Terpolymerization of propene and ethene with carbon monoxide catalyzed by [PdCl2(dppf)] in HCOOH–H2O as a solvent [dppf = 1,1′-bis(diphenylphosphino)ferrocene]
Research highlights
▶ [PdCl2(dppf)] efficiently catalyzes the propene–ethene–CO terpolymerization. ▶ The Pd(II) precursor is activated by the combined effects of H2O and HCOOH. ▶ H2O/HCOOH, propene concentration and reaction time influence the productivity. ▶ Polymer chain is formed mainly by ethene–CO and in low percent by propene–CO units.
Introduction
Pd(II)-chelating diphosphine complexes efficiently catalyze the strictly alternating co- and ter-polymerization of CO with aliphatic 1-olefins, providing access to a new family of engineering thermoplastics named polyketones (PK) [1], [2], [3].
In contrast to the intensive efforts devoted to ethene (E)–carbon monoxide (ECO) copolymerization, much less has been reported on terpolymerization of two olefins with carbon monoxide [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], although industries have shown more interest for terpolymers than for copolymers, as documented by the high number of patents in this field. This is because the ECO copolymer is a highly crystalline material with high melting point (Tm), whereas the terpolymers have lower melting points, making them more easily processable [13], [14], [15], [16], [17], [18], [19]. As matter of fact, the first commercialized PK, was the propene (P)–ethene–CO (PECO) terpolymer (trade name CARILON® by Shell [20] and KETONEX® by BP [21]). Currently, several industries are showing renovated keen interest in these terpolymers, searching for new catalytic systems more efficient and selective in particular for applications in the field of lubricants (oligomers) and/or in the field of fibers (high molecular weight). In general, for fiber production both high productivity of the catalyst and high average molecular weight of the polymer are required.
Among the main aspects that rule the Pd(II)-catalysis the nature of the chelating ligand, of the counter-anion and of the solvent plays a key role [1], [2], [3], [22], [23], [24], [25]. Methanol is the most used solvent in which case the counter-anions must be weakly coordinating in order to ensure high productivity.
We recently reported that by using reaction media such as H2O–MeOH, H2O–CH3COOH or H2O–HCOOH also Pd(II)-complexes having strongly coordinating ligands efficiently catalyze the reaction [26], [27], [28], [29], [30]. For example, we have found that the [PdCl2(dppp)] complex efficiently catalyzes the terpolymerization with propene (P), 1-hexene (Hex), 1-decene (D) or styrene (S) with CO (5000 gPECO/(gPd h), 5600 gHexECO/(gPd h), 5650 gDECO/(gPd h) and 4100 gSECO/(gPd h), in MeOH as a solvent containing H2O and TsOH as co-promoters [6].
In addition, it is reported that the CO–ethene copolymerization catalyzed by a cationic Pd(II)-dppf based catalyst, using MeOH as a solvent, gives PKs of low molecular weight, together with co-oligomers and other carbonylated products of even lower molecular weight such as dimethyl 4-oxoheptanoate, dimethyl succinate, methyl 4-oxohexanoate and methyl propanoate [31]. Under the same experimental conditions, but in CH3COOH–H2O as a solvent, we achieved a remarkable increase of the catalytic activity together with an increase of the average molecular weight [29].
More recently, interesting results have been obtained in HCOOH–H2O as a solvent in which the catalyst shows both a higher catalytic activity and a higher ECO molecular weight also using strongly coordinating counter anions. As matter of fact, we reported that the [PdCl2(dppf)] complex in such a solvent efficiently catalyzes the ethene–CO (ECO) copolymerization leading to high molecular weight polymer with a productivity of 20,200 gECO/(gPd h), at 90 °C, 45 atm (CO/E = 1/1), and H2O ca. 50 molar % [32]. It is worth to point out that both [Pd(OAc)2(dppf)] and [PdCl2(dppf)] are inactive in MeOH. This prompted us to extend the use of H2O–organic acid as a solvent to the terpolymerization of ethene and another olefin with CO.
In this paper, we report on the catalytic activity of the [PdCl2(dppf)] precursor in the PECO terpolymerization carried out in H2O–HCOOH as a solvent. The effect of the operative conditions on the productivity and on the viscosity average molecular weight has been studied. A reaction mechanism has been also proposed and discussed.
Section snippets
Reagents
Palladium(II) chloride was purchased from Engelhard Italy SRL; 1,1′-bis(diphenylphosphino)ferrocene (dppf), 1,1,1,3,3,3-hexafluoroisopropanol (99%), methanol (H2O = 100 ppm) and CDCl3 were Aldrich products. Carbon monoxide, ethene and propene were supplied by SIAD Company (‘research grade’, purity >99.9%).
The complex [PdCl2(dppf)] was prepared as reported in literature [33].
Equipment
The catalyst precursor was weighted on a Sartorius Micro balance.
Gas-chromatographic analysis of the liquid phase was
Results and discussion
The [PdCl2(dppf)] complex in H2O–HCOOH catalyzes the terpolymerization of propene and ethene with CO (Scheme 1) leading to a productivity of 7350 gPECO/(gPd h) under the typical experimental conditions reported in Section 2. This productivity is far superior to the productivity of [Pd(H2O)2(dppf)](TsO)2 in MeOH for the CO–ethene copolymerization [31]. No data have been reported for the terpolymerization with this catalyst. However, it may be foreseen that this catalyst is even less active for
Conclusions
The H2O–HCOOH solvent composition plays a fundamental role in the activation of the [PdCl2(dppf)] catalyst precursor toward the terpolymerization of ethene and propene with CO. Such complex was considered poorly active because has strong coordinating ligands (Cl), but high productivity has been obtained in H2O–HCOOH as a solvent, when H2O concentration ranges between 50 and 65 molar %. The productivity, the melting temperature and the viscosity average molecular weight of the terpolymer depend
Acknowledgements
Ca’ Foscari University of Venice is gratefully acknowledged for financial support (Ateneo 2009). A special thank to Dr. Enrico Militello for experimental assistance.
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