Molecular and Silica‐Supported Mo and W d0 Imido‐Methoxybenzylidene Complexes: Structure and Metathesis Activity

5‐Coordinated methoxybenzylidene complexes M(=NAr)(=CH?C₆H₄?o‐OMe)(O^tBu_F3)₂ (Ar=2,6‐ⁱPr₂C₆H₃; ^tBu_F3=CMe₂(CF₃)) of Mo ( 1m_Mo ) and W ( 1m_W ) were synthesized by cross‐metathesis from the corresponding neophylidene/neopentylidene precursors and o‐methoxystyrene. 1m_Mo and 1m_W were grafted onto the surface of silica partially dehydroxylated at 700 °C to give well‐defined silica‐supported alkylidenes (≡SiO)M(=NAr)(=CH?C₆H₄?o‐OMe)(O^tBu_F3) (M=Mo ( 1_Mo ), W ( 1_W )). Supported methoxybenzylidene complexes were tested in metathesis of cis‐4‐nonene, 1‐nonene, and ethyl oleate, and compared to their molecular precursors and supported classical analogs (≡SiO)M(=NAr)(=CHCMe₂R)(O^tBu_F3) (M=Mo, R=Ph ( 2_Mo ), M=W, R=Me ( 2_W )). Both grafted complexes 1_Mo and 1_W show significantly better performance as compared to their molecular precursors 1m_Mo and 1m_W but are less efficient than the classical 4‐coordinated alkylidenes 2_Mo and 2_W . Noteworthy, both 1_Mo and 1_W can reach equilibrium conversion in metathesis of cis‐4‐nonene at catalyst loadings as low as 50 ppm.     

Bulky Aryloxide Ligand Stabilizes a Heterogeneous Metathesis Catalyst

The reaction of [W(?O)(?CHCMe₂Ph)(dAdPO)₂], containing bulky 2,6‐diadamantyl aryloxide ligands, with partially dehydroxylated silica selectively yields a well‐defined silica‐supported alkylidene complex, [(?SiO)W(?O)(?CHCMe₂Ph)(dAdPO)]. This fully characterized material is a very active and stable alkene metathesis catalyst, thus allowing loadings as low as 50 ppm in the metathesis of internal alkenes. [(?SiO)W(?O)(?CHCMe₂Ph)(dAdPO)] also efficiently catalyzes the homocoupling of terminal alkenes, with turnover numbers exceeding 75 000 when ethylene is constantly removed to avoid the formation of the less reactive square‐based pyramidal metallacycle resting state.     

Neutral and Cationic Molybdenum Imido Alkylidene N‐Heterocyclic Carbene Complexes: Reactivity in Selected Olefin Metathesis Reactions and Immobilization on Silica

The synthesis and single‐crystal X‐ray structures of the novel molybdenum imido alkylidene N‐heterocyclic carbene complexes [Mo(N‐2,6‐Me₂C₆H₃)(IMesH₂)(CHCMe₂Ph)(OTf)₂] ( 3 ), [Mo(N‐2,6‐Me₂C₆H₃)(IMes)(CHCMe₂Ph)(OTf)₂] ( 4 ), [Mo(N‐2,6‐Me₂C₆H₃)(IMesH₂)(CHCMe₂Ph)(OTf){OCH(CF₃)₂}] ( 5 ), [Mo(N‐2,6‐Me₂C₆H₃)(CH₃CN)(IMesH₂)(CHCMe₂Ph)(OTf)]⁺ BAr_F^? ( 6 ), [Mo(N‐2,6‐Cl₂C₆H₃)(IMesH₂)(CHCMe₃)(OTf)₂] ( 7 ) and [Mo(N‐2,6‐Cl₂C₆H₃)(IMes)(CHCMe₃)(OTf)₂] ( 8 ) are reported (IMesH₂=1,3‐dimesitylimidazolidin‐2‐ylidene, IMes=1,3‐dimesitylimidazolin‐2‐ylidene, BAr_F^?=tetrakis‐[3,5‐bis(trifluoromethyl)phenyl] borate, OTf=CF₃SO₃^?). Also, silica‐immobilized versions I1 and I2 were prepared. Catalysts 3 – 8 , I1 and I2 were used in homo‐, cross‐, and ring‐closing metathesis (RCM) reactions and in the cyclopolymerization of α,ω‐diynes. In the RCM of α,ω‐dienes, in the homometathesis of 1‐alkenes, and in the ethenolysis of cyclooctene, turnover numbers (TONs) up to 100 000, 210 000 and 30 000, respectively, were achieved. With I1 and I2 , virtually Mo‐free products were obtained (<3 ppm Mo). With 1,6‐hepta‐ and 1,7‐octadiynes, catalysts 3 , 4 , and 5 allowed for the regioselective cyclopolymerization of 4,4‐bis(ethoxycarbonyl)‐1,6‐heptadiyne, 4,4‐bis(hydroxymethyl)‐1,6‐heptadiyne, 4,4‐bis[(3,5‐diethoxybenzoyloxy)methyl]‐1,6‐heptadiyne, 4,4,5,5‐tetrakis(ethoxycarbonyl)‐1,7‐octadiyne, and 1,6‐heptadiyne‐4‐carboxylic acid, underlining the high functional‐group tolerance of these novel Group 6 metal alkylidenes.     

