Alternating Copolymerization of Ethylene and Propylene:  Evidence for Selective Chain Transfer to Ethylene

Abstract

C1-symmetric ansa-metallocenes Me2E(Ind)(Flu)ZrCl2 (E ) Si (1) and C (2)), upon activation with methylaluminoxane (MAO), produce atactic alternating ethylene-propylene copolymers (up to 92% EP dyads) with low molecular weight (less than 20K). Quantitative 13 C NMR analysis of copolymer chain ends produced with 2/MAO indicates that the chain initiation/termination process is highly selective to generate a vinylidene as the unsaturated end group (terminating end) and an ethyl group as the saturated end group (initiating end). The high selectivity for sec-butyl end groups reveals a highly selective chaininitiation process where ethylene insertion is followed by propylene insertion at the initiating end. Chain transfer from a propylene-terminated polymer chain to a coordinated ethylene monomer is proposed to explain this selectivity. The facile cross-monomer chain-transfer process explains the much lower molecular weights obtained for the EP copolymers relative to the ethylene or propylene homopolymers.

Sequence and Stereoselectivity of the C1-Symmetric Metallocene Me2Si(1-(4,7-Me2Ind))(9-Flu)ZrCl2

“…The stereoselectivity of metallocene 2 in liquid propylene is similar at 0 and 20°C (Table 1); previous studies show that the stereoselectivity decreases modestly with increasing monomer concentration at 20°C. 26,28 These results for 2 are consistent with previously discussed enantiomorphic site control mechanism depicted in Scheme 1, where one of the coordination sites is stereoselective and the other is nonstereoselective. Insertion of propylene at the stereoselective site, followed by "site epimerization" (or backskip) from the nonstereoselective site prior to monomer insertion, leads to multiple insertions at the stereoselective site and a predominance of isotactic stereosequences.…”

Ethylene–Propylene Copolymerisations: Effect of Metallocene Structure on Termination Reactions and Polymer Microstructure

“…Catalyst 1 has less bulky side groups, which enables b- hydrogen transfer to the ethylene monomer, as discussed above. Fan and Waymouth [8] and Balboni et al [9] have reported that termination in EP copolymerisations take place by b-hydrogen transfer to coordinated ethylene monomers, in particular after an inserted propylene monomer. This termination mechanism may have been operative in our polymerisations as well, but our results are not detailed enough to confirm this statement.…”

“…One of the benefits of metallocene catalysts over the commercially widely used Ziegler-Natta catalysts is that they allow modification of the ligand structure and, thereby, of the coordination environment of the metal centre, which is essential for rapid catalyst evolution. [5] Although some studies on the termination mechanisms in EP copolymerisations have been reported, [6][7][8][9][10][11] further research is needed for a clear understanding of the relationship between the structure of the metallocene catalyst and the termination mechanisms.…”

Progress in advanced catalyst and their use in ethylene/propylene copolymerization

“…1998年, Jin等人 [45] , Amdt等人 [46] 和Leclerc等人 [47] [45] . 若将桥连 [48] . 将桥连的 -C(CH 3 ) 2 -变为-Si(CH 3 ) 2 -, 茂金属化合物13的催化 活性、 共聚产物的交替度和M n 均有所提高, 分别达到 2.05×10 7 g mol −1 h −1 , 90.4% (EPE+PEP=88%)和12300 g mol −1 [48] .…”

“…若将桥连 [48] . 将桥连的 -C(CH 3 ) 2 -变为-Si(CH 3 ) 2 -, 茂金属化合物13的催化 活性、 共聚产物的交替度和M n 均有所提高, 分别达到 2.05×10 7 g mol −1 h −1 , 90.4% (EPE+PEP=88%)和12300 g mol −1 [48] . 在茚基上增加取代基(化合物14), 由于空 [49] .…”