Understanding the Catalytic Activity of Single-Chain Polymeric Nanoparticles in Water

The structuring role of benzene-1,3,5-tricarboxamide (BTA) groups for the catalytic activity of single chain polymeric nanoparticles in water was investigated in the transfer hydrogenation of ketones. To this end, a set of segmented, amphiphilic copolymers was prepared, which comprised oligo(ethylene glycol) side chains to impart water solubility, BTA and/or lauryl side chains to induce hydrophobicity and diphenylphosphinostyrene (SDP) units in the middle part as a ligand to bind a ruthenium catalyst. All copolymers were obtained by reversible addition-fragmentation chain transfer (RAFT) polymerization and showed low dispersities (M-w/M-n=1.23-1.38) and controlled molecular weights (M-n=44-28 kDa). A combination of circular dichroism (CD) spectroscopy and dynamic light scattering (DLS) showed that all copolymers fold into a single chain polymeric nanoparticles (SCPNs) as a result of the helical self-assembly of the pendant BTA units and/or hydrophilic-hydrophobic phase separation. To create catalytic sites, RuCl2(PPh3)(3) was incorporated into the copolymers. The Cotton effects of the copolymers before and after Ru(II) loading were identical, indicating that the helical self-assembly of the BTA units and the complexation of SDP ligands and Ru(II) occurs in an orthogonal manner. DLS revealed that after Ru(II) loading, SDP-bearing copolymers retained their single chain character in water, while copolymers lacking SDP units clustered into larger aggregates. The Ru(II) loaded SCPNs were tested in the transfer hydrogenation of cyclohexanone. This study reveals that BTA induced stack formation is not crucial for SCPN formation and catalytic activity; SDP-bearing copolymers folded by Ru(II) complexation and hydrophobic pendants suffice to provide hydrophobic, isolated reaction pockets around Ru(II) complexes. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 12-20