Ethene/CO copolymerisation

From what is reported above, it is evident that the CO-ethene copolymerisation and the methoxycarbonylation of ethene are closely related. In principle the mechanisms discussed for the copolymerisation process are valid also for the case when termination occurs after the insertion of just one molecule of each monomer into the species that initiate the catalysis, Pd-OCH3+ or Pd - H+. These species can form as schematized by Eqs. 10-16. The copoly-... 

In CH2CI2, where the reaction of the bis-chelate complex (1) with Pd(OAc)2 to give (2) is much faster than in MeOH (Figure 7.5), no significant difference between dppe or dppp Pd" catalysts has been observed in CO/ethene copolymerisation [5e,f]. 

The P H HP NMR picture of CO/ethene copolymerisation in MeOH is exemplified by the sequence of variable-temperature spectra shown in Figure 7.6 relative to a reaction catalysed by [Pd(TFA)2(dppp)]. It has been observed that the intensity of the P NMR signal decreases with time, which is apparently due to the irreversible reductive degradation of Pd" species to Pd metal [5b, c]. 

In Situ High Pressure NMR Studies of CO/Ethene Copolymerisation in Aprotic Solvents... 

Several types of bidentate ligands, different from diphosphines, for example bipyridines and phenantrolines, have been proven to give active catalysts, particularly in the CO-styrene copolymerisation [25], but, particularly with ethene, diphosphines give higher performances. 

Thus, it has been found that H20 and TsOH have a beneficial effect on the catalytic system Pd(AcO)2/dppp/TsOH, first reported by Drent, as the copolymerisation rate significantly increases (with respect to the use of anhydrous MeOH) about five times and passes through a maximum in the presence of ca. 1000 ppm of H20 and when Pd/TsOH = 1/8 (ca. 12 000 g poly-mer(gPd h)-1 at 90 °C, 60 bar, CO/ethene = 1/1) [66]. 

Over the last ten years, several model studies of the ethene/CO copolymerisation have been carried out, aimed at investigating the reactions of isolated compounds with CO, ethene and other reactive components of the catalytic mixtures. Due to space limitations and the presence in the literature of excellent reviews on this sub-... 

The most active and selective catalysts for both the copolymerisation process and for the apparently simpler ethene carbonylation to monocarbonylated products MP or DEK are cationic square planar Pd(II) complexes in which the metal centre is czs-coordinated by a bidentate P - P ligand, by a Ugand involved in the initial step of the catalysis or in the process of forming the product and with the fourth vacant site coordinated by CO or ethene or a keto group of the growing chain or MeOH (or H2O, always present in the solvent even when not added on purpose) or even by a weakly coordinating anion. 

Of the reports that have appeared [37,72,80-90], only a few deal with more quantitative studies. In [86,89] the copolymerisation kinetics have been studied using the precursor [Pd(Ts0)(H20)(dppp)](Ts0) in MeOH over a temperature range of 70-90 °C and total pressure up to 70 bar. The rate increases linearly by increasing catalyst loading, the orders with respect to dissolved CO and ethene are 0.63 and 0.72, respectively the apparent activation energy is 11.7 kcal/mol. 

Alkenes and carbon monoxide are currently copolymerised in the presence of homogeneous Pd catalysts to give thermoplastic materials vith a perfectly alternating structure (Scheme 7.1a) [1, 2]. The non-perfect alternation of monomers (Scheme 7.1b) has been uniquely observed for ethene/CO copolymerisation reactions catalysed by Pd precursors vith anionic P-O ligands [3]. H NMR... 

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. 

Scheme 7.2 summarises the principal steps of the alternating ethene/CO copolymerisation in MeOH by Pd" catalysts modified with bidentate ligands [lb,c]. 

The factors that control the strictly alternating copolymer chain with no detectable errors (e. g., microstructures involving double insertion of ethene) have been the object of detailed studies since the discovery of the first Pd" catalysts for the alternating alkene/CO copolymerisation [11]. Sen was the first to demonstrate that double carbonylation is thermodynamically unfavorable and to suggest that the higher binding affinity of Pd" for CO relative to ethene inhibits multiple ethene insertions, even in the presence of very low concentrations of CO [12]. Therefore, once a palladium alkyl is formed, CO coordination ensures that the next monomer will be a CO molecule to generate the acyl complex. 

Formation of Active Pd" Sites and Initiation of Ethene/CO Copolymerisation... 

A quite similar picture has been reported for copolymerisation reactions catalysed by [Pd(TFA)2(Na2DPPPDS)] in water in the presence of an excess of TsOH (Figure 7.7) [5aj. Neither CO adducts nor ethene adducts were observed. Instead, a broad featureless resonance appeared at room temperature, which was assigned to several species containing trifluoroacetate, p-toluenesulfonate (the reaction was performed in the presence of a slight excess of TsOH), H2O, hydroxo and/or p-hy-droxo species, eventually in equilibrium with each other (Figure 7.7a). In contrast. 

Figure 7.9 Variable-temperature P H NMR study (sapphire tube, CD2CI2, 81.01 MHz) of ethene/CO copolymerisation (a) Dissolving [Pd(Me)(NCMe)(dppp)]PF6 in CD2CI2 under nitrogen at room temperature (b) after the tube was pressurized with 40 bar of ethene/CO (1 1) at room temperature (c) after 15 min at 50°C ...

Besides the isolation and characterisation of several catalytically relevant intermediates, model reaction studies, generally based on variable-temperature NMR experiments in CD2CI2, with isolated Pd" complexes have provided valuable kinetic and thermodynamic information on the mechanism of the alternating ethene/CO copolymerisation. 

Kinetic studies of migratory insertion reactions of the ligands that are involved as (P-P)Pd" fragments in either the propagation cycle of ethene/CO copolymerisation or ethene dimerisation to butenes have been reported by Brookhart [28] and Bian-chini [5e, fj. 

Using in situ NMR spectroscopy, Brookhart has also studied the activation barriers for the migratory insertion steps corresponding to chain growth in ethene/ CO copolymerisation catalysed by dppe-derived nickel(II) complexes [4a]. Activa-... 

In situ NMR analysis has also been used to determine the kinetic barriers for the migratory insertions of methyl carhonyl complexes [Pd CO) Me)(PPh2 CH2) PPh2)] (n = 2-4) relevant to propagation in ethene/CO copolymerisation. It was found that the steric bulk of the diphosphine has a significant effect on the insertion barriers with the most bulky ligand having the lowest barrier. 

Displacement of the chelate carbonyl from palladium by ethene has never been observed in model studies, which accounts for the virtual absence of double ethene insertions in actual copolymerisation reactions. Indeed, (5-chelate opening is actually brought about by CO to generate a six-membered metallacycle (y-chelate), while p-chelates of catalytically active systems generally react with CO to yield carbonyl acyl complexes, even at very low temperature. For the systems investigated by Bianchini [5e, f], the activation barriers for the conversion of the P-chelates... 

The discovery that the keto chelates (especially the p ones) are the species controlling the strict alternation of the monomers and the intrinsic copolymerisation rate in ethene/CO propagation has stimulated much research aimed at designing catalytic systems where the keto chelates can be readily opened by CO. These studies have allowed the development of a new generation of more efficient Pd" copolymerisation catalysts based on diphosphines with o-methoxyphenyl groups on the phosphorus atoms [13b, 31-34]. 

---