Organocatalysts in Polymer Chemistry E-Book

• Herausgeber: Wiley-VCH
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 804

Veröffentlichungsjahr: 2025

Beschreibung

Provides an up-to-date overview of organocatalysis in polymer chemistry, covers recent innovations and specific methodologies

Organocatalysis is revolutionizing polymer chemistry, offering a sustainable, cost-effective alternative to traditional metal-based catalysts. Organocatalysts in Polymer Chemistry: Synthesis and Applications presents a detailed summary of the development of organocatalysts and their transformative impact on polymer synthesis. Contributions by an international team of specialists present cutting-edge methodologies for creating precise macromolecular structures, covering a wide range of polymerization methods and practical applications.

Edited by Professor Zhibo Li, an acknowledged expert in polymer chemistry, the book covers the use of organocatalysts in processes such as ring-opening polymerization, controlled radical polymerization, and polymer depolymerization. It offers vital insights into the synthesis of advanced, biodegradable, and metal-free materials, making it a valuable resource of both foundational knowledge and the latest research breakthroughs in polymer chemistry.

Exploring the development, advantages, and applications of organocatalysts in polymer synthesis, this book:

• Presents advanced techniques for creating precise polymer architectures, including molecular weight, stereochemistry, and topology control
• Discusses applications of organocatalysts in ring-opening polymerization (ROP), controlled radical polymerization, and copolymerization techniques
• Highlights organocatalysis as a metal-free, cost-effective, and environmentally friendly alternative for polymer synthesis
• Examines the role of organocatalysts in recycling and depolymerizing commodity polymers such as PET and polycarbonate
• Addresses the synthesis of degradable polymers for biomedical, electronic, and environmental uses
• Summarizes advancements over the past two decades and explores emerging trends in polymer chemistry

Organocatalysts in Polymer Chemistry is essential reading for postgraduate students, researchers, and industrial professionals in polymer chemistry, materials science, and sustainable catalysis. It is also an essential reference for catalytic chemists, organic chemists, and chemical engineers engaged in the synthesis and application of polymers.

Leseprobe

Table of Contents

Cover

Table of Contents

Title Page

Copyright

Preface

1 Organocatalyzed Ring‐Opening Polymerization of Cyclic Esters Toward Degradable Polymers

1.1 General Introduction

1.2 Polymerization Mechanism

1.3 Recent Trends in Organocatalyst Development

1.4 Toward Higher Degradability and Recyclability

1.5 Summary and Outlook

References

2 Organocatalyzed Copolymerization of CO

2

with Epoxide Toward Polycarbonate Synthesis

2.1 Introduction

2.2 Discovery of TEB Catalyzed CO

2

/Epoxides Copolymerization

2.3 Development of TEB Catalyzed CO

2

/Epoxides Copolymerization

2.4 Synthesis of CO

2

‐Based Block Copolymers

2.5 Bifunctional Organoboron Catalysts

2.6 Conclusions

References

3 Organocatalyzed Ring‐Opening Copolymerization of Cyclic Anhydride and Cyclic Ether (Acetal)

3.1 Introduction

3.2 The Copolymerization of Epoxide with Cyclic Anhydride

3.3 The Copolymerization of Oxetane with Cyclic Anhydride

3.4 The Copolymerization of Tetrahydrofuran with Cyclic Anhydride

3.5 The Copolymerization of Cyclic Acetal (Aldehyde) with Cyclic Anhydride

3.6 Polyester‐Based Block Copolymer

3.7 Conclusions and Outlook

References

4 Organocatalysts for the Preparation of Degradable and Closed‐Loop Recyclable Polyesters

4.1 Introduction

4.2 Catalytic Mechanism of Various Organocatalysts

4.3 Organocatalytic ROP of Lactones or Lactides

4.4 Summary and Perspective

References

5 Organo‐Catalyst Catalyzed Ring Opening Polymerization of

N

‐Carboxyanhydrides

5.1 Introduction

5.2 Organo‐Catalysts for NCA Ring Opening Polymerization

5.3 Conclusions and Future Prospects

References

6 Organocatalyst Catalyzed Ring Opening Polymerization of O(S)‐Carboxyanhydride to Produce High Performance Poly(thio)esters

6.1 Introduction

6.2 Synthesis and Purification of O(S)‐Carboxyanhydride Monomers

6.3 The Racemization of α‐Proton

6.4 Organocatalyst for OCA Polymerization

6.5 Organocatalyst for SCA Polymerization

6.6 Conclusion and Perspective

References

7 Organocatalyzed Stereoselective Ring‐Opening (Co)Polymerization

7.1 Introduction

7.2 Mechanisms in the Organocatalyzed Stereoselective ROP

7.3 Organic Acids for Stereoselective ROP of

rac

‐LA

7.4 Organic Bases for Stereoselective ROP of

rac

‐LA

7.5 Dual Organocatalysts

7.6 Organocatalysts for Stereoselective ROP of Other Cyclic Monomers

7.7 Conclusion

Acknowledgements

References

8 Ring‐Opening Polymerization of Epoxides Through Organocatalysis

8.1 Introduction

8.2 Catalyst and Mechanism

8.3 Synthesis of Functionalized Polyether

8.4 Synthesis of Polyether‐Based Block Copolymer

8.5 Conclusion and Outlook

References

9 Organo‐Borane Catalysis for Ring Opening Polymerization of Epoxides

9.1 Introduction

9.2 Fundamentals of Borane Compounds

9.3 Et

3

B‐Mediated ROP of Epoxides

9.4 Modified Organoborane Mediated ROP of Epoxides

9.5 Chiral Organoborane Mediated Asymmetric ROP of Epoxides

9.6 Conclusion and Outlook

References

10 Organocatalyst for Ring‐Opening Polymerization of Cyclosiloxanes Toward Polysiloxanes

10.1 Introduction

10.2 Anionic ROP of Cyclosiloxanes

10.3 Phosphazene‐Catalyzed Anionic ROP of Cyclosiloxanes

10.4 Guanidines‐Catalyzed Anionic ROP of Cyclosiloxanes

10.5 Phosphorus Ylides‐Catalyzed Anionic ROP of Cyclosiloxanes

10.6 NHC‐Catalyzed Anionic ROP of Cyclosiloxanes

10.7 Conclusion

References

11 Organic Lewis Pair in Polymer Synthesis

11.1 Lewis Pair Catalytic Ring‐Opening Polymerization of Cyclic Esters

11.2 Mechanisms for Lewis Pair‐Catalyzed ROP of Cyclic Esters

11.3 Lewis Pairs Used for ROP of Cyclic Ester

References

12 Organocatalysts for Radical Polymerization

12.1 Photocontrolled Organocatalyzed Radical Polymerization

12.2 Organocatalyzed Atom Transfer Radical Polymerization (O‐ATRP)

12.3 Photoinduced Electron/Energy Transfer Reversible Addition‐Fragmentation Chain Transfer (PET‐RAFT)

12.4 Structure‐Property‐Performance Relationships

12.5 Outlook

References

13 Organocatalyst‐Catalyzed Non‐Radical Photopolymerizations

13.1 Introduction

13.2 Organocatalysts for Photocontrolled Cationic Polymerization

13.3 Organocatalysts for Photocontrolled Ring‐Opening Polymerization

13.4 Organocatalysts for Photocontrolled Ring‐Opening Metathesis Polymerization (ROMP)

13.5 Organocatalysts for Photocontrolled Step‐Growth Polymerization

References

14 Enzyme‐Catalyzed Controlled Radical Polymerization

14.1 Introduction

14.2 Enzymatic Deoxygenation in RAFT Polymerization

14.3 Enzymatic Initiation of RAFT Polymerization

14.4 Enzymatic Deoxygenation in ATRP

14.5 Enzyme‐Catalyzed ATRP

14.6 Conclusions

Acknowledgments

References

15 Organocatalyst‐Catalyzed Degradation of Polymers

15.1 Introduction

15.2 Chemical Degradation of Functional Polymers

15.3 Photocatalytic Degradation of Non‐Functional Polymers

15.4 Conclusion

References

Index

End User License Agreement

List of Tables

Chapter 2

Table 2.1 The productivity comparison of typical catalyst systems for CO

2

/P...

Chapter 5

Table 5.1 The timeline of initiators and organo‐catalysts for NCA ring‐open...

Chapter 8

Table 8.1 Selected ROP systems using two‐component organocatalysts.

List of Illustrations

Chapter 1

Figure 1.1 ROP of cyclic esters into polyesters.

Figure 1.2 Representative organocatalysts for the ROP of cyclic esters.

Figure 1.3 Representative cyclic esters with relatively high (a) and low (b)...

