There has been intense recent interest in the use of soluble early transition metal d°and lanthanide d°f!-based catalysts for polymerization of ethylene and -olefins.' These welldefined initiators serve as mechanistic models for traditional Ziegler-Natta catalysts, and, in addition, structural variations in these homogeneous catalysts allow control of polymer

No abstract

No abstract

Mechanistic aspects of palladium-catalyzed insertion copolymerizations of ethylene and R-olefins with methyl acrylate to give high molar mass polymers are described.dNAr, e.g., Ar t 2,6-C 6 H 3 (i-Pr) 2 , R t H (a), Me (b); Ar′ t 3,5-C 6 H 3 (CF 3 ) 2 ) with bulky substituted R-diimine ligands were used as catalyst precursors. The copolymers are highly branched, the acrylate comonomer being incorporated predominantly at the ends of branches as -CH 2 CH 2 C(O)OMe groups. The effects of reaction conditions and catalyst structure on the copolymerization reaction are rationalized. Lowtemperature NMR studies show that migratory insertion in the η 2 -methyl acrylate (MA) complex [(N ∧ N)-PdMe{H 2 CdCHC(O)OMe}] + (5) occurs to give initially the 2,1-insertion product [(N ∧ N)PdCH(CH 2 CH 3 )C-(O)OMe] + (6), which rearranges stepwise to yield 2 as the final product upon warming to -20°C. Activation parameters (∆H q ) 12.1 ( 1.4 kcal/mol and ∆S q ) -14.1 ( 7.0 eu) were determined for the conversion of 5a to 6a. Rates of ethylene homopolymerization observed in preparative-scale polymerizations (1.2 s -1 at 25°C, ∆G q ) 17.4 kcal/mol for 2b) correspond well with low-temperature NMR kinetic data for migratory insertion of ethylene in [(N ∧ N)Pd{(CH 2 ) 2n Me}(H 2 CdCH 2 )] + . Relative binding affinities of olefins to the metal center were also studied. For [(N ∧ N)PdMe(H 2 CdCH 2 )] + + MA h 5a + H 2 CdCH 2 , K eq (-95°C) ) (1.0 ( 0.3) × 10 -6 was determined. Combination of the above studies provides a mechanistic model that agrees well with acrylate incorporations observed in copolymerization experiments. Data obtained for equilibriashows that chelating coordination of the carbonyl group is favored over olefin coordination at room temperature. Formation of chelates analogous to 2 during the copolymerization is assumed to render the subsequent monomer insertion a turnover-limiting step.

Detailed investigations of the polymerization of ethylene by (R-diimine)nickel(II) catalysts are reported. Effects of structural variations of the diimine ligand on catalyst activities, polymer molecular weights, and polymer microstructure are described. The precatalysts employed were 6). Active polymerization catalysts were formed in situ by combination of 4-6 with modified methylaluminoxane. In general, as the bulk and number of ortho substituents increase, polymer molecular weights, turnover frequencies and extent of branching in the homopolyethylenes all increase. Effects of varying ethylene pressure and temperature on polymerizations are also reported. The degree of branching in the polymers rapidly decreases with increasing ethylene pressure but molecular weights are not markedly affected. Temperature increases result in more extensive branching and moderate reductions in molecular weights. Catalyst productivity decreases above 60 °C due to catalyst deactivation.

