Polymer chemists have long endeavored to gain control over the precise chemical structures of the polymers they synthesize. Polymers can have variable lengths and length distributions, chemically programmed units at each chain end, and different spatial arrangements of the pendant side chain atoms—a characteristic known as stereochemistry. Controlled polymerization techniques developed in the past three decades have provided excellent control over polymer length and chain end functionality (1). However, examples of stereocontrolled polymerizations are rare, and few methods have been developed to a sufficiently advanced level for commercialization. On page 1439 of this issue, Teator and Leibfarth show that an organocatalyst can be used to exact exquisite control over the stereochemistry and microstructure of several different poly(vinyl ethers) (PVEs) (2).
Perhaps the most successful example of stereocontrolled polymerization was the development of metal-organic polymerization catalysts in the 1950s (3). This led to the current dominance of polyolefins such as polyethylene (PE) (low-density and linear low-density PE are synthesized with metal-organic catalysts) and polypropylene (PP) in the plastics market (4). Since this discovery, several advances toward stereocontrolled copolymerization of functional monomers with ethylene or propylene have been made by using metal catalysts to expand the chemical diversity of stereo-controlled polymerization (5–10). Sawamoto and co-workers have reported preliminary studies on stereocontrolled polymerization of vinyl ethers (11, 12).
Although PE and PP have captured a majority of the plastics market, they have limited chemical diversity. Because both are entirely composed of carbon and hydrogen, they adhere poorly to materials such as glass, metal, or tissue, which limits their applications as adhesives or coatings and in biomedical devices. By contrast, PVEs adhere strongly to these materials due to their high oxygen content (13). But this improvement has come at a cost, because existing synthesis methods led to PVEs with low stereo-regularity and poor mechanical performance.
Teator and Leibfarth show that by precisely controlling the stereochemistry of PVEs, they can create highly functional materials with mechanical properties that are comparable to those of PE and PP. The authors report a catalyst-controlled approach for the preparation of PVEs with well-defined stereochemistry via cationic polymerization. They use an organocatalyst, which is cheaper and less toxic than the inorganic catalysts that are used to make PE and PP (14). The authors designed their catalyst to be stereoselective, taking account of how the geometry of the catalyst complex influences the orientation of monomers as they are polymerized. In this way, enchainment is directed toward a single face of the propagating chain end, resulting in the formation of polymers with high structural regularity (see the figure). The method is compatible with various vinyl ether monomers, suggesting broad scope for possible future industrial processes.
Traditional methods for making poly(vinyl ethers) are based on free-radical polymerization, which yields amorphous polymers without regular stereochemistry. Teator and Leibfarth instead use an organocatalyst that enables full control over the polymer’s stereochemistry. The resulting polymers are semi-crystalline, mechanically strong, and strongly adhesive.
GRAPHIC: JOSHUA BIRD/SCIENCE
The PVEs produced with Teator and Leibfarth’s catalytic method possess mechanical properties comparable to those of commercial low-density PE, while also exhibiting more than an order of magnitude stronger adhesion. Endowing PE-like materials with strong adhesion could lead to advanced, low-cost adhesives, coatings, and biomedical devices. These findings, coupled with the scalability of the polymerization procedure (multi-gram polymerizations were achieved without changing the procedure), the high processability of the resulting material (which can be melted, molded, and cooled), and the broad monomer scope signify an important breakthrough in the plastics field.
Translation of this simple and effective concept to other polymerization systems could rapidly push plastics manufacturing toward more highly functional materials. Indeed, the prospect of introducing control over polymer stereochemistry in non-hydrocarbon systems is a necessary and important step toward designing high-value, recyclable, degradable materials, because certain carbon-oxygen bonds can be rapidly broken down in the environment.
The findings of Teator and Leibfarth are a reminder that consideration of fundamental polymerization concepts can revitalize existing materials. Whereas polymer researchers have tended to focus on increasingly complex polymers, this work reinforces the importance of simple principles that underlie polymer chain behavior—in this case, the stereoregularity of repeating units—and their impact on the resulting material’s macroscopic properties.
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