Figure 1.4 Representative nucleophilic catalysts and their nucleophilicities...

Figure 1.5 General reaction mechanism of the ROP of cyclic esters using nucl...

Figure 1.6 Representative Brønsted base catalysts and their...

Figure 1.7 General reaction mechanism of the ROP of cyclic...

Figure 1.8 Representative Brønsted acid catalysts and their acidities.

Figure 1.9 General reaction mechanism of the ROP of cyclic...

Figure 1.11 Examples of salt and ionic catalysts.

Figure 1.10 Salt catalysts and general reaction mechanism of the ROP of cycl...

Figure 1.12 (a) Takemoto's catalyst; (b) dual‐activation mechanism in the RO...

Figure 1.13 Some bifunctional and multifunctional catalysts.

Figure 1.14 Representative structures of (a) ureas, thioureas and (b) bases....

Figure 1.15 (a) Anionic mechanism; (b) cooperative mechanism; (c) reaction m...

Figure 1.16 ROP of

rac

‐LA using achiral organocatalysts.

Figure 1.17 ROP of

rac

‐LA using chiral organocatalysts: enantiomer‐selective...

Figure 1.18 Representative chiral organocatalysts for the ROP of

rac

‐LA.

Figure 1.19 (a) Intrinsic difficulty in performing enantiomer‐selective ROP ...

Figure 1.20 Enantiomer‐selective polymerization of

rac

‐8DL.

Figure 1.21 Regioselective ROP of MG by the

t

‐BuP

2

catalyst.

Figure 1.22 Statistical ROP of LA and

ɛ

‐CL using a benzoic acid catalys...

Figure 1.23 (a) Self‐switchable polymerization of cyclic anhydrides, epoxide...

Figure 1.24 ROP of LLA in bulk catalyzed by DPP.

Figure 1.25 Density functional theory calculations of TBD, MSA, and the TBD:...

Figure 1.26 Representative acid–base mixed or salt organocatalysts for the R...

Figure 1.27 ROP of CL and representative truly safe catalysts.

Figure 1.28 ROP of LA and representative truly safe catalysts.

Figure 1.29 Degradation of polymers (a) from the chain ends; (b) from the la...

Figure 1.30 (a) Copolymerization of hemiacetal ester DOX with LLA; (b) incor...

Figure 1.31 Ring‐opening polymerization of cyclic hemiacetal esters via (a) ...

Figure 1.32 CROP of exocyclic hemiacetal esters.

Figure 1.33 ROP of novel cyclic esters.

Figure 1.34 Schematic of polymerization and depolymerization.

Figure 1.35 Polymerization of

γ

‐BL and depolymerization of P(

γ

‐BL)...

Figure 1.36 Cyclic ester monomers with bicyclic structures.

Figure 1.37 Cyclic ester monomers with various substituents.

Figure 1.38 Cyclic ester monomers with heteroatoms.

Chapter 2

Figure 2.1 Reported organocatalytically copolymerizable epoxide monomers wit...

Figure 2.2 Reactions involved in anionic copolymerization of CO

2

and epoxide...

Figure 2.3 The anionic copolymerization of epoxides and CO

2

using the cataly...

Figure 2.4 DFT calculation model of CHO/CO

2

copolymerization (kcal mol

−1

...

Figure 2.5 Organocatalytical selective coupling of oxetanes with CO

2

using (...

Figure 2.6 Multifunctional tetrabutylammonium carboxylates as initiators for...

Figure 2.7 Proposed mechanism for the TEB/TEA catalyzed zwitterionic CO

2

/PO ...

Figure 2.8 Cyclic phosphazene organobases used for copolymerization of CO

2

w...

Figure 2.9 Reported comonomers: COS, epoxides, anhydrides in organocatalytic...

Figure 2.10 Synthesis of poly(ester‐

b

‐carbonate) block copolymers through ta...

Figure 2.11 Synthesis of AB diblock copolymers using bifunctional initiator ...

Figure 2.12 All‐polycarbonate block copolymers prepared under metal‐free con...

Figure 2.13 AFM phase separation images (upper), and stress‐strain curves fo...

Figure 2.14 CO

2

‐based triblock copolymers prepared through sequential copoly...

Figure 2.15 (a) Representative stress‐strain curves of PDLA‐

b

‐PPPC‐

b

‐PDLA tr...

Figure 2.16 Stress‐strain curves of U‐PC‐U triblock copolymers and reference...

Figure 2.17 Representative effective bifunctional organoboron‐based catalyst...

Chapter 3

Figure 3.1 Illustration of the general reactions to prepare polyesters.

Figure 3.2 The scope of representative epoxides and anhydrides for copolymer...

Figure 3.3 The metalloporphyrin catalysts and diimine catalysts.

Figure 3.4 Representative

N, N′

‐Bis(salicylidene)ethylenediamine catal...

Figure 3.5 Dinclear and/or chiral metal complexes catalysts.

Figure 3.6 Bifunctional and multinuclear metal complexes catalysts.

Figure 3.7 The proposed mechanism catalyzed by tertiary amines. (X, R and R′...

Figure 3.8 The representative organophosphazene bases for PO/PA copolymeriza...

Figure 3.9 Representative binary organocatalysts for epoxides/cyclic anhydri...

Figure 3.10 (a) hexacoordinate metal species [(salen)M complexes], and tetra...

Figure 3.11 Represented (thio)ureas catalysts as the Lewis acid for epoxides...

Figure 3.12 Proposed mechanism for the alternating polymerization of epoxide...

Figure 3.13 Representative bifunctional organocatalysts for the copolymeriza...

Figure 3.14 (a) Copolymerization of OX with GA by titanium complexes as init...

Figure 3.15 The initial and catalytic systems for the copolymerization of 1 ...

Figure 3.16 Plausible mechanistic pathways for the copolymerization of 1 and...

Figure 3.17 Initiation and chain growth of (Nf)

2

NH‐catalyzed copolymerizatio...

Figure 3.18 The representative acetals and aldehydes monomer for copolymeriz...

Figure 3.19 The copolymerization of epoxides with acetals.

Figure 3.20 The copolymerization of aldehydes with cyclic anhydrides.

Figure 3.21 (a) One‐step approach to ester‐ether block copolymers, (b) A

z

C

y

(...

Chapter 4

Scheme 4.1 Electrophilic monomer activation mechanism for ROP of cyclic este...

Scheme 4.2 Nucleophilic monomer activation mechanism for ROP of cyclic ester...

Scheme 4.3 Chain‐end activation mechanism for ROP of cyclic esters.

Scheme 4.4 Bifunctional activation mechanism for ROP of cyclic esters.

Scheme 4.5 (a) Cooperative hydrogen bond activation mechanism and (b) (thio)...

Scheme 4.6 Different pathways for the ROP of γBL in the...

Scheme 4.7 The ROP of γBL catalyzed by the strong...

Figure 4.1 (a) The chemical structures of five‐membered lactones discussed i...

Scheme 4.8 (a) The proposed nucleophilic monomer activation mechanism for th...

Scheme 4.9 The proposed mechanism for zwitterionic ROP of...

Scheme 4.10 (a) Nucleophilic monomer activation mechanism by directly attack...

Figure 4.2 The chemical structures of organobases and (thio)ureas used for R...

Scheme 4.11 The activated thiourea mechanism and the dual thiourea activatio...

Scheme 4.12 The proposed bifunctional activation mechanism for the BA cataly...

Chapter 5

Figure 5.1 Proposed mechanism for NCA polymerization initiated by (a) normal...

Figure 5.2 Proposed mechanism of NCA polymerization catalyzed by frustrated ...

Figure 5.3 The binaphthyl phosphoric acids and primary amines used for the R...

Figure 5.4 Possible polymerization mechanism of Ala NCA catalyzed by RE comp...

Figure 5.5 Possible polymerization mechanism of Ala NCA catalyzed by NaBH

4

....

Figure 5.6 Proposed mechanism of LiHMDS‐initiated NCA polymerization.

Figure 5.7 (a) The normal amine mechanism (NAM) and (b) the proposed mechani...

Figure 5.8 Plausible mechanistic pathway of tetraalkylammonium carboxylate‐i...

Figure 5.9 The accelerated amine mechanism by monomer activation (AAMMA)....

Figure 5.10 The proposed mechanism of TU‐S catalyzed NCA ring opening polyme...

Figure 5.11 Proposed mechanism for living anionic ROP of NCAs enabled by tri...

Figure 5.12 Proposed mechanism of NCA polymerization catalyzed by the bifunc...

Figure 5.13 Organocatalytic ROP of NCA catalyzed by fluorinated alcohol. (a)...