Solvent‐Free Ring‐Opening Metathesis Polymerization of Norbornene over Silica‐Supported Tungsten–Oxo Perhydrocarbyl Catalysts

Ring opening metathesis polymerization (ROMP) of bicyclo[2.2.1]hept‐2‐ene (norbornene) is carried out over silica‐supported catalysts based on tungsten complexes bearing an oxo ligand ( 1 : [(SiO)W(O)(CH₂SiMe₃)₃, 2 : [(SiO)W(O)(CHCMe₂Ph)(dAdPO)], dAdPO  2,6 diadamantyl‐4‐methylphenoxide, 3 : [(SiO)₂W(O)(CH₂SiMe₃)₂]). The evaluation of the catalytic activities of the aforementioned materials in ROMP indicates that at low reaction time (0.5 min), the highest polymer yield is obtained with catalyst 2 . However, for longer reaction time (>2 min), complex 3 , a model of the industrial catalyst, exhibits a better monomer conversion. The polymers obtained are characterized. Moreover, these catalysts are shown to be rather preferentially selective to give the cis polynorbornene (>65%), characterized by high melting points (≈300 °C). The experimental values of the average molecular weight (M_n) of polynorbornenes are found to be close to the theoretical ones for the polymers prepared using catalyst 2 and higher for those originated from catalyst 3 .

     

Alkyne Metathesis with Silica‐Supported and Molecular Catalysts at Parts‐per‐Million Loadings

Improvement of the activity, stability, and chemoselectivity of alkyne‐metathesis catalysts is necessary before this promising methodology can become a routine method to construct C≡C triple bonds. Herein, we show that grafting of the known molecular catalyst [MesC≡Mo(OtBu_F6)₃] ( 1 , Mes=2,4,6‐trimethylphenyl, OtBu_F6=hexafluoro‐tert‐butoxy) onto partially dehydroxylated silica gave a well‐defined silica‐supported active alkyne‐metathesis catalyst [(≡SiO)Mo(≡CMes)(OtBu_F6)₂] ( 1 /SiO_2‐700). Both 1 and 1 /SiO_2‐700 showed very high activity, selectivity, and stability in the self‐metathesis of a variety of carefully purified alkynes, even at parts‐per‐million catalyst loadings. Remarkably, the lower turnover frequencies observed for 1 /SiO_2‐700 by comparison to 1 do not prevent the achievement of high turnover numbers. We attribute the lower reactivity of 1 /SiO_2‐700 to the rigidity of the surface Mo species owing to the strong interaction of the metal site with the silica surface.     

