Reports: DNI751030-DNI7: Carboxylic Acids as Latent Initiators of Radical Polymerizations Mediated by Hypervalent Iodine Compounds: Synthesis of Functional Polymers and Unimolecular Micelles for Emulsification of Oils in Water

Nicolay Tsarevsky, PhD, Southern Methodist University

Work during the second year of the project focused on i) employing exchange of ligands attached to hypervalent iodine atoms with carboxylates for the synthesis of dynamic polymers, including linear and star-shaped polymers; and ii) synthesis of graft copolymers derived from polymers with pendant carboxylate groups via in-situligand exchange followed by homolytic cleavage of the newly formed hypervalent bonds. The work was carried out by a graduate student supported by ACS-PRF (Hongzhang Han). In addition, an undergraduate student (Brendan Celii) was involved in the research related to star-shaped macromolecules with dynamic links in the cores. The work carried out during this period will be the subject of at least three publications, the first two of which, on dynamic polymers, have been written and will be submitted by the end of October 2013. The work on graft copolymers is still ongoing and will be finalized in the next several months. A request for extension was submitted to PRF and approved in August 2013. The period until August 2014 will be mostly dedicated to studies of the application of the prepared materials as dispersants of oils.

i) Synthesis of dynamic polymers

The I-O bonds in (diacyloxyiodo)arenes (ArI(O2CR)2) are labile and can be cleaved both homo- and heterolytically. These compounds can be reacted with dicarboxylic acids (examples are shown in Scheme 1a), affording polymers with multiple backbone hypervalent iodine (III) atoms. The obtained polymers participate in exchange reactions with monocarboxylic acids, which leads to reversible degradation, i.e., the polymers can be re-assembled, if the low-molecular weight acid is removed by extraction or evaporation under vacuum. The results will be a part of paper, which will be submitted shortly. Last year, we demonstrated that when polycarboxylic acids, such as poly(acrylic acid), were reacted with ArI(O2CR)2, networks were formed, which were also dynamic in nature. We continued these studies and prepared a number of gels derived from (diacetoxyiodo)benzene and tri-, tetra-, or various polycarboxylic acids. The mechanical properties of these gels and their reversible degradation behavior is currently being studied. When the polycarboxylic acid was part of a segment in a block copolymer, the reaction with the hypervalent iodine compound afforded star copolymers (Scheme 2b), which, when dissolved in a good solvent for both blocks, degraded in the presence of monocarboxylic acids but could be formed again if the monoacid was removed. Systematic studies were carried out with a 25-member library of block copolymers of styrene and acrylic acid. An undergraduate student was involved in these studies. A paper summarizing the results will be published shortly.

Scheme 1.

Synthesis of dynamic polymeric materials (linear and star polymers) via ligand exchange between DAIB and di- or polycarboxylic acids, and their reversible degradation.

ii) Synthesis of graft copolymers

The exchange of the acyloxy groups in ArI(O2CR)2 with a nucleophile L- (azide, carboxylate, etc.) yields a hypervalent iodine compound of the type ArIL2. The hypervalent bonds I-L can be cleaved homolyticially upon irradiation or heating, yielding the radicals L·. In other words, the ligand exchange is a convenient rout for the conversion of nucleophiles into radicals, which can then participate in a number of reactions, including initiation of polymerization. The exchange reaction between ArI(O2CCH3)2 and methacrylic acid was employed for the formation of hyperbranched polymers, as reported during the first year of this research (a paper was published in Polymer Chemistry). When the exchange reaction involves a polymeric acid (e.g., homo- or copolymers with units derived from acrylic, methacrylic, or 4-vinylbenzoic acid), the homolysis of the pendant I-O bonds leads the formation of a multifunctional macroinitiator, which, in the presence of a monomer, affords a graft copolymer (Scheme 2). Several ways approaches to control the polymerization and prepare polymers with grafts of controlled size were attempted, but the most suitable one was the degenerative transfer of iodine from alkyl halides, such as pefluorohexyl iodide. Currently, we are studying the possibility to carry out chain extension reactions at each grafted chain (derived from a hydrophobic monomer) with hydrophilic monomers. It is envisioned that the formed “core-shell” graft copolymers will act as unimolecular micelles and will be useful as dispersants of oils in water. The studies of the grafting reactions are near completion and, once a report is published, we will concentrate on the synthesis of graft polymers with segmented arms.

Scheme 2.

Exchange of ligands in a hypervalent iodine compound DAIB with polymer-attached carboxylate groups, followed by a controlled grafting-from reaction mediated by an alkyl iodide. The obtained macroinitiator can be chain extended with a second monomer to form a graft copolymer with segmented side chains. One graduate student (Mr. Hongzhang Han) was fully supported by the PRF grant. He employed a variety of analytical and characterization techniques (NMR, SEC, TGA, DSC, DLS, viscometry). During the first year of the research, he coauthored a paper published in Polymer Chemistry, describing the synthesis of hyperbranched polymers using exchange reactions at hypervalent iodine centers. He has already prepared two more papers on dynamic polymers and is currently finalizing work on graft copolymer synthesis. A publication will follow soon. Hongzhang Han is interested in working with undergraduate students and has trained one undergraduate student (Brendan Celii) to determine exchange equilibrium constants but also to prepare star polymers with dynamic cores and characterize them by SEC and DLS as well as to study their degradation in the presence of low-molecular weight carboxylic acids. Brendan Celii is a coauthor on the paper describing star polymers, which is to be submitted soon. In addition to research and training, Hongzhang Han presented his research at several local meetings, including an “ACS Meeting in Miniature”, where he won a Best Presentation Award. He will present his findings at the ACS National Meeting in Spring 2014 in Dallas, TX. The PI has presented the ACS-PRF sponsored work at several meetings and invited seminars. One collaboration has been initiated with colleagues from LSU, related to the applications of hypervalent iodine-based inimers/crosslinkers in frontal polymerization.