Chain walking mechanism

However, the practical, direct synthesis of functionalized linear polyolefins via coordination copolymerization olefins with polar monomers (CH2 = CHX) remains a challenging and industrially important goal. In the mid-1990s Brookhart et al. [25, 27] reported that cationic (a-diimine)palladium complexes with weakly coordinating anions catalyze the copolymerization of ethylene with alkylacrylates to afford hyperbranched copolymers with the acrylate functions located almost exclusively at the chain ends, via a chain-walking mechanism that has been meticulously studied and elucidated by Brookhart and his collaborators at DuPont [25, 27], Indeed, this seminal work demonstrated for the first time that the insertion of acrylate monomers into certain late transition metal alkyl species is a surprisingly facile process. It spawned almost a decade of intense research by several groups to understand and advance this new science and to attempt to exploit it commercially [30-33, 61]. 

Fig. 4 Dendritic materials made via chain-walking mechanism. Left processing aid, rheology modifier. Right ampiphilic core-shell...

The branched polymers produced by the Ni(II) and Pd(II) a-diimine catalysts shown in Fig. 3 set them apart from the common early transition metal systems. The Pd catalysts, for example, are able to afford hyperbranched polymer from a feedstock of pure ethylene, a monomer which, on its own, offers no predisposition toward branch formation. Polymer branches result from metal migration along the chain due to the facile nature of late metals to perform [3-hydride elimination and reinsertion reactions. This process is similar to the early mechanism proposed by Fink briefly mentioned above [18], and is discussed in more detail below. The chain walking mechanism obviously has dramatic effects on the microstructure, or topology, of the polymer. Since P-hydride elimination is less favored in the Ni(II) catalysts compared to the Pd(II) catalysts, the former system affords polymer with a low to moderate density of short-chain branches, mostly methyl groups. 

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-... 

These two chain-walking mechanisms have an impact upon the distribution of branches. Two consecutive methyl branches must have an even number of methylenes between them. This holds true for any odd-carbon branches because they can only be formed by having the metal center back down the chain by an odd number of carbon atoms. This mechanism would also seem to argue that there must be an odd number of methylenes between two even-carbon branches, but the fact that the catalyst has the ability to walk to any spot on the polymer backbone and insert one or more ethylenes means that this is not true. Thus, if there are an even number of methylene carbon atoms between two consecutive branches, at least one of them must contain an odd number of carbon atoms. This mechanistic argument has been nicely confirmed in the case of nickel-catalyzed... 

The level of alkene isomerization in the Heck reaction between an aryl bromide and 2,3-dihydrofuran was found to be controlled by the choice of neopentyl phosphine ligands (14JOC10837). Di-fert-butylneopentylphos-phine (DTBNpP) favored the formation of product A through double bond migration via a chain-walking mechanism. This selectivity was dramatically reversed when trineopentylphosphine (TNpP) was used, favoring product B without alkene isomerization. 

Post-metallocene developments include Brookhart catalysts based on nickel and palladium. These catalysts can incorporate polar monomers such as methyl acrylate into polyethylene chains. They possess chain-walking mechanisms that allow synthesis of various stmctures from HOPE to hyperbranched PE, as shown in Scheme Chain topology depends on the... 

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

Finally, we look at a mechanism-based explanation for the highly branched PE obtained with 6.18. In these catalytic systems, low-temperature NMR shows that the resting states of the catalysts are cationic complexes of the type 6.33, With these types of catalysts, the metal atom moves along the polymer chain via j3-hydride ehmination and readdition reactions. This is shown by reactions 6.5.3.3-6.5.3.5. Collectively, these reactions are called the chain walk mechanism. 

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