Mechanistic studies of olefin polymerizations catalyzed by aryl-substituted R-diimine-Pd(II) complexes are presented. Syntheses of several cationic catalyst precursors, [(N ∧ N)Pd(CH 3 )(OEt 2 )]BAr′ 4 (N ∧ N ) aryl-substituted R-diimine, Ar′ ) 3,5-(CF 3 ) 2 C 6 H 3 ), are described. X-ray structural analyses of [ArNd C(H)C(H)dNAr]Pd(CH 3 )(Cl) and [ArNdC(Me)C(Me)dNAr]Pd(CH 3 ) 2 (Ar ) 2,6-(iPr) 2 C 6 H 3 ) illustrate that o-aryl substituents crowd axial sites in these square planar complexes. Low-temperature NMR studies show that the alkyl olefin complexes, (N ∧ N)Pd(R)(olefin) + , are the catalyst resting states and that the barriers to migratory insertions lie in the range 17-19 kcal/mol. Following migratory insertion, the cationic palladium alkyl complexes (N ∧ N)Pd(alkyl) + formed are β-agostic species which exhibit facile metal migration along the chain ("chain walking") via β-hydride elimination/readdition reactions. Model studies using palladiumn-propyl and -isopropyl systems provide mechanistic details of this process, which is responsible for introducing branching in the polyethylenes made by these systems. Decomposition of the cationic methyl complexes (ArN ∧ NAr)Pd(CH 3 )(OEt 2 ) + (Ar ) 2,6-(iPr) 2 C 6 H 3 , 2-tBuC 6 H 4 ) occurs by C-H activation of β-C-H bonds of the ortho isopropyl and tert-butyl substituents and loss of methane. The rate of associative exchange of free ethylene with bound ethylene in (N ∧ N)Pd(CH 3 )(C 2 H 4 ) + is retarded by bulky substituents. The relationship of these exchange experiments to chain transfer is discussed.

Experimental; G eneral C onsiderations. A ll manipulations were performed using standard Schlenk techniques. Argon was purified by passage through columns o f BASF R 3-11 catalyst (Chemalog) and 4 A molecular sieves (Linde). Solid organometallic compounds were transferred and stored in a nitrogen-filled Vacuum Atmospheres dry box. NMR spectra were recorded with either a JEOL FX-90Q (89.60 MHz lH; 22.53 MHz ^C) or a QE-300 Plus (300.10 MHz lH; 75.49 MHz ^C) spectrometer. M aterials. Benzene and tetrahydrofuran were distilled or vacuum-transferred from sodium-benzophenone ketyl. Pentane was stirred over concentrated H2SO4 , dried over MgS(>4 and CaH2 , and then transferred onto sodium-benzophenone ketyl solubilized with tetraglyme. Benzene-^5 and THF-dg were dried over sodium-benzophenone ketyl. Methylene chloride-^ was dried over CaH2 , vacuum-transferred, and then degassed by three continuous freeze-pump-thaw cycles. Deuterium oxide was degassed by bubbling a stream of argon through the solvent for 15 minutes. E thanol-^ was dried over activated neutral alumina, vacuum-transferred and degassed by three continuous freeze-pump-thaw cycles. RuCi2 (PPh3)3, la RuCl2(PPh3)4 ,la and OsCl2 (PPh3)3 lb were synthesized according to literature procedures. 3,3-Diphenylcyclopropene was prepared following a modification o f a procedure described by Moore.2 (Pti3P)2 C l2R u = C H-C H = C P h 2 1. In a typical reaction, a 200 mL Schlenk flask equipped with a magnetic stirbar was charged with RuCl2 (PPh3)4 (6.00 g, 4.91 mmol) inside a nitrogen-filled drybox. Methylene chloride (40 mL) was added to dissolve the complex followed by 100 mL of benzene to dilute the solution. 3,3-Diphenylcyclopropene (954 mg, 1.01 equiv) was then added to the solution via pipet. The reaction flask was capped with a stopper, removed from the box, attached to a reflux condenser under argon and heated at 53 °C for 1 1 h. After allowing the solution to cool to RT, all the solvent was removed in vacuo to give a dark yellow-brown solid. Benzene (10 mL) was added to the solid and subsequent swirling of the mixture broke the solid into a fine powder. Pentane (80 mL) was then slowly added to the mixture via cannula while stirring vigorously. The mixture was stirred at RT for 1 h and allowed to settle before the supernatant was removed via cannula filtration. This washing procedure was repeated two more times to ensure the complete removal o f all phosphine by-products. The resulting solid was then dried under vacuum overnight to afford 4.28 g (98%) o f 1 as a yellow powder with a slight green tint. *H NMR (C6Dg): 5 17.94 (pseudo-quartet = two overlapping triplets. 1 H,

No abstract