Polymerization of ethene

Even more important is the stereoregular catalytic polymerization of ethene and other alkenes to give high-density polyethene ( polythene ) and other plastics. A typical Ziegler-Natta catalyst can be made by mixing TiCU and Al2Eti in heptane partial reduction to Ti " and alkyl transfer occur, and a brown suspension forms which rapidly absorbs and polymerizes ethene even at room temperature and atmospheric pressure. Typical industrial conditions are 50- 150°C and 10 atm. Polyethene... 

Stereoregular polymerization of ethene and propene by catalysts developed by K. Ziegler and by G. Natta (shared Nobel Prize 1963). 

Examine the sequence of structures corresponding to Ziegler-Natta polymerization of ethene, or more specifically, one addition step starting from a zirconocene-ethene complex where R=CH3. Plot energy (vertical axis) vs. frame number (horizontal axis). Sketch Lewis structures for the initial complex, the final adduct and the transition state. Indicate weak or partial bonding by using dotted lines. 

High-temperature and -pressure free radical polymerization of ethene, to produce low-density polyethene (LDPE). 

In the early 1950s there was the quite contemporary discovery—in three different laboratories—of processes for the polymerization of ethene at low pressure using solid catalysts The catalyst used by the Standard Oil of Indiana was Mo(VI) oxide supported on aluminum oxide the one by Phyllips Petroleum was Cr(VI) oxide still supported on silica/alumina the catalyst studied by Ziegler and his co-workers at the Max Planck Institute at Miihlheim... 

Many other metal-catalysed polymerizations may be carried out in water including the copper-catalysed polymerization of methacrylates, the palladium-and nickel-catalysed polymerization of ethene and other alkenes and the rhodium-catalysed polymerization of butadiene [22],... 

Complexes 88—91 catalyzed the polymerization of ethene upon activation with MAO. Every dendritic catalyst displayed a lower activity than its reference complex Cp2ZrCl2 or Cp2TiCl2. It was found that the replacement of a C5HJ ligand by a dendritic cyclopentadienide (88 or 89) caused a moderate decrease in activity (4,320 2,064 and 1,720 kg/mol/h for CpTiCls, 88, and 89, respectively), which could be attributed to steric hindrance. The bis-dendritic Cp system 90 gave a lower activity (576kgmor h ). Interaction between a pair of dendritic wedges may restrict the conformations in which they are kept far away from the metal center. 

The quasi living polymerization of ethene and norbornene has been reviewed, among other topics in living polymerization of alkenes (19). Specifically, arylimido-aryloxo-vanadium(V) complexes with methylaluminoxane or Et2AlCl as co-catalyst have been used as catalyst systems. The polymers exhibit a low polydispersity and a high molecular weight (20). 

Although both linear polyethene and isotactic polypropene are crystalline polymers, ethene-propene copolymers prepared with the aid of Ziegler catalysts are excellent elastomers. Apparently, a more or less random introduction of methyl groups along a polyethene chain reduces the crystallinity sufficiently drastically to lead to an amorphous polymer. The ethene-propene copolymer is an inexpensive elastomer, but having no double bonds, is not capable of vulcanization. Polymerization of ethene and propene in the presence of a small amount of dicyclopentadiene or 1,4-hexadiene gives an unsaturated heteropolymer, which can be vulcanized with sulfur in the usual way. 

Alkenes are also one main feedstock for preparation of polymers (i.e., plastics). Again, the double-bond electrons play a critical role. In the presence of a suitable catalyst, the double-bond electrons can be used to stitch together a large number of alkene molecules into immensely long alkanes (as seen in Figure 11.13). For example, polymerization of ethene gives polyethylene. 

The complex Cp2Sm(THF)2 reduces Al(Me)3 in toluene giving rise to AlMe4 bridged complex Cp Sm[(/r-Me)AlMe2(/r-Me)]2SmCp which metallates C-H bonds of benzene, hexane and pyridine and catalyzes the polymerization of ethene. The structure of the complex shows [157] bent Cp Sm units, connected to two tetrahedral (/r-Me)2AlMe2 units... 

Breslow and Newburg [14] used bis-cyclopentadienyltitaniumdichloride (Cp2TiCl2) together with diethylaluminumchloride for the polymerization of ethene. Subsequent research on this and other metallocene systems with various alkyl groups has been performed by Natta et al. [15], Belov et al. [16], Dyachkovskii et al. [17], Patat and Sinn [18], Chien and Hsieh [19], Clauss and Blstian [20], Henrici-Olive and Olive [21], Reichert and Schoetter [22], and... 

Figure 2 shows some of the classes of metallocene catalysts used for the polymerization of ethene. In order to compare the reactivities and molecular masses, the polymerizations are carried out under the same conditions (30 °C, 2 bar ethene pressure, toluene as solvent) or calculated to these parameters by data from the literature [57-60]. 

It undergoes ready insertion of alkenes into the Ln-C bond and is a very active catalyst for the polymerization of ethene. 

Figure 6.2-24 Near-infrared absorbance spectra, recorded during free-radical polymerization of ethene at 190 °C and 2630 bar initial pressure (the arrows indicate the direction of the absorbance change with the reaction time).

The square-planar complex (70) incorporating a C2 symmetric diphosphine catalyzes the polymerization of ethene, which is somewhat surprising as diphosphine... 

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