Figure 5.14 The proposed mechanism for (a) PhS‐TMS and (b) S‐Sn Lewis pair m...

Figure 5.15 Reaction pathway of ROP of NCA using 3‐phenyl‐1propanol as initi...

Figure 5.16 Proposed mechanism for ROP of Sar‐NCA catalyzed by carboxylic ac...

Figure 5.17 (a) NCA polymerization initiated by NB and PNB

n

. (b) conversion ...

Figure 5.18 Proximity‐induced cooperative polymerization in “hinged” helical...

Figure 5.19 Schematic illustration of the catalytic power of an α‐helix.

Figure 5.20 Accelerated polymerization rate with localized initiators.

Figure 5.21 CE‐catalyzed polymerization of NCA by small‐molecular amine init...

Figure 5.22 The proposed mechanism of water‐initiated NCA‐ROP on UiO‐66 NP....

Chapter 6

Figure 6.1 (a) Synthesis of sustainable poly(thio)esters via O(S)CAs polymer...

Figure 6.2 Synthesis of sustainable polyesters via organocatalytic ROP of OC...

Figure 6.3 Epimerization of α‐proton in OCAs ROP.

Figure 6.4 Various Organocatalysis for OCAs Polymerization.

Figure 6.5 The nucleophilic activation monomer (Path A) and basic activation...

Figure 6.6 Preparation and ROP of

L

‐Lys(Cbz)OCA (

L

‐8).

Figure 6.7 The application of acid/base adducts to suppress the epimerizatio...

Figure 6.8 Thiourea‐based organocatalysis for OCA polymerization.

Figure 6.9 Bifunctional organocatalysts for OCAs polymerization.

Figure 6.10 Proposed mechanism for OCAs polymerization. Reproduced with perm...

Figure 6.11 DMAP‐thiourea cocatalyzed ring‐opening polymerization.

Figure 6.12 (a) Structures of initiators and chain transfer agent. (b) Open‐...

Figure 6.13 (a) Postulated propagation and transfer intermediates (INT) and ...

Figure 6.14 Plausible mechanistic pathway for the [PPN]OBz/BzOH‐mediated ROP...

Chapter 7

Scheme 7.1 Lactide stereochemistry and PLA microstructures from stereo‐contr...

Scheme 7.2 Stereoselective ROP of

rac

‐LA via the (i) enantiomorphic site con...

Scheme 7.3 Enantioselective ROP of

rac

‐LA catalyzed by (a) binaphthol (BINOL...

Scheme 7.4 Stereoselective ROP of

rac

‐LA and

meso

‐LA by various chiral and a...

Scheme 7.5 Typical amidines and guanidines used for the stereoselective ROP ...

Scheme 7.6 Phosphazene bases used for the stereoselective ROP of

rac

‐LA.

Scheme 7.7 Mono‐component bifunctional organocatalysts for the stereoselecti...

Scheme 7.8 Epimerization of

meso

‐LA followed by stereoselective ROP of

rac

‐L...

Scheme 7.9 Bi‐component organocatalysts based on DBU, (–)‐sparteine, benzyl ...

Scheme 7.10 PMDETA/azobenzene‐based thiourea bi‐component organocatalyst for...

Scheme 7.11 NHC and (thio)urea bi‐component organocatalysts (X = S, 17a; X =...

Scheme 7.12 Bi‐component organocatalysts based on (a) NHO and (b) (thio)urea...

Scheme 7.13 Bi‐component organocatalysts based on (a) phosphazene and (b) (t...

Scheme 7.14 (a) Enantioselective ROP of

rac

‐LA by (b) diastereomeric densely...

Scheme 7.15 Dynamic achiral rotaxanes as organocatalysts for the enantiosele...

Scheme 7.16 Chiral phosphoric acid organocatalysts for the stereoselective R...

Scheme 7.17 Chiral phosphoric acid organocatalysts for the stereoselective R...

Chapter 8

Figure 8.1 General schematic illustration of the organobase‐catalyzed ROP of...

Figure 8.2 Representative boron‐based bifunctional catalysts used for ROP of...

Figure 8.3 Examples of synthesis and post‐transformation of end‐functionaliz...

Figure 8.4 ROP of PO catalyzed/initiated by PBB‐Br with representative CTAs ...

Figure 8.5 Chemoselective ROP catalyzed by organic Lewis pair using function...

Figure 8.6 Chemical structures of functional monosubstituted epoxides used f...

Figure 8.7 Chemoselective ROP of GB and copolymerization of GB with other ep...

Figure 8.8 One‐pot two‐step synthesis of polyacetal‐

b

‐polyether block copoly...

Figure 8.9 Synthetic approach to polyolefin‐

b

‐PEO hybrid amphiphilic block c...

Figure 8.10 One‐pot continuous synthesis of polyether‐

b

‐polyester block copo...

Figure 8.11 One‐step block copolymerization of EO and Az by (a) cesium alkox...

Figure 8.12 Kinetically controlled one‐step approaches to polyester‐

b

‐polyet...

Figure 8.13 Three routes to polyacrylamide‐

b

‐polyether block copolymers by m...

Chapter 9

Figure 9.1 Chemical structures of trimethylborane and borohydride ion.

Figure 9.2 Chemical structure of borane Lewis acid.

Figure 9.3 Regio‐chemistry of S

N

2‐type epoxide ring opening.

Figure 9.4 Et

3

B‐mediated polymerization catalysis featured a distinctive fun...

Figure 9.5 Proposed polymerization mechanism for Et

3

B‐mediated epoxide homop...

Figure 9.6 Amphiphilic block copolyether series developed using PEO macroini...

Figure 9.7 Homopolymerization of GA and (R)‐GB catalyzed by Et

3

B binary syst...

Figure 9.8 Synthesis of well‐defined poly(ester‐ether) multiblock copolymers...

Figure 9.9 Chemical structures of bifunctional mono‐, di‐, tri and tetranucl...

Figure 9.10 Bifunctional borane initiators/catalysts for epoxide homopolymer...

Figure 9.11 Dinuclear organoboron catalysts evaluated by Yang and Wu for epo...

Figure 9.12 Chemical structure of phosphonium‐bridged bisborane initiators/c...

Figure 9.13 The chemical structure of silicon‐centered organoborane.

Figure 9.14 Synthetic route of PVP‐1B and PVP‐2B.

Figure 9.15 A schematic diagram proposed for macromolecular Lewis acid catal...

Figure 9.16 Different polyethers generated by 9‐BBN ammonium borane system....

Figure 9.17 The chemical structures of bifunctional borinane compounds B2, B...

Figure 9.18 Syntheis of diverse

α

,

ω

‐difunctionalized PPOs catalyze...

Figure 9.19 Rationale for the research (top), two‐step synthetic pathway (mi...

Figure 9.20 The binary and bifunctional organoboron compounds employed in th...

Chapter 10

Figure 10.1 Different routes of thermodynamically controlled polymerization ...

Figure 10.2 The mechanism for the anionic ROP of cyclosiloxanes.

Figure 10.3 Representative Schwesinger's and cyclic phosphazene bases and th...

Figure 10.4 Phosphazene base

t

‐BuP

4

conversion to active hydroxide or silano...

Figure 10.5 Reaction of

t

‐BuP

4

hydroxide with CO

2

/H

2

O [26].

Figure 10.6 Conversion of P

5

Cl to P

5

OH by anion exchange [26].

Figure 10.7 Possible T and Q branches in the ROP of D

4

and P

4

.

Figure 10.8 CTPB‐catalyzed copolymerization of methyl and vinyl cyclosiloxan...

Figure 10.9 Synthetic route of

p

‐PDMS prepolymer via CTPB‐catalyzed ROP.

Figure 10.10 Synthesis of PDES and PMES copolymers.

Figure 10.11 C

3

N

3

‐Me‐P

3

‐mediated polymerization of D

3

.

Figure 10.12 Structure of HTGCP and the ROP of cyclic siloxanes.

Figure 10.13 TBD‐catalyzed ROP of carbosiloxane.

Figure 10.14 TMnPG‐mediated polymerization of cyclic trisiloxanes.

Figure 10.15 TMnPG‐catalyzed ROP of PT3.

Figure 10.16 Guanidines‐catalyzed polymerization of monofunctional cyclic tr...

Figure 10.17 TMMG‐catalyzed polymerization of CNPD2.

Figure 10.18 Preparation of alkynylsilyl end‐capped polysiloxanes with well‐...

Figure 10.19 Preparation of PDMS bi‐terminated with bromoaryl groups.

Figure 10.20 Phosphonium alcoholates for the polymerization of cyclic siloxa...

Figure 10.21 Propagation/backbiting equilibrium modified by the effect of co...

