skip to main content

Attention:

The NSF Public Access Repository (NSF-PAR) system and access will be unavailable from 5:00 PM ET until 11:00 PM ET on Friday, June 21 due to maintenance. We apologize for the inconvenience.


Title: Photocatalyst-Independent Photoredox Ring-Opening Polymerization of O-Carboxyanhydrides: Stereocontrol and Mechanism
Photoredox ring-opening polymerization of O -carboxyanhydrides allows for the synthesis of polyesters with precisely controlled molecular weights, molecular weight distributions, and tacticities. While powerful, obviating the use of precious metal-based photocatalysts would be attractive from the perspective of simplifying the protocol and enabling unexpected reactivity. Herein, we report the Co and Zn catalysts that are activated by external light to mediate efficient ring-opening polymerization of O -carboxyanhydrides, without the use of exogenous precious metal-based photocatalysts. Our methods allow for the synthesis of isotactic polyesters with high molecular weights (>200 kDa) and narrow molecular weight distributions ( M w / M n < 1.1). Mechanistic studies indicate that light activates the oxidative status of Co III intermediate that is generated from the regioselective ring-opening of the O -carboxyanhydride. We also demonstrate that the use of Zn or Hf complexes together with Co can allow for stereoselective photoredox ring-opening polymerizations of multiple racemic O -carboxyanhydrides to synthesize syndiotactic and stereoblock copolymers, which vary widely in their glass transition temperatures.  more » « less
Award ID(s):
1807911
NSF-PAR ID:
10211210
Author(s) / Creator(s):
; ; ; ; ; ;
Date Published:
Journal Name:
Chemical Science
ISSN:
2041-6520
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    RecentlyO‐carboxyanhydrides (OCAs) have emerged as a class of viable monomers which can undergo ring‐opening polymerization (ROP) to prepare poly(α‐hydroxyalkanoic acid) with functional groups that are typically difficult to achieve by ROP of lactones. Organocatalysts for the ROP of OCAs, such as dimethylaminopyridine (DMAP), may induce undesired epimerization of the α‐carbon atom in polyesters resulting in the loss of isotacticity. Herein, we report the use of (BDI‐IE)Zn(OCH(CH3)COOCH3) ((BDI)Zn‐1, (BDI‐IE)=2‐((2,6‐diethylphenyl)amino)‐4‐((2,6‐diisopropylphenyl)imino)‐2‐pentene), for the controlled ROP of various OCAs without epimerization. Both homopolymers and block copolymers with controlled molecular weights, narrow molecular weight distributions, and isotactic backbones can be readily synthesized. (BDI)Zn‐1 also enables controlled copolymerization of OCAs and lactide, facilitating the synthesis of block copolymers potentially useful for various biomedical applications. Preliminary mechanistic studies suggest that the monomer/dimer equilibrium of the zinc catalyst influences the ROP of OCAs, with the monomeric (BDI)Zn‐1 possessing superior catalytic activity for the initiation of ROP in comparison to the dimeric (BDI)Zn complex.

     
    more » « less
  2. Abstract

    Ring‐opening polymerization (ROP) of lactones or cyclic (di)esters is a powerful method to produce well‐defined, high‐molecular‐weight (bio)degradable aliphatic polyesters. While the ROP of lactones of various ring sizes has been extensively studied, the ROP of the simplest eight‐membered lactone, 7‐heptanolactone (7‐HL), has not been reported using metal‐based catalysts. Accordingly, this contribution reports the ROP of 7‐HL via metal‐catalyzed coordinative‐insertion polymerization to the corresponding high‐molecular‐weight polyester, poly(7‐hydroxyheptanoate) (P7HHp). The resulting P7HHp is a semi‐crystalline material, with aTmof 68 °C, which is ~10 °C higher than poly(ε‐caprolactone) derived from the seven‐membered lactone. Mechanical testing showed that P7HHp is a hard and tough plastic, with elongation at break >670%. P7HHp‐based polyesters with higherTmvalues have been achieved through stereoselective copolymerization of 7‐HL with an eight‐membered cyclic diester, racemic dimethyl diolide (rac‐8DLMe), known to lead to highTmpoly(3‐hydroxyburtyrate) (P3HB). Notably, catalyst's strong kinetic preference for polymerizingrac‐8DLMeover 7‐HL in the 1/1 comonomer mixture rendered the formation of di‐block copolymer P3HB‐b‐P7HHp, showing two crystalline domains withTm1 ~ 65 °C andTm2 ~ 160 °C. Semi‐crystalline random copolymers withTmup to 164 °C have also been obtained by adjusting copolymerization conditions. Mechanical testing showed that P3HB‐b‐P7HHp can synergistically combine the high modulus of isotactic P3HB with the high ductility of P7HHp.

