Polymerization of ethylene derivatives

The polymerization of ethylene derivatives has recently become extremely important for the manufacture of plastics. Certain substituents in ethylene — those that increase the polarization — increase both the extent and the rate of polymerization. Such substituents are aromatic groups (as in styrene), oxygen-containing groups (as in acrolein, acrylic esters, and vinyl esters and ethers), and halogens (as in vinyl chloride). Multiplication of these substituents, however, depresses or completely suppresses the tendency to polymerize for instance, stilbene gives only a dimer when illuminated in benzene.14 

Polymerization can be accelerated by ultraviolet irradiation or by peroxides, and also by acid or basic catalysts. 

Kirmse, Carbene Chemistry, Academic Press, New York, London, 1964. 

A radical chain mechanism must be assumed for polymerization on irradiation or in presence of peroxides  

Peroxides also initiate telomerization, which is a process also occurring by way of radicals.15 In this reaction compounds such as halomethanes participate to form short-chain polymers with both ends of the chain defined. In the overall reaction the taxogen (e.g., ethylene) combines with the telogen (e.g., carbon tetrachloride) to afford a telomer  

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Table 20-6. Syndiotactic Diad Fractions and Transition Probabilities p for the Formation of Iso- and Syndiotactic Diads in the Free Radical Polymerization of Ethylene Derivatives, CHj CRR ...

Since the polymerization of ethylene derivatives, particularly styrene, has been studied most exhaustively from the kinetic standpoint, it may suffice for the purpose of the present work to confine ourselves to this case and to cite it as exemplifying the treatment of a polymerization reaction. 

The resulting polymer, polyethylene, consists essentially of a long chain of repeating -CH2-CH2- groups (the structural units) in a typical commercial material n has a value of the order of 1000. Further examples of polymerizations of ethylene derivatives are as follows ... 

Section 14 15 Coordination polymerization of ethylene and propene has the biggest eco nomic impact of any organic chemical process Ziegler-Natta polymer ization IS carried out using catalysts derived from transition metals such as titanium and zirconium tt Bonded and ct bonded organometallic com pounds are intermediates m coordination polymerization... 

Substituents in the allyl group of a catalyst have a marked effect on the polymerization efficiency (9,12). This is shown in Table IV for the polymerization of ethylene with chromium and zirconium allyls and for the polymerization of methyl methacrylate with chromium allyls. Introducing a methyl group into the allyl ligand increases the activity by a factor of 2 to 7. In some polymerizations of ethylene Cr(2-Me-allyl)3 compounds are ten times more effective than the simple allyl derivatives. The introduction of... 

The diimine palladium compounds are less active than their nickel analogs, producing highly branched (e.g., 100 branches per 1,000 carbons) PE. However, they may be used for the copolymerization of Q-olefins with polar co-monomers such as methyl acrylate.318,319 Cationic derivatives, such as (121), have been reported to initiate the living polymerization of ethylene at 5°C and 100-400 psi.320 The catalyst is long-lived under these conditions and monodisperse PE (Mw/Mn= 1.05-1.08) may be prepared with a linear increase in Mn vs. time. 

Kaminsky W, Engehausen R, Zoumis K (1992) Standardized polymerizations of ethylene and propene with bridged and unbridged metallocene derivates a comparison. Makromol Chem 193 1643-1651... 

Long (81) showed that the complex from biscyclopentadienyltitanium dichloride and methylaluminum chloride or a simply derived product from it, was an active ethylene polymerization catalyst. There have been a number of attempts to determine the exact nature of initiation in polyethylene. However, by any techniques available until now, it has not been possible to determine the actual ionic nature of the active catalyst which polymerizes ethylene. Karapinka and Carrick (82) studied the polymerization of ethylene with biscyclopentadienyltitanium dichloride and various alkylaluminum compounds. They found that the alkyl group exchanged so readily between the aluminum and titanium, that the location of the initiating site could not be determined. All that could be concluded was that an ethyl group initiated the polymerization more easily than the phenyl. 

Titanocene dichloride (8) is readily reduced to the [(ir-C5H5)2Ti(III)]+ species which in turn can be converted into alkyl and hydrido derivatives (18-20). These titanocene derivatives have been found to be active catalysts in the low-pressure polymerization of ethylene (21, 22) and the... 

Ziegler catalysis involves rapid polymerization of ethylene and a-ole-fins with the aid of catalysts based on transition-element compounds, normally formed by reaction of a transition-element halide or alkoxide or alkyl or aryl derivative with a main-group element alkyl or alkyl halide (1,2). Catalysts of this type operate at low pressures (up to 30 atm), but often at 8-10 atm, and, in special cases, even under reduced pressure, and at temperatures up to 120°C, but often as low as 20-50°C. Approximately 2,200,000 tons of polyethylene and 2,900,000 tons of polypropylene are produced per year with the aid of such catalysts. The polyeth-... 

As mentioned in Chapter 1, ethylene is always the more reactive olefin in systems used to produce copolymers involving a-olefins (LLDPE and VLDPE). An important process consideration for copolymerizations is the reactivity ratio. This ratio may be used to estimate proportions needed in reactor feeds that will achieve the target resin. However, fine tuning is often required to obtain the density or comonomer content desired. Reactivity ratios were discussed previously (Chapter 2) in the context of free radical polymerization of ethylene with polar comonomers. Reactivity ratios are also important in systems that employ transition metal catalysts for copolymerization of ethylene with a-olefins to produce LLDPE. Discussions of derivations and an extensive listing of reactivity ratios for ethylene and the commonly used a-olefins are provided by Krentsel, et al. (1). 

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