Figure 10.22 (a) Organocatalysts for ROP of TMOSC and (b) scrambling product...

Figure 10.23 NHC‐catalyzed polymerization of cyclosiloxanes.

Chapter 11

Figure 11.1 Mechanism of LPP of cyclic esters operating in different modes. ...

Figure 11.2 Epimerization of

meso

‐lactide into

rac

‐lactide catalyzed by the ...

Figure 11.3 Proposed mechanism for the controlled ROP of NCAs by B/N LP.

Figure 11.4 Proposed initiation mechanism for the living ROP of...

Figure 11.5 Proposed initiation mechanism for the controlled ROP of

rac

‐lact...

Figure 11.6 ROP of

β

‐BL,

rac

‐lactide, and TMC was mediated by NHC‐Zn(C

6

Figure 11.7 Synthesis of cyclic polyesters from lactide or

ɛ

‐CL by Zn(C

Figure 11.8 Synthesis of “polyethylene‐like” copolyester by a cascade of rin...

Figure 11.9 NHC − metal chloride adducts employed for the ROP of cyclic este...

Figure 11.10 Structures of NHOs employed in this LPP study and inactive NHO‐...

Figure 11.11 Proposed equilibrium between dissociated NHO/metal halide pairs...

Chapter 12

Scheme 12.1 Typical Catalytic Cycles of Photo‐RDRP.

Scheme 12.2 Typical Chromophore Cores in Organocatalysts Used for Photo‐RDRP...

Scheme 12.3 Typical Functional Groups in Organocatalysts Used for Photo‐RDRP...

Scheme 12.4 Unfunctionalized PAHs and Functionalized PAHs.

Scheme 12.5 Phenothiazine Derivatives.

Scheme 12.6 Dihydrophenazine Derivatives.

Scheme 12.7 Phenoxazine Derivatives.

Scheme 12.8 Dimethylacridine Derivatives.

Scheme 12.9 Donor‐acceptor Scaffolds.

Scheme 12.10 Heteroatom‐doped Anthanthrene Derivatives.

Scheme 12.11 Thienothiphene Derivatives.

Scheme 12.12 1,1'‐Bisnaphthol Derivatives.

Scheme 12.13

N

‐unsubstituted Diketopyrrolopyrrole Derivatives.

Scheme 12.14 2‐Phenylpyrimidine Derivatives.

Scheme 12.15 Porphyrin Derivatives.

Scheme 12.16 Fluorescein Derivatives.

Scheme 12.17 Phthalocyanine Derivatives.

Scheme 12.18 Jablonski Diagram of Excited‐State Evolutions.A: absorption. ...

Scheme 12.19 Representations for Electron Transitions in Molecular Orbitals....

Chapter 13

Figure 13.1 The application of photocatalyst (PC) for the development of pho...

Figure 13.2 Visible light‐initiated metal‐free living cationic polymerizatio...

Figure 13.3 Photocontrolled cationic polymerization of vinyl ethers using py...

Figure 13.4 A mechanistic proposal for the photocontrolled cationic RAFT pol...

Figure 13.5 Pyrylium photocatalyst derivatives examined in the photocontroll...

Figure 13.6 Bisphosphonium‐mediated photocontrolled cationic polymerization ...

Figure 13.7 Phosphonium photocatalysts developed for photocontrolled cationi...

Figure 13.8 Acridinium photocatalysts for photocontrolled cationic polymeriz...

Figure 13.9 Temporal control evaluation of the photomediated cationic polyme...

Figure 13.10 Tritylium photocatalysts for the photocontrolled cationic RAFT ...

Figure 13.11 Temporal control and proposed mechanism for the photocontrolled...

Figure 13.12 Stereoselective photocontrolled living cationic polymerization ...

Figure 13.13 Sulfonium‐type PAGs and their application in photoinduced ring‐...

Figure 13.14 Several typical mechanisms employed for the development of phot...

Figure 13.15 Photochromic catalyst for ring‐opening polymerization.

Figure 13.16 Photo‐switchable (co)catalyst for photopolymerization of

L

‐lact...

Figure 13.17 Photo‐switchable azobenzene‐based thiourea catalysts, and the r...

Figure 13.18 Reversible MEH‐type photoacids for ring‐opening polymerization....

Figure 13.19 ArOH as ESPT catalysts for ring‐opening polymerization of lacto...

Figure 13.20 1‐Hydroxypyrene as a photocatalyst for the visible light‐regula...

Figure 13.21 Various organic photocatalysts suitable for the development of ...

Figure 13.22 Metal‐free electrochemical olefin metathesis and the pathways t...

Figure 13.23 Organic photocatalyst‐enabled metal‐free ROMP and the light on/...

Figure 13.24 The mechanism of metal‐free ROMP under photoredox catalysis.

Figure 13.25 Pyrylium and thiopyrylium photocatalysts examined in the metal‐...

Figure 13.26 Homo‐ and copolymerizations between norbornene and functionaliz...

Figure 13.27 Step‐growth polymerizations of dithiols and bisolefins under me...

Figure 13.28 Step‐growth polymerization of bis(silane)s and bisolefins under...

Figure 13.29 Mechanistic proposal for the step‐growth polymerization of bis(...

Figure 13.30 Organocatalytic visible light‐regulated solution [2 + 2] photop...

Chapter 14

Figure 14.1 Enzyme‐catalyzed CRP.

Figure 14.2 Enzymes that have been used for deoxygenation in RAFT polymeriza...

Figure 14.3 (a) Photoenzymatic RAFT polymerization, (b) proposed mechanism, ...

Figure 14.4 The concentration of the formate affects oxygen consumption, H

2

O

Figure 14.5 ICAR ATRP of OEOMA with GOx deoxygenation.

Figure 14.6 ICAR ATRP catalyzed by a GOx/HRP cascade.

Figure 14.7 Enzyme‐catalyzed ATRP‐PISA with myoglobin as ATRPase.

Chapter 15

Figure 15.1 (a) Hydrogen bond activation mechanism of TBD, and (b) The impac...

Figure 15.2 Selective organocatalyst for specific PLA depolymerization.

Figure 15.3 The degradation process of poly(CHO/NA)‐

block

‐PCHC copolymer und...

Figure 15.4 TGA thermograms for various TBD‐based salts: dynamic heating at ...

Figure 15.5 Two possible routes for the depolymerization of BPA‐PC using 1,3...

Figure 15.6 Synthesis of imidazolium‐based self‐supported elastomeric ionene...

Figure 15.7 Degradation of PS catalyzed by organocatalysts. (a) Reisner et a...

Figure 15.8 HAT cycle and single oxygen cycle mechanism.

Guide

Cover

Table of Contents

Title Page

Copyright

Preface

Begin Reading

Index

End User License Agreement

Pages

iii

iv

xiii

xiv

xv

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

45

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

61

62

63

64

65

66

67

68

69

70

71

72

73

74

75

76

77

79

80

81

82

83

84

85

86

87

88

89

90

91

92

93

94

95

96

97

98

99

100

101

102

103

104

105

106

107

108

109

110

111

112

113

115

116

117

118

119

120

121

122

123

124

125

126

127

128

129

130

131

132

133

134

135

136

137

138

139

140

141

142

143

144

145

147

148

149

150

151

152

153

154

155

156

157

158

159

160

161

162

163

164

165

166

167

168

169

170

171

173

174

175

176

177

178

179

180

181

182

183

184

185

186

187

188

189

190

191

192

193

194

195

196

197

198

199

200

201

202

203

204

205

206

207

208

209

210

211

212

213

214

215

216

217

219

220

221

222

223

224

225

226

227

228

229

230

231

232

233

234

235

236

237

238

239

240

241

242

243

244

245

246

247

248

249

250

251

252

253

254

255

256

257

258

259

260

261

262

263

264

265

266

267

268

269

270

271

272

273

274

275

276

277

278

279

280

281

282

283

284

285

286

287

288

289

290

291

292

293

294

295

296

297

298

299

300

301

302

303

304

305

306

307

308

309

310

311

312

313

314

315

316

317

318

319

320

321

322

323

324

325

326

327

328

329

330

331

332

333

334

335

336

337

338

339

340

341

342

343

344

345

346

347

348

349

350

351

352

353

354

355

356

357

358

359

360

361

362

363

364

365

366

367

368

369

370

371

372

373

374

375

376

377

378

379

380

381

382

383

384

385

386

387

388

389

390

391

392

393

394

395

396

397

399

400

401

402

403

404

405

406

407

408

409

410

411

412

413

414

415

416

417

419

420

421

422

423

424

425

426

427

428

429

430

431

433

434

435

436

437

438

439

440

441

442

443

444

445

446

447

448

449

Organocatalysts in Polymer Chemistry

Synthesis and Applications

Edited by Zhibo Li

Editor

Prof. Zhibo Li

Qingdao University of Science and Technology

Qingdao, 266042

China

Cover Image: Courtesy of Xiaoyu Guo

All books published by WILEY‐VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

Library of Congress Card No.: applied for

British Library Cataloguing‐in‐Publication Data A catalogue record for this book is available from the British Library.

Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at <http://dnb.d‐nb.de>.

© 2025 WILEY‐VCH GmbH, Boschstraße 12, 69469 Weinheim, Germany

The manufacturer's authorized representative according to the EU General Product Safety Regulation is WILEY‐VCH GmbH, Boschstr. 12, 69469 Weinheim, Germany, e‐mail: [email protected].

All rights reserved (including those of translation into other languages, text and data mining and training of artificial technologies or similar technologies). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.

Print ISBN: 978‐3‐527‐35241‐8

ePDF ISBN: 978‐3‐527‐84305‐3

ePub ISBN: 978‐3‐527‐84306‐0

oBook ISBN: 978‐3‐527‐84307‐7

Preface

Catalysts not only play a crucial role in increasing the reaction rate, but also have a great impact on the selectivity of desirable reaction over side reactions. In particular, catalysts have been a powerful tools in polymer synthesis, including the ability to control the molecular weight, dispersity, terminal groups, the architecture, stereochemistry and topology of the macromolecule (linear, branched, cyclic, bottle‐brush, cross‐linking, etc) as well as the composition and monomer sequence of the copolymers. Organometallic catalysts consisting of a metal center and organic ligands are the most extensively investigated catalysts for polymer synthesis considering the ability to finely adjust the catalytic activity and selectivity by almost limitless combination of metals and ligands. Organocatalysts have emerged as an alternative to organometallic complexes, especially for the biomedical and electronic applications where metal residues may be detrimental to the material performance. Compared to organometallic catalysts, the advantages of organocatalysts include simple handle, low cost, better stability to oxygen and water, good solubility and easy removal from the polymers. In addition, many organocatalysts are commercially available, thus promoting their widespread applications for polymer synthesis.

Organocatalysis can be traced back to 1894 when Knoevenagel promoted the condensation reaction of formaldehyde and malonate using ethylenediamine as the catalyst [1]. Organocatalysts have been greatly developed, especially in the field of enantioselective organocatalysis. The earliest attempt that can be verified is the enantioselective synthesis of cyanohydrin with quinine alkaloids reported by Bredig and Fiske in 1911 [2]. In 2021, the Novel Prize in Chemistry was awarded to Benjamin List and David W.C. MacMillan for their pioneering contributions in the development of asymmetric organocatalysis. Compared to the organocatalysis on the synthesis of small molecules, the organocatalysis for polymer synthesis is still in its infancy. In 2001, James L. Hedrick et al. reported the first organocatalytic “living” ring‐opening polymerization of lactide using ethanol as an initiator and 4‐(dimethylamino)pyridine (DMAP) or 4‐pyrrolidinopyridine (PPY) as the catalyst [3]. Since then, the field of organocatalytic polymerization has undergone a renaissance. In the past two decades, organocatalysts have been greatly developed and achieved extensive applications in various polymer synthesis, including ring‐opening polymerization (ROP), ring‐opening copolymerization (ROCP), controlled radical polymerization as well as the depolymerization of commodity polymers.

Some excellent reviews have been published to summarize the recent advancements on organocatalytic polymerization, especially for the ROP of cyclic monomers [4–13]. A book with the title of “Organic Catalysis for Polymerisation” edited by Andrew Dove, Haritz Sardon and Stefan Naumann summarized the achievements of organocatalytic polymerization until 2018 [14]. Given the rapid development of this field, expecially the binary organocatalyst in polymer synthesis, organocatalyst catalyzed photo polymerization, as well as organocatalyst catalyzed depolymerization, we believe it is a suitable time to edit a book to review the state‐of‐the‐art examples for organocatalytic polymerization.

All chapters in this book are written by the outstanding scholars in their field. This book include following topics: ROP of cyclic esters toward degradable polymers, ROCP of epoxide and anhydrides, ROCP of CO2/COS/CS2 with epoxides, ROP of cyclicsiloxane, ROP of N‐carboxyanhydride (NCA) to prepare polypeptides, stereoselective ring‐opening (co)polymerization and controlled ATRP and RAFT. We will also cover the applications of organocatalysts in the depolymerization of commodity polyesters such as PET and polycarbonate. Recent progresses of binary organocatalyst and organic Lewis pairs for polymer synthesis will also be discussed. This book will cover the most recent research progresses regarding the applications of organocatalysts in polymer synthesis, particularly in degradable polymers. It will summarize the advances of organocatalysis in polymer chemistry in the past two decades. We hope this book will offer a great guideline for postgraduate students, professors, technicians as well as industrial experts.

December 2024

Zhibo Li

References

1

Knoevenagel, E. (1894). Ueber eine Darstellungsweise der Glutarsäure.

Ber. Dtsch. Chem. Ges.

27: 2345–2346.

2

Bredig, G. and Fiske, P.S. (1912). Durch Katalysatoren bewirkte asymmetrische synthese.

J. Plankton Res.

46: 7.

3

Nederberg, F., Connor, E.F., Möller, M. et al. (2001). New paradigms for organic catalysts: the first organocatalytic living polymerization.

Angew. Chem. Int. Ed.

40: 2712–2715.

4

Fukushima, K. and Nozaki, K. (2020). Organocatalysis: a paradigm shift in the synthesis of aliphatic polyesters and polycarbonates.

Macromolecules

53: 5018–5022.

5

Kiesewetter, M.K., Shin, E.J., Hedrick, J.L., and Waymouth, R.M. (2010). Organocatalysis: opportunities and challenges for polymer synthesis.

Macromolecules

43: 2093–2107.

6

Kamber, N.E., Jeong, W., Waymouth, R.M. et al. (2007). Organocatalytic ring‐opening polymerization.

Chem. Rev.

107: 5813–5840.

7

Liu, S., Ren, C., Zhao, N. et al. (2018). Phosphazene bases as organocatalysts for ring‐opening polymerization of cyclic esters.

Macromol. Rapid Commun.

39: 1800485.

8

Hu, S., Zhao, J., Zhang, G., and Schlaad, H. (2017). Macromolecular architectures through organocatalysis.

Prog. Polym. Sci.

74: 34–77.

9

Xu, J., Wang, X., Liu, J. et al. (2022). Ionic H‐bonding organocatalysts for the ring‐opening polymerization of cyclic esters and cyclic carbonates.

Prog. Polym. Sci.

125: 101484.

10

Hong, M., Chen, J., and Chen, E.Y.X. (2018). Polymerization of polar monomers mediated by main‐group Lewis acid–base pairs.

Chem. Rev.

118: 10551–10616.

11

Dove, A.P. (2012). Organic catalysis for ring‐opening polymerization.

ACS Macro Lett.

1: 1409–1412.

12

Jehanno, C., Pérez‐Madrigal, M.M., Demarteau, J. et al. (2019). Organocatalysis for depolymerisation.

Polym. Chem.

10: 172–186.

13

Naumann, S. and Dove, A.P. (2015).

N

‐heterocyclic carbenes as organocatalysts for polymerizations: trends and frontiers.

Polym. Chem.

6: 3185–3200.

14

Dove, A., Sardon, H., and Naumann, S. (ed.) (2018).

Organic Catalysis for Polymerisation

. The Royal Society of Chemistry

https://doi.org/10.1039/9781788015738

.

1Organocatalyzed Ring‐Opening Polymerization of Cyclic Esters Toward Degradable Polymers

Feng Li1, Takuya Isono1, and Toshifumi Satoh12

1Division of Applied Chemistry, Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan

2List Sustainable Digital Transformation Catalyst Collaboration Research Platform (List-PF), Institute for Chemical Reaction Design and Discovery (ICReDD), Hokkaido University, Sapporo 001-0021, Japan

1.1 General Introduction

The discovery of ring‐opening polymerization (ROP) of cyclic esters to afford polyesters dates back to the 1930s. The hydrolyzable nature of the ester functional group in the polymer chain endows the chain with degradability (e.g. thermal, chemical, bio), rendering polyesters as promising candidates for biomedical applications and as environmentally benign polymer materials. In addition, cyclic esters exhibit polymerizability with an extremely broad scope of catalysts. The ROP of cyclic esters can occur via anionic, cationic, and coordination mechanisms, using different types of catalysts such as transition‐metal catalysts, enzymes, and organocatalysts (Figure 1.1). Thus, the ROP of cyclic esters is the first and the most investigated organocatalyst‐based polymerization reaction to date.

