Ethylene, polymerization mechanisms

Keywords Ethylene polymerization mechanisms Heterogeneous model catalysts Homogeneous model catalysts Molecular modeling Phillips Cr/silica catalyst Polyethylene Polymerization kinetics... 

The initial complex in Equation 2.3 forms extremely rapidly, while the alkyl exchange reaction in Equation 2.4 to form the catalyst initiation complex takes place more slowly, but provides the titanium-carbon bond necessary for the ethylene propagation reaction to take place, leading to high molecular weight polyethylene. Therefore, the examination of these soluble titanium complexes provided the important data to explain the ethylene polymerization mechanism, which was detailed in a series of papers by Cossee [22-24]. 

Because the ethylene polymerization mechanism for the manufacture of high-pressure polyethylene involves a growing free-radical active site, the structure of this type of polyethylene is unique in several aspects ... 

Groppo E, Lamberti C, Bordiga S, Spoto G, Zecchina A The structure of active centers and the ethylene polymerization mechanism on the Cr/Si02 catalyst a frontier for the characterization methods, Chem Rev 105(1) 115—184, 2005a. 

Figure 14 5 outlines a mechanism for ethylene polymerization m the presence of Cp2ZrCl2 Step 1 describes the purpose of the MAO promoter which is to transfer a methyl group to the metallocene to convert it to its catalytically active form This methyl group will be incorporated into the growing polymer chain—indeed it will be the end from which the rest of the chain grows... 

Figure 9.21. The Cossee-Arlman mechanism of chain growth in ethylene polymerization involves the insertion of ethylene in the...

Scheme 6 Scheme of the initiation mechanism in ethylene polymerization according to a Ziegler-Natta-like behavior... 

All mechanisms proposed in Scheme 7 start from the common hypotheses that the coordinatively unsaturated Cr(II) site initially adsorbs one, two, or three ethylene molecules via a coordinative d-7r bond (left column in Scheme 7). Supporting considerations about the possibility of coordinating up to three ethylene molecules come from Zecchina et al. [118], who recently showed that Cr(II) is able to adsorb and trimerize acetylene, giving benzene. Concerning the oxidation state of the active chromium sites, it is important to notice that, although the Cr(II) form of the catalyst can be considered as active , in all the proposed reactions the metal formally becomes Cr(IV) as it is converted into the active site. These hypotheses are supported by studies of the interaction of molecular transition metal complexes with ethylene [119,120]. Groppo et al. [66] have recently reported that the XANES feature at 5996 eV typical of Cr(II) species is progressively eroded upon in situ ethylene polymerization. 

The stoichiometry of this conversion is in accordance with a carbene starting structure. An alternating alkyUdene/metallacyclobutane mechanism [102, 131-133], which has precedent in the ethylene polymerization catalyzed by a Ta(III) neopentilydene complex [134], has been proposed where the chro-miiun alkyUdenes undergo [2-1-2] cycloaddition to give chromacyclobutane intermediates (mechanism III in Scheme 7). 

Petitjean, L., Pattou, D., Ruiz-Lopez, M. F., 1999, Theoretical Study of the Mechanisms of Ethylene Polymerization with Metallocene-Type Catalysts , J. Phys. Chem. B, 103, 27. 

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

Scheme 4 Mechanism of chain growth for a all Pd(II) polymerizations and ethylene polymerizations with Ni(II), and b a-olefin polymerizations with Ni(II). Specific kinetic data shown for Ni catalyst 1.15b [63]...

Block copolymerization was carried out in the bulk polymerization of St using 18 as the polymeric iniferter. The block copolymer was isolated with 63-72 % yield by solvent extraction. In contrast with the polymerization of MMA with 6, the St polymerization with 18 as the polymeric iniferter does not proceed via the livingradical polymerization mechanism,because the co-chain end of the block copolymer 19 in Eq. (22) has the penta-substituted ethane structure, of which the C-C bond will dissociate less frequently than the C-C bond of hexa-substituted ethanes, e.g., the co-chain end of 18. This result agrees with the fact that the polymerization of St with 6 does not proceed through a living radical polymerization mechanism. Therefore, 18 is suitably used for the block copolymerization of 1,1-diubstituted ethylenes such as methacrylonitrile and alkyl methacrylates [83]. 

Poly(Ethylene Terephthalate) Polymerization - Mechanism, Catalysis, Kinetics, Mass Transfer and Reactor Design... 

Huang, B. and Walsch, J. J., Solid-phase polymerization mechanism of poly(ethylene terephthalate) affected by gas flow and particle size, Polymer, 39, 6991-6999 (1998). 

Figure 5.5 Effect of temperature on the SSP reaction rate for (a) small, and (b) large chips of PET [13]. Reprinted from Polymer, 39, Huang, B. and Walsh, J. J., Solid-phase polymerization mechanism of poly(ethylene terephthalate) affected by gas flow velocity and particle size, 6991-6999, Copyright (1998), with permission from Elsevier Science...

After activation with MAO (molar ratios [Al] [Zr] = 1000) the polymerization of ethylene has been successfully carried out using the zirconocene functionalized dendrimer at 40 bar ethylene pressure and 70 °C. We obtained high activity and productivity values for the ethylene polymerization and polymers with very high molecular masses in the range of 2 x 10 g/mol. The polydispersity of the polymer is quite low (3.0) indicating the single site character of the catalytically active species. Optimization of this system and study of the mechanism are stiU under investigation. Nevertheless, these preliminary results reveal the suitability of polyphenylene dendrimers as supports for zirconocene catalysts. 

Figure 2 shows the profile of the 27-29 ppm spectral region of three polymers which served as models (1J ) for ethylene propylene rubber. The better agreement between the observed spectrum and the five-parameter model strongly suggests the three-parameter model is less realistic as an explanation for the polymerization mechanism. Table VII compares the observed profiles of EPDM rubbers made with a Ziegler catalyst system. The ratio of... 

In a similar fashion, the dynamic behavior of Ni(p-agostic alkyl)(a-diimines) cationic complexes, e. g. 102, which are models for the intermediates involved in Ni catalyzed polymerization of ethylene, has been studied by LSA and provides insight into the mechanism of ethylene polymerization [44]. Eq. (6), shows the exchange between two of these species. 

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. 

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