Polymerization allyl compounds

Recently some information became available on a new type of highly active one-component ethylene polymerization catalyst. This catalyst is prepared by supporting organometallic compounds of transition metals containing different types of organic ligands [e.g. benzyl compounds of titanium and zirconium 9a, 132), 7r-allyl compounds of various transition metals 8, 9a, 133), 7r-arene 134, 185) and 71-cyclopentadienyl 9, 136) complexes of chromium]. 

The application of these catalysts in the initial state (without any special treatment of the surface organometallic complexes of such cata-lysts) for ethylene polymerization has been described above. The catalysts formed by the reaction of 7r-allyl compounds with Si02 and AUOj were found to be active in the polymerization of butadiene as well (8, 142). The stereospecificity of the supported catalyst differed from that of the initial ir-allyl compounds. n-Allyl complexes of Mo and W supported on silica were found to be active in olefin disproportionation (142a). 

The choice of diallylphtalate as the cross-linker is somewhat surprising, because allylic compounds are not very reactive in radical polymerizations due to the stability of the allyl radicals. 

Wilke s allyl compounds were found to be very poor catalysts indeed, e.g., Ti(2 Me-allyl)4, was found only to have an activity equal to 0.5 gm/m.M Ti/atm/C2H4/hr. For this reason there has been considerable dispute that transition metal alkyls can be the intermediates in Ziegler polymerization. 

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 activity of transition metal allyl compounds for the polymerization of vinyl monomers has been studied by Ballard, Janes, and Medinger (10) and their results are summarized in Table II. Monomers that polymerize readily with anionic initiators, such as sodium or lithium alkyls, polymerize vigorously with allyl compounds typical of these are acrylonitrile, methyl methacrylate, and the diene isoprene. Vinyl acetate, vinyl chloride, ethyl acrylate, and allylic monomers do not respond to these initiators under the conditions given in Table II. 

Some transition metal ir-allyl compounds are not catalysts for polymerization. For example, Zr (allyl) 4 will not polymerize methyl methacrylate. Spectroscopic and other studies have shown that this allyl compound, unlike those of chromium, react with the carbonyl group of the monomer giving compounds of the type... 

Polymerization of Substituted Transition Metal Allyl Compounds Temperature 50°C, Ethylene Pressure Jfi aims, Ethylene. Catalyst Concentration 24 X 10 M... 

The compounds of the type L2L1Mn+[CH2M (CH3) ]n, where M is a transition metal, M is Si, Ge and Sn, and L2Li are ligands such as CO, 7r-Cpd, etc., have been synthesized by Collier, Lappert, and Truelock (22). These, like the x-allyl compounds, were inactive for polymerization (Table V). When the ligands CO and x-Cpd were removed, however, the isoleptic compounds Mn+[CH2M/(CH3) were found to be weak polymerization catalysts (16, 28). 

The results of polymerizing ethylene using varying sigma-bonded transition metal alkyl compounds are summarized in Table VII. It is evident that none of the catalysts are very active and are comparable with the simple allyl compounds listed in Table I. 

Sigma-bonded transition metal complexes are able to polymerize a range of vinyl monomers, the only limitation being that the monomer should not have groups that react chemically with the transition metal compound. An important observation is that styrene and its derivatives are polymerized by the sigma complexes. In this respect they differ from the jr-allyl compounds that show no reactivity at all toward these monomers. A reasonable explanation for this is that the mechanism of the initiation is different... 

Initial studies of the polymerization of propylene with transition metal allyl compounds suggested that this monomer could not be polymerized by any of the soluble catalysts available. Subsequent work (16) has revealed, however, that the propylene polymerization is much more susceptible to impurities, in particular traces of ether which compete with the monomer for the coordination sites. When this and other impurities are removed, weak activity is detected. These results are summarized in Table XIII. 

It has been suggested however that isotacticity derives from polymerization occurring on colloidal particles formed by thermal decomposition of the catalysts. As stated previously, in the presence of the monomer even the allyl compounds are stable at 65°C and none of the thermal decomposition products (black to yellow solids) could be detected. As a check on these results a polymerization of propylene was carried out with Zr (benzyl) 4 in toluene at 0°C in a sealed tube. The reaction was very slow and analytical quantities of polymer could be obtained only after 312 hr. NMR analysis showed peaks assignable to isotactic sequences, and these were much stronger than the peaks assignable to syndiotactic diads. It was concluded... 

Many recent publications have described the stereospecific polymerization of dienes by ir-allyl compounds derived from Cr, Nb, Ni, etc. Of particular interest is the work of Durand, Dawans, Teyssie who have shown that ir-allyl nickel catalysts (XXI) in the presence of certain additives polymerize butadiene stereospecifically (87, 38). The active center results from reaction of acidic additives with the transition metal. 

A detailed study of the mechanism of the insertion reaction of monomer between the metal-carbon bond requires quantitative information on the kinetics of the process. For this information to be meaningful, studies should be carried out on a homogeneous system. Whereas olefins and compounds such as Zr(benzyl)4 and Cr(2-Me-allyl)3, etc. are very soluble in hydrocarbon solvents, the polymers formed are crystalline and therefore insoluble below the melting temperature of the polyolefine formed. It is therefore not possible to use olefins for kinetic studies. Two completely homogeneous systems have been identified that can be used to study the polymerization quantitatively. These are the polymerization of styrene by Zr(benzyl)4 in toluene (16, 25) and the polymerization of methyl methacrylate by Cr(allyl)3 and Cr(2-Me-allyl)3 (12)- The latter system is unusual since esters normally react with transition metal allyl compounds (10) but a-methyl esters such as methyl methacrylate do not (p. 270) and the only product of reaction is polymethylmethacrylate. Also it has been shown with both systems that polymerization occurs without a change in the oxidation state of the metal. 

