Late transition metal-catalyzed polymerization

Late Transition Metal-catalyzed Polymerization of Ethylene... 

Recent Progress in Late Transition Metal-Catalyzed Polymerization... 

Triggered by the developments in late transition metal catalyzed polymerization, new catalyst systems were described very recently for the oligomerization of ethene. Nickel and palladium complexes based on a-diimine ligands 13 and imi-nophosphines 14 were reported to be very active and selective oligomerization catalysts [57, 58], Activation of the Ni(II) diimine halides with a large excess of MAO (210 equiv.) leads to oligomerization catalysts with activities of between... 

Eq. 1 Late transition metal-catalyzed polymerization reaction of MeH2SiSiH2Me. 

Scheme 23 Chain walking mechanism responsible for branch fonmation in a-diimine-based late transition metal-catalyzed polymerization reactions.

MMA can function as a chain-transfer agent for late transition metal-catalyzed ethylene polymerization. Gibson and Tomov revealed that Ni complexes with bulky phosphino-enolate ligands, F13-17, were capable of polymerizing ethylene in the presence of MMA to afford MMA-end-capped PEs through immediate /3-H transfer after MMA 337... 

Mixtures of Ni and Co salts with organoaluminum reagents catalyze the polymerization of 1,3-butadiene, whereas these catalysts are not effective for alkene polymerization due to facile chain transfer by /1-hydrogen elimination of the growing polymer. The polymerization of 1,3-butadiene usually proceeds via the intermediate n-allyl complexes, which are more stable than the alkyl metal complexes and hardly cause any /1-hydrogen elimination [56]. The late transition metal compounds polymerize 1,3-butadiene even in... 

Surprisingly there have been no reports of late transition metal-catalyzed addition polymerization of cyclobutenes. Since the homopoiymerization of cyclobutene using metallocene catalysts is exemplified in the literature [16] it is only a matter of time before a report of a cationic palladium or nickel catalyst for the polymerization appears in the literature. 

Not only the highly Lewis acidic early transition metal-based polymerization catalysts suffer from poisoning by coordination of functional groups. Even in late transition metal-based complexes, the possible o-coordination in certain functional groups has a negative impact on polymerization reactions. The prominent example here is the still ongoing search for active acrylonitrile (AN) copolymerization catalysts. This reaction can serve as an ideal example to illustrate the challenges in late transition metal-catalyzed insertion polymerizations with polar functionalized comonomers. The metal-mediated copolymerization of AN has numerous appearances in literature however, in most cases, the reaction mechanism seems to be of ionic or radical nature. 

In general, several pathways can lead to active polymerization complexes. These possibilities can serve as a common principle with possible transfer to other late transition metal-catalyzed... 

In addition to the reactions discussed above, there are still other alkyne reactions carried out in aqueous media. Examples include the Pseudomonas cepacia lipase-catalyzed hydrolysis of propargylic acetate in an acetone-water solvent system,137 the ruthenium-catalyzed cycloisomerization-oxidation of propargyl alcohols in DMF-water,138 an intramolecular allylindination of terminal alkyne in THF-water,139 and alkyne polymerization catalyzed by late-transition metals.140... 

The Key Steps in Olefin Polymerization Catalyzed by Late Transition Metals... 

In 1996, Brookhart and co-workers developed a remarkable class of Pd complexes with sterically encumbered diimine ligands (Scheme 4, S4-1, S4-2, S4-4, and S4-5). These examples are capable of mediating the co-polymerization of ethylene with methyl acrylate (MA) to furnish highly branched PE with ester groups on the polymer chain ends by a chain-walking mechanism (Scheme 10). " This represents the first example of transition metal-catalyzed ethylene/MA co-polymerization via an insertion mechanism. The mechanism for co-polymerization is by 2,1-insertion of MA and subsequent chelate-ring expansion, followed by the insertion of ethylene units. The discovery of these diimine Pd catalysts has stimulated a resurgence of activity in the area of late transition metal-based molecular catalysis. Recently, the random incorporation of MA into linear PE by Pd-catalyzed insertion polymeriza-... 

Michalak A, Ziegler T, The Key Steps in Olefin Polymerization Catalyzed by Late Transition Metals. In Computational Modeling of Homogeneous Catalysis, edited by F Maseras, A Lledos (Kluwer Academic Publishers, 2002)... 

Michalak A, Ziegler T, Theoretical Studies on the Polymerization and Copolymerization Processes Catalyzed by the Late-Transition Metal Complexes, in Beyond Metallocenes Next-Generation Polymerization Catalysts ACS Symp Series 857, edited by AO Patil, GG Hladky (American Chemical Society, Washington 2003), pp 154-172... 

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