Metathesis reactions are widely used for chemical syntheses, e.g. in the form of ring-closing metatheses (RCM), cross-metatheses (CM) or ring-opening metatheses (ROMP). Metathesis reactions are employed, for example, for the synthesis of olefins, for the depolymerization of unsaturated polymers and for the synthesis of telechelic polymers.
Metathesis catalysts are known, inter alia, from WO-A-96/04289 and WO-A-97/06185. They have, the following in-principle structure:
where M is osmium or ruthenium, the radicals R are identical or different organic radicals having a wide range of structural variation, X1 and X2 are anionic ligands and L are uncharged electron donors. The customary term “anionic ligands” is used in the literature regarding such metathesis catalysts to describe ligands which are always negatively charged with a closed electron shell when regarded separately from the metal centre.
Metathesis reactions have recently also become increasingly important for the degradation of nitrile rubbers.
Nitrile rubber, also referred to as “NBR” for short, is rubber which is a copolymer or terpolymer of at least one α,β-unsaturated nitrile, at least one conjugated diene and, if appropriate, one or more further copolymerizable monomers.
Hydrogenated nitrile rubber, also referred to as “HNBR” for short, is produced by hydrogenation of nitrile rubber. Accordingly, the C═C double bonds of the copolymerized diene units have been completely or partly hydrogenated in HNBR. The degree of hydrogenation of the copolymerized diene units is usually in the range from 50 to 100%.
Hydrogenated nitrile rubber is a specialty rubber which has very good heat resistance, an excellent resistance to ozone and chemicals and also an excellent oil resistance.
The abovementioned physical and chemical properties of HNBR are associated with very good mechanical properties, in particular a high abrasion resistance. For this reason, HNBR has found wide use in a variety of applications. HNBR is used, for example, for seals, hoses, belts and damping elements in the automobile sector, also for stators, oil well seals and valve seals in the field of oil extraction and also for numerous parts in the aircraft industry, the electronics industry, mechanical engineering and shipbuilding.
Commercially available HNBR grades usually have a Mooney viscosity (ML 1+4 at 100° C.) in the range from 55 to 105, which corresponds to a weight average molecular weight Mw (method of determination: gel permeation chromatography (GPC) against polystyrene equivalents) in the range from about 200 000 to 500 000. The polydispersity index PDI (PDI=Mw/Mn, where Mw is the weight average molecular weight and ML the number average molecular weight), which gives information about the width of the molecular weight distribution, measured here is frequently 3 or above. The residual double bond content is usually in the range from 1 to 18% (determined by IR spectroscopy).
The processability of HNBR is subject to severe restrictions as a result of the relatively high Mooney viscosity. For many applications, it would be desirable to have an HNBR grade which has a lower molecular weight and thus a lower Mooney viscosity. This would decisively improve the processability.
Numerous attempts have been made in the past to shorten the chain length of HNBR by degradation. For example, the molecular weight can be decreased by thermomechanical treatment (mastication), e.g. on a roll mill or in a screw apparatus (EP-A-0 419 952). However, this thermomechanical degradation has the disadvantage that functional groups such as hydroxyl, keto, carboxyl and ester groups, are incorporated into the molecule as a result of partial oxidation and, in addition, the microstructure of the polymer is substantially altered.
The preparation of HNBR having low molar masses corresponding to a Mooney viscosity (ML 1+4 at 100° C.) in the range below 55 or a number average molecular weight of about Mn<200 000 g/mol was for a long time not possible by means of established production processes since, firstly, a step increase in the Mooney viscosity occurs in the hydrogenation of NBR and, secondly, the molar mass of the NBA feedstock used for the hydrogenation cannot be reduced at will since otherwise the work-up can no longer be carried out in the industrial plants available because the product is too sticky. The lowest Mooney viscosity of an NBR feedstock which can be processed without difficulties in an established industrial plant is about 30 Mooney units (ML 1+4 at 100° C.). The Mooney viscosity of the hydrogenated nitrile rubber obtained using such an NBR feedstock is in the order of 55 Mooney units (ML 1+4 at 100° C.). The Mooney viscosity is determined in accordance with ASTM standard D 1646.
