Polyketones alternating

Palladium(II) complexes possessing bidentate ligands are known to efficiently catalyze the copolymerization of olefins with carbon monoxide to form polyketones.594-596 Sulfur dioxide is an attractive monomer for catalytic copolymerizations with olefins since S02, like CO, is known to undergo facile insertion reactions into a variety of transition metal-alkyl bonds. Indeed, Drent has patented alternating copolymerization of ethylene with S02 using various palladium(II) complexes.597 In 1998, Sen and coworkers also reported that [(dppp)PdMe(NCMe)]BF4 was an effective catalyst for the copolymerization of S02 with ethylene, propylene, and cyclopentene.598 There is a report of the insertion reactions of S02 into PdII-methyl bonds and the attempted spectroscopic detection of the copolymerization of ethylene and S02.599... 

Drent, E. van Broekhoven, J. A. M. Doyle, M. J. Wong, P. K. Palladium Catalyzed Copolymerization of Carbon Monoxide with Alkenes to Alternating Polyketones and Polyspiroketals. Fink, G. Muelhaupt, R. Brintzinger, H. H. Eds. Ziegler Catal. Springer, Berlin, 1995, pp 481 496. 

The copolymerization of carbon monoxide and a-olefins is one of the most challenging problems in polymer synthesis. Sen and his coworkers discovered that some cationic palladium compounds catalyze this alternative copolymerization, giving polyketones (Eq. 13). 

The discovery in the early 1980s that cationic palladium-phosphine complexes catalyse the copolymerisation of carbon monoxide with ethene or a higher a-olcfin to yield perfectly alternating polyketones has since attracted continuous increasing interest [1,2]. This is because the monomers are produced in large amounts at a low cost and because polyketones represent a new class of thermoplastics of physical-mechanical and chemical properties that have wide applications [3-6]. In addition, easy functionalisation can open the way to a large number of new materials [7]. The copolymerisation has... 

Although sequential B-oxidation from the carboxyl end of fatty acids was believed to be the mechanism for their breakdown, other schemes had been proposed, notably by Hurtley in 1915, who suggested multiple alternate oxidation—this idea was not widely accepted because the probable intermediates, polyketonic or polyunsaturated fatty acids, had never been detected. The abnormally high levels of acetoacetate produced by various liver preparations, however, caused multiple... 

In this chapter we will discuss some aspects of the carbonylation catalysis with the use of palladium catalysts. We will focus on the formation of polyketones consisting of alternating molecules of alkenes and carbon monoxide on the one hand, and esters that may form under the same conditions with the use of similar catalysts from alkenes, CO, and alcohols, on the other hand. As the potential production of polyketone and methyl propanoate obtained from ethene/CO have received a lot of industrial attention we will concentrate on these two products (for a recent monograph on this chemistry see reference [1]). The elementary reactions involved are the same formation of an initiating species, insertion reactions of CO and ethene, and a termination reaction. Multiple alternating (1 1) insertions will lead to polymers or oligomers whereas a stoichiometry of 1 1 1 for CO, ethene, and alcohol leads to an ester. 

The most common alkenes employed in the Pd-catalysed synthesis of alternating polyketones are ethene, styrene, propene and cyclic alkenes such as norbomene and norbornadiene. Even though the mechanism does not vary substantially with the alkene, the reactions of the various co-monomers are here reported and commented on separately, starting with the ethene/CO copolymerisation, which is still the most studied process. As a general scheme, the proposed catalytic cycles are presented first, then the spectroscopic experiments that have allowed one to elucidate each single mechanistic step. 

Besides proving the formation of p-chelates [Pd(CH7CH7C(0)Me)(P-P)] at room temperature, the spectra showed the occurrence of chain-transfer by protonolysis with adventitious water to give the p-hydroxo compounds cis/trans [Pd(p-OH)(P-P)]2 as well as the conversion of the latter compounds into cis/trans bis-chelates [Pd(P-P)2] (Chart 7.2) [5f]. Independent experiments with isolated compounds showed that the p-OH and bis-chelate complexes are not dead ends, and can reenter the catalysis cycle to give alternating polyketones. 

Alternating co-polymerization of olefins with carbon monoxide (CO), typically by using Pd-based catalysts, produces 1,4-polyketones (y-polyketones) that exhibit unique material properties (e.g., high crystallinity, excellent mechanical properties, and high chemical resistance). Whereas Shell suspended their efforts to commercialize these polyketones. 

Strictly speaking, condensation polymers such as polyesters could be considered to belong to this category. Also polyketone ( Carilon ) would be a strictly alternating copolymer, if the regularity would not have been disturbed by a third comonomer, propylene. 

Palladium catalysts with simple monodentate phosphine ligands (e.g. PPh3) can catalyze the methoxycarbonylation of ethylene. However, the Lucite process employs a bulky diphosphine, 1,2-( Bu2PCH2)2CgH4, and is highly active and selective under quite mild conditions (10 bar/80°C). Two alternative catalytic cycles are possible, based either upon a palladium hydride or a palladium methoxide complex (Figure 9), and mechanistic and spectroscopic studies indicate that the hydride cycle is dominant. The alkene and CO insertion steps are the same as those in the Pd-catalyzed co-polymerisation of CO and alkenes to polyketones (Section 4.4). 

Figure 10 Chain growth steps for alternating COjethylene co-polymerization in polyketone formation.

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