Polyketone Formation

The change of selectivity from alkoxycarbonylation to oligomerization or polymerization when changing from monophosphines to chelating diphosphines was first rationalized in terms of a bite angle effect [33]. With monophosphines, a trans orientation of the phosphine ligands is more stable for the acyl or alkyl species. Therefore, immediately after an insertion, a cis-trans isomerization occurs. The new species formed opposes further insertions and chain growth. Thus, the acyl-palladium species will eventually terminate by alcoholysis of the Pd-acyl bond. 

Ligand Pn (° ) Product H(CH2CH2C0) 0CH3 Reaction rate (gg Pdh ) 

However, when diphosphines are used, in which the phosphorus donor atoms are always cis to one another (all the ligands assayed were cis coordinating), the growing chain and monomer are in cis positions as well - a prerequisite for insertion reactions. As a result, diphosphines vhth natural bite angles close to 90° (dppp) stabilize the transition state for insertion reactions (chain growth), explaining also the higher activity and polymer selectivity of dppp when compared to monophosphines. 

The trend for the bidentates in Table 1.4 together with those of other series of diphosphine ligands [35] will be discussed below. Later, this explanation for the difference between mono- and diphosphines has been reconsidered. 

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Since 1985, several thousands of publications have appeared on complexes that are active as catalysts in the addition of carbon monoxide in reactions such as carbonylation of alcohols, hydroformylation, isocyanate formation, polyketone formation, etc. It will therefore be impossible within the scope of this chapter to review all these reports. In many instances we will refer to recent review articles and discuss only the results of the last few years. Second, we will focus on those reports that have made use explicitly of coordination complexes, rather than in situ prepared catalysts. Work not containing identified complexes but related to publications discussing well-defined complexes is often mentioned by their reference only. Metal salts used as precursors on inorganic supports are often less well defined and most reports on these will not be mentioned. 

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

Figure 26 Proposed steps in Pdfllj-catalyzed polyketone formation from ethylene and CO in MeOH.

Carbon monoxide insertion in a palladium-carbon bond is a fairly common reaction [21]. Under polymerization conditions, CO insertion is thought to be rapid and reversible. Olefin insertion in a palladium-carbon bond is a less common reaction, but recent studies involving cationic palladium-diphosphine and -bipyridyl complexes have shown that olefin insertion also, particularly in palladium-acyl bonds, appears to be a facile reaction [22], Nevertheless, it is likely that olefin insertion is the slowest (rate-determining) and irreversible step vide infra) in polyketone formation. 

At low temperatures (below —85 °C), the majority of the product molecules are keto-esters, with only small but balancing quantities of diesters and diketones. At higher temperatures, the same product molecules are produced in a 2 3 4 ratio close to 2 1 1. These observations have been explained [13] by assuming two initiation and two termination mechanisms for polyketone formation. 

Scheme 5. Competition between CO and ethene coordination in polyketone formation.

However, there is also a major difference between the two types of catalysts. The olefin polymerization metallocene catalysts (cf. Section 2.3.1.1) are much more electrophilic, due to the higher positive charge of the metal ion, than the pal-ladium(II) complexes discussed above. For polyketone formation, electrophilicity needs to be balanced so that olefins can still compete with carbon monoxide for coordination to the metal cation. 

Although the basic principles of polyketone formation are now reasonably well understood, further studies, both of polymerization characteristics and of the elementary steps underlying polyketone catalysis, will be needed to exploit fully the potential of these selective polymerizations. 

After CO insertion, CO and C2H4 compete for adsorption on the Pd-center. Although the adsorption equilibrium clearly favors the adsorption of CO, this does not lead to insertion in the polymer, as this reaction step, sketched in Figure 6.31a appears to be strongly endothermic. The ethylene insertion step (Figure 6.31b), however, is thermodynamically favored. Thus, selective polyketon formation by... 

N. M. (2007) Ester versus polyketone formation in the palladium-diphosphine catalyzed carbonylation of ethene. 

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