Olefin Polymerization DOI: 10.1002/anie.201003918 Hydridoboranes as Modifiers for Single-Site Organochromium Catalysts: From Low- to Ultrahigh-Molecular-Weight Polyethylene** Stefan Mark, Alexander Kurek, Rolf Mülhaupt, Rong Xu, Günter Klatt, Horst Köppel, and Markus Enders* Molecular single-site catalysts allow the precise synthesis of polyolefins with a range of specific properties. Organochro- mium complexes show advantageous properties in the polymerization and copolymerization of ethylene. [1] Depend- ing on the catalyst system used, low-molecular-weight up to ultrahigh-molecular-weight polyethylenes (UHMW-PEs) with a narrow molecular weight distribution can be obtained. The molecular weight of the polyethylene produced by a single catalyst system can be reduced by the use of chain- transfer reagents. [2, 3] We now report on the use of hydrido- boranes as additives, which in the presence of single-site cyclopentadienylchromium complexes [4] are able to increase the molecular weight of polyethylene accurately up to the UHMW-PE range. These observations are in contrast to the situation when boranes are used as chain-transfer reagents in metallocene-catalyzed olefin polymerization. [5] The influence of several modifiers on ethylene polymer- ization was investigated by using A and B as catalyst precursors and methylaluminoxane (MAO) as the activator and compared with the results obtained with zirconocene dichloride C. [6, 7] Although 200–300 equivalents of the respec- tive modifier were used, this amount did not result in a considerable decrease in the catalytic activity (see Table 1). This finding indicates that the chosen modifiers readily allow the coordination of the incoming ethylene monomer. As expected, organoaluminum compounds tend to lead to lower molecular weight (MW), as a result of chain-transfer reactions from Cr to Al. Whereas trimethylaluminum (AlMe 3 ) has little effect (Table 1, entries 2 and 8), triethyla- luminum (AlEt 3 ) leads to a strong decrease in the PE chain lengths. A similar difference in chain-transfer efficiency has been observed recently for catalytic carboalumination reac- tions in the presence of complex A. [8] Although phenylsilane does not have a significant effect (Table 1, entries 4 and 10), boranes display considerable influence. Whereas trialkylboranes lead, as expected, to lower MW, the use of 9-borabicyclo[3.3.1]nonane (9-BBN) as an additive produces high-molecular-weight polyethylenes up to the ultrahigh range (Table 1, entries 6 and 12). We were able to control the increase in MW by using different amounts of 9- BBN (Figure 1). In contrast, different amounts of alkylalu- minum compounds produce correspondingly lower molecular weights. Thus, by varying the amount and nature of the modifiers we are able to selectively control the MW over a broad range from 50 000 up to 5 000 000 g mol À1 without changing the reaction conditions or the catalytic system. The role of 9-BBN compared to established chain-transfer reagents was investigated in a series of control experiments: B-benzyl-9-BBN leads to lower MW. Since other trialkylbor- anes react in a similar way, the B ÀH function is essential. Therefore, other hydridoboranes (BH 3 and Et 2 BH) and also hydridoalanes (diisobutylaluminum hydride) were tested, but Figure 1. Gel-permeation chromatography (GPC) plots of polyethylenes synthesized with A/MAO and different amounts of 9-BBN to control the MW. [9] [*] Dr. S. Mark, Prof. Dr. M. Enders Anorganisch-Chemisches Institut Ruprecht-Karls Universität Heidelberg Im Neuenheimer Feld 270, 69120 Heidelberg (Germany) Fax: (+ 49) 6221-541-616-247 E-mail: markus.enders@uni-hd.de Dr. R. Xu, Dr. G. Klatt, Prof.Dr. H. Köppel Physikalisch-Chemisches Institut Ruprecht-Karls Universität Heidelberg Im Neuenheimer Feld 229, 69120 Heidelberg (Germany) Dr. A. Kurek, Prof. Dr. R. Mülhaupt Freiburger Materialforschungszentrum and Institut für Makromolekulare Chemie Albert-Ludwigs-Universität Freiburg Stefan-Meier-Strasse 31, 79104 Freiburg (Germany) [**] This work was supported by the Deutsche Forschungsgemeinschaft (DFG, SFB 623), Basell Polyolefine GmbH, and Bundesministerium für Bildung und Forschung (BMBF project no. 03X2006). We thank Basell Polyolefine GmbH, especially Dr. Marc Oliver Kristen and Dr. Shahram Mihan, for placing the chromium complex B at our disposal and for the good collaboration. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201003918. A ngewandte Chemi e 8751 Angew. Chem. Int. Ed. 2010, 49, 8751 –8754  2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim