Synthesis, characterization, and purification of cyclic polystyrene, poly(ε-caprolactone), and various polyethers
Description
The synthesis of cyclic polymers has become popularized due to improved synthetic chemistry increasing access to architecturally well-defined polymers. Since their discovery, many studies have been performed describing their physical properties in solution and bulk, and as confined thin films. Also, their electronic, biomedical, and self-assembly behavior has been characterized for advanced biomedical and industrial applications. Herein, some of the ongoing shortcomings within the cyclic polymer field are addressed by a detailed analysis of cyclic polymers synthesized through: 1) the copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) “click” ring closure method, and; 2) the electrophilic zwitterionic ring expansion polymerization (eZREP). First, cyclic polystyrene synthesized via CuAAC is investigated to source potential impurities using a series of analytical tools including HPLC for fractionation and MALDI-ToF MS for characterization of the linear precursor and cyclic product. It was ascertained that the linear precursor undergoes uncatalyzed azide alkyne dimerization during storage, resulting in a linear dimer that retains an azide and alkyne group. Consequently, post-cyclization the sample has a high cyclic architectural purity with small amounts of cyclic dimers. With this same CuAAC cyclization, cyclic polycaprolactone was generated to fabricate thin films (~100 nm). The CuAAC cyclization was performed using an optimized synthesis, where the azide was coupled to the linear precursor directly prior to cyclization to minimize the opportunity for dimerization. Both the linear and cyclic polymers made stable as-cast films, but only the cyclic polymer maintained a stable film after recrystallization as demonstrated by its resistance to dewetting. Moreover, the linear polymer dewet after recrystallization regardless of end group, suggesting that architecture provides a larger influence on thin film stability than end group effects. Finally, towards understanding the ring expansion method, eZREP has been investigated to generate pure cyclic polyethers by polymerizing monosubstituted epoxides with B(C6F5)3. While the major component was the cyclic structure, in most cases, there was still a contribution of impurities arising from non-cyclic structures (e.g., tadpole and linear architectures). Using primarily MALDI-ToF MS, it was ascertained that most impurities had free hydroxyl groups that could be alkynylated, “clicked” onto a solid phase azidified resin, and then filtered to remove resin-bound impurities. This allowed for a facile method of making pure cyclic materials in two reactions (i.e. polymerization and “click” scavenging purification). Finally, the origin of the impurities was further explored, elucidating that when glycidyl ether-based monomers were polymerized, there was competition between the boron coordinating with the epoxide oxygen to polymerize, and boron coordination with the ether oxygen to generate non-cyclic derivatives. The polymerization was improved by using monomers that either did not contain the glycidyl ether oxygen (e.g. alkyl groups) or adding electron withdrawing groups to deactivate the glycidyl ether oxygen, yielding greater amounts of cyclic polymer and fewer side reactions. The goal of this work is to garner interest in cyclic polymers by increasing accessibility of these compounds by addressing the primary deficiencies of the CuAAC and eZREP methods for the synthesis of cyclic polymers, while bringing attention to the concern of cyclic polymer purity, arguably the largest concern within the cyclic polymer community. Through these advancements, continued efforts will be made to make novel cyclic materials for unique biomedical and industrial applications.