Figure 1.1 ROP of cyclic esters into polyesters.

After extensive investigations over the past two decades, various organocatalysts have been reported to exhibit catalytic activity in the ROP of cyclic esters (Figure 1.2). The typical reaction mechanism for various catalysts is introduced in Section 1.2.

Figure 1.2 Representative organocatalysts for the ROP of cyclic esters.

After the numerous initial investigations of novel organocatalysts, driven by the scientific interest in transition‐metal‐free catalysts, several factors such as catalytic efficiency, selectivity, thermal stability, and safety have been considered in recent works toward meeting the requirement for industrial application. However, metal complexes, such as tin(II) 2‐ethylhexanoate (Sn(Oct)2), are still used in industries to produce polyesters. An increasing number of recent studies have indicated the promising future of organocatalysts, even under industrially relevant conditions. In Section 1.3, the paradigm shifts in the organocatalyzed ROP of cyclic esters are illustrated.

β‐Butyrolactone (β‐BL), lactide (LA), δ‐valerolactone (VL), and ɛ‐caprolactone (CL) are among the most studied cyclic ester monomers because of their relatively high ring strain, good polymerizability, biodegradability, and biocompatibility of their corresponding polyesters, poly(β‐butyrolactone) (P(β‐BL)), poly(lactic acid) (PLA), poly(δ‐valerolactone) (PVL), and poly(ɛ‐caprolactone) (PCL) (Figure 1.3a). However, five‐member and large‐ring lactones such as γ‐butyrolactone (γ‐BL) [1, 2] and ω‐pentadecalactone (DL) [3, 4] cannot be polymerized easily owing to their low ring strain (Figure 1.3b). The successful demonstration of controlled polymerization of low ring strain γ‐BL under low temperatures by highly active catalysts, including metallic catalysts and organocatalysts, and depolymerization of P(γ‐BL) back to γ‐BL monomer at an elevated temperature signified the impact of the closed‐loop polymerization methodology [1, 2]. Since this seminal work of Hong and Chen, chemically recyclable polyesters with novel monomer designs, especially the ones that can be easily derived from renewable biomass resources, have become an emerging research topic, and the number of research reports has increased rapidly in recent years. Although their research was largely focused on the monomer design, polymer properties, and recyclability, organocatalysts have been used extensively. Polyesters that can degrade easily under environmental conditions are also important, aside from the chemically recyclable polyesters. Therefore, the introduction of other facile functional groups to the main chain of polyesters for enhancing their degradability has also been an important topic in recent years. In Section 1.4, breakthroughs in achieving improved degradability and recyclability are discussed.

Figure 1.3 Representative cyclic esters with relatively high (a) and low (b) ring strains.

This chapter focuses on the features of organocatalysts, cyclic ester monomers, and the corresponding polymers. Utilizing the organocatalyzed ROP of cyclic esters for the synthesis of block copolymers and tailoring a highly complicated polymer architecture design for synthesizing advanced degradable materials are also important research directions; however, they are not the topics of this chapter.

Alkali‐metal salts, such as the salts of carboxylic acids, vitamin C, and (thio)ureas, are not completely organic compounds. The reaction mechanisms of these catalysts are similar to those of common organocatalysts, rather than transition‐metal catalysts. In addition, sodium and potassium ions are safe and essential for the human body; therefore, they have been categorized as organocatalysts herein.

1.2 Polymerization Mechanism

1.2.1 Nucleophilic Catalysts

Nucleophilic catalysts, e.g. 4‐dimethylaminopyridine (DMAP) and N‐heterocyclic carbenes (NHCs), are widely used in organic chemistry. The ROP of lactide by DMAP, as reported by Hedrick and coworkers in 2001, is recognized as the landmark that initiated the era of organocatalyzed polymerization [5]. Since this seminal work, many other nucleophilic catalysts, such as phosphines [6], amidines [7], and NHCs [8, 9], have been investigated.

The reaction conditions and polymerizable monomers largely depend on the catalysts employed. A quantitative comparison of the nucleophilicities of these catalysts can provide a better understanding. The Mayr reactivity parameters provide a scale for quantitatively evaluating and comparing the nucleophilicities of various nucleophilic catalysts. Four representative nucleophilic organocatalysts are shown in Figure 1.4, whose Mayr nucleophilic parameter N increases in the order of triphenyl phosphines, DMAP, 1,8‐diazabicyclo[5.4.0]‐7‐undecene (DBU), and NHCs [10].

Figure 1.4 Representative nucleophilic catalysts and their nucleophilicities evaluated using the Mayr reactivity parameters.

Regarding the reaction mechanism of nucleophilic catalysts for the ROP of cyclic esters, the catalytic cycle typically commences with a nucleophilic attack from the catalysts on the carbonyl group of the cyclic esters to open the ring and generate a zwitterionic intermediate. If the reaction is conducted in the presence of an alcohol chain‐transfer agent, i.e. ROH, the hydroxyl group can be activated by the anionic site of the zwitterionic intermediate, thus inducing an intramolecular nucleophilic attack and releasing the nucleophilic catalyst (Figure 1.5, upper reaction route). The iteration of this process affords linear polyesters. In the absence of ROH, the anionic site, i.e. alkoxide, of the generated zwitterionic intermediate continues to attack other cyclic ester monomers, and at a certain stage, the anionic chain end can nucleophilically attack the cationic activated carbonyl group and release the nucleophilic catalysts, affording cyclic polyesters (Figure 1.5, lower reaction route).

Figure 1.5 General reaction mechanism of the ROP of cyclic esters using nucleophilic catalysts in the presence and absence of alcohol initiators.

Nucleophilic catalysts typically exhibit a strong basicity; however, their reaction mechanism may not be identified easily. For example, DBU and 1,5,7‐triazabicyclo[4.4.0]dec‐5‐ene (TBD) are moderate/strong Brønsted bases, but they exhibit relatively high nucleophilicity. Therefore, they could follow the mechanisms of either nucleophilic catalysts [11] or Brønsted base catalysts [12].

1.2.2 Base Catalysts

Organobase catalysts constitute an important type of organocatalysts for the ROP of cyclic esters. Base catalysts can be divided into Lewis bases and Brønsted bases. This section focuses on Brønsted bases, which function as proton acceptors. Lewis bases with a high nucleophilicity have been categorized as nucleophilic catalysts in Section 1.2.1. In the ROP of cyclic esters, commonly used Brønsted bases catalysts include amines, amidines, guanidines, and phosphazenes [13–15]. Pyridines and other N‐containing heterocycles are Brønsted bases as well; however, considering their weak basicity and medium‐to‐high nucleophilicity, they have been introduced in Section 1.2.1 as nucleophilic catalysts (e.g. DMAP).

The basicity of the abovementioned Brønsted bases spans a wide range across 24 orders of magnitude – from the relatively weak triethylamine (pKBH+ 18.8 in acetonitrile) to the super strong base t‐BuP4 (pKBH+ 42.7 in acetonitrile) (Figure 1.6) [16–18]. Hence, the basicity of the catalyst can be tuned according to the requirements of the corresponding polymerization reaction. Catalysts with medium‐to‐strong basicity, e.g. DBU, TBD, and phosphazenes, are most commonly used.

Figure 1.6 Representative Brønsted base catalysts and their basicity (pKa of the conjugated acid in MeCN).

Regarding the reaction mechanism, the chain end activation mechanism is typically considered in the ROP of cyclic esters with alcohols as initiators (Figure 1.7a) [19]. Through hydrogen bonding, the base activates the –OH group of the alcohol initiators or the propagating chain ends to enhance their nucleophilicity and attack the carbonyl group of the cyclic esters. The higher the basicity of the catalyst, the more prone it is to complete deprotonation toward generating naked alkoxide anions; thus, its reactivity is higher.

Figure 1.7 General reaction mechanism of the ROP of cyclic esters using Brønsted base catalysts.

When a bifunctional organobase, such as TBD, is used, the dual‐activation mechanism via double hydrogen bonding interactions is usually considered (Figure 1.7b) [20]. The Brønsted basic site of the guanidine activates the –OH group, whereas the Brønsted acidic site (N–H) interacts with the carbonyl group of the cyclic esters. When cyclic esters are polymerized without using alcohol initiators, the polymerization could initiate from the monomer via the deprotonation of the α‐proton of the esters, thus forming nucleophilic enolate species (Figure 1.7c). In this case, highly basic catalysts, such as phosphazenes, are commonly used [2].