New Microphase‐Separated Diblock Copolymers Carrying Semifluorinated Side Groups Prepared by ROMP

We prepared new varied diblock copolymers by ring‐opening metathesis polymerization of functionalized norbornenes and cyclooctene in the presence of Schrock‐type initiators, either [Mo(CHCMe₂Ph)(N‐2,6‐iPr₂Ph)(OCCH₃(CF₃)₂)₂] or [Mo(CHCMe₂Ph)(N‐2,6‐iPr₂Ph)(OC(CH₃)₃)₂]. The block copolymers were microphase separated and presented the individual phases of each polymer block constituent, that were amorphous/amorphous, amorphous/semicrystalline, or semicrystalline/liquid‐crystalline. One example of such a block copolymer is shown.

     

Synthesis of laterally attached side‐chain liquid‐crystalline polynorbornenes with high mesogen density by ring‐opening metathesis polymerization

(±)‐exo,endo‐5,6‐Bis{[[11′‐[2″,5″‐bis[2‐(3′‐fluoro‐4′‐n‐alkoxyphenyl)ethynyl]phenyl]undecyl]oxy]carbonyl}bicyclo[2.2.1]hept‐2‐ene (n = 1–12) monomers were polymerized by ring‐opening metathesis polymerization in tetrahydrofuran at room temperature with Mo(CHCMe₂Ph)(N‐2,6‐ⁱPr₂Ph)(O^tBu)₂ as the initiator to produce polymers with number‐average degrees of polymerization of 8–37 and relatively narrow polydispersities (polydispersity index = 1.08–1.31). The thermotropic behavior of these materials was independent of the molecular weight and therefore representative of that of a polymer at approximately 15 repeat units. The polymers exhibited an enantiotropic nematic mesophase when n was 2 or greater. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4076–4087, 2006     

Switching on the Metathesis Activity of Re Oxo Alkylidene Surface Sites through a Tailor‐Made Silica–Alumina Support

Re oxo alkylidene surface species are putative active sites in classical heterogeneous Re‐based alkene‐metathesis catalysts. However, the lack of evidence for such species questions their existence and/or relevance as reaction intermediates. Using Re(O)(=CH‐CH=CPh₂)(OtBu_F6)₃(THF), the corresponding well‐defined Re oxo alkylidene surface species can be generated on both silica and silica–alumina supports. While inactive on the silica support, it displays very good activity, even for functionalized olefins, on the silica–alumina support.     

Super‐Bulky Penta‐arylcyclopentadienyl Ligands: Isolation of the Full Range of Half‐Sandwich Heavy Alkaline‐Earth Metal Hydrides

Hydrogenolysis of the half‐sandwich penta‐arylcyclopentadienyl‐supported heavy alkaline‐earth‐metal alkyl complexes (Cp^Ar)Ae[CH(SiMe₃)₂](S) (Cp^Ar=C₅Ar₅, Ar=3,5‐ⁱPr₂‐C₆H_3; S=THF or DABCO) in hexane afforded the calcium, strontium, and barium metal–hydride complexes as the same dimers [(Cp^Ar)Ae(μ‐H)(S)]₂ (Ae=Ca, S=THF, 2‐Ca ; Ae=Sr, Ba, S=DABCO, 4‐Ae ), which were characterized by NMR spectroscopy and single‐crystal X‐ray analysis. 2‐Ca , 4‐Sr , and 4‐Ba catalyzed alkene hydrogenation under mild conditions (30 °C, 6 atm, 5 mol % cat.), with the activity increasing with the metal size. A variety of activated alkenes including tri‐ and tetra‐substituted olefins, semi‐activated alkene (Me₃SiCH=CH₂), and unactivated terminal alkene (1‐hexene) were evaluated.     

