Polymerization transition metal-carbon compounds

Pi and Sigma Transition Metal Carbon Compounds as Catalysts for the Polymerization of Vinyl Monomers and Olefins... 

Ballard DGH (1973) n and n transition metal carbon compounds as catalysts for the polymerization of vinyl monomers and olefins. Adv Catal 23 263... 

The choice of a labeled compound, able to react with the active transition metal-carbon bond. This compound should have an inhibiting effect strong enough to result in completely stopping polymerization on its addition in a quantity comparable with that of the transition metal compound in the polymerization system. 

The specific behavior of surface compounds, being the propagation centers of polymerization catalysts, are mainly determined by two of their features the coordinative insufficiency of the transition metal ion and the presence of the transition metal-carbon bond. 

Unfortunately, at present the information characterizing the properties of the active bond in polymerization catalysts is very scant. The analogy between the features of the active bonds in the propagation centers and those of the transition metal-carbon bond in individual organometallic compounds is sure to exist, but as in the initial form the latter do not show catalytic activity in olefin polymerization this analogy is restricted to its limits. 

The processes of reversible adsorption of the coordination" inhibitors (including the adsorption of organometallic compounds) result in an increase in the lifetime of the transition metal-carbon bond. It is possible that due to this, in the case of propylene polymerization by two-component catalysts based on TiCU, at low temperatures a long-term increase of molecular weight with time was observed (192,193). 

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 has been shown (p. 266) that transition metal alkyl compounds containing Cpd and C6H6 groups, ir-bonded to the metal inactivate the metal center for polymerization. It has also been shown by Nyholm and Aresta (45), in the platinum series, that five- or six-membered rings containing only sigma and ir-carbon-to-metal bonds are very stable compounds. These observations add chemical plausibility to reaction (29). 

Combustion of transition metal organometallic compounds produces a mixtures of simple compounds (metal oxides, carbon oxides, water, nitrogen) which is subject to exact analysis. Thermal decomposition or high temperature iodination of the same compounds cannot necessarily be expected to produce simple materials, with the result that identification is often a difficult problem. This is typified by diene derivatives of iron carbonyl10, where side reactions of the dienes (e.g. polymerization) follow disruption of the iron-diene bonds. The oligomeric mixture can be parti-... 

Many transition metals and their compounds with organic ligands initiate the polymerization of alkenes and/or dienes. Some of them do not need any special treatment to this end while others require the presence of some organic or mineral compound or a special physical modification. In contrast to ZN catalysts, they are active without an organometal of Groups I—III. They are commonly known as metal alkyl free (MAF) catalysts. Many of their features are, of course, in common with ZN catalysts. MAF catalysts initiate stereoselectively controlled polymerization. Even less is known of their operating mechanism than that of ZN catalysts. It is assumed that propagation also occurs on the transition metal-carbon bond. 

The propagation reaction is the main step of catalytic polymerization. The study of its mechanism implies the elucidation of its elementary steps including the determination of the rate-determining step. The most important properties of AC, necessary for the reaction to proceed include i) the presence of a metal-carbon bond ii) coordinative unsaturation of a metal ion in the AC. Here, we discuss the data on the mechanism of the propagation reaction with respects to the AC containing the transition metal-carbon bond and also for organoaluminium compounds, i.e. when the AC contains a non-transition metal carbon bond. 

The essential characteristic of Ziegler-Natta catalysis is the polymerization of an olefin or diene, using a combination of a transition metal compound and a base metal alkyl cocatalyst, normally an aluminum alkyl. The function of the cocatalyst is to alkylate the transition metal, generating a transition metal-carbon... 

Several routes to transition metal-carbon bond exist. Transition metal carbon bonds may be generated by alkylation of a transition metal compound with a metal alkyl. Low-valent transition metal compounds, per se, that is, TiCl2i may function as catalysts. Reduction by the olefin that occurs with the Cr02/Si02 catalyst may also provide sites for polymerization. Finally, transition metal compounds in solution (3A) or supported may function as polymerization sites. 