1.2.3 Acid Catalysts

Acid catalysts are among the most important organocatalysts for the ROP of cyclic esters. Based on their characteristics, acid catalysts can be divided into Brønsted acids and Lewis acids. In terms of the organocatalyzed ROP of cyclic esters, Brønsted acid catalysts have been widely reported. Herein, the representative mechanisms and examples are introduced. Simple strong inorganic acids, such as HOTf and HCl, can catalyze the ROP of cyclic esters; however, these reactions were typically not considered organocatalyzed polymerizations at the time of the study [21–25].

Carboxylic, phosphoric, and sulfonic acids are commonly used in the organocatalyzed ROP of cyclic esters based on Brønsted acid catalysts (Figure 1.8). On the basis of the structure and acidity of the catalyst and the polymerization conditions, the reaction mechanisms can be divided into three categories (Figure 1.9). The activated monomer (AM) mechanism is a typical polymerization mechanism for the ROP of cyclic esters (Figure 1.9a). The Brønsted acid activates the carbonyl group of the cyclic ester, rendering it more prone to a nucleophilic attack from the hydroxyl group of either the initiator or the propagating chain end. The nucleophilic attack could also occur at the sp3 carbon next to the ester group, thus forming an ion‐pair intermediate with the counteranion of the Brønsted acid catalysts (Figure 1.9b) [28, 29]. This chain end structure is highly reactive and can accept a nucleophilic attack from another molecule of the cyclic ester monomer. This polymerization mechanism is usually referred to as the activated chain end (ACE). The activated chain ends are usually terminated by quenching the polymerization using alcohols or other nucleophiles. The AM and ACE polymerization mechanisms were proposed a long time ago; however, the dual‐activation mechanism of the ROP of cyclic esters by acid catalysts was not recognized until the recent decades (Figure 1.9c) [30, 31]. In the dual‐activation mechanism, the Brønsted acidic site of the catalyst activates the carbonyl group of the cyclic ester monomer and renders it more electrophilic; meanwhile, the Brønsted basic site of the catalyst activates the alcohol initiator or chain end, rendering it more nucleophilic. This dual‐activation mechanism ensures proximity of the monomer and propagating chain end, thus allowing for smooth progress of the polymerization. Considering the structural features and acidities of the catalysts, the catalysis by sulfonic acids is more likely to proceed via the AM or ACE mechanism because of their high acidities. In the case of carboxylic and phosphoric acids, the polymerization usually proceeds via the dual‐activation mechanism because of the weak‐to‐medium acidities of these two types of catalysts.

Figure 1.8 Representative Brønsted acid catalysts and their acidities.

Source: Bordwell [26] and Christ [27].

Figure 1.9 General reaction mechanism of the ROP of cyclic esters using Brønsted acid catalysts.

1.2.4 Ionic Catalysts

During the initial research on organocatalyzed polymerization, catalysts without charges were used in most studies. In the past decade, salt or ionic organocatalysts have been developed for the ROP of cyclic esters [32]. They exhibit better catalytic activities and thermal stabilities than conventional organocatalysts. These aspects are introduced in Sections 1.3.1 and 1.3.3.

Figure 1.11 Examples of salt and ionic catalysts.

When a Brønsted acid and base are mixed in a ratio of 1 : 1, a salt is formed (Figure 1.10a). However, the produced salt exhibits a different acidity or basicity, depending on the relative strengths of the acidic and basic parts, which in turn determines the reaction mechanism governing the ROP of cyclic esters. A dual‐activation mechanism has been typically proposed – The anion activates the alcohol initiator or propagating chain end, whereas the cation activates the cyclic ester monomer (Figure 1.10b). This mechanism is usually observed for a mixture of a weak/moderate acid and weak/moderate base, such as 1 : 1 mixtures of DBU and benzoic acid, DMAP and diphenyl phosphate (DPP), and DMAP and saccharin (Figure 1.11a–c) [33–35]. The conjugated bases and acids of strong acids and bases are weak, respectively; hence, they exhibit weak or negligible interactions with the monomer or propagating chain end. Therefore, the AM mechanism is typically proposed when a salt catalyst constituted by a strong acid and a weak/moderate base is used, e.g. a 1 : 2 mixture of bipyridine and camphorsulfonic acid (Figures 1.10c and 1.11d) [36]. In theory, the chain end activation mechanism can also be considered for combinations of strong bases and weak/moderate acids (Figure 1.10d). However, to the best of our knowledge, this activation has not been reported [32]. Ureas, thioureas, and carboxylic acids have relatively weak acidities. Their alkali‐metal salts and salts with strong organic bases, such as phosphazenes, have been reported as effective catalysts for the ROP of cyclic esters (Figure 1.11e–g) [37–39]. The dual‐activation mechanism has been proposed in these cases because of the bifunctional nature of the anions thereof.

Figure 1.10 Salt catalysts and general reaction mechanism of the ROP of cyclic esters for different types of salt catalysts.

In addition to the salts formed using a 1 : 1 ratio mixture of an acid and a base, mixtures with an unequal ratio, i.e. an excess amount of either an acid or a base, have been used for fine‐tuning the reactivity and catalytic performance. For example, the ionic catalyst DMAP: methanesulfonic acid (MSA) (1 : 2) has been used for catalyzing the ROP of L‐lactide (LLA) in bulk at an acceptable reaction rate without epimerization (Figure 1.11h) [40]. Tetraalkylammonium halides, which are representative of onium salts, can also function as effective catalysts for the ROP of cyclic esters (Figure 1.11i) [41, 42].

Zwitterions are also referred to as inner salts, in which the cations and the anions are covalently linked together in the same molecule. To the best of our knowledge, zwitterionic organocatalysts have not been used for the ROP of cyclic esters thus far. However, for the ROP of trimethylene carbonate (TMC), trimethyl glycine, which is a natural betaine sourced from sugar beets, has been demonstrated to be an effective and environmentally benign organocatalyst [43].

1.2.5 Bifunctional and Multifunctional Catalysts

Bifunctional and multifunctional organocatalysts are molecules in which two or more catalytically active moieties are linked covalently. Dual‐activation is the commonly accepted reaction mechanism by which the ROP of cyclic esters is promoted efficiently. Although the dual‐activation model can be realized using monofunctionalized catalysts such as DPP and TBD, the use of bifunctional and multifunctional catalysts could be beneficial for fine‐tuning the interactions with both the propagating chain ends and cyclic ester monomers. Thus, both the active propagating chain end and the activated cyclic ester monomer can adopt suitable conformations in the transition state, thereby decreasing the activation energy. Bifunctional and multifunctional catalysts are viewed as the mimicry of enzymes, which catalyze reactions using multiple amino acid residues cooperatively.

Regarding the catalyst design, Takemoto's catalyst, which comprises a thiourea moiety and trialkyl amine group, is one of the most representative bifunctional organocatalysts (Figure 1.12a) [44]. It has been successfully used in the ROP of LA [45]. Because Takemoto's catalyst is chiral, it has also been employed for the stereoselective polymerization of racemic lactides (rac‐LA) [46].

Figure 1.12 (a) Takemoto's catalyst; (b) dual‐activation mechanism in the ROP of LA.

In the case of Takemoto's catalyst, the weakly acidic thiourea functions as a hydrogen‐bond donor that interacts with the carbonyl group of the cyclic esters, thus activating the electrophile; the basic amine group partially deprotonates the –OH group at the propagating chain end or initiators, thus activating the nucleophile (Figure 1.12b). Owing to its efficiency, Takemoto's catalyst has been used as a prototype for developing bifunctional and multifunctional catalysts. Various bifunctional catalysts have been developed by varying the components of either the hydrogen‐bond donor or the Brønsted base. For example, upon changing the amine moiety in the thiourea–amine catalyst to a more basic iminophosphorane moiety, its catalytic activities in the ROP of LA, VL, and CL are enhanced, while maintaining good control over the dispersity (Đ) (Figure 1.13a) [47]. The change in the hydrogen‐bond donor moiety from thiourea to squaramide also affords an effective bifunctional catalyst for the ROP of LA (Figure 1.13b) [48]. The borane–thiourea–amine trifunctional organocatalyst can catalyze the synthesis of block copolymers, with PLA as one block, from a monomer mixture in a one‐pot, one‐step manner (Figure 1.13c), as per a recent report [49]. Mimicry of enzymes by introducing multiple functional groups into a single catalyst molecule could be a promising strategy. However, this approach significantly increases the synthetic complexity of the catalysts and the number of possible structures to be explored to obtain an optimized catalyst candidate. Furthermore, a novel catalyst design concept of using more than one hydrogen‐bond donor unit, such as urea or thiourea, has been reported for the ROP of cyclic esters (Figure 1.13d) [50, 51].

