Poly organometallic catalysts

The concept of transesterifications was used for polymerization reactions by Hedrick and colleagues [76]. Various biodegradable polyesters were synthesized with the l,3-dimethylimidazol-2-ylidene carbene in THF at 25 °C. Polymers such as poly(e-caprolactone) were obtained with no need of organometallic catalysts, as in classical methods. Poly(ethylene terephthalate) (PET) 97 was synthesized in the ionic liquid 98, which functions as the reaction medium and, at the same time, as a precatalyst that is activated (99) with KOt-Bu. Dimethyl terephthalate (DMT) 100 was condensed with an excess of ethylene glycol 101 to generate 102. The melt condensation of 102 was performed under vacuum using a heating ramp to 280 °C. 

For instance, in the last decade synthesis of poly(ester-alt-ether) was intensively studied. A common enzyme used in these syntheses is CALB. Polymerization of l,5-dioxepan-2-one (DXO) was performed by enzyme-catalyzed ROP in order to avoid contamination of product polymers by toxic organometallic catalysts [92], High molecular weight of poly(DXO) was obtained (Mn = 56000 Mw = 112000, 97% yield) at 60 °C for 4h. The polymerization had the characteristics of a living polymerization, as indicated by the linearity of plots between M and monomer conversion, meaning that the product molecular weight could be controlled by the stoichiometry of the reactants. Similarly, Nishida et al. [91] carried out enzymatic ROP of l,4-dioxan-2-one at 60 °C catalyzed by Novozym 435 that resulted in a polymer with Mw = 41000 in 77% yield. 

The first synthesis of a soluble and processible poly(2,7-fluorene) was reported by Yoshino and coworkers in 1989 [44,45]. Their approach involved coupling fluorene monomers with substituents at the C9 position by chemical oxidation with FeCls (see P3 in Scheme 1.2). However, the polymerization process was not regiospecific and the polymer backbone contained structural defects that influence the electronic delocalization. Thus, the synthesis of defect-free poly(fluorene)s became a major challenge and advances in carbon-carbon bond forming reactions promoted by organometallic catalysts provided the... 

Copolymers of p-dioxanone and alkylene oxides have also been prepared (Bezwada and Shalaby, 1991) (Figure 11). These crystalline copolymers can be either blocky or branched. Diblock (AB) and triblock copohaners (ABA) are derived from monofunctional and difunctional poly(alkylene oxides), respectively, while branched copolymers, are formed from pohdunctional poly(alkylene oxides). Polymerization occurs by reacting p-dioxanone with the desired type and amount of poly(alkylene oxide) in the presence of an organometallic catalyst at elevated temperatures. 

Therefore It seems to be interesting to study such effects in the selective hydrogenation of a poly-unsaturated molecule as carvone. It was reported that the partial hydrogenation of this molecule is very sensitive to different homogeneous catalysts organometallic compounds (refs. 8-10), Zn/OH (ref. 11), NaBH (ref.12), and Zn-NiCl (ref. 13) as examples. 

It is an axiom of modern organometallic chemistry that the pursuit of late transition metal complexes is ultimately driven by the need to formulate ever more efficient catalysts and reagents for chemical synthesis. In this respect, the field of poly(pyrazolyl)borate chemistry is no different from any other, albeit that in the case of the group 10 triad the breadth of study is perhaps more limited than for other metals and/or ligands. This section provides an overview of prominent results in respect of both catalysis and the C—H activation processes that underpin them. 

Mu, Y. Harrod, J. F. Synthesis of Poly(methylsilyene) by Catalytic Dehydrocoupling with Cp2MMe2 (M = Ti, Zr) Catalysts, (J. F. Harrod, R. M. Laine, Eds.) Inorganic and Organometallic Oligomers and Polymers, Kluwer Academic Publishers, Netherlands, 1991. p. 23. 

A final example of a stereoselective heterogeneous catalytic system is the work of Laycock, Collacott, Skelton and Tchir.17 Layered double hydroxide (LDH) synthetic hydrotalcite materials were used to stereospecifically polymerize propylene oxide [PO] to crystalline isotactic and liquid atactic poly(propyleneoxide) [PPO]. These authors suggest that the LDH surface acts as other inorganic or organometallic coordination initiators or catalysts by providing specific surface orientations for propylene oxide monomer. X-ray powder diffraction showed some loss of crystallinity after calcination and X-ray photoelectron spectroscopy showed an enhancement of Mg/Al content due to restructuring of the Mg and A1 surface atoms. The surface was also rich in Cl ... 

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