Example polymerization of ethylene

Bohm polymerized ethylene [48] on a highly active catalyst, formed by the interaction of Mg(OEt)-, with TiCl4. Et,Al was used as cocatalyst. The activity of the catalytic system depended on the ratio between catalyst and cocatalyst (see Fig. 9)f. After a certain mixing rate ( 500 rev. min 1) has 

The maximum of the dependence from Fig. 9 is usually placed to much lower values of the ratio 

The yield of the catalyst, 0, was measured at various ethylene concentrations (see Fig. 10). According to the results, initiation is rapid and the catalytic system maintains full capacity for a long time, for at least 1 h. In this interval, the polymeric particles increase their size 5-10 fold. Thus the monomer supply into the pores of the particles by diffusion cannot be hindered. In the subsequent phase, activity already decreases. Either the conditions for monomer transport to the centres by diffusion are deteriorating, and/or the centres are slowly decaying. The polymerization rate, i pol, can be determined from the slopes of the curves in Fig. 10. The determined values of the initial rates are directly proportional to monomer concentration (except for the lowest values of [M]), as shown in Fig. 11. 

The polymerization rate, upol, will predominantly depend on Ara. The rate--determining step in this case is the formation of the complex between the monomer and the active centre. When kd kp, the approximation 

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Substance made of giant molecules formed by the union of simple molecules (monomers) for example, polymerization of ethylene forms a polyethylene chain, or condensation of phenol and formaldehyde (with production of water) forms phenol-formaldehyde resins. 

For example, polymerization of ethylene or propylene at 200-400 C, 1-250 atm gives only liquefied polymers. However, with Ziegler type eatalysts of tetraethyllead and metal halides (halides of Ti, Mo or V), produces the polymerization at 90-180 °C form an atmospheric pressure to a high pressure gives erystalline polymers [39], Organolead compounds have been investigated as catalysts for polyesters, polysulfones, polyurethanes, etc. [9,39]. 

Thus a polymer is a substance consisting of large molecules formed by the joining together of simple molecules that are known as monomers where two or more monomers are involved the resultant product is known as a copolymer. As an example, polymerization of ethylene forms a... 

Similar phenomena, although less effective, take place in the polymerization of all monomers leading to macromolecules with heteroatoms in the chains. A related phenomenon is chain transfer to the same macromolecule, called backbiting and bearing a resemblance to the intramolecular transfer in, for example, polymerization of ethylene. However, in the latter, branching results from backbiting and in ROP/ROMP formation of cydic molecules is the dominating process. 

Macromolecules are formed by linking together monomer molecules through chemical reactions, the process by which this is achieved being known as polymerization. For example, polymerization of ethylene yields polyethylene, a typical sample of which may contain molecules with 50000 carbon atoms linked together in a chain. It is this long chain nature which sets polymers apart from other materials and gives rise to their characteristic properties. 

Boron Bromide. Approximately 30% of BBr produced in the United States is consumed in the manufacture of proprietory pharmaceuticals (qv) (7). BBr is used in the manufacture of isotopicaHy enriched crystalline boron, as a Etiedel-Crafts catalyst in various polymerization, alkylation, and acylation reactions, and in semiconductor doping and etching. Examples of use of BBr as a catalyst include copolymerization of butadiene with olefins (112) polymerization of ethylene and propylene (113), and A/-vinylcarbazole (114) in hydroboration reactions and in tritium labeling of steroids and aryl rings (5). 

An example of an emulsifier for the polymerization of ethylenically unsaturated monomers is a 22 88 mixture of disodium a-sulfolaurate and sodium methyl a-sulfolaurate. The emulsion is stable for much longer than an emulsion with n-dodecylbenzenesulfonate as the emulsifier [92]. Ester sulfonates are also used as emulsifiers in the continuous manufacture of vinyl chloride polymers [93]. 

Small olefins, notably ethylene (ethene), propene, and butene, form the building blocks of the petrochemical industry. These molecules originate among others from the FCC process, but they are also manufactured by the steam cracking of naphtha. A wealth of reactions is based on olefins. As examples, we discuss here the epoxida-tion of ethylene and the partial oxidation of propylene, as well as the polymerization of ethylene and propylene. 