Evaluation of Several Molybdenum and Ruthenium Catalysts for the Metathesis Homocoupling of 3‐Methyl‐1‐Butene

We synthesized Mo(NC ₆F₅)(CHCM e₂Ph)(TPPO )(PP hMe₂)Cl (TPPO = 2,3,5,6‐tetraphenylphenoxide), Mo(NC ₆F₅)(CHCM e₂Ph)(TTBTO )(PP hMe₂)Cl (TTBTO = 2,6‐di(3′,5′‐di‐tert‐butylphenyl)phenoxide), and Mo(NC ₆F₅)(CHCM e₂Ph)(TPPO )(PP hMe₂)(CF ₃Pyr) (CF ₃Pyr = 3,4‐bistrifluoromethylpyrrolide), in order to evaluate them as catalysts for the homocoupling of 3‐methyl‐1‐butene. They were compared with Mo(NC ₆F₅)(CHCM e₂Ph)(HMTO )(PP hMe₂)Cl (HMTO = 2,6‐dimesitylphenoxide), Mo(NC ₆F₅)(CHCM e₂Ph)(HIPTO )(PP hMe₂)Cl (HIPTO = 2,6‐di(2′,4′,6′‐triisopropylphenyl)phenoxide), and several other Mo and Ru catalysts. In the best cases turnover numbers (TON s) of 400 – 700 were observed for the homocoupling of 3‐methyl‐1‐butene in a closed vessel (ethylene not removed).     

Boron Lewis Acid‐Catalyzed Hydroboration of Alkenes with Pinacolborane: BArF3 Does What B(C6F5)3 Cannot Do!

The transition‐metal‐free hydroboration of various alkenes with pinacolborane (HBpin) initiated by tris[3,5‐bis(trifluoromethyl)phenyl]borane (BAr^F₃) is reported. The choice of the boron Lewis acid is crucial as the more prominent boron Lewis acid tris(pentafluorophenyl)borane (B(C₆F₅)₃) is reluctant to react. Unlike B(C₆F₅)₃, BAr^F₃ is found to engage in substituent redistribution with HBpin, resulting in the formation of Ar^FBpin and the electron‐deficient diboranes [H₂BAr^F]₂ and [(Ar^F)(H)B(μ‐H)₂BAr^F₂]. These in situ‐generated hydroboranes undergo regioselective hydroboration of styrene derivatives as well as aliphatic alkenes with cis diastereoselectivity. Another ligand metathesis of these adducts with HBpin subsequently affords the corresponding HBpin‐derived anti‐Markovnikov adducts. The reactive hydroboranes are regenerated in this step, thereby closing the catalytic cycle.     

Methylaluminoxane‐free ethylene polymerization with in situ activated zirconocene triisobutylaluminum catalysts and silica‐supported stabilized borate cocatalysts

Heterogenization of tris(pentafluorophenyl)borane [B(C₆F₅)₃] on a silica support stabilized with chlorotriphenylmethane (CICPh₃) and N,N‐dimethylaniline (HNMe₂Ph) creates the following supported borane cocatalysts: [HNMe₂Ph]⁺[B(C₆F₅)₃‐SiO₂]^? and [CPh₃]⁺[B(C₆F₅)₃‐SiO₂]^?. These supported catalysts were reacted with Cp₂ZrCl₂ TIBA in situ to generate active metallocene species in the reactor. Triisobutylaluminum (TIBA) was a good coactivator for dichloro‐zirconocene, acting as the prealkylating agent to generate cationic zirconocene (Cp₂ZrC₄H₉⁺). The catalytic performances were determined from the kinetics of ethylene‐consumption profiles that were independent of the time dedicated to the activation of the catalysts. The scanning electron microscopy‐energy dispersive X‐ray measurements showed that B(C₆F₅)₃ dispersed uniformly on the silica support. Under our reaction conditions, the [CPh₃]⁺[B(C₆F₅)₃‐SiO₂]^? system had higher productivity and weight‐average molecular weight than the [HNMe₂Ph]⁺[B(C₆F₅)₃‐SiO₂]^? system. For the [CPh₃]⁺[B(C₆F₅)₃‐SiO₂]^? system, the productivity increased with the amount catalyst; however, the polydispersity index of polyethylene synthesized did not change. The final shape of polymer particles was a larger‐diameter version of the original support particle. The polymer particles synthesized with supported [CPh₃]⁺[B(C₆F₅)₃‐SiO₂]^? catalysts had larger diameters. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3240–3248, 2002     

Copolymerization of amino acid and amino ester functionalized norbornenes via living ring‐opening metathesis polymerization

