Main Group Alkyl Research Articles

Analysis of Polymeryl Chain Transfer Between Group 10 Metals and Main Group Alkyls during Ethylene Polymerization

The ability of various group 10 α-diimine and salicylaldimine polymerization catalysts to undergo chain transfer with main group metal alkyls during ethylene polymerization has been investigated in depth. The catalyst systems with the most efficient chain transfer were found to be cationic (α-diimine)Ni catalysts paired with dialkyl zinc chain-transfer reagents, in which all growing polymeryl chains were transferred to Zn on the basis of 13C NMR analysis. In these systems, chain transfer was found to be dependent on the sterics of both the catalyst and the chain-transfer reagent (CTR). When less sterically encumbered catalysts or CTRs were utilized, the relative rate of bimetallic chain transfer to chain propagation was increased; however, in cases where chain termination via β-H elimination was extremely rapid, chain transfer to Zn was kinetically not viable. Importantly, chain transfer from (α-diimine)Ni catalysts to Zn alkyls is also very sensitive to the strength of the Zn–C bond: ZnMe2 (186 kJ/mol) i...

Investigations of Chain Shuttling Olefin Polymerization Using Deuterium Labeling

Chain shuttling among olefin polymerization catalysts with differing monomer responses has been developed for the preparation of block copolymers with enhanced performance. The chain shuttling agent (CSA) is usually a main group alkyl complex that is able to exchange polymeryl chains with the catalysts. We have performed a diverse yet complementary set of polymerization experiments using a hafnium pyridyl−amide catalyst (1) and Oct3Al as CSA. Ethylene/1-hexene copolymerization experiments using different amounts of CSA were quenched with D2O after varied reaction times. In ethylene homopolymerizations with fairly constant polymer yields, Mw/Mn is less than 2 and decreases monotonically with the addition of CSA, indicative of chain shuttling. However, less than half of the available Al sites are occupied by polymer chains. This indicates that chain shuttling (polymeryl−polymeryl exchange) is kinetically competitive with chain transfer (polymer−alkyl exchange) in this system. This approach also allows a more complete accounting of the fates of each polymeryl and alkyl chain by using multiple characterization techniques (GPC, GC, and 2H NMR). The mass balance agreement among these techniques is excellent, which validates our techniques and our basic understanding of chain shuttling processes. The results indicate that chain transfer to aluminum is the dominant termination mechanism under these conditions. Furthermore, 2H and 13C NMR experiments indicate that chain transfer from 1 to Oct3Al occurs significantly faster after an ethylene insertion than after a hexene insertion.

Role of B(C6F5)3 in catalyst activation, anion formation, and as C6F5 transfer agent

Abstract The versatile reactivity of B(C6F5)3 in alkene polymerization reactions is summarized. Adduct formation with basic anions such as CN– and NH2– gives extremely weakly co- ordinating diborates, which are the basis of some of the most active polymerization catalysts known to date. By contrast, the reaction of B(C6F5)3 with zirconium half-sandwich complexes leads to extensive C6F5 transfer, including the surprising formation of borole-bridged triple decker complexes. Main group alkyls undergo such C6F5 exchange reactions very readily unless donor ligands are present. Borate salts of new three-coordinate zinc alkyl cations proved to be highly effective catalysts for the ring-opening polymerization of epoxides and lactones.

Dehalogenation of alkyl halides catalyzed by single-enantiomer complexes of titanium

Racemic alkyl bromides were dehalogenated with a stoichiometric quantity of a main group alkyl metal reductant in the presence of a chiral, single-enantiomer titanium catalyst. The reaction was monitored by chiral gas chromatography of samples from the reaction mixture. The chiral titanium compounds examined proved to be effective catalysts. However, there was no detectable difference in the rate of reduction between the two enantiomers of the alkyl halide. An enantiomerically pure secondary bromide was reduced under the same conditions without racemization during the course of the reaction. This indicates that the secondary alkyl bromide is stereochemically stable to the reaction conditions. Radical probe reactions suggest a radical mechanism.

Mono(pentamethylcyclopentadienyl) complexes of cerium(III). Synthesis, molecular structure, thermal stability, and reactivity of (C5Me5)CeX2 (X = 2,6-di-tert-butylphenoxo, CH(SiMe3)2, and N(SiMe3)2) complexes

Reaction of Ce(OAr)3 (1) with LiR (R = C5Me5, 1,3-diphenyl-2-methylindene) affords novel RCe(OAr)2 (R = C5Me5, 2; R = 1,3-diphenyl-2-methylindene, 3) compounds. Metathesis of the remaining aryloxide ligands in 2 with bulky main-group alkyl and amide compounds (LiCH(SiMe3)2 and NaN(SiMe3)2) provide neutral C5Me5CeR2 complexes (R = CH(SiMe3)2, 5; R = N(SiMe3)2, 6). Compound 5 is a polymerization catalyst for ethylene but not for propylene. Compounds 2 and 6 disproportionate in solution with formation of equilibrium mixtures containing C5Me5CeX2, (C5Me5)2CeX, and CeX3 complexes. Structural studies on 2, 5, and 6 show monomeric, formally five-coordinate compounds in the solid state. Compound 2 crystallizes in the monoclinic space group P21 with a = 9.575 (1) A, b = 17.026 (2) A, c = 11.530 (1) A, β = 112.02 (1)°, and Z = 2. Compound 5 crystallizes in the hexagonal space group P63 with a = 18.477 (2) A, c = 15.991 (3) A, and Z = 6. Compound 6 crystallizes in the monoclinic space group P21/c with a = 19.252 (6) A, b = 13.430 (4) A, c = 12.503 (3) A, β = 90.28 (2)° , and Z = 4. All compounds display interesting secondary interactions of the metal with C-H and/or Si-C bonds in the solid state.