Figure 1.13 Some bifunctional and multifunctional catalysts.

1.3 Recent Trends in Organocatalyst Development

Regarding the recent trends in organocatalyst development for the ROP of cyclic esters, the following four aspects are considered to be significant: enhancements in the catalytic efficiency, selectivity, heat tolerance, and safety. These four aspects are introduced in this section.

1.3.1 Higher Catalytic Efficiency

When the ROP of cyclic esters approaches a high percentage of monomer conversion, transesterification can easily occur, leading to a broad dispersity. Before the period of extensive research on organocatalyzed polymerization, strong inorganic bases were used for catalyzing or initiating the ROP of cyclic esters via anionic polymerization. For instance, when the ROP of lactide is initiated by lithium diisopropylamide (LDA) at room temperature in a dioxane solvent, the monomer conversion can reach over 95% within a few minutes, affording PLA with a broad dispersity (Đ = 1.9) [52]. In the organocatalyzed ROP of cyclic esters, high selectivity has been considered as a common research objective instead of high reactivity, for a relatively long time.

In recent years, organocatalysts have been successfully developed to achieve fast, selective, and well‐controlled ROP of cyclic esters. (Thio)urea anions are good examples. The (thio)urea anions with alkaline‐metal cations, such as Na+ or K+, were first reported to catalyze the ROP of various cyclic esters in a fast and controlled manner [37, 53]. By fine‐tuning the molecular structure of (thio)ureas and the countercation, the ROP of LA, δ‐VL, ɛ‐CL, etc., could be completed within seconds, with the monomer conversion reaching >90% and narrow dispersity (Đ < 1.1). The reaction proceeds via the dual‐activation mechanism, with the (thio)urea anion (N−) activating the propagating chain end (–OH), whereas the N–H moiety of the (thio)urea anion interacts with the carbonyl group of the cyclic esters (Figure 1.14a). In addition to alkaline‐metal bases, various strong organobases, including DBU, MTBD, cyclopropenimine [54], and phosphazene bases, have been investigated in conjunction with (thio)urea for the ROP of cyclic esters (Figure 1.14b). Consequently, when the acidity of (thio)ureas and the basicity of bases attain equilibrium – pKa of (thio)urea ≈ pKa of base‐H+ – the best catalytic activity is achieved [55]. Regarding the reaction mechanism, the ROP is proposed to proceed via an anionic mechanism when pKa of (thio)urea < pKa of base‐H+ (Figure 1.15a). When pKa of (thio)urea > pKa of base‐H+, a cooperative mechanism is proposed (Figure 1.15b).

Figure 1.14 Representative structures of (a) ureas, thioureas and (b) bases.

Figure 1.15 (a) Anionic mechanism; (b) cooperative mechanism; (c) reaction mechanism using the 2,2′‐bisindole anion.

After these pioneering works, investigations have been expanded by further tuning the catalyst structures, synthesizing statistical and block copolymers, and using them in flow chemistry. For instance, the urea salt with the tetra‐n‐butyl ammonium cation is an effective catalyst for the rapid, selective, and versatile ROP of lactides [56]. Further expansion of the investigation scope of the phosphazene bases in forming urea salts enabled the efficient synthesis of poly(γ‐butyrolactone) [57], random poly(lactic‐co‐glycolic acid) [58], and poly(lactic acid)‐b‐poly(alkyl‐δ‐lactone)‐b‐poly(lactic acid) triblock copolymer as thermoplastic elastomers [38] and pressure‐sensitive adhesives [59]. Aside from (thio)urates, the 2,2′‐bisindole anion can function as an excellent catalyst to promote the rapid ROP of cyclic esters (Figure 1.15c) [60].

The progress in rapid and controlled ROP for cyclic esters has opened up new avenues for developing high‐throughput synthetic platforms of polymer libraries using flow chemistry techniques [61]. Moreover, by employing continuous‐flow reactors, ultrafast ROP of cyclic esters can be achieved using conventional strong inorganic base catalysts, such as KOtBu, affording the polyester products in a well‐controlled manner, which cannot be achieved under batch polymerization conditions [62].

1.3.2 Higher Selectivity

High selectivity has been a common, important research goal in the development of novel catalytic reactions and polymerizations. In the ROP of cyclic esters, selectivity can include various aspects. This section focuses on enantiomer‐selective polymerization and briefly introduces some examples of other aspects.

Among all reports on enantiomer‐selective ROP of cyclic esters, the ROP of racemic lactide (rac‐LA) and racemic β‐butyrolactone (rac‐β‐BL) have been chosen as two representative examples [63–66]. Chiral metal‐complex catalysts have been extensively investigated for these two ROPs, and highly selective catalysts have been developed. In the past decade, reports on organocatalysts, especially chiral ones, have emerged.

In the early stages, the ROP of rac‐LA by achiral organocatalysts was mainly reported, wherein isotactic PLA or isotactic‐rich PLA was obtained. The selectivity relies on the chirality of the chain end structure that is combined with the achiral catalyst. The L‐LA and D‐LA molecules were consumed at the same rate, and the obtained isotactic‐rich PLA was typically a multiblock copolymer consisting of PLLA and PDLA stereoblocks (Figure 1.16). Therefore, the formation of stereocomplexes was observed in some cases. NHCs and phosphazenes are representative achiral organocatalysts used for this polymerization [67–69]. However, to achieve a high level of isotacticity, in which the meso dyads (Pm) ≥ 0.90, the polymerization was usually performed at a low temperature (≤ −70 °C).

Figure 1.16 ROP of rac‐LA using achiral organocatalysts.

Figure 1.17 ROP of rac‐LA using chiral organocatalysts: enantiomer‐selective polymerization.

Compared to the use of achiral organocatalysts in the synthesis of isotactic PLA, the enantiomer‐selective polymerization of rac‐LA based on chiral organocatalysts is more challenging (Figure 1.17). Cinchona alkaloids were the first organocatalysts reported for this polymerization [70]. However, their stereoselectivity factor is low (s ≤ 4.4) (Figure 1.18a). 1,1′‐Bi‐2‐naphthol (BINOL)‐derived chiral phosphoric acids are more effective in controlling the enantioselectivity of rac‐LA. The (R)‐catalyst polymerizes L‐LA prior to D‐LA, and the stereoselectivity factor is as high as 28.3 (Figure 1.18b) [71]. Takemoto's catalyst is a commonly used chiral bifunctional organocatalyst for the ROP of LA [45]. The use of enantiopure Takemoto's catalyst in catalyzing the ROP of rac‐LA results in a relatively high isotacticity (Pm ≥ 0.80) but low stereoselectivity (s ≤ 5.0) (Figure 1.18c) [46]. The combination of enantiopure Takemoto's catalyst and a phosphazene base can further increase the isotacticity and reactivity [72]. Other novel chiral bifunctional organocatalysts synthesized by combining the moieties, including cinchona alkaloids, 1,1′‐binaphthyl, thiourea, and trans‐1,2‐cyclohexanediamine, have also been reported for the ROP of rac‐LA (Figure 1.18d,e) [73, 74]. Using these two catalysts shown in Figure 1.18d,e, stereoselectivity factor values of 53 and 17.5, respectively, were obtained, in addition to the high isotacticity of the PLA products. Ionic catalysts consisting of a densely substituted proline‐type amino acid and DBU have also been reported to be effective for the ROP of rac‐LA (Figure 1.18f) [75]. In the abovementioned cases, the highest stereoselectivity factor is 53. Therefore, the obtained PLA should be either L‐LA‐rich or D‐LA‐rich, instead of perfect PLLA or PDLA.

Figure 1.18 Representative chiral organocatalysts for the ROP of rac‐LA.

Unlike the emerging reports on the organocatalyzed ROP of rac‐LA, there are no reports on the organocatalyzed ROP of rac‐β‐BL, to the best of our knowledge. It can be attributed to the intrinsic difficulties in distinguishing the enantiomers of β‐BL. The carbonyl group activated by the catalyst is opposite to the chiral center (Figure 1.19a). Therefore, enantiomer‐selective polymerization of rac‐β‐BL is highly challenging, even when metal‐complex catalysts are used [76]. In addition, the ROP of β‐BL can proceed via two different pathways, which further increases the difficulty in controlling the stereoselectivity in the ROP of rac‐β‐BL. A nucleophilic attack on the lactone carbonyl carbon results in stereoretention (Figure 1.19b), whereas that on the methine carbon next to the ester group results in stereoinversion (Figure 1.19c).

Figure 1.19 (a) Intrinsic difficulty in performing enantiomer‐selective ROP of rac‐β‐BL; ROP of β