Phosphine-Functionalized Core-Crosslinked Micelles and Nanogels with an Anionic Poly(styrenesulfonate) Shell: Synthesis, Rhodium(I) Coordination and Aqueous Biphasic Hydrogenation Catalysis

Abstract

Stable latexes containing unimolecular amphiphilic core-shell star-block polymers with a triphenylphosphine(TPP)-functionalized hydrophobic core and an outer hydrophilic shell based on anionic styrenesulfonate monomers have been synthesized in a convergent three-step strategy by reversible addition-fragmentation chain-transfer (RAFT) polymerization, loaded with [RhCl(COD)]2 and applied to the aqueous biphasic hydrogenation of styrene. When the outer shell contains sodium styrenesulfonate homopolymer blocks, treatment with a toluene solution of [RhCl(COD)]2 led to undesired polymer coagulation. Investigation of the interactions of [RhCl(COD)]2 and [RhCl(COD)(PPh3)] with smaller structural models of the polymer shell functions, namely sodium p-toluenesulfonate, sodium styrenesulfonate, and a poly(sodium styrenesulfonate) homopolymer in a biphasic toluene/water medium points to the presence of equilibrated Rh-sulfonate interactions as the cause of coagulation by inter-particle cross-linking. Modification of the hydrophilic shell to a statistical copolymer of sodium styrenesulfonate and poly(ethylene oxide) methyl ether methacrylate (PEOMA) in a 20:80 ratio allowed particle loading with the generation of core-anchored [RhCl(COD)TPP] complexes. These Rh-loaded latexes efficiently catalyze the aqueous biphasic hydrogenation of neat styrene as a benchmark reaction. The catalytic phase could be recovered and recycled, although the performances in terms of catalyst leaching and activity evolution during recycles are inferior to those of equivalent nanoreactors based on neutral or polycationic outer shells.

Polymeric nanoreactors for catalytic applications

“…No catalytic application was described for these CCMs. Furthermore, three generations of CCMs with amphiphilic unimolecular polymer-based nanoreactors [88] having neutral (CCM-N), cationic (CCM-C) and anionic (CCM-A) shells, respectively, have been prepared by RAFT PISA and used for aqueous biphasic catalysis (Figure 9) [118]. In all cases, special attention was devoted to the catalyst recovery using an optimized procedure.…”

“…To address the substrate scope limitation of the CCM-C nanoreactors, a third generation of phosphine-functionalized CCM nanoreactors was developed with a polyanionic poly(styrene sulfonate)-based shell (CCM-A) [12,118]. The synthesis route was identical to those used for the syntheses of CCM-N and CCM-C described above, except for using the water-soluble sodium styrene sulfonate monomer in the first step (Scheme 10, Figure 13).…”

“…This bottleneck was circumvented by "dilution" of the shell with a neutral monomer (PEOMA), forming a mixed polyanionic-neutral CCM, which allowed Rh I complexation in the core. Nevertheless, the mixed polyanionic-neutral CCM nanoreactors had inferior performance compared to equivalent nanoreactors with neutral and polycationic shells, which seemed to stem from catalyst alterations induced by migra-tion of the Rh centers towards the shell sulfonate groups [12,118]. Thus, despite significant progress in the development of CCMs, further improvements are still needed to obtain efficient and durable nanoreactor systems for aqueous biphasic hydrogenation catalysis.…”

Molecular Insights into Photochemical Hydrogen Evolution Dual Catalysis within the Shell of Polymer Micelle-Based Nanoreactors

“…Polymer micelles, which are self-assembled core–shell nanomaterials comprising amphiphilic block copolymers, provide distinguished nanoenvironments within their confined spaces in solution. − Compared to conventional molecular micelles, polymer micelles possess several advantages, including high designability and loading capacity for functional moieties, owing to the excellent structural diversity of block copolymers, and high kinetic stability. , Therefore, the tailor-made nanospaces within polymer micelles are promising candidates for constructing synthetic nanoreactors with precisely controlled reaction environments. − Polymer micelle-based nanoreactors enable efficient catalysis via catalyst protection, catalytic site isolation, high substrate selectivity, and catalyst/substrate accumulation, depending on their compositions . Extensive efforts have been devoted to construct polymer micelle-based nanoreactors with a wide variety of catalysts immobilized onto the block copolymers. ,− …”

Acetate ion addition to and exchange in (1,5-cyclooctadiene)rhodium(i) acetate: relevance for the coagulation of carboxylic acid-functionalized shells of core-crosslinked micelle latexes

“…1b), even when using the lower-permittivity toluene medium. 21 In the latter case, molecular control experiments gave evidence in favour of [Rh(COD)] + -sulfonate interactions, which are enhanced by the proximity of sulfonate groups (chelate effect), thus presumably leading to shellanchored, crosslinking [Rh 2 (COD) 2 (O 3 S-polymer) 2 ] or [Rh (COD)(O 3 S-polymer) 2 ] − moieties. 21 A number of complexes with the [Rh I (alkene) 2 (O 2 CR)] stoichiometry, all of them adopting a dinuclear structure with two bridging carboxylates (Scheme 1A), have been described in the literature (alkene = C 2 H 4 22 or dialkene = COD [23][24][25][26][27][28][29][30][31][32] or norbornadiene (NBD); 28,33,34 R = various alkyl and aryl substituents).…”