Terpolymerisation of 1-olefin and ethene with CO catalysed by the [PdCl2(dppp)] complex in methanol as a solvent [dppp = 1,3-bis(diphenylphosphino)propane]
Graphical abstract
The catalytic system [PdCl2(dppp)]/TsOH (1/8) catalyses the terpolymerisation of propene (P), 1-hexene (Hex), 1-decene (D) and styrene (S) with ethene (E) and CO, in methanol (H2O = 1000 ppm) as a solvent. At 90 °C and 45 atm (E/CO = 1/1), the best productivity were 5000 g PECO/(g Pd h), 5600 g HexECO/(g Pd h), 5650 g DECO/(g Pd h), and 4100 g SECO/(g Pd h). It has been studied the effect of olefin concentration on the productivity, average molecular weight and melting temperature and a mechanism of reaction has been proposed and discussed.
Introduction
Pd(II)-chelating diphosphine complexes catalyse efficiently the strictly alternating copolymerisation of ethene (E) with CO (ECO) leading to a high molecular weight polymer [1], [2]. This class of catalysts is also active for the co- and terpolymerisation of CO with olefins other than ethene thus providing access to a new family of polymers named polyketones (PK). ECO is a highly crystalline material with high melting point (Tm = 256 °C), whereas copolymers of CO with alkene others than ethene or terpolymers of CO with two olefins may have different properties depending on the relative characteristics of both alkenes and catalysts [2], [3], [4], [5], [6], [7]. Several industries have been showed a keen interest in this new group of polymers, and the propene (P)–E–CO (PECO) terpolymer, which has reduced melting transition (Tm) and a much more favorable behavior in blow-molding or in extrusion applications, was the first PK that found entrance into industrial applications, commercialized under the trade name of CARILON® by Shell [8] and of KETONEX® by BP [9].
Various aspects of the PK catalysis have been reviewed [2], [10], [11], [12], [13], [14], and the nature of chelating ligand appears fundamental to determine the catalytic activity which is influenced also by the nature of the counter-anions and of the solvent. By comparing the activity of several Pd(II) complexes, it has been found that bidentate bisphosphines are effective for the copolymerisation of ethene, propene, or higher 1-alkenes with CO [15], [16], [17], [18], [19], [20], on one hand. On the other hand, nitrogen-based ligands, such as diimine [21], [22], [23], [24], [25], bisoxazoline [26], [27], phosphine-imine [28], [29], and phosphine–phosphite [30], [31], [32] have been utilized for the copolymerisation of vinylarenes with CO.
In contrast to the intensive efforts devoted to the copolymer synthesis, however, much less has been reported on terpolymer synthesis. Most of these deal with the terpolymerisation of CO with two aliphatic olefins catalysed by Pd–diphosphine complexes [33], [12], [13], [14], [15]. The nitrogen-based ligands appear more effective in the terpolymerisation of CO with two aromatic olefins [34] or in the terpolymerisation of CO with one aliphatic and one aromatic olefin [35], [28] and only few reports appeared on terpolymerisation of CO with one aliphatic and one aromatic olefin in which are used diphosphine [36], [37] or phosphine–phosphite [38] based ligands.
Even if protic or non-protic organic solvents can be used [39], [40], [41], [42], [43], [44], [45], in most olefin/CO copolymerisation and terpolymerisation reactions, methanol is employed as a solvent [10], [11], [12], [13], [14]. Moreover, in the most relevant cases, there are weakly coordinating anions coordinated to metal, being the strong coordinating anions (for instance Cl) considered less effective [10], [11], [12], [13], [14].
Only recently in some reports it has been claimed that also the [PdX2(dppp)] complexes having strong coordinating ligands (X = Cl, OAc) are highly active in the ECO copolymerisation, if the appropriate solvent is used, for instance H2O–CH3COOH or H2O–MeOH [46], [47]. In these papers it has been pointed out the fundamental role played by H2O: at 90 °C and 45 atm (CO/E = 1/1), the [PdCl2(dppp)] complex, which is inactive in MeOH without addition of H2O, catalyses the ECO copolymerisation leading to a maximum of productivity [4000 g ECO/(g Pd h)] when H2O is 20% (molar), which increases up to 6000 g ECO/(g Pd h) when the precursor is used in the presence of p-toluenesulfonic acid (TsOH) [47]. The same precursor leads to a productivity of 27,500 g ECO/(g Pd h) when the solvent is H2O–CH3COOH (H2O = 55%, mol/mol) [47].
In the present paper, it has been studied the catalytic activity of [PdCl2(dppp)] complex, having strong coordinating ligands (Cl), in 1-olefins/CO copolymerisation and in 1-olefins/E/CO terpolymerisation, in MeOH as a solvent containing H2O and TsOH as co-promoters.
Since in literature, there are only few reports on terpolymerisation of ethene, vinylarenes and CO, in which are used phosphine-based ligands, and no data concerning the productivity were reported [35], [37], it appear interesting to study also the catalytic activity of [PdCl2(dppp)] in the styrene (S)–E–CO (SECO) terpolymerisation. A mechanism of reaction has been also proposed and discussed.
Section snippets
Ethene/CO and 1-olefin/CO copolymerisations
The [PdCl2(dppp)] complex has been used as catalyst precursor in the copolymerisation of CO with ethene, propene, 1-hexene (Hex), and 1-decene (D). It has been reported that [PdCl2(dppp)] catalyses efficiently the ECO copolymerisation in H2O–MeOH (H2O = 20%) as a solvent pointing out the fundamental role played by H2O and the acid (TsOH) to determinate the catalytic activity (see Introduction [47]). Unfortunately, in such a solvent the olefins tested form two immiscible phases forcing us to use
Reagents
Palladium(II) chloride was purchased from Engelhard Italiana SRL; 1,3-bis(diphenylphosphino)propane (dppp), 1,1,1,3,3,3-hexafluoroisopropanol (99%), methanol (H2O = 100 ppm), CDCl3, 1-hexene, 1-decene and styrene were Aldrich products. Carbon monoxide, ethene and propene were supplied by SIAD Company (’research grade’, purity >99.9%).
The complex [PdCl2(dppp)] was prepared as reported in the literature [65].
Equipment
The catalyst precursor was weighted on a Sartorious Micro balance (precision 0.001 mg).
Acknowledgments
The financial support of MIUR (Rome) PRIN 2004 is gratefully acknowledged. A special thank to Dr. Enrico Militello for its technical support.
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