A new method allows chemists to precisely place single monomers at any position within a polymer chain made of other types of monomers. With this single monomer, chemists can add various types of functionality such as chromophores to produce sensors or another polymer chain to create novel polymer architectures.
Previous methods for adding single monomers have lacked precision, either adding multiple monomers or none where only one was desired. In the new method, Yan Xia and coworkers at Stanford University use cyclopropene ester derivatives with ring-opening metathesis polymerization to add individual cyclopropenes at multiple locations in a polymer otherwise made from norbornenes (Chem 2019, DOI: 10.1016/j.chempr.2019.07.017).
For examples, if they want to make a 100-unit polymer with the special monomer at about 30% of the total length, they first make a 30-unit polymer and then add the cyclopropene monomer. They then continue the norbornene polymerization with another 70-unit segment.
The challenging part of getting this precise method to work turned out to be restarting the norbornene polymerization after inserting the cyclopropene monomer. In their early attempts, about half of the polymers wouldn’t continue to grow.
Former grad student Benjamin R. Elling optimized the conditions so that the polymerization would pick up where it left off. He suspected that the ring-opened cyclopropene was complexing with the ruthenium catalyst that drives the polymerization, preventing complete reinitiation. To free up the catalyst, he lowered the temperature of the reaction to −30 °C and added an excess of 3-bromopyridine, which weakly binds to the catalyst. Both modifications shifted the equilibrium back toward the norbornene polymerization. Adding 3-bromopyridine once to the reaction mixture is enough for the rest of the polymerization, even if the chemists add other cyclopropene monomers at other locations along the polymer.
Xia was surprised that they could add just one cyclopropene to the chain without it polymerizing. “Cyclopropene may be the last monomer you would think of not being able to polymerize,” he says. “When you have a lot of strain, it’s really hard to stop the polymerization.” The simple ester groups added to the rings effectively prevented the addition of more cyclopropene monomers, Xia explains.
Xia and his team are exploring other cyclopropene derivatives to understand what’s required for single monomer addition and for running the reaction at room temperature. They are also exploring this new synthetic capability to more precisely control polymer architectures and properties.
“[Xia] and coworkers are bringing the structural control that we normally associate with proteins to the synthetic arena,” says Craig J. Hawker, a polymer chemist at the University of California, Santa Barbara. “Coupled with the versatility of ring-opening metathesis polymerization, I see numerous researchers worldwide taking full advantage of this exciting discovery.”