Ethylene polymerization reactivity

A monomer is a reactive molecule that has at least one functional group (e.g. -OH, -COOH, -NH2, -C=C-). Monomers may add to themselves as in the case of ethylene or may react with other monomers having different functionalities. A monomer initiated or catalyzed with a specific catalyst polymerizes and forms a macromolecule—a polymer. For example, ethylene polymerized in presence of a coordination catalyst produces a linear homopolymer (linear polyethylene) ... 

In Figure 23—6, polymer grade ethylene and any comonomers are blown into the-base of a fluidized bed reacton A very reactive catalyst (based on-titanium and magnesium chlorides) is injected and admixes with the ethylene. Polymerization takes place at 150-212 F and 300 psi, and polymer particles stay in the fluidized state as the ethylene swirls through the reactor. Since the temperature is controlled at or below the melting point, the particles form a white powder. 

In a quest to increase the efficiency of olefin polymerization catalysts and their selectivity in the orientation of the polymerization, the highly effective Group IV metallocene catalysts, M(Cp)2(L)2, have been studied, since they all display high fluxionality. Following methide abstraction, the metallocene catalysts of general formula M(Cp-derivatives)2(CH3)2 (M= Ti, Zr, Hf), were turned into highly reactive M+-CH3 cationic species. The activation parameters for the methide abstraction, derived from variable temperature NMR experiments, establish a correlation between the enthalpies of methide abstraction, the chemical shift in the resulting cation, and the ethylene polymerization activities [149]. 

An increase in the alkene-alkane ratio results in a significant decrease in single-labeled propane ethylene polymerization-cracking and hydride transfer become the main reaction. This labeling experiment carried out under conditions where side reactions were negligible is indeed unequivocal proof for the direct alkylation of an alkane by a very reactive carbenium ion. 

Modem methods based on density-functional theory (DFT) can describe relative activation barriers of organometallic reactions, i.e. relative reactivities, as well as the transition-metal NMR chemical shifts of the reactant complexes involved. It is thus possible to reproduce or rationalize observed correlations between these properties or to predict new ones. NMR/reactivity correlations that could be reproduced theoretically ("intrinsic correlations") are summarized. Newly predicted NMR/ reactivity correlations are discussed for the ethylene polymerization with V(=0-X)R3 or V(=Y)R3 catalysts. When X or Y are varied (X = A1H3, Li+, SbF5, H+ Y = NH, O, S, Se), both... 

Arylimido derivatives, on the other hand, are deshielded with respect to alkyl imido complexes, cf. V(NTol)(CH2SiMe3)3 (Tol = tolyl), 8 = 1048 (21). Provided the NMR/reactivity correlations hold also when the substituents at the imido nitrogen are varied, one might speculate that suitably derivatized aryl rests (for instance, by introducing electron-withdrawing groups) could produce more deshielded 51V resonances and, at the same time, more active catalysts for ethylene polymerization. Further experimental and theoretical studies in that direction could be rewarding. 

It is remarkable that a reduced Ziegler type of catalyst that is no longer reactive for ethylene polymerization causes the exchange between 14C-labeled ethylene and the alkyl groups of the triethylaluminum until the equilibrium point is reached. The intermediate structures mentioned were only detected spectroscopically because of their low stability, they were not isolated. 

A particular subset of these ligated dialkyls are those in which the ligand is tethered to the cyclopentadienyl ligand. Thus, chromacyclopentane and cycloheptane derivatives stabilized by the (7/ 7/ -Me4C5CH2CH2NMe2) ligand have been prepared and their reactivity supports the intermediacy of such metallacycles in the catalytic trimerization of ethylene. A variety of donor-ligand-substituted cyclopen-tadienylchromium(III) complexes with amino and phosphino substituents has been prepared and screened for an ethylene polymerization activity. ... 

A limited kinetic investigation has been carried out on promotors for vanadium catalysts (VO(Ot-Bu)3/Al2Et3Cl3) in ethylene polymerization [234]. It was shown that esters of trichloroacetic acid, added continuously during the polymerization, reactivated the catalyst and permitted polymerization to be carried out at 120°C. Under these circumstances over 250 polymer chains were produced per vanadium atom and the polymers had /Mn ratios close to 2.0, which would be anticipated for a single catalytic entity. 

A major disadvantage of the highly reactive ethylene polymerization catalyst 4 was the decreased turnover number and decreased efficiency at prolonged reaction... 

In a similar way, n-butyl acrylate was copolymerized by ATRP with methacrylate macromonomers containing highly branched polyethylene prepared by Pd-catalyzed living ethylene polymerization. The observed reactivity ratios depend on the molecular weight and concentration of the macromonomer. The resulting graft copolymers showed microphase separation by AFM [304]. 

CrA sites, the most numerous of the three in this preparation, were identified as the most reactive species and comprise the sites that are active in ethylene polymerization, whereas Crc sites were found to be inactive. CrB sites were also thought to exhibit polymerization activity, but distinctions were made. The concentration of these surface species could be varied with the chromium content and with the conditions of the thermal pretreatments [279]. Heating the catalyst under vacuum at 700 °C, which caused a deactivation for ethylene polymerization, was found to convert CrA into Crc whereas CrB remained unaffected. 

Butene and other secondary olefins do not readily copolymerize with ethylene, and they may even tend to inhibit ethylene polymerization. This observation probably indicates steric crowding. For example, secondary olefins might be able to coordinate to the catalytic site, but not insert in the polymer chain. Alternatively, they may insert but give chains that are resistant to further insertion of ethylene. Similarly, a-olefins with a branch near the double bond, such as isobutylene or a-olefins with branches in the third position, also react poorly [403]. Examples of the differences in reactivity of the various a-olefins are shown in Table 8. 

Reaction 1 appears to result solely in termination. In hydrogenolysis experiments with various chelates we have observed precipitation of lithium hydride in all cases at room temperature. Attempts to generate chelated LiH in situ by adding hydrogen during ethylene polymerization also caused a rapid, irreversible loss of activity. Since there is no evidence that lithium hydride can add to ethylene under moderate polymerization conditions, it is unlikely that any significant chain transfer occurs via this mechanism. Potassium alkyls readily eliminate olefin with the formation of metal hydride, and sodium alkyls do so at elevated temperatures (56). It was noted earlier that chelation of lithium alkyls makes them more like sodium or potassium compounds, so it is quite probable that some termination occurs by eliminating LiH. It is conceivable that this could be a chain transfer mechanism with more reactive monomers than ethylene because addition to lithium hydride would be more favorable. 

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