Dimerization and polymerization of ethylene

Coordination polymerization of ethylene by late transition metals is a rather slow process especially when the catalyst is dissolved in water. In a study of the interaction of CH2=CH2 and [Ru(H20)6](tos)2 (tos = tosylate), both [Ru(CH2=CH2)(H20)5](tos)2 and [Ru(CH2=CH2)2(H20)4](tos)2 were isolated by evaporation of the aqueous phase which had been previously pressurized with 60 bar ethylene at room temperature for 6 and 18 hours, respectively. Tonger reaction times (72 h) led to the formation of butenes with no further oligomerization. This aqueous catalytic dimerization was not selective, the product mixture contained Z-2-butene, E-2-butene and 1-butene in a 112.2122 ratio [3]. 

The facially coordinating l,4,7-trimethyl-l,4,7-triazacyclononane (Cn) ligand forms stable methylrhodium(III) complexes, such as [Rh(Me)3Cn], [Rh(Me)2Cn]OTf and [Rh(Me)Cn](OTf)2 (OTf=trifluoromethanesulfonate) and the latter two have rich aqueous chemistry. When dissolved in water, [Rh(Me)Cn]2+ readily coordinates two water molecules to form the 

At 24 °C and 15-60 bar ethylene, [Rh(Me)(OH)(H20)Cn]+ catalyzed the slow polymerization of ethylene [4], Propylene, methyl acrylate and methyl methacrylate did not react. After 90 days under 60 bar CH2=CH2 (the pressure was held constant throughout) the product was low molecular weight polyethylene with Mw =5100 and a polydispersity index of 1.6. This is certainly not a practical catalyst for ethylene polymerization (TOF 1 in a day), nevertheless the formation and further reactions of the various intermediates can be followed conveniently which may provide ideas for further catalyst design. For example, during such investigations it was established, that only the monohydroxo-monoaqua complex was a catalyst for this reaction, both [Rh(Me)3Cn] and [Rh(Me)(H20)2Cn]2+ were found completely ineffective. The lack of catalytic activity of [Rh(Me)3Cn] is understandable since there is no free coordination site for ethylene. Such a coordination site can be provided by water dissociation from [Rh(Me)(OH)(H20)Cn]+ and [Rh(Me)(H20)2Cn]2+ and die rate of this exchange is probably the lowest step of the overall reaction.The hydroxy ligand facilitates the dissociation of H20 and this leads to a slow catalysis of ethene polymerization. 

---

Dimerization and Polymerization of Ethylene with Nickelocene and Dibenzenechromium... 

A soluble titanium-based modified Ziegler-Natta catalyst [Ti(OR)4-Et3Al, R = n-Bu, isoPr] is employed in the reaction.42 Since similar catalysts may be used for the oligomerization and polymerization of ethylene, the nature and oxidation state of the metal and reaction conditions determine selectivity. Ti4+ was found to be responsible for high dimerization selectivity, whereas polymerization was shown to be catalyzed by Ti3+. According to a proposed mechanism,42,43 this catalyst effects the concerted coupling of two molecules of ethylene to form a metal-lacyclopentane intermediate that decomposes via an intramolecular p-hydrogen transfer ... 

The mechanisms for dimerizing ethylene by nickelocene and polymerizing of ethylene by dibenzenechromium (0) are of course only speculative, based on the products obtained. 

Constrained geometry chromium alkyls catalyzed the polymerization of ethylene however, the reaction was relatively slow, and elevated pressures (PC2H4 = 500 psi) were required to generate significant amounts of polymer. Not surprisingly then, no homopolymoization or copolymerization of a-olefins was observed. Instead, catalytic isomerization and dimerization of the alkyl-substituted olefins was found. 

Quite often in the ring-opening polymerization, the polymer is only the kinetic product and later is transformed to thermodynamically stable cycles. The cationic polymerization of ethylene oxide leads to a mixture of poly(ethylene oxide) and 1,4-dioxane. In the presence of a cationic initiator poly(ethylene oxide) can be almost quantitatively transformed to this cyclic dimer. On the other hand, anionic polymerization is not accompanied by cyclization due to the lower affinity of the alkoxide anion towards linear ethers only strained (and more electrophilic) monomers can react with the anion. 

Practical Applications. IFP s Alphabutol process is used to dimerize ethylene selectively to 1-butene.43,85 The significance of this technology is the use of 1-butene as a comonomer in the polymerization of ethylene to produce linear low-density polyethylene (see Section 13.2.6). Under the reaction conditions applied in industry (50-60°C, 22-27 atm), the selectivity of 1-butene formation is higher than 90% at the conversion of 80-85%. Since no metal hydride is involved in this system, isomerization does not take place and only a small amount of higher-molecular-weight terminal alkenes is formed. 

The olefinic group of vinylpyridines undergoes a variety of reactions, which include reduction to the ethane, addition reactions, dimerizations, and polymerizations. There are few voltammetry studies on the isomeric vinylpyridines (26). The reduction of the bisquaternary salt of 1,2-di(2-pyridyl)-ethylene has been studied, and the role of adsorption and autoinhibition during reduction of l,2-di(4-pyridyl)ethylene was also determined.49,50... 

Crassous et al.177) studied the polymerization of ethylene using n-butyllithium in conjunction with the tertiary diamines TMEDA, TEEDA (tetraethylethylenedi-amine) and PMDT (pentamethyldiethylenetriamine). In contrast to the situation with TMEDA the rate of polymerization was found to show a first order dependence upon the complexed chain end concentration. Steric hindrance seems to prevent the dimerization of the chain ends. Examination of the n-BuLi. TEEDA complex by -NMR shows that the displacement to high field of the protons a to the lithium induced by complexation is much smaller with TEEDA than with TMEDA or PMDT. With [TEEDA] < [RLi] time-averaged signals were obtained showing that the exchange process... 

