Copolymerisation ethene

As said in the introduction there are many more polymers than can be discussed within the limits of this chapter, but we want to add just one example of a group of high-value polymers that is made using the same principles of coordination polymerisation as shown above for the commodity polymers. We mentioned metallocene catalysts that can be used to copolymerise ethene and norbomene to give Topas type products. 

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... 

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. 

Hereafter, the factors ruling the activity and selectivity of Pd(II)-phosphine catalysts for the carbonylation of ethene in MeOH are presented. In order to make the exposition clearer some of the concepts already discussed in other reviews will be shortly resumed. It will deal first with copolymerisation because it includes more general aspects, several of which are involved also in the catalysis to monocarbonylated non-polymeric products. The literature search covers all up to 2004. 

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. 

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]. 

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. 

The overall energy barrier for chain growing is lower than that for chain termination, thus, if a lower molecular weight product is desired, the copolymerisation should be carried out at higher temperature. In the copolymerisation process, the insertion of ethene is the slow step [13-15], thus upon increasing the pressure of the olefin, as well as the total pressure, it is reasonable to expect an increase in the molecular weight. The nature of the... 

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-... 

Another way to recover the catalyst from the dormant site is the copolymerisation of ethene, but this is slower and less attractive than the use of hydrogen. Furthermore the use of ethylene inevitably results in the formation of propylene-ethylene copolymers with all the consequent effects on polymer properties. 

Highly branched ethene-methyl acrylate polymers. The cationic palladium diimine complexes are remarkably tolerant towards functional groups, although the rates decrease somewhat when polar molecules are added. In ETM catalysis addition of polar molecules or monomers kills the catalyst and therefore it was very interesting to see what the new palladium catalysts would do in the presence of polar monomers. Indeed, using methyl acrylate a copolymerisation... 

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... 

A number of ex situ spectroscopic techniques, multinuclear NMR, IR, EXAFS, UV-vis, have contributed to rationalise the overall mechanism of the copolymerisation as well as specific aspects related to the nature of the unsaturated monomer (ethene, 1-alkenes, vinyl aromatics, cyclic alkenes, allenes). Valuable information on the initiation, propagation and termination steps has been provided by end-group analysis of the polyketone products, by labelling experiments of the catalyst precursors and solvents either with deuterated compounds or with easily identifiable functional groups, by X-ray diffraction analysis of precursors, model compounds and products, and by kinetic and thermodynamic studies of model reactions. The structure of some catalysis resting states and several catalyst deactivation paths have been traced. There is little doubt, however, that the most spectacular mechanistic breakthroughs have been obtained from in situ spectroscopic studies. 

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. 

Replacing MeOH with water does not significantly affect the catalytic cycle of copolymerisation, which involves the usual steps of insertion of ethene into... 

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

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]. 

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

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]. 

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 ...

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-... 

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