In the more recent prior art, this problem is solved by reducing the molecular weight of the nitrile rubber prior to hydrogenation by degradation to a Mooney viscosity (ML 1+4 at 100° C.) of less than 30 Mooney units or a number average molecular weight of Mn<70 000 g/mol. The decrease in the molecular weight is achieved by metathesis in which low molecular weight 1-olefins are usually added. The metathesis of nitrile rubber is described, for example, in WO-A-02/100905, WO-A-02/100941 and WO-A-03/002613. The metathesis reaction is advantageously carried out in the same solvent as the hydrogenation reaction (in situ) so that the degraded nitrile rubber does not have to be isolated from the solvent after the degradation reaction is complete before it is subjected to the subsequent hydrogenation. Metathesis catalysts which have a tolerance towards polar groups, in particular towards nitrile groups, are used for catalyzing the metathetic degradation reaction.
WO-A-02/100905 and WO-A-02/100941 describe a process which comprises degradation of nitrile rubber starting polymers by olefin metathesis and subsequent hydrogenation to form HNBR having a low Mooney viscosity. Here, a nitrile rubber is reacted in a first step in the presence of a coolefin and specific catalysts based on osmium, ruthenium, molybdenum or tungsten complexes and hydrogenated in a second step. Hydrogenated nitrile rubbers having a weight average molecular weight (Mw) in the range from 30 000 to 250 000, a Mooney viscosity (ML 1+4 at 100° C.) in the range from 3 to 50 and a polydispersity index PDI of less than 2.5 can be obtained by this route.
The metathesis of nitrile rubber can be carried out using, for example, the catalyst bis(tricyclohexylphosphine)benzylideneruthenium dichloride shown below.

After metathesis and hydrogenation, the nitrile rubbers have a lower molecular weight and also a narrower molecular weight distribution than the hydrogenated nitrile rubbers which have hitherto been able to be prepared according to the prior art.
However, the amounts of Grubbs (I) catalyst employed for carrying out the metathesis are large. In the experiments in WO-A-03/002613, they are, for example, 307 ppm and 61 ppm of Ru based on the nitrile rubber used. The reaction times necessary are also long and the molecular weights after the degradation are still relatively high (see Example 3 of WO-A-03/002613, in which Mw=180 000 g/mol and Mn=71 000 g/mol).
US 2004/0127647 A1 describes blends based on low molecular weight HNBR rubbers having a bimodal or multimodal molecular weight distribution and also vulcanizates of these rubbers. To carry out the metathesis, 0.5 phr of Grubbs I catalyst is used according to the examples. This corresponds to the large amount of 614 ppm of ruthenium based on the nitrile rubber used.
Furthermore, WO-A-00/71554 discloses a group of catalysts which are known in the technical field as “Grubbs (II) catalysts”.
If such a “Grubbs(II) catalyst”, e.g. the catalyst 1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene)(tricyclohexylphosphine)ruthenium(phenylmethylene)dichloride, is used for the metathesis of NBR (US-A-2004/0132891), this can be carried out successively even without the use of a coolefin.

After the subsequent hydrogenation, which is preferably carried out in situ, the hydrogenated nitrile rubber has lower molecular weights and a narrower molecular weight distribution (PDI) than when using catalysts of the Grubbs (I) type. In terms of the molecular weight and the molecular weight distribution, the metathetic degradation thus proceeds more efficiently when using catalysts of the Grubbs II type than when using catalysts of the Grubbs I type. However, the amounts of ruthenium necessary for this efficient metathetic degradation are still relatively high. Long reaction times are also still required for carrying out the metathesis using the Grubbs II catalyst.
In all the abovementioned processes for the metathetic degradation of nitrile rubber, relatively large amounts of catalyst have to be used and long reaction times are required in order to produce the desired low molecular weight nitrile rubbers by means of metathesis.
The activity of the catalysts used is also of critical importance in other types of metathesis reactions.
In J. Am. Chem. Soc. 1997, 119, 3887-3897, it is stated that in the ring-closing metathesis of diethyl diallylmalonate,
the activity of the catalysts of the Grubbs I type can be increased by additions of CuCl and CuCl2. This increase in activity is explained by a shift in the dissociation equilibrium resulting from a phosphane ligand which is eliminated reacting with copper ions to form copper-phosphane complexes.
However, this increase in activity due to copper salts in the abovementioned ring-closing metathesis cannot be carried over to any desired other types of metathesis reactions.
Our studies have unexpectedly shown that although the addition of copper salts leads to an initial acceleration of the metathesis reaction in the metathetic degradation of nitrile rubbers, a significant decrease in the efficiency of the metathesis is observed: the molecular weights which can ultimately be achieved for the degraded nitrile rubbers are substantially higher than when the metathesis reaction is carried out in the presence of the same catalyst but in the absence of the copper salts.