Poly(isobutylene-co-isoprene), or IIR, is a synthetic elastomer commonly known as butyl rubber which has been prepared since the 1940's through the random cationic copolymerization of isobutylene with small amounts of isoprene (1-2 mole %). As a result of its molecular structure, IIR possesses superior air impermeability, a high loss modulus, oxidative stability and extended fatigue resistance.
Butyl rubber is a copolymer of an isoolefin and one or more, preferably conjugated, multiolefins as comonomers. Commercial butyl comprises a major portion of isoolefin and a minor amount, not more than 2.5 mol %, of a conjugated multiolefin.
Butyl rubber or butyl polymer is generally prepared in a slurry process using a suitable polymerization solvent, such as methyl chloride, and a Friedel-Crafts catalyst, such as AlCl3, as the polymerization initiator. The methyl chloride offers the advantage that AlCl3, a relatively inexpensive Friedel-Crafts catalyst, is soluble in it, as are the isobutylene and isoprene comonomers. Additionally, the butyl rubber polymer is insoluble in the methyl chloride and precipitates out of solution as fine particles. The polymerization is generally carried out at temperatures of about −90° C. to −100° C. See U.S. Pat. No. 2,356,128 and Ullmanns Encyclopedia of Industrial Chemistry, volume A 23, 1993, pages 288-295. The low polymerization temperatures are used to achieve molecular weights which are sufficiently high for rubber applications.
Other compounds that have been found to be active as catalysts for polymerizing isoolefins include organometallic compounds in combination with a cation-generating agent, for example C5Me5TiMe3/B(C6F5)3 (WO-00/04061-A1), Cp2AlMe/B(C6F5)3 (U.S. Pat. No. 5,703,182), and combinations of zirconocenes and related complexes with either B(C6F5)3 or CPh3[B(C6F5)4] (WO-95/29940-A1, DE-A1-198 36 663), Song, X.; Thornton-Pett, M.; Bochmann, M. Organometallics 1998, 17, 1004, Carr, A. G.; Dawson, D. M.; Bochmann, M. Macromol. Rapid Commun. 1998, 19, 205.
Nuyken, in collaboration with M. Bohnenpoll (Chem. Eur. J. 2004, 10, 6323), published a system based on [Mn(NCMe)6]2+ salts of non-coordinating borate anions which was active at room temperature:

This system operated at +30° C. in IB/CH2Cl2 but showed no activity ≦0° C. Polymerizations were generally slow (55-110 h for IB homopolymerizations), and there was some doubt about the mechanism. IB homopolymers and copolymers had Mn=8,000-10,000. Conversion was rapidly depressed at higher IP feed.
Zinc compounds have not commonly been used as catalysts for isoalkene polymerizations. Indeed, ZnCl2 in the presence or absence of alkyl halide activators (such as Me3CCl or MeCOCl) and used either in neat isobutene or in isobutene/methyl chloride mixtures, proves to be inactive, and no polymer is obtained. Recently however, Bochmann and coworkers filed a patent on the use of Zn(C6F5)2/ButCl system for IB homo- and IB/IP copolymerizations (Canadian patent application 2,441,079, filed Sep. 16, 2003). Zinc had never been used as an initiator for cationic polymerizations before. This patent teaches that this system possessed particularly good copolymerization characteristics and allowed the formation of IB/IP copolymers in neat IB solutions (no solvent). The polymers had up to 15 mol-% IP, with little gel content. However, monitoring the reaction of Zn(C6F5)2 with tert-butyl chloride (tBuCl) always found substantial amounts of C6F5H together with insoluble precipitates. In addition, the Zn(C6F5)2 is expensive to use in a commercial scale process and lower cost alternatives are therefore being sought.
The need therefore remains for improved polymerization processes using zinc-based initiators.