Rubbing a venerable polymer the right way
Rubber polymers have been capturing the attention of chemists and the wider public since at least 1770, when pioneering chemist Joseph Priestley coined the term for a material that could remove lead pencil marks from paper. From humble school supply to vital component for millions of wheel-bound vehicles, this versatile substance is now poised to get even more interesting as 21st century chemists begin to tweak key parts of its molecular anatomy.
The chemical backbone of butyl rubber — used in everything from car tires to basketballs — is isobutylene (C4H8). But University of British Columbia chemistry professor Derek Gates is looking beyond a carbon-hydrogen paradigm to other elements that could be substituted within this formulation. As a hint of what might be possible, Gates points to the development of the once-popular toy Silly Putty, which spun off wartime research into synthetic rubber substitutes in the 1940s. Silly Putty contains no carbon but instead features silicon in its structure, a substitution that results in flow characteristics that differ sharply from those seen in natural rubber, as every kid who played with the stuff can testify.
With that inspiration, Gates focused on substituting one of butyl rubber’s carbon-carbon double bonds with a phosphorus-carbon double bond. Recently he introduced two of the resulting materials, 1-phosphaisoprene and 1-phospha-1,3- butadiene, in an article for Angewandte Chemie.
His aim was dictated by the dynamics of group chemistry. “Instead of having four groups surrounding the phosphorus, as silicon and carbon do, phosphorus only has three,” Gates says. “That leaves room to do chemical transformations. Our hypothesis is that the phosphorus is going to lead to unique chemical functionality, the type of functionality that’s not possible in a carbon-containing polymer. We believe that changing elements will revolutionize the properties.”
Spectrometric analysis confirmed as much, showing how the polymerization of these materials still occurs primarily through remaining C=C bonds, with most of the phosphorus atoms winding up in side chains where they are available for other reactions or manipulations. Gates suggests that these atoms offer the prospect of further derivatization and cross-linking, which he and his colleagues demonstrated through the extraordinary step of binding these polymers to gold (I) ions. “We’re pretty excited about what we could do,” he says. “We also imagine things like catalysis, where you have a material that incorporates with the phosphorus group and you can then bind to metals and the metals can be used in a catalytic organic transformation.”
Gates isn’t content to stop with phosphorus. He is already looking forward to nurturing a whole family of functional groups by inserting boron, silicon and/or nitrogen into the diene monomer. The properties of such “unnatural” rubbers, Gates acknowledges, will remain a mystery until they are available on a lab bench, a feature that makes this kind of discovery research all the more compelling for him. “Above all, it’s fundamentally intriguing,” he says.