1. Characterization of SCNPs Summary and Conclusions
Fabrication of Polymer Nanoparticles via Intrachain Ring-Opening Metathesis Polymerizations (ROMP)
Sarah J. Benware, Isabelle M. Crawford-Eng, Ruiwen Chen, Erik B. Berda. Department of Chemistry, University of New Hampshire.
Introduction
Synthesis of SCNP
Characterization of SCNPs
The function of biomolecules is heavily dependent on structure and specific placement of functional groups. The formation of these precise structures is often the result of perfectly controlled polymerizations. Many polymer chemists study fundamentals of these complex natural processes and attempt to synthesize similarly complex and feature-rich macromolecules. To this end, research in the Berda group involves the synthesis of polymers with narrow molecular weight distributions using controlled radical polymerizations. These polymers are designed to incorporate cross-linkable functionalities which may be reacted to form three-dimensional nano-objects. Intramolecular cross-linking induces a collapse observed by the reduction of the hydrodynamic radius of the polymer, producing single chain nanoparticles (SCNPs) (Fig. 1)1.
Fig. 1 Linear polymer chains are decorated with functional groups that will promote intra-chain interactions when triggered in dilute solution.
Fig. 5 GPC of parent polymer and SCNP detected by multi-angle light scattering and Refractive Index. Increase in retention time from parent polymer to SCNP.
Fig. 6 GPC of parent polymer and SCNP detected by multi-angle light scattering and refractive index. No increase in retention time of SCNP, coupled with a broad, higher molecular weight distribution.
The norbornene imide ethanol was synthesized through the reaction of the cyclic anhydride with 2-aminoethanol and triethyl amine. A dicyclohexylcarbodiimide (DCC) coupling converted the product to NBI- methacrylate, through the use of dimethylamino pyridine (DMAP) and methacrylic acid (Scheme 1).
The random copolymer was synthesized with a target of 20% incorporation of NBI-methacrylate in methyl methacrylate. The resulting copolymer was found to contain 15.9% incorporation by nuclear magnetic resonance (NMR) analysis. The polymer was cross linked into single chain nanoparticles (SCNPs) in dilute solution via ring opening metathesis polymerization (ROMP) through the use of Grubb’s 3rd and 2nd generation catalysts (Scheme 2).
Summary and Conclusions
The results of gel permeation chromatography support the conclusion of the synthesis of SCNPs through the collapse of the functional polymer with Grubb’s 3rd generation catalyst. The reaction caused an increase in retention time, suggesting decrease in the hydrodynamic radius of the particles. A shift towards lower retention time after reaction of the parent polymer with Grubb’s 2nd generation catalyst in tandem with a high molecular weight aggregation suggests the presence of intermolecular crosslinking, which is unfavorable for this study. This collapse experiment with Grubb’s 2nd generation catalyst will be repeated to determine if these results persist, and if so, the reaction will be carried out in a more dilute solution to decrease the probability of intermolecular crosslinking. Once efficient reaction conditions and catalyst species are identified, random copolymers will be synthesized to contain varying percentages of the di-functional monomer, 15, 30, and 50 percent, to observe the collapse efficiency and effect on retention time as seen by characterization through GPC.
The characterization of SCNPs is greatly facilitated by gel permeation chromatography (GPC). This process separates molecules by their effective size in solution, by injecting the prepared sample into a continually flowing mobile phase through a column packed with rigid, porous beads (Fig. 2)2.
Fig. 2 Cross sectional view of porous particle2
A dually-functional norbornene imide (NBI) monomer was synthesized. The design features two orthogonal polymerizable units, the first of which can undergo reversible addition fragmentation chain-transfer (RAFT) polymerization to produce a primary polymer structure with a flexible backbone, and the second of which can undergo ring opening metathesis polymerization (ROMP) to produce a secondary structure through cross-linking. The monomer was then polymerized with methyl methacrylate in varying incorporations to observe the effect of incorporation on the efficiency of the collapse of the parent polymer to SCNP.
Acknowledgements
Fig. 3 Stepwise nuclear magnetic resonance analysis
The authors would like to graciously thank both the Army Research Office and the Army Educational Outreach Program for the support, as well as the program for its funding which allowed research to be conducted and presented. We would also like to thank Dr. Erik Berda, for sharing his time and expertise.
References
Lyon, C. K., Prasher, A., Hanlon, A. M., Tuten, B. T., Tooley, C. A., Frank, P. G., & Berda, E. B. (2015). A brief user's guide to single-chain nanoparticles. Polymer Chemistry, 6(2),
Waters Corporation
Fig. 4 Evolution from Polymer to SCNP shown through nuclear magnetic resonance