In many polymerizations cyclic material is produced by a concurrent backbiting reaction as linear polymer is formed. For example dioxan Is formed In the cationic polymerization of ethylene oxide to polyoxyethylene, and polyoxyethylene can be degraded to... 

The above example outlines a general problem in immobilized molecular catalysts - multiple types of sites are often produced. To this end, we are developing techniques to prepare well-defined immobilized organometallic catalysts on silica supports with isolated catalytic sites (7). Our new strategy is demonstrated by creation of isolated titanium complexes on a mesoporous silica support. These new materials are characterized in detail and their catalytic properties in test reactions (polymerization of ethylene) indicate improved catalytic performance over supported catalysts prepared via conventional means (8). The generality of this catalyst design approach is discussed and additional immobilized metal complex catalysts are considered. 

Boratabenzene complexes of Group 3 and Group 6 metals serve as effective catalysts for the oligomerization/polymerization of ethylene. For example, [(C5H5B-Ph)2ScPh]2, pretreated with H2, oligomerizes ethylene to furnish 1-alkenes.17a In the case of a Cr(III)-boratabenzene complex, ethylene is polymerized to afford... 

Table 2. Examples of the catalytic activity of selected metallocene and related complexes for the polymerization of ethylene...

At 24 °C and 15-60 bar ethylene, [Rh(Me)(0H)(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 (TOP 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] 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] and the rate of this exchange is probably the lowest step of the overall reaction.The hydroxy ligand facilitates the dissociation of H2O and this leads to a slow catalysis of ethene polymerization. 

On the industrial scale, suspension polymerizations are not only carried out in the aqueous phase, but also in aliphatic hydrocarbons using Ziegler-Natta catalysts, as for example, in the polymerization of ethylene and propylene (see also Sect. 3.3.1). 

The initiation of polymerizations by metal-containing catalysts broadens the synthetic possibilities significantly. In many cases it is the only useful method to polymerize certain kinds of monomers or to polymerize them in a stereospecific way. Examples for metal-containing catalysts are chromium oxide-containing catalysts (Phillips-Catalysts) for ethylene polymerization, metal organic coordination catalysts (Ziegler-Natta catalysts) for the polymerization of ethylene, a-olefins and dienes (see Sect. 3.3.1), palladium catalysts and the metallocene catalysts (see Sect. 3.3.2) that initiate not only the polymerization of (cyclo)olefins and dienes but also of some polar monomers. 

In 1996, Brookhart and co-workers developed a remarkable class of Pd complexes with sterically encumbered diimine ligands (Scheme 4, S4-1, S4-2, S4-4, and S4-5). These examples are capable of mediating the co-polymerization of ethylene with methyl acrylate (MA) to furnish highly branched PE with ester groups on the polymer chain ends by a chain-walking mechanism (Scheme 10). " This represents the first example of transition metal-catalyzed ethylene/MA co-polymerization via an insertion mechanism. The mechanism for co-polymerization is by 2,1-insertion of MA and subsequent chelate-ring expansion, followed by the insertion of ethylene units. The discovery of these diimine Pd catalysts has stimulated a resurgence of activity in the area of late transition metal-based molecular catalysis. Recently, the random incorporation of MA into linear PE by Pd-catalyzed insertion polymeriza-... 

The polymerization of ethylene oxide (epoxyethane, EO) with 17 also proceeded by irradiation with visible light. For example, the polymerization with the mole ratio [EO]o/[17]o of 190 in benzene at room temperature, where the monomer conversion after 205 min was very low (<2%, determined by H NMR) in the dark, proceeded to 97% conversion in only 80 min under irradiation. The Mn of the polymer, as estimated from the GPC chromatogram, was 8700, which is in excellent agreement with the expected value of 8100 provided that the numbers of the molecules of the produced polymer and 17 (X=SPr) are equal [81]. The Mw/Mn of the polymer (1.05) was close to unity, indicating the livingness of the visible Hght induced polymerization of EO initiated with (NMTPP)ZnSPr (17). 

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