Ethene oligomers

Table 10.2. Ethene oligomer formation measured in laboratory batch reactor at 100°C and 80 atm pressure with a SHOP-type catalyst.

Fig. 2.18 GC chromatograms of the ethene oligomers, resulting from complexes 15x...

The difference of absorbance between the spectrum (b) and the spectrum (c) at a wavelength, for example, SOOnm, denoted by A, must be proportional to the amount of Nb ion. The shaded part shown in the figure must result from the adsorbed ethene oligomers. The change in the value of A along the irradiation time is displayed in Fig.3. The linear increase of A or the... 

In the early 1980s, Sen reported the synthesis of CO-ethene oligomers using a combination of dicationic palladium complexes [Pd(NCMe)J(BF ), and triphenylphosphine as precatalysts, Tliese catalysts induced polymerization under milder conditions than the previous catalysts for this process. Although the material reported by Sen possessed relatively low molecular weight, and the catalysts were less reactive than those known today, tliis finding showed that cationic palladium complexes were more reactive than the neutral systems studied previously. 

Important for the polymerization activity is the substituent 1 which has to be a bulky aryl group. The task of this substituent is to fill up the coordination spheres below and above the square plane of the complex and thus enable the growing polymer chain to stay coordinated to the metal center. This is one of the main differences to the well-known SHOP catalysts invented by Keim et al. [164] and Ostoja-Starzewski and Witte [165] which produces mainly ethene oligomers. 

Long-chain branched polyethylenes can be also obtained by copolymerization of ethene with ethene oligomers by tandem polymerization in one step [100] or with ethene/propene oligomers in two steps [101]. In the latter case, polymers are obtained with crystalline polyethylene backbone chains and amorphic ethene/ propene copolymer side chains [102]. 

In this chapter we will discuss some aspects of the carbonylation catalysis with the use of palladium catalysts. We will focus on the formation of polyketones consisting of alternating molecules of alkenes and carbon monoxide on the one hand, and esters that may form under the same conditions with the use of similar catalysts from alkenes, CO, and alcohols, on the other hand. As the potential production of polyketone and methyl propanoate obtained from ethene/CO have received a lot of industrial attention we will concentrate on these two products (for a recent monograph on this chemistry see reference [1]). The elementary reactions involved are the same formation of an initiating species, insertion reactions of CO and ethene, and a termination reaction. Multiple alternating (1 1) insertions will lead to polymers or oligomers whereas a stoichiometry of 1 1 1 for CO, ethene, and alcohol leads to an ester. 

It was Ayusman Sen [8] who discovered in 1982 that the use of weakly coordinating anions and phosphines as the ligands together with palladium yielded much more stable and active catalysts for the formation of polyketone from CO and ethene in alcoholic solvents. Cationic palladium-(triphenylphosphine)2(BF4)2 gave a mixture of oligomers having methoxy ester... 

When chain transfer is very fast, the reaction observed is the alkoxycarbonylation of ethene, which is nothing but a perfect chain transfer after the insertion of just two monomers. In recent years several very fast catalysts for this reaction have been reported as we will see in the next section. The first effective catalysts reported by Sen [8] (Pd(BF4)2 and PPh3) also give relatively fast chain transfer as the Flory-Schulz constant for their product was only 0.3-0.4 (see Chapter 9.2.1 on oligomerisation) and the product is an oligomer rather than a polymer. 

Oligomers of perfluorohexyl-ethene fulfilled these expectations in all preclinical studies, in vitro tests as well in animal tests. A radical polymerisation, followed by ultra-purification steps, created a crystal clear gel-like substance. The behaviours of the mixture of dimeric, trimeric and tetrameric star-shaped species with an inner core of hydrocarbon bonds and an outer layer of perfluoro-alkyl chains could be adjusted by the ratio of the dimeric, trimeric and tetrameric species, using a thin layer distillation. In dependence on this ratio, the viscosity could be adjusted in the range between 90 mPas and 1700 mPas, the specific density between 1.60 g/ml and 1.66 g/ml and the interfacial tension against water between... 

---