Ethylene radical polymerization

A different behavior is observed if TMA is replaced by TEA. Even though the final catalysts are equally active for ethylene polymerization, radicals have never been observed. This is in line with expectations based on indirect evidence [126-128] proposing a disproportionation reaction for the activation with TEA according to ... 

Tetrafluoroethylene of purity suitable for granular or dispersion polymerizations is acceptable for copolymerization with ethylene. Polymerization-grade ethylene is suitable for copolymerization with tetrafluoroethylene. Modifying termonomers, eg, perfluorobutylethylene and perfluoropropylene, are incorporated by free-radical polymerization. 

An independent development of a high pressure polymerization technology has led to the use of molten polymer as a medium for catalytic ethylene polymerization. Some reactors previously used for free-radical ethylene polymerization at a high pressure (see Olefin polymers, low density polyethylene) have been converted to accommodate catalytic polymerization, both stirred-tank and tubular autoclaves operating at 30—200 MPa (4,500—30,000 psig) and 170—350°C (57,83,84). CdF Chimie uses a three-zone high pressure autoclave at zone temperatures of 215, 250, and 260°C (85). Residence times in all these reactors are short, typically less than one minute. 

Addition polymerization is employed primarily with substituted or unsuhstituted olefins and conjugated diolefins. Addition polymerization initiators are free radicals, anions, cations, and coordination compounds. In addition polymerization, a chain grows simply hy adding monomer molecules to a propagating chain. The first step is to add a free radical, a cationic or an anionic initiator (I ) to the monomer. For example, in ethylene polymerization (with a special catalyst), the chain grows hy attaching the ethylene units one after another until the polymer terminates. This type of addition produces a linear polymer ... 

Conjugated dienes can be polymerized just as simple alkenes can (Section 7.10). Diene polymers are structurally more complex than simple alkene polymers, though, because double bonds remain every four carbon atoms along the chain, leading to the possibility of cis-trans isomers. The initiator (In) for the reaction can be either a radical, as occurs in ethylene polymerization, or an acid. Note that the polymerization is a 1,4-addition of the growing chain to a conjugated diene monomer. 

For /-butyl peresters there is also a variation in efficiency in the series where R is primary secondary>tertiary. The efficiency of /-butyl peroxypentanoate in initiating high pressure ethylene polymerization is >90%, that of /-butyl peroxy-2-ethylhexanoate ca 60% and that of/-butyl peroxypivalate ca 40%.196 Inefficiency is due to cage reaction and the main cage process in the case where R is secondary or tertiary is disproportionation with /-butoxy radicals to form /-butanol and an olefin.196... 

Radical Polymerization. Radical chain polymerization involves initiation, propagation, and termination. Consider the polymerization of ethylene. Initiation typically involves thermal homolysis of an initiator such as benzoyl peroxide... 

Ethylene polymerizes by a radical mechanism when it is heated at a pressure of 1000 atm with a small amount of an organic peroxide. 

It was discovered by Ziegler in Germany and Natta in Italy in the 1950s that metal alkyls were very efficient catalysts to promote ethylene polymerization at low pressures and low temperatures, where free-radical polymerization is very slow. They further found that the polymer they produced had fewer side chairrs because there were fewer growth mistakes caused by chain transfer and radical recombination. Therefore, this polymer was more crystalline and had a higher density than polymer prepared by free-radical processes. Thus were discovered linear and high-density polymers. 

Taylor in 1925 demonstrated that hydrogen atoms generated by the mercury sensitized photodecomposition of hydrogen gas add to ethylene to form ethyl radicals, which were proposed to react with H2 to give the observed ethane and another hydrogen atom. Evidence that polymerization could occur by free radical reactions was found by Taylor and Jones in 1930, by the observation that ethyl radicals formed by the gas phase pyrolysis of diethylmercury or tetraethyllead initiated the polymerization of ethylene, and this process was extended to the solution phase by Cramer. The mechanism of equation (37) (with participation by a third body) was presented for the reaction, - which is in accord with current views, and the mechanism of equation (38) was shown for disproportionation. Staudinger in 1932 wrote a mechanism for free radical polymerization of styrene,but just as did Rice and Rice (equation 32), showed the radical attack on the most substituted carbon (anti-Markovnikov attack). The correct orientation was shown by Flory in 1937. In 1935, O.K. Rice and Sickman reported that ethylene polymerization was also induced by methyl radicals generated from thermolysis of azomethane. 

