Polymerization, olefin insertion

In the propagation centers of chromium oxide catalysts as well as in other catalysts of olefin polymerization the growth of a polymer chain proceeds as olefin insertion into the transition metal-carbon tr-bond. Krauss (70) stated that he succeeded in isolating, in methanol solution from the... 

In the propagation process of Ziegler-Natta polymerization, the insertion of olefin into a metal-carbon bond is the most important basic step, but many questions concerning to this process remained unanswered for a long time. 

By performing excellent model reactions [144], Grubbs and his co-workers demonstrated direct olefin insertion into an M-C bond. Thus, complex 115 was treated with AlEtCl2 to give complex 116, whose decomposition afforded methylcyclopentane. Under the same conditions, the polymerization of ethylene took place. In this way, the insertion of a-olefins into a Ti-C single bond in a model Ziegler-Natta catalyst system was directly observed (Eq. 9). 

Stereoselective alkylations, 12 165-166 Stereoselective hydrolysis, 16 400 Stereoselective propene polymerization, catalyst symmetries for, 16 104 Stereoselectivity, in a-olefin insertion, 16 99-102... 

Since Zr-H is able both to (i) activate the C-H bonds of alkanes (via cr-bond metathesis) [15, 48] and to carry out their hydrogenolysis (transfer of a least two carbons via a P-alkyl transfer) and (ii) polymerize olefins (via insertion), the ability of such supported Zr-H was tested in the homologation of propane. 

Of course, it is dangerous to exclude the possibility of aluminium polymerization on the basis of calculations. Reality is invariably more complicated than the simplified models put into computers. However, in view of the uncertainties surrounding existing systems, and the doubts thrown by calculations, any well-defined aluminium alkyl claimed to be active should at least be checked, as an isolated complex, for its propensity to olefin insertion vs. chain transfer, e.g,. using the Al-i-butyl/ethene experiment reported by Jordan [15], as explained above. 

The relatively high tendency of aluminium alkyls to undergo j -hydrogen transfer, coupled with olefin insertion barriers that are much higher than those for transition metals, makes it unlikely that very efficient high-MW olefin polymerization catalysts based on aluminium will be discovered. Even for the reaction of aluminium alkyls with ketones, using ligands to tune towards insertion selectivity will be difficult. 

The Ziegler polymerizations of olefins (92, 5) and the aluminum (108), gallium, beryllium, and indium (ill) alkyl growth reactions also seem to be examples of olefin insertion reactions of metal-carbon compounds. Despite great effort concerning the mechanism of the polymerization reactions, relatively little has been learned about the actual catalytic species involved. 

Recently, Spange et al. (19,20) have successfully achieved cationic graft polymerizations of vinyl ethers, vinyl furan, and cyclopentadiene onto silica, initiated by a stable ion pair formed from silanol and aiylmethyl halide, such as di(p-methoxy-phenyl)methyl chloride. The grafting of the polymer onto silica is proposed to take place via the propagation based on olefin insertion to a cation center being in a rapid equilibrium with the ion pair, as shown in Scheme 12.1.3. 

The temperature-dependent stereoregular MAO-promoted polymerization of a-olefins502 has been explained by /i-hydrogen interactions in the olefin insertion and formation of a six-membered — 0 —Cp—D — Al—O—A1 ring TS. The stereoselective isotactic product formation occurs as a result of the substituent orientation at the -carbon (R1 vs CH2CHR2R4 in the conformationally restricted 426 equation 255). 

This linear polymerization represents one unusual case of diazoacetophenone oxidation. For instance, upon the action of copper oxide, diazoacetophenone gives the ketocar-bene, which is involved in typical carbene reactions—dimerization, addition to olefins, insertion in the O—H bonds of alcohols, etc. If the amine cation radical is used as an oxidant instead of copper oxide, only the polymer is formed. The ketocarbene was not observed, despite careful search (Jones 1981). 

The involvement of the a-elimination reaction in this cycle has been in question following experiments on cyclopentadienyl cobalt complexes, where evidence for olefin insertion for Ziegler-Natta polymerization catalysis has been obtained by labelling experiments, using C2H4 with a deuterated cobalt complex (70) ... 

Figure 3 Propylene polymerization by successive olefin insertions into metal-alkyl bonds.

In other cases, for example in syndiotactic propylene polymerization with certain unbridged 0,N-chelated Ti complexes, chain-end chirality can control the stereochemistry of 2,1-olefin insertions by adjusting the chirality of a ligand framework so as to minimize mutual repulsions [P. Corradini, G. Guerra, L. Cavallo, Acc. Chem. Res. 2004, 37, 231]. 

Comparative data of the mechanism of polymerization on active centers containing non-transition metals. Olefin insertion into metal-carbon bond is also a key step not only for catalytic polymerization but also for olefin oligomerization with organoaluminium compounds. The latter process includes the same steps as the catalytic polymerization in the presence of transition metal compounds... 

The catalytic oligomerization of olefins in the presence of OAC and the olefin polymerization in the presence of transition metals are based on similar olefin insertions into the metal-carbon and metal-hydrogen bonds (see Section 3.2). However, in organoaluminium compounds, the structure of the active center is defined more simply and more reliably. Data on its coordination state, thermodynamic and kinetic parameters have been reported (e.g. Table 13). 

C5Mes)2Lu(R) complexes undergo all of the key reactions involved in olefin polymerization including olefin insertion (chain propagation), )7-H and ) -alkyl elimination, and Lu—R bond hydrogenolysis (chain transfer/ termination) (23). 

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