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Ethylene pressure has been used to control the competition between isomerization (chain walking) and monomer insertion processes for ethylene coordination polymerization catalyzed by a palladium-o-diimine catalyst. The topology of the polyethylene varies from linear with moderate branching to "hyperbranched" structures. Although the overall branching number and the distribution of short-chain branching change very slightly, the architecture or topology of the polyethylene changes from linear polyethylene with moderate branches at high ethylene pressures to a hyperbranched polyethylene at low pressures.
Vinyl polymers are an important category of synthetic materials-millions of tons are produced every year for broad applications such as fibers, plastics, and elastomers (1). Most vinyl polymers are synthesized by addition polymerization of vinyl monomers. In various addition polymerization mechanisms (anionic, cationic, free radical, group transfer, coordination, and so on), the active growing site usually stays at the chain end, and the unidirectional monomer addition to the chain end leads to the formation of linear polymers (2). The need to synthesize polymers with unusual properties has driven the effort to design polymers with new architectures (3). One approach is to introduce branches into the polymer backbone to produce architectures such as dendritic (4) and its practical analog, the hyperbranched structure (5). Most of the hyperbranched polymers reported are made from condensation polymerization of AB 2 type of monomers, and recently an elegant "self-condensing" approach was applied to polymerize vinyl monomers to give hyperbranched structures (6, 7). In both approaches, branching is introduced by the structure of the monomer. Each addition of a monomer increases the number of active sites from one to two (Fig. 1, A and B). Despite the elegance of these approaches, most of them require the synthesis of a specially designed monomer. It is highly desirable to identify methods that can directly polymerize existing commodity monomers to give controllable architecture. Here we report a fundamentally different approach in which the polyethylene (PE) topology is controlled simply by pressure for ethylene polymerization catalyzed by a palladium (Pd) catalyst.
Our approach is to introduce branching by using a polymerization system in which the active growing site isomerizes to an internal position on the polymer backbone during propagation, so that the next monomer unit can be assembled onto the polymer backbone instead of at...