Ethylene polymerization significance

Compounds of the type Zr(7r-Cpd)2, Ti(Tr-Cpd)2, and Cr(CaH6)2, were found to be completely inactive with all monomers whereas a significant number of transition metal allyl compounds were found to have weak activity for ethylene polymerization. The latter results are summarized in Table I. Despite the fact that many transition metal allyl compounds are unstable above 0°C, in the presence of monomer, the metal allyl structure... 

The FI ligand structure has a significant influence on the ethylene polymerization activity in particular, modification of the R2 substituent has a dramatic effect on the activity (Table 1) [12, 54, 55], Namely, R2 substituents that are sterically smaller than a f-Bu group [i.e., t -Pr (6), Me (7)] significantly reduce the activity (activity < 1 kg mmoF1 Ir1). By contrast, R2 substituents that are sterically larger than the f-Bu group markedly enhance the activity. The activity is thus directly correlated to the steric bulk of the R2 substituent. For example, in the sequence f-Bu (1, 8) < adamantyl (9) < cumyl (10) < 1,1-diphenylethyl (11), the activity increases from 519 (1 R1 = phenyl, R2 = f-Bu) to 2383 kg mmol-1 h-1 (11 R1 = phenyl, R2 = 1,1-diphenylethyl). 

We demonstrated that a series of Ti-FI catalysts 40 (Fig. 25) and 44-47 (Fig. 29) possessing a t-Bu, cyclohexyl, i-Pr, Me, and H ortho to the phenoxy-O (thus having various steric environments in close proximity to the active site) all initiate room temperature living ethylene polymerization, though, for the non-fluorinated congeners, the steric bulk of the substituent ortho to the phenoxy-O significantly influences product molecular weight (Table 6) [28, 33]. 

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. 

The Ziegler-Natta polymerization of ethylene and propylene is among the most significant industrial processes. Current processes use heterogeneous catalysts formed from Ti(IH)Cl3 or MgCl2-supported Ti(IV)Cl4 and some otganoaluminum compounds. The widely accepted Cossee mechanism of ethylene polymerization is illustrated in Scheme 62. 

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. 

This second reaction leads to the small amount of branching (usually less than 5%) observed in the alcohol product. The alpha olefins produced by the first reaction represent a loss unless recovered (8). Additionally, ethylene polymerization during chain growth creates significant fouling problems which must be addressed in the design and operation of commercial production facilities (9). 

For titanium trichloride k, estimated in ref. by taking into account kf obtained by use of CO, is practically independent of the cocatalyst nature. is strongly influenced by the monomer nature. For propylene polymerization k is much lower (almost 20 times) than for ethylene polymerization (Table 11). kf, however, differs more significantly (by two orders of magnitude). Thus, under similar reaction conditions, the polymer molecular mass is apparently lower in propylene than in ethylene polymerization. The rate constant of the chain transfer with hydrogen, k, in the case of ethylene and propylene polymerization differs only by the factor four (Table 11) this is much lower than the differences in kf. Hence, for a similar decrease of the molecular mass of polypropylene the hydrogen concentration should be much lower than in ethylene polymerization. 

Another commonly observed decomposition pathway involves C-H bond activation. For example, the reaction of (l,3-But2Gp)2ZrMe2 with B(C6F5)3 yields a C-H activated 77s,771- tuck-in cation 710 (Scheme 173), which is inert with respect to ethylene polymerization. There are also significant metal-alkyl group effects on the thermodynamic stability and stereochemical mobility of the B(C6F5)3-derived Zr and Hf metallocenium ion pairs.538... 

An example of this behavior is shown in Table 11. In the reported experiments, a typical Cr/silica catalyst was tested for ethylene polymerization with small amounts of butene added to the reactor. Three different butene isomers were used in three series of experiments 1-butene, 2-butenes (cis and trans), and isobutylene. In the first series, as 1-butene was added to the reactor, the density of the polymer declined significantly, indicating the presence of ethyl branches on the chains from the incorporation of the comonomer (branching disrupts crystallinity and creates more amorphous polymer, which lowers the average density). The MI values of the polymers in this series went up as 1-butene was added, as would be expected from the greater ease with which a (3-hydride can be abstracted from the tertiary carbon resulting from 1-butene incorporation. This is the behavior typical of all a-olefin comonomers. 

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. 

Kinetics of addition of the tetrameric t-BuLi to 1,1-diphenyl ethylene in benzene was investigated by Evans et al.169. This reaction was found to be first order in the ethylene but, significantly, Zt order in /-butyl lithium. The Zt order dependence of the initiation induced by the tetrameric sec-butyl lithium was observed in the polymerization of styrene or isoprene proceeding in benzene159,164. This is shown in Fig. 25. Both observations lend further support to the schemes involving monomeric alkyl lithiums as the active, initiating species. 

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