Transition metal catalyzed polymerization

Ionic liquids have been widely used as solvents for transition metal-catalyzed reactions (see Chapter 5). They can act simply as the solvent or sometimes as a co-catalyst or catalyst activator. They are often used in biphasic systems, with the catalyst retained in the ionic liquid phase and the products separated in an organic solvent phase. 

The previous sections show that certain ionic liquids, namely the chloroalumi-nate(III) ionic liquids, are capable of acting both as catalyst and as solvent for the polymerization of certain olefins, although in a somewhat uncontrolled manner, and that other ionic liquids, namely the non-chloroaluminate(III) ionic liquids, are capable of acting as solvents for free radical polymerization processes. In attempts to carry out polymerization reactions in a more controlled manner, several studies have used dissolved transition metal catalysts in ambient-temperature ionic liquids and have investigated the compatibility of the catalyst towards a range of polymerization systems. 

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Ring-Opening Polymerization. As with most other inorganic polymers, ring-opening polymerization of cyclotetrasilanes has been used to make polysilanes (109,110). This method, however, has so far only been used for polymethylphenylsilane (eq. 12). Molecular weights (up to 100,000) are higher than from transition-metal catalyzed polymerization of primary silanes. 

Late Transition Metal-catalyzed Polymerization of Ethylene... 

ADMET is quite possibly the most flexible transition-metal-catalyzed polymerization route known to date. With the introduction of new, functionality-tolerant robust catalysts, the primary limitation of this chemistry involves the synthesis and cost of the diene monomer that is used. ADMET gives the chemist a powerful tool for the synthesis of polymers not easily accessible via other means, and in this chapter, we designate the key elements of ADMET. We detail the synthetic techniques required to perform this reaction and discuss the wide range of properties observed from the variety of polymers that can be synthesized. For example, branched and functionalized polymers produced by this route provide excellent models (after quantitative hydrogenation) for the study of many large-volume commercial copolymers, and the synthesis of reactive carbosilane polymers provides a flexible route to solvent-resistant elastomers with variable properties. Telechelic oligomers can also be made which offer an excellent means for polymer modification or incorporation into block copolymers. All of these examples illustrate the versatility of ADMET. 

Cunningham, M. Dumas, C. Dusseault, J.J.A. Hsu, C.C. International Symposium on Transition Metal Catalyzed Polymerizations R.P. Quirk, Ed. in press. 

Fig. 5. Mechanism of the transition metal-catalyzed polymerization of a silacyclobutane.

In 1992/1994, Grubbs et al. [29] and MacDiarmid et al. [30] described an improved precursor route to high molecular weight, structurally regular PPP 1, by transition metal-catalyzed polymerization, of the cyclohexa-1,3-diene derivative 14 to a stereoregular precursor polymer 16. The final step of the reaction sequence is the thermal, acid-catalyzed elimination of acetic acid, to convert 16 into PPP 1. They obtained unsupported PPP films of a definite structure, which were, however, badly contaminated with large amounts of polyphosphoric acid. 

Quirk RP (ed) (1988) Transition metal catalyzed polymerization Ziegler-Natta and metathesis polymerizations. Cambridge University Press, Cambridge... 

Polycondensation pol5mers, like polyesters or polyamides, are obtained by condensation reactions of monomers, which entail elimination of small molecules (e.g. water or a hydrogen halide), usually under acid/ base catalysis conditions. Polyolefins and polyacrylates are typical polyaddition products, which can be obtained by radical, ionic and transition metal catalyzed polymerization. The process usually requires an initiator (a radical precursor, a salt, electromagnetic radiation) or a catalyst (a transition metal). Cross-linked polyaddition pol5mers have been almost exclusively used so far as catalytic supports, in academic research, with few exceptions (for examples of metal catalysts on polyamides see Ref. [95-98]). 

Synthesis of Block Copolymers by Transition Metal-Catalyzed Polymerization... 

Zinc compounds have recently been used as pre-catalysts for the polymerization of lactides and the co-polymerization of epoxides and carbon dioxide (see Sections 2.06.8-2.06.12). The active catalysts in these reactions are not organozinc compounds, but their protonolyzed products. A few well-defined organozinc compounds, however, have been used as co-catalysts and chain-transfer reagents in the transition metal-catalyzed polymerization of olefins. 

The transition metal-catalyzed polymerization of olefins yields high molecular weight polymers as the result of the successive insertion of monomer into the metal-carbon bond of the growing polymer chain. This chain growth is... 

The basic assumptions common to most mechanism studies relative to transition metal catalyzed polymerizations are as follows (i) The mechanism is essentially monometallic and the active center is a transition metal-carbon bond.13-15,18,19 (ii) The mechanism is in two stages coordination of the olefin to the catalytic site followed by insertion into the metal-carbon bond through a cis opening of the olefin double bond.13,20,21... 

Finally, when deahng with transition metal-catalyzed polymerizations, the efficiency of metal removal from the monolith after polymerization needs to be addressed. Investigations revealed that the remaining ruthenium concentrations... 

At lower temperatures (or in solution) and at high monomer concentration, a second chain termination process that could occur is direct j -hydrogen transfer to a second molecule of monomer. This kind of chain transfer step is now generally accepted for many transition-metal-catalyzed polymerizations, where direct /1-elimination would be too much uphill to explain the observed molecular weights, for olefin oligomerization at aluminium, a similar situation applies. Since insertion and j -hydrogen transfer have an identical concentration dependence, their ratio does not depend much on the reaction conditions (except temperature) and hence limits the molecular weight attainable in the Aufbau reaction. 

Anions, too, are not always innocent. The borate anion B(C6F5)4 is very popular in early-transition-metal catalyzed polymerization, where it acts as a rather inert and non-coordinating anion. Examples of decomposition of the anion by, e.g., CeFs transfer exist but are not very common. In aluminium chemistry, transfer of CeFs groups from B(C6Fs)3, MeB(C6F5)3 and B(C6F5)4 to the metal appears to be rather easy [14, 15], and it may be that other, even more innocent anions will be required here. 

Transition metal-catalyzed polymerizations have been reported to give high-purity, regioregular poly-3-alkylthio-phenes suitable for use in electronic devices. Thus, the Negishi protocol using a nickel catalyst and Rieke zinc was employed for the polymerization of 63 to produce the product as a low molecular weight polymer <2003CC2548>. 

Dehydrogenative Coupling. Transition-metal catalyzed polymerization of silanes appears to hold promise as a viable route to polysilanes. A number of transition-metal complexes have been investigated, with titanium and zirconium complexes being the most promising (105—108). Only primary silanes are active toward polymerization, and molecular weights are rather low. The dehydrogenative polymerization is depicted in reaction 11, where Cp = cyclopentadienyl ... 

Quirk, R. P. (ed.) Transition Metal Catalyzed Polymerizations, Part A and B, New York, Harwood Academic Publ., 1983... 

Kaminsky W (1983) In Quirk RP (ed) Transition metal catalyzed polymerizations -alkenes and dienes, vol 4. Harwood, New York, p 225... 

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