     
    more » « less
  3. null (Ed.)
    Poly(α-hydroxy acids), as a family of biodegradable polyesters, are valuable materials due to their broad applications in packaging, agriculture, and biomedical engineering. Herein we highlight and explore recent advances of catalysts in controlled ring-opening polymerization of O-carboxyanhydrides towards functionalized poly(α-hydroxy acids), especially metal catalyst-mediated controlled polymerization. Limitations of current polymerization strategies of O-carboxyanhydrides are discussed. 
    more » « less
  4. Abstract

    Transforming renewable resources into functional and degradable polymers is driven by the ever‐increasing demand to replace unsustainable polyolefins. However, the utility of many degradable homopolymers remains limited due to their inferior properties compared to commodity polyolefins. Therefore, the synthesis of sequence‐defined copolymers from one‐pot monomer mixtures is not only conceptually appealing in chemistry, but also economically attractive by maximizing materials usage and improving polymers’ performances. Among many polymerization strategies, ring‐opening (co)polymerization of cyclic monomers enables efficient access to degradable polymers with high control on molecular weights and molecular weight distributions. Herein, we highlight recent advances in achieving one‐pot, sequence‐controlled polymerizations of cyclic monomer mixtures using a single catalytic system that combines multiple catalytic cycles. The scopes of cyclic monomers, catalysts, and polymerization mechanisms are presented for this type of sequence‐controlled ring‐opening copolymerization.

     
    more » « less
  5. Photoinduced atom transfer radical polymerization (photo-ATRP) has risen to the forefront of modern polymer chemistry as a powerful tool giving access to well-defined materials with complex architecture. However, most photo-ATRP systems can only generate radicals under biocidal UV light and are oxygen-sensitive, hindering their practical use in the synthesis of polymer biohybrids. Herein, inspired by the photoinduced electron transfer-reversible addition–fragmentation chain transfer (PET-RAFT) polymerization, we demonstrate a dual photoredox/copper catalysis that allows open-air ATRP under green light irradiation. Eosin Y was used as an organic photoredox catalyst (PC) in combination with a copper complex (X–Cu II /L). The role of PC was to trigger and drive the polymerization, while X–Cu II /L acted as a deactivator, providing a well-controlled polymerization. The excited PC was oxidatively quenched by X–Cu II /L, generating Cu I /L activator and PC˙ + . The ATRP ligand (L) used in excess then reduced the PC˙ + , closing the photocatalytic cycle. The continuous reduction of X–Cu II /L back to Cu I /L by excited PC provided high oxygen tolerance. As a result, a well-controlled and rapid ATRP could proceed even in an open vessel despite continuous oxygen diffusion. This method allowed the synthesis of polymers with narrow molecular weight distributions and controlled molecular weights using Cu catalyst and PC at ppm levels in both aqueous and organic media. A detailed comparison of photo-ATRP with PET-RAFT polymerization revealed the superiority of dual photoredox/copper catalysis under biologically relevant conditions. The kinetic studies and fluorescence measurements indicated that in the absence of the X–Cu II /L complex, green light irradiation caused faster photobleaching of eosin Y, leading to inhibition of PET-RAFT polymerization. Importantly, PET-RAFT polymerizations showed significantly higher dispersity values (1.14 ≤ Đ ≤ 4.01) in contrast to photo-ATRP (1.15 ≤ Đ ≤ 1.22) under identical conditions. 
    more » « less