Chain walk

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]. 

These materials have possible utility in a number of specialty applications and are being explored by Guan et al. [37], They have used these catalysts, and their unique chain-walking characteristics to synthesize a variety of dendritic materials (Fig. 4), which could find potential application as processing aids, rheological modifiers, and amphiphilic core-shell nanoparticles for drug delivery and dye formulation. 

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. 

Scheme 5 The chain walking process. The formation of a methyl branch is shown...

The P-alkoxy elimination pathway is important during the incorporation of oxygen-containing monomers. Therefore, it is often necessary to provide distance between the olefin and the polar group, or to prevent chain walking close to the group that can be eliminated by the placement of a quaternary carbon spacer [87], The incorporation of acrolein dimethyl acetal is accompanied by reduced activity and full catalyst... 

In the absence of external forces the all directions of the end of chain walking are equiprobable accordingly to condition ni = N / d so... 

Another typical feature of these catalysts is the socalled chain walking . Prior to insertion of the next ethene molecule a series of P-hydride elimination and re-insertions can take place, which looks like a metal atom running along the chain. Insertion of ethene in a secondary alkyl chain leads to the formation of branches. During the isomerisation process palladium can even cross tertiary carbon atoms, since branches on branches are obtained. 

When the ethene pressure is raised the number of branches decreases, while the productivity of the catalyst remains the same (the reaction is zero order in ethene pressure). Chain walking requires an open site at the metal and obviously the competition between ethene complexation and chain walking determines the number of branches formed. 

Thus, it has been shown that, in SOMC-catalyzed hydrogenolysis on an oxide at low temperature, side phenomena due to adsorption or chain walking could occur. These results with linear alkanes and polymers bring a better understanding of the catalytic activity of the ZrH catalyst. 

Figure 3.25 Chain walking hypothesis in polyolefin hydrogenolysis. After a first cleavage in the polymer chain, the polymer moves from the cleavage position (along the chain) and undergoes successive cleavages.

Scheme 4 Initiation, propagation, chain-transfer, and chain-walking reactions in ethylene polymerization catalyzed with group 10 a-diimine catalysts.

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-... 

Complex 17a displayed moderate catalytic activity toward the polymerization of ethylene (3.3 x 105 g/molh 1). In addition, higher molecular weight distributions were observed (Mw/Mn = 12.8). The 13C NMR analysis of the polyethylene showed that methyl branches predominate (with ca. 3.4 methyl branches per 1000 carbon atoms), suggesting that chain walking does not affect polymerization to a high degree. When only the pyridine moiety (and not the imidazolium salt) is ligated (17b) [48], ethylene polymerization occurs twice as effectively (6 x 105 g PE/(mol of Ni) h 1) under similar conditions (only 30 min rather than 60 min). 

Shultz LH, Brookhart M, Measurement of the Barrier to -Hydride Elimination in a -Agostic Palladium-Ethyl Complex A Model for the Energetics of Chain-Walking in (-Diimine)PdR+ Olefin Polymerization Catalysts, Organometallics, 20, 3975-3982 (2001)... 

Figure 15 Formation of branches in polyethylene by chain walking of diimine-nickel catalyst.

Because of the relative rates of chain propagation versus chain walking, polymers from the bis(imine) catalysts can be quite different depending on the metal. Nickel complexes form polymers with mostly shorter-chain branches and more crystallinity while polyethylene from the palladium analogs is more highly branched, to the point it can be amorphous. The palladium complexes also have the abihty to incorporate remarkably high (1 10 mole percent) amounts of polar monomers such as methyl acrylate and methyl vinyl ketone, though at considerable loss in activity. ... 

The palladium diuuine catalysts can be used to prepare polymers of a-olefins as well as cyclic internal olefins. For a-olefins, the rate of polymerization is slower, compared to ethene, but high molecular weigh polymers are formed. Chain walking takes place with a-olefins, but the a-olefin does not insert into secondary methylene carbons. Copolymers of a-olefins and ethene give polymers with complex microstructures since ethene will insert into secondary... 

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