My research group focuses on polymer and materials chemistry, particularly with an emphasis on cyclic polymers. We work in a collaborative and interdisciplinary atmosphere, moving from the synthesis of complex macromolecular systems to the study of their physical properties. With guidance from theory and physics, we design, synthesize, and characterize new polymer structures and create polymer-based materials. We aim to understand the mechanisms of polymerization that lead to the formation of cyclic polymers, as well as the property-structure relationship of these intriguing structures.
We typically employ ring-opening polymerization techniques combined with post-polymerization functionalization reactions to prepare a range of materials. Synthesizing these materials constitutes the primary focus of our research program, providing students with exposure to numerous synthetic techniques. Various methods for chemical and physical characterization of polymers and materials are utilized.
Below, we outline representative research projects developed to date:
Investigation of the synthesis and characterization of novel cyclic polymers for advanced material applications
Cyclic polymers are currently at the forefront of macromolecular science. This is because cyclic polymers exhibit intriguing physical and chemical properties due to the absence of end groups and their circular architecture. The extensive research conducted on synthetic polymers has been crucial in understanding their property-structure relationship. This progress has been made possible thanks to innovative advancements in organic synthesis and catalysis, which have led to the development of various pathways for cyclization of preformed chains through ring-closure (RC) strategies and ring-expansion polymerization (REP) of various monomers. As a result, there is now a wide array of synthetic methods available for producing various types of cyclic polymers. Our review on cyclic polymers covers recent advances in the synthesis and purification of cyclic (bio)polymers, together with a very comprehensive historical revision of the synthesis methods used to date.
We have studied the dielectric behaviour of cyclic polymers with different orientation of the dipolar moment along the chain contour; with inverted-dipole microstructure and with cancellation of the dipolar moment. The control over the dipolar microstructure enables the monitoring of single-chain behavior in an electric field, providing dielectric spectroscopy with essential structural and dynamical information for synthesized cyclic polymers.
The synthesis of cyclic polyethers by zwitterionic REP (ZREP) was reported in 2014, where we proved that the reaction of monosubstituted epoxides with B(C6F5)3 at high concentratation leads to the formation of a diversity of cyclic polyether structures, including copolymers with THF . Following studies were centered in the investigation of non-cyclic impurities of ZREP using MALDI-ToF MS and their purification in collaboration with Prof. Scott Grayson (Tulane University). Our most recent works on ZREP are devoted to the formation of water-soluble branched cyclic polyglycerol structures either by reaction of glycidol in toluene or bulk , as well as the generation of cyclic polyethers from protected-glycidol monomers with B(C6F5)3.
In collaboration with Prof. Francesca Re (University of Milano-Bicocca), we have rencently evaluated the ability of branched cyclic polyglycerol structures to cross the blood-brain barrier (BBB), a highly selective and protective physiological barrier composed of specialized endothelial cells that tightly regulate the passage of substances from the bloodstream into the brain, by using in vitro transwell model made by hCMED/D3 cells. We demonstrated potential applications of our synthesized branched cyclic polyglycerol structures in brain delivery systems, and highlighted the importance of polymer topology in the BBB crossing in vitro.
In collaboration with Dr. Marek Grzelczak (Materials Physics Center, CSIC-UPV/EHU) we have studied flat gold surfaces and gold nanoparticles (AuNPs) chemically grafted with cyclic PEO (CPEO) brushes and have shown that polymer topology really matters. For example, surface wettability, AuNP colloidal stability in water and in ethanol are some of the properties measured. The differences found between CPEO brushes and their linear analogues can be attributed to the fact that the cyclic brushes have to fit into less space near the surface than the linear chains, resulting in an increased concentration of polymer segments near the surface. As a result, the cyclic chains are forced to stretch more than the linear chains. In ethanol, AuNPs modified with linear PEO (LPEO) suffer from reversible phase separation upon temperature drop over the course of a few hours. However, the use of a polymer brush with cyclic topology as a stabilizer prevents sedimentation and ensures colloidal stability in ethanol at -25 ºC for several months. We postulated that temperature-induced collapse of chain brushes promotes interpenetration of linear chains, causing progressive sedimentation of AuNPs, a process that is unfavorable for cyclic polymer brushes whose topology prevents chain interpenetration.
In collaboration with Prof. Luca Salassa (DIPC) and Dr. Ana Pizarro (IMDEA, Madrid) we have synthesized the first prototype of a Ruthenium-polypyridyl cyclic macromolecule based on PEO, featuring long-lived emission in the red region of the visible spectrum and the capacity of accumulating in MCF7 cancer cells.
Graphene-based compounds as sieve materials for the separation of cyclic and linear polymers
The purity of cyclic polymers is a major concern in polymer physics. Small amounts of linear impurities in cyclic polymers are known to affect important physical properties such as rheological and crystallization behavior. However, large-scale purification of cyclic polymers from linear polymer impurities remains a challenging task. In our current studies, we have shown that cyclic oligomers of polyethylene oxide (PEO) exhibit much slower intercalation kinetics in graphene oxide than their linear analogues due to entropic penalties. This enormous difference in intercalation rate was used to selectively exclude cyclic molecules from the linear ones, thus revealing the potential of graphene oxide for the
separation of cyclic and linear oligo(ethylene oxide)s. The differences in intercalation rate become smaller with increasing molecular weight. By introducing fixed pillars into the GO structure, which act as physical barriers against polymer diffusion, we have shown that it is possible to restrict the intercalation of cyclic PEOs into graphene oxide while allowing the linear analog to diffuse through the graphene oxide layers. This important finding could be the basis for the development of a method for the purification of cyclic polymers.
Polymers under confinement
We obtained well-defined polymer monolayers of 3.4 Angstroms thickness confined in two dimensions. This was achieved by intercalating poly(ethylene oxide) (PEO) into graphite oxide. In this extreme confinement, the PEO chains showed a striking absence of crystallization and suppression of the glass transition, as well as an absence of alpha relaxation and a slowing of beta relaxation in the dielectric response. The vibrational density of states of 2D-confined PEO was obtained by vibrational spectroscopy with neutrons revealing unprecedented details of the PEO structure such as the suppression of certain vibrational modes and the distinct formation of zigzag planar structures in contrast to the characteristic helical conformation of the bulk polymer crystal. These results were shown to be largely insensitive to molecular size due to the probable emergence of a new set of chain length scales, primarily dictated by the presence of anchoring points on the graphite oxide upon intercalation. A different picture emerged for PEO chains adsorbed on graphene sheets. In this case, the PEO exhibited a physical behavior closer to the bulk phase, albeit with glass transition and melting events at significantly lower temperatures.
We studied the 3D confinement of PEO in pores of: carbon nanoparticles and resorcinol-formaldehyde resin nanoparticles. The results showed that the glass transition of PEO confined in pores is primarily driven by geometrical constraints imposed by the small micropore size (diameters smaller than 2 nm) rather than by specific polymer-substrate interactions at the mesopore. Once again, vibrational spectroscopy with neutrons revealed the emergence of planar zigzag conformations in the confined PEO phase, demonstrating the effects of confinement on the polymer's fisical properties.