Ad-C3-Ad and poly-VAd exhibited the monomeric fluorescence at about 320 nm in ethanol. This finding suggests that the excimer was not formed for both dimeric and polymeric compounds in ethanol solution. Furthermore, the excimer could not be obtained either in the case of dimers or polymers of 6-methylaminopurine derivatives, though excimer emissions were observed in water-ethylene glycol. The absence of the excimer may be explained by the fact that the stacked forms of the nucleic acid bases are unstable in ethanol solution where the bases appear to be solvated with ethanol molecules23. ... 

Cationic polymerization of ethylene oxide exemplifies this behavior [91]. Due to the lack of strain of 6-membered 1,4-dioxane ring, this cyclic dimer of ethylene oxide is formed readily by back-biting. Its polymerization would involve AH — 0 and negative AS thus it is thermodynamically prohibited (equilibrium concentration of 1,4-dioxane is higher, than the highest attainable concentration, i.e., concentration in bulk). 

On the other hand, the cationic bis(phosphine) complexes [IndNi(PR3)2] and the highly electrophilic, in situ generated cations [Ind(PR3)]+ catalyze the dimerization of ethylene and oligo- and polymerization of other olefins (Scheme... 

The first observations of chain transfer come from studies of cyclic oxides. As described in Sect. 5.1, the formation of a cyclic dimer (1,4-dioxane) was observed in the polymerization of ethylene oxide and involves intramolecular attack within a peiHiltimate unit of the cludn ... 

Among the most significant developments in the field of catalysis in recent years have been the discovery and elucidation of various new, and often novel, catalytic reactions of transition metal ions and coordination compounds 13, 34). Examples of such reactions are the hydrogenation of olefins catalyzed by complexes of ruthenium (36), rhodium (61), cobalt (52), platinum (3, 26, 81), and other metals the hydroformylation of olefins catalyzed by complexes of cobalt or rhodium (Oxo process) (6, 46, 62) the dimerization of ethylene (i, 23) and polymerization of dienes (15, 64, 65) catalyzed by complexes of rhodium double-bond migration in olefins catalyzed by complexes of rhodium (24,42), palladium (42), cobalt (67), platinum (3, 5, 26, 81), and other metals (27) the oxidation of olefins to aldehydes, ketones, and vinyl esters, catalyzed by palladium chloride (Wacker process) (47, 48, 49,... 

Despite the early use of phosphonium salt melts as reaction media [12, 18, 25], the use of standard ionic liquids of type 1 and 2 as solvents for homogeneous transition metal catalysts was described for the first time in the case of chloroaluminate melts for the Ni-catalyzed dimerization of propene [5] and for the titanium-catalyzed polymerization of ethylene [6]. These inherently Lewis-acidic systems were also used for Friedel-Crafts chemistry with no added catalyst in homogeneous [7] as well as heterogeneous fashion [8], but ionic liquids which exhibit an enhanced stability toward hydrolysis, i. e., most non-chloroaluminate systems, have been shown to be of advantage in handling and for many homogeneously catalyzed reactions [la]. The Friedel-Crafts alkylation is possible in the latter media if Sc(OTf)3 is added as the catalyst [9]. 

The oligomerization of olefins is mostly catalyzed by cationic complexes which are very soluble in ionic liquids. The Pd-catalyzed dimerization of butadiene [36] and the Ni-catalyzed oligomerization of short-chain olefins [5, 37], which is also known as the Difasol process [1 d] if chloroaluminate melts are used, can be mn in imidazolium salts 1 [38, 39]. Here, the use of chloroaluminate melts and toluene as the co-solvent is of advantage in terms of catalyst activity, product selectivity, and product separation. Cp2TiCl2 [6] and TiCU [40] in conjunction with alkylaluminum compounds were used as catalyst precursors for the polymerization of ethylene in chloroaluminate melts. Neither Cp2ZrCl2 nor Cp2HfCl2 was catalytically active under these conditions. The reverse conversion of polyethylene into mixtures of alkanes is possible in acidic chloroaluminate melts without an additional catalyst [41]. 

The dimeric yttrium and samarium hydride complexes are converted into monomers in the first step of the reaction. The preparation of block-co-polymers of 1-hexene or 1-pentene with MMA and e-caprolactone, respectively, has also been reported. The polymerization of ethylene with MMA, e-caprolactone and 2,2-dimethyltri-methylenecarbonate was studied in detail racemic Me2Si(CsH2-2-SiMe3-4-But)2Sm(THF)2 or meso-Me2Si(Me2SiOSiMe2)(CsH2-3-But)Sm(THF) were active in the ABA-type triblock-co-polymerization.986... 

Co-polymerization of ethylene with cyclic dienes such as 1,3-cyclopentadiene, dicyclopentadiene, and 4-vinyl-l-cyclohexene using rac-C2H4(Ind)2ZrCl2 showed that dicyclopentadiene was the most reactive co-monomer. 1,3-Cyclopentadiene rapidly dimerizes to dicyclopentadiene, and thus ethylene/l,3-cyclopentadiene co-polymerization actually resulted in ethylene/l,3-cyclopentadiene terpolymers with dicyclopentadiene. Co-polymers with more than 9mol% of the co-monomer did not show a melt transition.1058... 

---