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Reports: ND754596-ND7: Synthesis, Functionalization, and Characterization of Crosslinked Porous Elastomers

Robert B. Grubbs, State University of New York at Stony Brook

Overview: This project explores the polymerization of petroleum-derived cycloolefins for the preparation of porous crosslinked elastomers (PCEs). (Scheme 1) This class of monomers is amenable to synthetic manipulations and thus offers a facile route to a variety of functionalized PCEs. Such materials have not been extensively explored, but would be valuable in applications involving tissue engineering, substrates for flexible electronics, and low impact cleaning materials for fine art. In this regard, we have proposed to (1) optimize PCE synthesis (2) prepare functionalized PCEs and (3) prepare materials suitable for model applications.

Scheme 1.

During the grant funding period (9/2014-8/2015), we have accomplished:
- Ring opening metathesis copolymerizations of cyclopentene (CP) and dicyclopentadiene (DCPD); cyclooctene (COE) and DCPD; CP, COE, and DCPD; and CP with novel crosslinkers have been explored and targeted for desired elasticity.
- New norbornene based crosslinkers and antioxidant functionalized monomers have been synthesized.
- Qualitative assessment of material properties based on systematic variation of polymerization conditions (monomer concentration and composition, catalyst concentration, as well as porogen selection) was made in regard to resulting material properties.
- Small amplitude oscillatory shear (SAOS) rheological characterization of PCEs was conducted.  Dynamic moduli were related to polymer compositions.

Key Findings: Our initial synthetic efforts focused on the copolymerization of cyclopentene (CP) with dicyclopentadiene (DCPD) under precipitating conditions. It was found that this approach does not easily yield materials with elastomeric properties suitable for our target applications. Several synthetic parameters were investigated as a means of exploring the character of the polymerization, including monomer feed ratio, catalyst concentration and reaction media. Monomer conversion and resulting polymer properties were found to be sensitive to all reaction parameters. Pore generation was primarily influenced by solvent (s) to monomer (m) ratio. In particular, a 10:1 (s:m) mole ratio was able to induce macropore formation at monomer conversions of ~ 60%. It is presumed that the lower reactivity of CP limited its incorporation into the copolymer, an effect which was amplified at low monomer conversions. 
Early difficulties in optimization of the CP/DCPD system led to exploration of new monomer/cross-linker systems. The most promising results have been obtained working with cyclooctene (CO) based terpolymer systems as well as polycyclopentenamers crosslinked with tetraethylene glycol dinorbornenyl acetate (Nb2-TEG) (Scheme 2). Cyclopentene conversions appear to be improved in these systems. For example, while only ~20% CP conversion was obtained in the CP/DCPD copolymerizations, ~75% CP conversions have been attained in CP/COE/DCPD terpolymers and ~70% CP conversions in CP/Nb2-TEG copolymers under similar polymerization conditions with identical DCPD concentrations. The resulting materials have significantly improved elasticity (Figure 1). The rheological properties of these new PCEs have been investigated in order to establish structure-property relationships in the materials.

Scheme 2.


Figure 1. New PCE systems under investigation: a) polypentenamer crosslinked with 5 mol% Nb2-TEG, and CP/COE/DCPD terpolymers incorporating 5 mol% DCPD with b) 75:25 CP/COE and c) 20:80 CP/COE. Polymers are shown being deformed under the weight of a glass microscope slide.

A further difficulty encountered in the preparation of these unsaturated polymers was their susceptibility to oxidation. As a result some effort has been invested into the preparation of antioxidant functionalized monomers, particularly crosslinkers, that could be incorporated into the network systems. The greatest success to date was realized by using a norbornene derivative incorporating a butylated hydroxyl toluene (BHT) moiety. A CP/COE/DCPD terpolymer polymerized with 2 mol% of the antioxidant monomer showed minimal oxidative damage by IR after 30+ days storage under ambient conditions. It is worth noting that the CP/Nb2-TEG polymer also exhibits enhanced resistance to oxidation without incorporation of antioxidant. Future work will include optimization and measurement of porosity in the synthesized PCEs with favorable material properties (elasticity) as well as investigation and implementation of functionalization strategies appropriate for target applications.

Impact.
The PRF award has afforded an opportunity to pursue a new direction of research exploring the preparation of porous elastomers via ROMP methodologies. It has provided training and support for one postdoctoral researcher, as well as training for one graduate student and two undergraduate students in the general field of polymer synthesis and materials characterization. Additionally, this project has fostered collaboration with conservators / art conservation scientists at the Metropolitan Museum of Art in NY.