The 1,2-insertion of alkenes into transition metal-carbon o-bond leads to C-C bond formation under mild conditions, as described in Chapter 6. This reaction is considered to be a crucial step in the coordination polymerization and carbometalation of alkenes catalyzed by transition metal complexes. A common and important carbometalation is the Heck-type arylation or vinylation of alkene catalyzed by Pd complexes [118], The arylation of alkene, most typically, involves the formation of arylpalladium species and insertion of alkene into the Pd-aryl bond as shown in Scheme 5.20. The arylpalladium species is formed by the oxidative addition of aryl halides to Pd(0) complexes or the transmetalation of aryl compounds of main group metals with Pd(II) complexes. Insertion of alkene into the Pd-aryl bond produces 2-arylalkylpalladium species whose y6-hydrogen elimination leads to the arylalkene. Oxidative chlorination of the 2-arylalkylpalladium intermediate forms chloroalkanes as the product. 

Most of transmetalation between main group metal compounds and transition metal complexes leads to the formation of a transition metal-carbon bond. The reaction which causes alkyl or aryl ligand transfer from transition metal to main group element is much less common. Olefin polymerization catalyzed by a metallocene catalyst is sometimes accompanied by chain transfer caused by the transfer of the growing polymer end from Ti or Zr to an A1 compound that is used as the cocatalyst (Scheme 5.23) [139,140]. 

It should be noted that the monomer coordination step shown in Eq. (2.82) may not be a distinct step as discussed previously. An important feature of this mechanism which affects the stereospecificity of olefin polymerizations using these types of soluble catalysts is the fact that the insertion of the monomer into the transition metal-carbon bond involves a secondary insertion reaction, i.e., the more substituted carbon of the double bond in the monomer becomes bonded to the transition metal (Corradini et al., 1985). In contrast, a primary insertion mechanism to form a transition metal bond to the less substituted carbon on the double bond of the monomer Ti-CH2CHR-P is involved in polymerizations using typical heterogeneous catalysts, e.g., from titanium halides and alkylaluminum compounds (Boor, 1979). 

Polymer Chain Growth. The essential characteristic of Ziegler-Natta catalysis is the polymerization of an olefin or diene using a combination of a transition-metal compound and a base-metal alkyl cocatalyst, normally an aluminum alkyl. The function of the cocatalyst is to alkylate the transition metal, generating a transition-metal-carbon bond. It is also essential that the active center contains a coordination vacancy. Chain propagation takes place via the Cossee-Arlman mechanism (23), in which coordination of the olefin at the vacant coordination site is followed by chain migratory insertion into the metal-carbon bond, as illustrated in Figure 1. 

Dithiocarbamates can also be prepared from diamines. For example, as early as 1872, Hofmann (97) reported the reaction of carbon disulfide with 1,2-diaminoethane. Later in the 1960s, addition of 2 equiv of sodium hydroxide and carbon disulfide to 1,2-diaminoethane was shown to afford the bis(dithiocarba-mate) compound in an exothermic reaction (98-102) 1,6-diaminohexane behaving in a similar manner (98). A wide range of polymeric transition metal complexes have been prepared using these salts (98). All are insoluble in water and common organic solvents, and some examples have been developed as fungicides (14, 31). 

Other miscellaneous compounds that have been used as inhibitors are sulfur and certain sulfur compounds (qv), picryUiydrazyl derivatives, carbon black, and a number of soluble transition-metal salts (151). Both inhibition and acceleration have been reported for styrene polymerized in the presence of oxygen. The complexity of this system has been clearly demonstrated (152). The key reaction is the alternating copolymerization of styrene with oxygen to produce a polyperoxide, which at above 100°C decomposes to initiating alkoxy radicals. Therefore, depending on the temperature, oxygen can inhibit or accelerate the rate of polymerization. 

Olefin polymerization by catalysts based on transition metal halogenides is usually designated as coordinated anionic, after Natta (194). It is believed that the active metal-carbon bond in Ziegler-Natta catalysts is polarized following the type M+ - C. The polarization of the active metal-carbon bond should influence the route of its decomposition by some compounds ( polar-type inhibitors), e.g. by alcohols. When studying polymerization by Ziegler-Natta catalysts tritiated alcohols were used in many works to determine the number of metal-polymer bonds. However, as it was noted above (see Section IV), in two-component systems the polarization of the active bond cannot be judged by the results of the treatment of the system by alcohol, as the radioactivity of the polymer thus obtained results mainly from the decomposition of the aluminum-polymer bonds. 

---