Cyclocopolymerization: A Mechanistic Probe for Dual-Site Alternating Copolymerization of Ethylene and r-Olefins Tong Neo Choo and Robert M. Waymouth* Department of Chemistry, Stanford UniVersity, Stanford, California 94305-5080 Received December 7, 2001 Metallocene catalysts have proven versatile for the synthesis of random ethylene/R-olefin copolymers with homogeneous and statistical sequence distributions. 1 Recently, we and others have sought to exploit the two heterotopic coordination sites of C 1 - symmetric metallocenes for the copolymerization of ethylene and R-olefins to control the sequence distribution in ethylene/R-olefin copolymers (Scheme 1). 2,3 This strategy is based on the premise that the process of migratory insertion results in the movement of the polymer chain from one coordination site to the other. 4 If the kinetic selectivities of the two insertion sites are sufficiently different to discriminate the two comonomers, then alternating copolymers can be formed (Scheme 1). This dual-site hypothesis was recently invoked to explain the alternating copolymerization of ethylene and R-olefins with the C 1 - symmetric metallocenes 2 and 3. 2,3 This mechanism provides an appealing and potentially general strategy for the synthesis of copolymers of defined sequence distribution; nevertheless, recent reports that metallocenes 4 and 5 (possessing homo- or enantiotopic sites) can also produce alternating copolymers indicate that a dual- site mechanism is not necessary to generate alternating copoly- mers. 1b,2b,5 To evaluate the viability of the two-site mechanism, we sought an independent experimental test for the involvement of both heterotopic sites in the alternating copolymerization of ethylene and R-olefins from metallocene 3. To this end, we have investigated the copolymerization of ethylene with 1,5-hexadiene, 6 an R-olefin which, when it cyclopolymerizes, requires two insertion events to be incorporated into the chain (Scheme 2). 7-9 The copolymerization of ethylene (E) with 1-hexene (H) or 1,5- hexadiene (HD) was carried out at 0.54 atm of ethylene at 20 °C in toluene solution at a variety of R-olefin concentrations. 13 C NMR analysis of the copolymer composition reveals that 1,5-hexadiene and 1-hexene are readily copolymerized with ethylene in the presence of metallocenes 1, 3, and 4. Under these conditions ([HD] < 2.0 M), 1,5-hexadiene cyclopolymerizes almost exclusively to give methylene-1,3-cyclopentane units in the copolymers, with only traces of uncyclized 1,2-inserted hexadiene. The diastereoselectivity of the cyclocopolymerization favors the formation of trans-1,3- cyclopentane rings for metallocenes 1, 3, and 4 (74% trans, 81% trans, and 66% trans respectively). For metallocenes 1 and 4, the E/H and E/HD copolymerization yielded copolymers with similar comonomer compositions and sequence distributions 10 over a range of feed ratios (Table 1, entries 1-3 vs 11-13 and 8-10 vs 18-20). This result suggests that for catalysts derived from 1 and 4, the copolymerization behavior of 1,5-hexadiene is similar to that of 1-hexene, even though the nature of the propagating polymer chain end after the insertion of the R-olefin is slightly different (i.e. a butyl branch vs a 1,3- cyclopentane unit). In contrast, the E/H and E/HD copolymers derived from metallocene 3 show very different compositions and sequence distributions over the range of feed ratios investigated. At comparable feed ratios, the E/HD copolymers obtained were enriched in the hexadiene comonomer and deficient in ethylene when compared to the corresponding E/H copolymers (Table 1, entries 4-7 vs 14-17). * To whom correspondence should be addressed. E-mail: waymouth@ stanford.edu. Scheme 1. Proposed Alternating-Site Mechanism Table 1. Copolymerization of Ethylene with 1-Hexene and 1,5-Hexadiene a run metallocene olefin [olefin] (M) [Zr] (μM) yield (g) %E b 1 1 1-H 0.38 7.5 0.67 72 2 1 1-H 0.75 9.4 0.90 63 3 1 1-H 1.21 9.4 0.55 57 4 3 1-H 0.38 9.4 0.52 69 5 3 1-H 0.75 9.4 0.50 62 6 3 1-H 1.21 9.4 1.83 56 7 3 1-H 1.89 4.7 1.09 53 8 4 1-H 0.38 9.4 0.61 70 9 4 1-H 0.75 9.4 1.06 59 10 4 1-H 1.21 9.4 1.11 57 11 1 1,5-HD 0.38 7.5 0.22 75 12 1 1,5-HD 0.75 9.4 0.28 63 13 1 1,5-HD 1.21 9.4 0.23 60 14 3 1,5-HD 0.38 7.5 0.78 57 15 3 1,5-HD 0.75 9.4 0.56 47 16 3 1,5-HD 1.21 9.4 0.31 38 17 3 1,5-HD 1.89 7.5 1.44 21 18 4 1,5-HD 0.38 9.4 0.10 70 19 4 1,5-HD 0.75 7.1 0.10 61 20 4 1,5-HD 1.21 7.1 0.17 55 a Polymerization conditions: T ) 20 °C, Volume ) 53 mL, MAO ) 120 mg, 0.54 atm ethylene, polymerization time ) 10 min. b Determined by 13 C NMR. Published on Web 03/27/2002 4188 9 J. AM. CHEM. SOC. 2002, 124, 4188-4189 10.1021/ja017707l CCC: $22.00 © 2002 American Chemical Society