and 3 display significant photo‐ROMP activity with NBE whereas compound 2, although active in the ROMP of NBE, shows virtually no latency at all. The results are discussed with respect to the structural features of the novel catalysts.

Group 4 Dimethylsilylenebisamido Complexes Bearing the 6‐[2‐(Diethylboryl)phenyl]pyrid‐2‐yl Motif: Synthesis and Use in Tandem Ring‐Opening Metathesis/Vinyl‐Insertion Copolymerization of Cyclic Olefins with Ethylene

Two novel Zr(IV)- and Hf(IV)-based bisamido complexes bearing the 6-[2-(diethylboryl)phenyl]pyrid-2-yl motif, that is, [ZrCl(2){Me(2)Si(DbppN)(2)}(thf)] (9) and [HfCl(2){Me(2)Si(DbppN)(2)}(thf)(2)] (10) (DbppN=6-[2-(diethylboryl)phenyl]pyridine-2-amido) have been prepared. Their reactivities have been compared with that of a model precatalyst that does not bear the aminoborane motif. Upon activation with methylalumoxane, precatalysts 9 and 10 are active in the homopolymerization of ethylene (E) yielding high-density polyethylene (HDPE). In the copolymerization of E with cyclopentene (CPE), for example by the action of 9, the presence of CPE resulted in a dramatic increase in the polymerization activity of E, while CPE incorporation remained close to or at zero. In the vinyl-insertion copolymerization of norborn-2-ene (NBE) with E by the action of 9, statistical cyclic olefin copolymers of these two monomers were obtained. At higher NBE concentrations, however, 9 gave rise to reversible ring-opening metathesis (ROMP)/vinyl-insertion polymerization (VIP) of NBE with E, resulting in the formation of multi-block copolymers of the general formula poly(NBE)(ROMP)-co-poly(NBE)(VIP)-co-poly(E). This particular feature of precatalyst 9, that is, the ability to induce a reversible α-H elimination/α-H addition reaction, is attributed to the unique role of the 6-[2-(diethylboryl)phenyl]pyrid-2-yl ligand. Accordingly, a model precatalyst lacking this ligand does not have the ability to induce α-H elimination/α-H addition reactions. The different (11)B NMR shifts of various diethylborylphenylpyrid-2-ylamines and -amides permit a ranking of the strengths of the B-N bonds in these compounds. This strength of the B-N bond is correlated with the propensity of 9/MAO to produce poly(NBE)(ROMP)-co-poly(NBE)(VIP)-co-poly(E) at different temperatures.

Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes: Structure–Productivity Correlations and Mechanistic Insights

The syntheses and single‐crystal X‐ray structures of a series of Mo–imido alkylidene N‐heterocyclic carbene (NHC) complexes (1–15) and of the first complexes containing bidentate NHC‐phenolate ligands (16–18) are reported. Mo(N‐2,6‐Me2‐C6H3)((1‐R‐phenethyl)‐3‐mesitylimidazolidin‐2‐ylidene)(CHR)(OTf)2 (R=CMe2Ph, 1) is the first enantiomerically pure Mo–imido alkylidene NHC catalyst. With [Mo(N‐2,6‐Me2‐C6H3)(IMes)(CHR)(CH3CN)(OTf)(CH3CN)+ B(ArF)4−] (7), turnover numbers up to 545 000 were achieved in the homometathesis (HM) of 1‐octene and 1‐nonene (≤95 % E). With 7 and 1‐nonene, a turnover frequency (TOF4 min) of 8860 min−1 was determined. Productivity and E/Z‐selectivity were correlated with catalyst structure. For 1, Mo(N‐3,5‐Me2‐C6H3)(IMesH2)(CHR)(OTf)2 (9) and Mo(N‐3,5‐Me2‐C6H3)(IMes)(CHR)(OTf)2 (10), productivity was correlated with the coalescence temperature of the two triflates, determined by variable‐temperature 19F NMR spectroscopy. The square‐planar conformer is postulated to be the most relevant for the catalyst activation.

Stereo- and regioselective cyclopolymerization of chiral 1,7-octadiynes

A series of diastereomeric 4,5-diethoxycarbonyl-1,7-octadiynes were synthesized from which the pure diastereomers were obtained by selective Soxhlet extraction in CHCl3. The concept of “small alkoxides” was used with varying size of the alkoxide ligands and all the monomers were cyclopolymerized with two different Schrock-type initiators. As a result, poly(1,7-octadiyne)s consisting virtually solely of six membered repeat units were obtained via regioselective α-insertion. The structure of the polymers and the regioselectivity of insertion were further confirmed by comparing the 13C NMR shifts of model compounds with those of the corresponding polymers. The living character of the polymerization was demonstrated by kinetic studies and end group analysis via MALDI-ToF MS; diblock copolymers were successfully prepared from 1,7-octadiynes and 1,6-heptadiynes. Finally, the tacticity of the cyclopolymers was elucidated from NMR of poly(1,7-octadiyne) synthesized from 1,7-octadiyne-4,4-dicarboxylic esters containing enantiomerically pure menthyl ester groups. These investigations revealed the formation of highly tactic polymers (>86%) with at least predominantly trans-configured exocyclic double bonds.