The block and random copolymerization of a series of amino acid and amino ester functionalized norbornenes by ring‐opening metathesis polymerization (ROMP) induced by the well‐defined molybdenum [Mo(?N‐2,6‐ⁱPr? C₆H₃)(?CHCMe₂)Ph)(OCMe₃)₂] or ruthenium [Ru(PCy)₂Cl₂(?CHPh)] based initiators is described. The monomers are derived from the amino acids glycine, alanine, and isoleucine or the methyl esters of these amino acids and either endo‐ or exo‐norborn‐5‐ene‐2,3‐dicarboxylic anhydride. Enantiomerically pure monomers afforded optically active polymers, and the mechanism and kinetics of the copolymerizations are investigated. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7985–7995, 2008     

Cationic Platinum(II) σ‐SiH Complexes in Carbon Dioxide Hydrosilation

The low‐electron‐count cationic platinum complex [Pt(ItBu’)(ItBu)][BAr^F], 1 , interacts with primary and secondary silanes to form the corresponding σ‐SiH complexes. According to DFT calculations, the most stable coordination mode is the uncommon η¹‐SiH. The reaction of 1 with Et₂SiH₂ leads to the X‐ray structurally characterized 14‐electron Pt^II species [Pt(SiEt₂H)(ItBu)₂][BAr^F], 2 , which is stabilized by an agostic interaction. Complexes 1 , 2 , and the hydride [Pt(H)(ItBu)₂][BAr^F], 3 , catalyze the hydrosilation of CO₂, leading to the exclusive formation of the corresponding silyl formates at room temperature.     

Syntheses of Molybdenum and Tungsten Imido Alkylidene Complexes that Contain a Bidentate Oxo/Thiolato Ligand

3,3′,5,5′-Tetra-tert-butyl-2′-sulfanyl[1,1′-biphenyl]-2-ol (H₂[^tBu₄OS]) was prepared in 24 % yield overall from the analogous biphenol using standard techniques. Addition of H₂[^tBu₄OS] to Mo(NAr)(CHCMe₂Ph)(2,5-dimethylpyrrolide)₂ led to formation of Mo(NAr)(CHCMe₂Ph)[^tBu₄OS], which was trapped with PMe₃ to give Mo(NAr)(CHCMe₂Ph)[^tBu₄OS](PMe₃) ( 1 (PMe₃)). An X-ray crystallographic study of 1 (PMe₃) revealed that two structurally distinct square pyramidal molecules are present in which the alkylidene ligand occupies the apical position in each. Both 1 (PMe₃)_A and 1 (PMe₃)_B are disordered. Mo(NAd)(CHCMe₂Ph)(^tBu₄OS)(PMe₃) ( 2 (PMe₃); Ad=1-adamantyl) and W(NAr)(CHCMe₂Ph)(^tBu₄OS)(PMe₃) ( 3 (PMe₃)) were prepared using analogous approaches. 1 (PMe₃) reacts with ethylene (1 atm) in benzene within 45 minutes to give an ethylene complex Mo(NAr)(^tBu₄OS)(C₂H₄) ( 4 ) that is isolable and relatively stable toward loss of ethylene below 60 °C. An X-ray study shows that the bond distances and angles for the ethylene ligand in 4 are like those found for bisalkoxide ethylene complexes of the same general type. Complex 1 (PMe₃) in the presence of one equivalent of B(C₆F₅)₃ catalyzes the homocoupling of 1-decene, allyltrimethylsilane, and allylboronic acid pinacol ester at ambient temperature. 1 (PMe₃), 2 (PMe₃), and 3 (PMe₃) all catalyze the ROMP of rac-endo,exo-5,6-dicarbomethoxynorbornene (rac-DCMNBE) in the presence of B(C₆F₅)₃, but the polyDCMNBE that is formed has a random structure.     

