Coordination copolymerization of polar vinyl monomers H2C=CHX.

Of all polymerization methods, catalytic polymerization offers the broadest scope of microstructure control. Stereoregularity, comonomer incorporation and sequences, molecular weights, and molecular weight distributions can be controlled by the catalyst structure. Polyolefins are produced on a vast scale of nearly 100 million tons annually, predominantly by catalytic polymerization. High-density polyethylene (HDPE) and isotactic polypropylene are examples. These polymers are hydrocarbons without any heteroatom-containing functional groups, such as ester moieties. Catalysts employed in industrial polyolefin production are based on early transition metals such as Ti, Zr, Cr, or V. Owing to their high oxophilicity, the polymerization-active metal centers interact strongly with oxygenor nitrogencontaining moities, rendering them inactive for insertion polymerization. Thus, polar-substituted vinyl monomers H2C=CHX, of which vinyl acetate (VA), acrylates, and acrylonitrile (AN) are the major representatives, are homoand copolymerized by routes other than insertion polymerization, usually by free-radical polymerization. Polyolefins containing polar groups are prepared on a large scale in the form of ethylene–VA copolymers by high-pressure freeradical polymerization. Since the discoveries of Ziegler and Natta, the coordination copolymerization of polar-substituted vinyl monomers H2C=CHX has been a challenge. Incorporation in polyolefins is of broad interest, for example, to increase interactions with polar surfaces, such as metals, or to achieve stability towards hydrocarbon solvents. A major advance was achieved in the mid-1990s with Brookhart and co-workers7 studies of olefin polymerization by cationic Ni and Pd diimine complexes. In comparison to the aforementioned early-transition-metal complexes, these late-transition-metal complexes are much less oxophilic. For the first time, ethylene or 1-olefins were copolymerized with acrylates in a catalytic fashion. Owing to the propensity of the Pd complexes employed to “chain walk” [Eq. (1)] on the growing chain during polymerization, highly branched amorphous polyethylenes with the acrylate repeat units located predominantly at the ends of branches are obtained (see also Scheme 2).