Stereospecific Ring-Opening Metathesis Polymerization (ROMP) of endo-Dicyclopentadiene by Molybdenum and Tungsten Catalysts

Abstract

We report the stereospecific ring-opening metathesis polymerization (ROMP) of endodicyclopentadiene (DCPD) by various well-defined molybdenum-based and tungsten-based alkylidene initiators. Tungsten MAP (MonoAryloxide Pyrrolide) initiators with the general formula W(X)(CHCMe 2 Ph)(Me 2 Pyr)(OAr) (X = arylimido, alkylimido, or oxo; Me 2 Pyr = 2,5dimethylpyrrolide; OAr = an aryloxide) yield cis,syndiotactic-poly(DCPD), while biphenolate alkylidene complexes with the general formula M(NR)(CHCMe 2 Ph)(biphen) (M = Mo or W; R = alkyl or aryl, biphen = (e.g.) 3,3′-(t-Bu) 2 -5,5′-6,6′-(CH 3 ) 4 -1,1′-biphenyl-2,2′-diolate) yield cis,isotactic-poly(DCPD). Subtle changes in the initiator can greatly alter the structure of the poly(DCPD)s that are formed. Cis,syndiotactic or cis,isotactic poly(DCPD)s (made with 50-1000 equiv of DCPD) are accessible within seconds to minutes in dichloromethane at room temperature. No isomerization or cross-linking reactions are observed, and addition of a chain transfer reagent (1-hexene) or the use of THF as a solvent does not decrease the stereospecificity of the polymerizations. Cis,syndiotactic and cis,isotactic poly(DCPD)s can be distinguished readily from each other by 13 C NMR spectroscopy. Hydrogenation of each stereoregular poly(DCPD) produces crystalline H-poly(DCPD)s that have melting points between 270 and 290 °C.

Syntheses of Molybdenum Oxo Alkylidene Complexes through Addition of Water to an Alkylidyne Complex

“…3 However, imido alkylidene complexes were chosen for development of olefin metathesis chemistry 4 because an imido ligand is less likely than an oxo ligand to bridge between metals 5 or to be attacked by an electrophile and removed from the metal. 6,7 A new approach to tungsten oxo alkylidenes 8 allowed several examples that contain sterically demanding ligands to be prepared and their reactions explored, 9 but isolable molybdenum oxo alkylidene complexes that are active for metathesis of olefins have remained elusive.…”

“…11 Unfortunately, the steric and electronic properties of the [NP(t-Bu) 3 ] − ligand prevent facile initiation of olefin metathesis reactions, even upon “activating” Mo(O)(CHSiMe 3 )[NP(t-Bu) 3 ] 2 through addition of B(C 6 F 5 ) 3 to the oxo ligand. 9b,11 (Addition of B(C 6 F 5 ) 3 to an oxo ligand has been shown to accelerate ROMP reactions with tungsten-based oxo alkylidene initiators by at least two orders of magnitude. 9c )…”

Metathesis by Molybdenum and Tungsten Catalysts

“…38 Enantiomorphic site control of polymer formation is complemented by the more recent development of MAP initiators (e.g., 1, 3, or 4) that promote formation of cis C=C bonds (vide supra) and in the process, formation of cis,syndiotactic polymers as a consequence of the chirality at the metal switching with each insertion of monomer into the M=C bond (stereogenic metal control). 39 Hydrogenation of pure cis,isotactic or cis,syndiotactic polymers made from norbornene 40 or dicyclopentadiene 41 yields isotactic or syndiotactic hydrocarbon polymers that are crystalline, high melting, relatively stable to oxygen, and therefore of commercial value. Tungsten oxo alkylidene complexes, especially when activated with B(C 6 F 5 ) 3 , recently have been found to polymerize norbornenes and norbornadienes that are difficult to polymerize stereoselectively, or in some cases, to polymerize at all at 22 °C with traditional Mo or W imido alkylidene initiators.…”

Molybdenum imido alkylidene and tungsten oxo alkylidene N-heterocyclic carbene complexes for olefin metathesis

“…The high functional group tolerance also holds for ROMP: catalyst 3b polymerized norborn-5-ene-2,3-dimethanol (M6) in 80 % isolated yield (M n = 2800 g/mol, PDI = 1. 12 Fig. 1 Monomers used in ROMP and cyclopolymerization (Cy cyclohexyl) (theor.)…”

“…Also, it should be regio-and stereoselective, whether in the synthesis of small molecules or polymers. Mo-and W-alkylidenes [10][11][12][13][14][15][16] clearly fulfill the requirements in terms of regio-and stereoselectivity; however, so far they are not competitive to Ru-based catalysts, which clearly outrival these group 6 metal-based catalysts in terms of functional group tolerance [1,17]. Unfortunately, the latter cannot fully compete with the high stereo-and regioselectivity of group 6 metal alkylidenes and if, often on the expense of activity/stability or functional group tolerance [18][19][20][21][22].…”