One‐Electron Oxidation of a Disilicon(0) Compound: An Experimental and Theoretical Study of [Si2]+ Trapped by N‐Heterocyclic Carbenes

One‐electron oxidation of the disilicon(0) compound Si₂(Idipp)₂ ( 1 , Idipp=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene) with [Fe(C₅Me₅)₂][B(Ar^F)₄] (Ar^F=C₆H₃‐3,5‐(CF₃)₂) affords selectively the green radical salt [Si₂(Idipp)₂][B(Ar^F)₄] ( 1 ‐[B(Ar^F)₄). Oxidation of the centrosymmetric 1 occurs reversibly at a low redox potential (E_1/2=?1.250 V vs. Fc⁺/Fc), and is accompanied by considerable structural changes as shown by single‐crystal X‐ray structural analysis of 1 ‐B(Ar^F)₄. These include a shortening of the Si?Si bond, a widening of the Si‐Si‐C_NHC angles, and a lowering of the symmetry, leading to a quite different conformation of the NHC substituents at the two inequivalent Si sites in 1⁺ . Comparative quantum chemical calculations of 1 and 1⁺ indicate that electron ejection occurs from the symmetric (n₊) combination of the Si lone pairs (HOMO). EPR studies of 1 ‐B(Ar^F)₄ in frozen solution verified the inequivalency of the two Si sites observed in the solid‐state, and point in agreement with the theoretical results to an almost equal distribution of the spin density over the two Si atoms, leading to quite similar ²⁹Si hyperfine coupling tensors in 1⁺ . EPR studies of 1 ‐B(Ar^F)₄ in liquid solution unraveled a topomerization with a low activation barrier that interconverts the two Si sites in 1⁺ .     

NMR Spectroscopy and X‐Ray Characterisation of Cationic N‐Heteroaryl‐Pyridylamido ZrIV Complexes: A Further Level of Complexity for the Elusive Active Species of Pyridylamido Olefin Polymerisation Catalysts

New [(N^?,N,N^?)ZrR₂] dialkyl complexes (N^?,N,N^?=pyrrolyl‐pyridyl‐amido or indolyl‐pyridyl‐amido; R=Me or CH₂Ph) have been synthesised and tested as pre‐catalysts for ethene and propene polymerisation in combination with different activators, such as B(C₆F₅)₃, [Ph₃C][B(C₆F₅)₄], [HNMe₂Ph][B(C₆F₅)₄] or solid AlMe₃‐depleted methylaluminoxane (DMAO). Polyethylene (M_w>2 MDa and M_w/M_n = 1.3–1.6) has been produced if pre‐catalysts were activated with 1000 equivalents of DMAO (based on Al) [activity >1000 kg_PE (mol_[Zr] h mol atm)^?1] or by using a higher pre‐catalyst concentration and a mixture of [HNPhMe₂][B(C₆F₅)₄] (1 equiv) and AliBu₂H (60 equiv). In the case of propene polymerisation, activity has been observed only if pre‐catalysts were treated with an excess of AliBu₂H prior to addition of DMAO, which led to highly isotactic polypropylene ([mmmm]>95 %). Neutral pre‐catalysts and ion pairs derived from their activation have been characterised in solution by using advanced 1D and 2D NMR spectroscopy experiments. The detection and rationalisation of intercationic NOEs clearly showed the formation of dimeric species in which some pyrrolyl or indolyl π‐electron density of one unit is engaged in stabilising the metal centre of the other unit, which relegates the counterions in the second coordination sphere. The solid‐state structure of the dimeric indolyl‐pyridyl‐amidomethylzirconium derivative, determined by X‐ray diffraction studies, points toward a weak Zr???η³‐indolyl interaction. It can be hypothesised that the formation of dimeric cationic species hampers monomer coordination (especially of less reactive α‐olefins) and that addition of AliBu₂H is crucial to split the homodimers.     

