Bifunctional and Supramolecular Organocatalysts for Polymerization

Abstract

Bimolecular, H-bond mediated catalysts for ring-opening polymerization (ROP)—thiourea or urea plus base, squaramides and protic acid/base pairs, among others—are unified in a conceptual approach of applying a mild Lewis acid plus mild Lewis base to effect ROP. The bimolecular, and other supramolecular catalysts for ROP, produce among the best-defined materials available via synthetic polymer chemistry through a delicately balanced series of competing chemical reactions by interacting with substrate at an energy of <4 kcal mol−1. These catalysts are among the most controlled available for ROP. Part of this arises from the modular, highly tunable nature of dual catalysts, which conduct extremely controlled ROP of a host of cyclic monomers. The broader field of organocatalytic polymerization is a bridge between the disparate worlds of the materials chemist (ease of use) and the synthetic polymer chemist (mechanistic interest). The cooperative and collegial nature of the organocatalysis for the ROP community has facilitated the synergistic evolution of new mechanism to new abilities—in monomer scope, polymer architecture and level of reaction control.

Bisurea and Bisthiourea H-Bonding Organocatalysts for Ring-Opening Polymerization: Cues for the Catalyst Design

“…The ROP are living in behavior, both 2-S5 and 2-O5 plus MTBD produce linear evolution of M n versus conversion (Figure ) and M n that is predictable by [M] o /[I] o (Table S1). In C 6 D 6 (and other non-polar solvents), an H-bond-mediated mechanism of enchainment has been proposed; ,, urea plus base-mediated ROP has been shown to display faster rates than the analogous thiourea (Table ). , …”

“…H-bonding organocatalysts for ring-opening polymerization (ROP) are a facile means of generating precisely tailored macromolecules. − Among the larger class of H-bonding organocatalysts, (thio)­urea H-bond donors stand out for the remarkable level of control they can render in polymer synthesis. , The thiourea plus base-mediated ROP of lactone and carbonate monomers are thought to effect enchainment by H-bond activation of monomer by thiourea and initiating/propagating chain end by base; these catalysts are most active in a nonpolar solvent . The urea plus base class of H-bonding catalysts are among the most active catalysts for the ROP of lactones. − Several mechanistic studies by our group and others have shown that (thio)­urea/base-mediated ROP can proceed by one of two mechanisms: neutral H-bonding or (thio)­imidate-mediated ROP (Scheme ).…”

“…The urea plus base class of H-bonding catalysts are among the most active catalysts for the ROP of lactones. − Several mechanistic studies by our group and others have shown that (thio)­urea/base-mediated ROP can proceed by one of two mechanisms: neutral H-bonding or (thio)­imidate-mediated ROP (Scheme ). − Which mechanism is operative depends largely on reaction conditions (high temperature, , polar solvent, ,, strong electron-withdrawing groups on H-bond donor, high monomer concentration, and strong bases favor imidate) , though generally ureas are more active than thioureas and imidate-mediated ROP is far more active than neutral H-bonding . Remarkably, these “hyperactive” catalysts for ROP remain controlled.…”

“…The synthetic addition of one or more (thio)­urea H-bond donating arms to the parent (thio)­urea has been shown to substantially increase the activity of (thio)­urea H-bond donors. , Our group first disclosed bis- and tris-(thio)­urea H-bond donors for ROP, , and other intramolecular Lewis acid donors have been used . In general, the bis-(thio)­ureas ( 2-O and 2-S , Figure ) are more active than mono-(thio)­urea ( 1-O and 1-S ), and (bis)­ureas are more active than (bis)­thioureas for the bis-(thio)­urea plus base-mediated ROP of lactone and carbonate monomers. ,, However, this rule of thumb does not apply for the (thio)­urea plus base-mediated ROP of lactide (LA), where the higher rates are displayed by (bis)-thioureas (vs (bis)-ureas) of like substitution . Again, the high rates exhibited by 2-O and 2-S plus base for ROP occur without the reduction of reaction control.…”

Organocatalytic ring-opening polymerization of thionolactones: anything O can do, S can do better

“…[2][3][4][5] This class of catalysts, often an H-bond donating (thio)urea plus an H-bond accepting organic base, are believed to effect ROP by H-bond activation of a lactone monomer and alcohol chain-end/initiator. 5,6 More recently, (thio)urea/base-mediated ROP has been found to access an alternate mechanism of enchainment whereby proton transfer from (thio)urea to the base produces a highly active (thio) imidate that is among the most active and controlled catalysts for the ROP of lactones, carbonates and other cyclic monomers. [7][8][9][10] Of particular importance here is the highly controlled aspect of the (thio)urea/base-mediated ROP, which allows for the polymerization of functionalized and heteroatom-containing monomers while retaining polymerization control.…”