We have observed that the radioactive contamination is practically independent of the temperature A9). We believe that this radioactive contamination is due to the presence of traces of radioactive polyethylene resulting from ethylene polymerization. Ethylene can result, in fact, from the disproportionation of C2Hs radicals released by decomposition of ethyl titanium compounds, which derive from the reaction between ethylalu-minum and traces of titanium tetrachloride or other tetravalent titanium compounds that are sometimes present as impurities in the a-titanium trichloride. 

LDPE was occasionally found in 1933 by R.O. Gibson and E.W. Fawcett, when they tried to perform reactions with ethylene [1]. Based on their invention. Imperial Chemicals Ltd (ICI), Great Britain, developed a process with a stirred autoclave in which ethylene was radically polymerized under high pressure [2], Later, BASF AG in Germany designed a tubular reactor to produce LDPE under similar high-pressure conditions [3]. 

Write the steps in the mechanism of ethylene polymerization catalyzed by a free radical. 

Polythene is difficult to make and was discovered only when chemists at ICI were attempting to react ethylene with other compounds under high pressure. Even with the correct reagents, radical initiators like AIBN or peroxides (Chapter 39), high pressures and temperatures are still needed. At 75 °C and 1700 atmospheres pressure ethylene polymerization, initiated by dibenzoyl peroxide, is a radical chain reaction. The peroxide is first cleaved homolytically to give two benzoate radicals. 

Historically, ethylene polymerization was carried out at high pressure (lUOO-3000 atm) and high temperature (100-250 °C) in the presence of a catalyst such as bciizoyl peroxide, although other catalysts and reaction conditions are now more often used. The key step is the addition of a radical to the ethylene double bond, a reaction similar in many respects to what takes place in the addition of an electrophile. In writing the mechanism, recall that a curved halfarrow, or "fishhook" A, is used to show the movement of a single electron, as opposed to the full curved arrow used to show the movement of an electron pair in a polar reaction. 

As a result of the advances in catalyst discovery for aqueous ethylene polymerization, silica-polyethylene nancomposites have been prepared with structures that vary with changing catalyst structure and silica composition." It is likely that many more advances in the area of high-tech composites with potential biological and nanotechnology applications will be made in the next few years through aqueous polymerization processes. In addition to free radical polymerizations and catalytic polymerizations, it should be noted that oxidative polymerizations can also be performed in aqueous media to yield conducting polymers. Recently, this has been used to prepare polypyrrole-coated latex particles that are expected to be interesting synthetic mimics for micrometeorites. 

Even though vinyl chloride was discovered in 1835, polyvinyl chloride was not produced until 1912. It is now one of our most common polymers production in 1984 was over 6 billion pounds. The monomer is made by the pyrolysis of 1,2-dichloroethane, formed by chlorination of ethylene. Free radical polymerization follows Markovnikov s rule to give the head-to-tail polymer with high specificity ... 

Figure 2.2 Backbiting mechanism in free radical ethylene polymerization 27...

The initiator can be a radical, as in ethylene polymerization (Section 7.10), an acid, as in isobutylene polymerization (Section 7.10), or an anion. Radical polymerization is the most common and can be carried out with practically any vinyl monomer. Acid-catalyzed (cationic) polymerization, however, is effective only with vinyl monomers that contain an electron-donating group (EDG) capable of stabilizing the chain-carrying carbocation intermediate. Thus, isobutylene polymerizes rapidly under cationic conditions, but ethylene, vinyl chloride, and acrylonitrile do not. Isobutylene polymerization is carried out commercially at -80 C, using BFy and a small amount of water to generate BF3OH" catalyst. 

An interesting method for producing plastic microspheres is disclosed by Bayer AG (13). The microspheres consist of vinyl chloride-ethylene copolymers. This method involves the use of the difference of monomer reactivity between vinyl chloride and ethylene. Upon radical polymerization of the two monomers at 50°C, vinyl chloride reacts faster than does ethylene, and unreacted monomeric ethylene then remains in the resulting copolymer jn-oduced by the pearl polymerization. The monomer can then act as the blowing agent. 

---