Synthesis of homopolymers and multiblock copolymers by the living ring‐opening metathesis polymerization of norbornenes containing acetyl‐protected carbohydrates with well‐defined ruthenium and molybdenum initiators

The ring‐opening metathesis polymerization (ROMP) of norbornenes containing acetyl‐protected glucose [2,3,4,6‐tetra‐O‐acetyl‐glucos‐1‐O‐yl 5‐norbornene‐2‐carboxylate ( 1 )] and maltose [2,3,6,2′,3′,4′,6′‐hepta‐O‐acetyl‐maltos‐1‐O‐yl 5‐norbornene‐2‐carboxylate ( 2 )] was explored in the presence of Mo(N‐2,6‐ⁱPr₂C₆H₃)(CHCMe₂Ph)(O^tBu)₂ ( A ), Ru(CHPh)(Cl)₂(PCy₃)₂ ( B ; Cy = cyclohexyl), and Ru(CHPh)(Cl)₂(IMesH₂)(PCy₃) ( C ; IMesH₂ = 1,3‐dimesityl‐4,5‐dihydromidazol‐2‐ylidene). The polymerizations promoted by B and A proceeded in a living fashion with exclusive initiation efficiency, and the resultant polymers possessed number‐average molecular weights that were very close to those calculated on the basis of the monomer/initiator molar ratios and narrow molecular weight distributions (weight‐average molecular weight/number‐average molecular weight < 1.18) in all cases. The observed catalytic activity of B was strongly dependent on both the initial monomer concentration and the solvent employed, whereas the polymerization initiated with A was completed efficiently even at low initial monomer concentrations. The polymerization with C also took place efficiently, and even the polymerization with 1000 equiv of 1 was completed within 2 h. First‐order relationships between the propagation rates and the monomer concentrations were observed for all the polymerization runs, and the estimated rate constants at 25 °C increased in the following order: A > C > B . On the basis of these results, we concluded that ROMP with A was more suitable than ROMP with B or C for the efficient and precise preparation of polymers containing carbohydrates. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4248–4265, 2004     

Polymerization of methyl methacrylate and other polar monomers with alkylaluminum initiators bearing bidentate and tridentate N‐ and O‐donor ligands

Dimethylaluminum complexes bearing bidentate amidate, oxypyridine, and salicylaldimine N,O‐ligands and tridentate N,N′,N″‐pyridyliminoamide ligands were synthesized and spectroscopically characterized. The complexes were investigated in both neutral and borane‐activated cationic forms, along with bidentate N,N′‐ligated aluminum amidinates, as catalysts for the polymerization of methyl methacrylate, ?‐caprolactone, and propylene oxide. The neutral complexes generally did not carry out polymerization, but the polymerization/oligomerization of all three monomers was achieved when the various catalysts were activated with B(C₆F₅)₃ or [Ph₃C]⁺[B(C₆F₅)₄]^?. The N,O‐ligated cations were much less active for polymerization than the analogous, more stable N,N′‐ligated amidinate cations; both types of cationic complexes catalyzed the ring‐opening cationic polymerization of tetrahydrofuran. B(C₆F₅)₃ and [Ph₃C]⁺[B(C₆F₅)₄]^? also independently carried out the oligomerization of propylene oxide. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1633–1651, 2002     

Ring‐opening metathesis polymerization of cis‐cyanocyclooct‐4‐ene: Search for active catalysts,variation of monomer to catalyst ratios and monomer concentrations

The ring‐opening metathesis polymerization (ROMP) of cis‐cyanocyclooct‐4‐ene initiated by ruthenium‐based catalysts of the first, second, and third generation was studied. For the polymerization with the second generation Grubbs catalyst [RuCl₂(?CHPh)(H₂IMes)(PCy₃)] (H₂IMes = N,N′‐bis(mesityl)‐4,5‐dihydroimidazol‐2‐ylidene), the critical monomer concentration at which polymerization occurs was determined, and variation of monomer to catalyst ratios was performed. For this catalyst, ROMP of cis‐cyanocyclooct‐4‐ene did not show the features of a living polymerization as M_n did not linearly increase with increasing monomer conversion. As a consequence of slow initiation rates and intramolecular polymer degradation, molar masses passed through a maximum during the course of the polymerization. With third generation ruthenium catalysts (which contain 3‐bromo or 2‐methylpyridine ligands), polymerization proceeded rapidly, and degradation reactions could not be observed. Contrary to ruthenium‐based catalysts of the second and third generation, a catalyst of the first generation was not able to polymerize cis‐cyanocyclooct‐4‐ene. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011