Polymerization metal-catalyzed

In all these cases and in contrast to starting a polymerization by initiators, there are no fragments of the starting molecule incorporated in the polymer chain. Consequently, the mechanisms are different to those of radical, anionic, or cationic polymerizations. 

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 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  

Pd(PPh3)2Cl2 [Stille poly condensation] Pd(PPli3)4 [SuzuM polycondensation] 

FIGURE 3.21 Synthesis of polyarylenes via C—C coupling by A +B Stille and Suzuki polycondensation (Mj, Mj=arene like phenyl). 

Furthermore, a number step growth metal-catalyzed C—C combining reactions like Stille, Kumada, or Suzuki coupling, Heck and Sonogashira-Hagihara reactions, a.o., are employed especially for the preparation of highly aromatic and conjugated polymers (see Fig. 3.21). 

An example of C—C coupling that shows characteristics of chain growth for the preparation of conjugated polymers will be given in Section 3.2.5 (Fig. 3.26). 

---

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... 

With no sufficient hydrogen present, the molecules get stuck on the surface. Owing to purely statistical reasons (Scheme I), this is more probable in an elongated position. Such molecules may combine with each other to give high molecular weight polymers ( coke ). Metal-catalyzed polymerization has actually been observed with lower molecular weight hydrocarbons (61). Such reactions are responsible for more rapid deactivation of the catalyst by trans isomers (Table III). 

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... 

---