It is more difficult to study equilibria between transition metal allyl compounds and bases, olefins, etc. In the case of Zr (allyl) 4 and pyridine, a valency change occurs as shown by Eq. (8), and the process is irreversible. The polymerization is considered to be preceded by displacement of one allyl group by the monomer (12) as shown in Eq. (1). In the methyl methacrylate/Cr(allyl)3 system it was not possible to detect any interaction between the olefin and catalyst with infrared radiation, even with equimolar concentrations because of the strong absorption by the allyl groups not involved in the displacement processes. Due to the latter, evidence for equilibrium between monomer and catalyst is less likely to be found with these compounds than with the transition metal benzyl compounds. 

In addition to these supported transition-metal catalysts, some soluble transition-metal compounds exhibit considerable activity in polymerization without added aluminum alkyls.292 The most active compounds are a-organometallics of Ti and Zr with methyl, benzyl, and halogen ligands. jt-Allyl compounds of Ti, Zr, and Cr are also useful catalysts. 

The more cationic halogen containing compounds produced other products. Cobalt bis-allyliodide produced cis-polybutadiene and the even more cationic chromium, produced cyclododecatriene. Only with the more cationic system which introduced trans-structures, was cyclization and reduction of the metal able to intercept the polymerization reaction. Cyclization was not possible in the less cationic cobalt which produces all cis-polybutadiene nor was the hydride transfer possible with the less anionic chromium tris-allyl compound. 

Bartlett, P. D., and K. Nozaki Polymerization of allyl compounds. III. The peroxide-induced copolymerization of allyl acetate with maleic anhydride. J. Amer. chem. Soc. 68, 1495 (1946). 

In scientific and patent literature on diene polymerization the following Nd(III)-salts are most often cited as catalyst precursors halides, carboxylates, alcoholates, phosphates, phosphonates, allyl compounds, cyclopentadienyl derivatives, amides, boranes and acetylacetonates. The catalyst systems based on these Nd-sources are reviewed in the following subsections. 

Subsequent studies on BD polymerization by the solvent-free allyl compound Nd( ]3-C3H5)3 (without cocatalysts added) showed that—depending on monomer concentration—2 to 3 poly(butadiene) chains are generated per Nd-atom [291], By the addition of cocatalyst the number of chains generated per Nd-atom is reduced to one. 

It has to be mentioned that as early as in 1991 Porri et al. reported on the reaction of NdCl3 with Mg(C3H5)Cl in THF yielding an undefined Nd allyl compound which was successfully tested in diene polymerizations [141, 167,294]. For the polymerization experiments the cocatalysts TIBA, TMA, TIBAO and MAO were used and no halide donor was added. The undefined Nd allyl compound + TIBA yields a catalyst system that is reported to be at least three times more active than the system NdzO/TIBA/DEAC. The application of MAO with the Nd allyl compound increases catalytic activity 30-fold. 

Presently, the importance of Nd allyl compounds as intermediates in Nd carboxylate- and other Nd-based catalyst systems is widely accepted. As various Nd allyl compounds have been synthesized, characterized and successfully tested as polymerization catalysts this view is supported by solid experimental evidence (Sect. 2.1.1.5 and the references therein). Selected Nd allyl compounds exhibit significant polymerization activities without the addition of cocatalysts. In these cases the active species is neutral. But also cationic active Nd species are taken into consideration (Sect. 2.1.1.5) [288,291]. Cationic species also prevail in the presence of non-coordinating anions. 

In a detailed study Maiwald et al. used the Nd allyl complex Nd(rj3- 3115)3 for the polymerization of BD in toluene. A linear relationship between - ln(l - x) and polymerization time was obtained. An intercept at 10 to 12 min was attributed to an induction period [291]. Similar results were obtained for a variety of catalyst mixtures based on Nd allyl compounds [293]. 

The structure and chemical properties of metal-allyl compounds (ir-allylic, dynamic and a-allylic) which can be considered as models of a living polymer chain in butadiene polymerization have been studied. The polymerization of dienes proceeds only in dynamic allylic systems through the metal-ligand ir-bond in a-isomers. 

Butenyllithium and butenylmagnesium chloride were used as "dynamic allylic compounds. The former was selected because of the ability of lithium catalysts to provide high rates of diene polymerization and to give stereoregular polymers the latter was selected for its availability and simplicity of synthesis. 

The solutions are nonconducting, and the compounds display no tendency to undergo chemical reactions (hydrolysis, polymerization, etc.). However, as shown later, heating these compounds in the presence of donor reagents causes significant changes in most of their properties (NMR spectra, chemical activity, electrical conductivity, etc.), thus making them similar in properties to the typical dynamic allylic compounds. 

Butadiene Polymerization Initiated by Model Metal-Allylic Compounds... 

Metal allylic compounds, whose structure and reactions have been described in the preceding section, were used as catalysts for diene polymerization. 

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