Star‐Polymer‐Catalyzed Living Radical Polymerization: Microgel‐Core Reaction Vessel by Tandem Catalyst Interchange

Abstract

The design and functionalization of reaction space around catalytic centers may promote catalysis. [1][2][3] With their unique three-dimensional globular structures, microgel-core star polymers [4][5][6][7][8] are intriguing as scaffolds that enclose catalysts: the central core is not only compartmentalized by linear-arm polymers but is also locally heterogeneous (cross-linked network), while the molecule as a whole is completely homogeneous and soluble through its soluble surrounding arms. Given these features, core-functionalized star polymers [5][6][7][8] have been developed as a new type of macromolecularly supported catalysts with a unique reaction space. For example, we have synthesized such star polymer catalysts by metal-catalyzed living radical polymerization [9,10] (Scheme 1) by in situ direct encapsulation ("tandem catalyst interchange") of ruthenium complexes into the microgel core that carries multiple phosphine ligands.[5] These "metalbearing" star polymers efficiently catalyzed hydrogen-transfer reactions with high activity, versatility, and recyclability, in comparison to their homogeneous or polymer-supported counterparts.[3]Herein, we report living radical polymerization in microgel-core reaction vessels of metal-bearing star polymers obtained by tandem catalyst interchange (Scheme 1). These star catalysts are well soluble, but the metal complexes are caged (i.e., protected) within the microgel network and lead to high activity and stability, functionality tolerance, and catalyst recycling, among others. This work is to demonstrate that the star-catalyzed system enables a novel homogeneous compartmentalized polymerization in a catalyst-embedded core, which is to be clearly distinguished from the polymerizations with conventional polymer-supported insoluble catalysts [11] and those in heterogeneous dispersed or emulsion systems particularly in terms of catalytic activity, functionality tolerance, and recyclability, among other feartures expected. [12] To successfully apply star polymer catalysts to the polymerization, PPh 3 -star S3 was synthesized by a one-pot tandem route consisting of: 1) the synthesis of RuCl 2 -Star S1 by the ruthenium-catalyzed linking reaction of linear-arm polymers with a binfunctional linker 1 and a ligand comonomer 2; [5] 2) the in situ hydrogenation [13] of the corebound chlorine and olefin units within S1 to give hydrogenated star S2, and 3) the removal of the core-bound ruthenium complexes from S2 leading to an empty-core star S3 with nonligating phosphines. Then, new metal complexes were introduced into the core of S3, giving metal-star catalysts such as RuCp*-Star S4 to be directly employed for living polymerization. Hence, the original polymerization catalyst [RuCl 2 (PPh 3 ) 3 ] is first core-bound and then interchanged with a new complex [e.g., RuCp* or FeBr 2 ], all in situ and in one pot. Note that step (2) serves to eliminate the potentially growth-active or reactive units (halogen and olefin, respectively) from the core. The products were characterized by si...

Catalytic polymeric nanoreactors: more than a solid supported catalyst

“…Furthermore, the same star polymer catalysts showed high activity, versatility, functionality tolerance, and recyclability as a catalyst for ATRP reactions. [54] …”

Polymeric pseudo-crown ether for cation recognition via cation template-assisted cyclopolymerization

“…The key for selective vinyl-type cyclopolymerization is to bring the two olefins in a bifunctional monomer to close proximity for effective intramolecular cyclization. This suppresses intermolecular propagation with only one of the two alkene units into pendent olefin-bearing polymers; otherwise the dangling olefin further causes intermolecular crosslinking of polymers to give insoluble gels and/or branched polymers [40][41][42][43] . So far, cyclopolymers with relatively large in-chain rings (at most 20-membered rings) generally required the elaborate design of bifunctional monomers that bring two olefins close, typically through a rigid spacer [29][30][31][32] , except for counterparts with small five-or six-membered rings from 1,6-dienes and 1,6-diynes [33][34][35][36] .…”

Polymer Microgels for Catalysis

“…Focusing on ruthenium catalysts in living radical polymerization, the author's group discovered a novel strategy to directly synthesize metal-bearing microgel-core star polymer catalysts (Fig. 2), that is, "direct transformation of ruthenium polymerization catalysts into star polymer catalysts" (19,(56)(57)(58)(59)(60)(61)(62). The key is to utilize diphenylphosphinostyrene (SDP) as a ligand monomer in ruthenium-catalyzed polymerization, where SDP is dynamically coordinated to a ruthenium catalyst during the arm-linking reaction to capture the ruthenium into a star polymer core.…”

“…In-core metal catalysts can be interchanged from an original ruthenium complex [RuCl 2 (PPh 3 ) 3 ] in RuCl 2 -Star (SC1) to others via the removal of core-bound ruthenium and the introduction of new metals ( Fig. 3) (61,62). For this, SC1 is first treated with a large excess of a basic (1) in situ hydrogenation of the in-core chlorine and olefins of SC1 with the in-core ruthenium coupled with K 2 CO 3 and 2-propanol, (2) removal of core-bound ruthenium via ligand exchange reaction with P(CH 2 OH) 3 to give PPh 3 -Star (SL1) with an empty core, and (3) introduction of metal salts and complexes (M: Ru, Fe, Ni) into SL1 to lead to various metal-stars.…”