Synthesis of New Polymers and Block Copolymers

All of our work hinges crucially on the ability to synthesize new polymers of well-defined and controllable macromolecular structure. Usually these syntheses rely on so-called “living” polymerization chemistries: chain-growth polymerizations where there is negligible chain transfer or spontaneous termination of the active sites, that allow one to build the final macromolecule through a series of sequential steps. Typically these polymerizations are conducted in a controlled environment (glove box or vacuum line) to exclude adventitious impurities that might “kill” the active sites, and solvents and monomers must be rigorously purified.

Mike Showak
Undergraduate Mike Showak ’13 using the Schlenk (dual vacuum/nitrogen) to purify ROMP monomers. Just out of frame to the left is the nitrogen-atmosphere glove box used for ROMP.

Currently, we are employing three different kinds of polymerization chemistries (anionic, ROMP, Pd-catalyzed; see links below), each of which is effective for a particular class of monomers. In past work, we have also combined polymers made by these orthogonal polymerization chemistries to create block copolymers that cannot be made by any single mechanism.

We often modify as-synthesized polymers via post-polymerization chemistries, to alter their local monomer structure and hence manipulate their physical properties, such as glass transition temperature or crystallizability. Modifications can include ionomerization (sulfonation, carboxylation), as well as removal of protecting groups on monomers, but currently, the most commonly practiced post-polymerization reaction in our laboratory is catalytic hydrogenation. Elimination of olefinic unsaturation greatly improves a polymer’s thermo-oxidative stability, but it can also have a dramatic effect on physical properties, including miscibility with other polymers. We employ a variety of molecular and supported catalysts with differing selectivities for olefinic and aromatic unsaturation; this allows the preparation of several different “daughter” polymers, each with its own set of properties, from a single “parent” polymer, as exemplified in the figure below.

Modification of repeat unit structure in ROMP
Modification of repeat unit structure in ROMP polymers via catalytic hydrogenation. The two chemical structures in the top line are polymerized via Mo- or Ru-initiated ROMP to yield the two polymers in the second line. Hydrogenation of PPhNb with two catalysts of different selectivity leads to the two polymers in the third line. Thus, with these two monomers and two hydrogenation catalysts, all four combinations of backbone and sidegroup (un)saturation are accessible, yielding the glass transition temperatures indicated (Li et al., Macromol. Chem. Phys., 213, 2027 (2012)).

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