“An Elastomer,” as first defined in 1940, refers to a synthetic thermosetting advanced polymer with properties similar to those of vulcanized natural rubbers, for example, capable of being elongated at least two times its original length and very quickly contracting into the almost same length as the original one when being released.
Among the most commonly available polyolefin elastomers prepared in these days are a copolymer of ethylene and propylene (EP) and a hybrid terpolymer of ethylene, propylene, and a diene monomer (EPDM).
In the past, soluble vanadium catalysts such as VCl4, VOCl3, and VO(AC)3 were used for the production of polyolefinic elastomers. However, the vanadium catalyst has a low catalytic activity, which becomes even lower, especially, in the presence of a diene monomer as included. In addition, if the residual amount of vanadium (V) in the polymer is equal to or greater than 10 ppm, it can lead to coloration of the polymers and the toxicity thereof, necessitating a further step of de-ashing the catalyst during the process. In contrast, metallocene catalysts are composed of a transition metal atom as inserted between cyclic structures. Polymers prepared from the metallocene catalysts are advantageous in that they are superior in impact strength and toughness and are likely to show good melting properties and improved transparency in films. Although the metallocene catalyst itself is more expensive than the conventional Ziegler-Natta catalyst, not only does it have a superb productivity, making the de-ashing step unnecessary, but also it can provide a higher degree of copolymerization for propylene or a diene monomer, allowing the production of a wide range of copolymers. Moreover, the polymerization can be conducted at a higher temperature in comparison with the vanadium catalyst such that the metallocene catalysts are very advantageous in the production of EPDM from a process-relating perspective.
In J. Polm. Sci. vol. 23, pp. 2151-64 (1985), Kaminsky disclosed a use of a soluble catalyst system of bis(cyclopentadienyl)zirconium dimethyl aluminoxane for a solution polymerization of EP and EPDM elastomers. U.S. Pat. No. 5,229,478 disclosed a slurry polymerization method by using a similar catalyst system of bis (cyclopentadienyl)zirconium.
The conventional catalyst system has drawbacks that preparing EP and EPDM elastomers in the presence of such a catalyst system requires an enhanced reactivity of the diene monomers, which were mentioned in U.S. Pat. No. 5,229,478 but have yet to be resolved. Factors having an effect on the availability of EP and EPDM include the production costs as well as the cost of diene monomers. Diene monomers are typically more expensive than ethylene or propylene. Moreover, the reactivity between diene monomers and the previously known metallocene catalyst is lower than that of ethylene or propylene. Therefore, in order for the diene monomers to be incorporated to such an extent required for the preparation of the EPDM at a high curing rate, they should be used at a substantially higher concentration than the diene content as aimed to be actually incorporated into the final EPDM product. Considerable amount of the unreacted diene monomers should be recovered from the effluent of the polymerization reactor for their reuse, and this incurs an unnecessary additional cost for the production.
Moreover, what makes the cost of EPDM even higher is the fact that when the catalyst for an olefinic polymerization is exposed to the diene monomer, especially, of an increased concentration as required for incorporating the diene to the desired extent in the final EPDM product, it may often suffer a decrease in a rate or an activity for carrying out the polymerization of the ethylene and propylene monomers. Therefore, EPDM has a lower productivity and requires a longer time for the polymerization reaction than the ethylene-propylene elastomers or other α-olefinic elastic copolymers.
Since the aluminoxane-activated metallocene catalyst was introduced for producing polyethylene, polypropylene, and a ethylene/α-olefin copolymer, many efforts have been made to employ such catalyst for the production of elastomers. Yet, there has been no known method of producing a high-molecular weight elastomer with a high yield within a reasonable time for the polymerization reaction.