For eword
The ACS Symposium Series was first published 1974 provide a mechanism for publishing symposia quickly book The purpose the series publish timely , comprehensive books developed from the ACS sponsored symposia based current scientific Occasionally , books are developed from symposia sponsored other ganizations when the topic keen interest the chemistry audience.
Before agreeing publish a the proposed table contents reviewed for appropriate and comprehensive coverage and for interest the audience. Some papers may excluded better focus the book; others may added provide comprehensiveness. When appropriate, overview introductory chapters are added. Drafts chapters are peer - reviewed prior final acceptance rejection, and manuscripts are prepared camera - ready
a rule, only original research papers and original review papers are included the V erbatim reproductions previous published papers are not accepted.
ACS Books Department
eface
Contents
Sivaprakash Shanmugam and Krzysztof Matyjaszewski
Mechanistic Insights into Lewis Acid Mediated Sequence - and Ster - Contr Radical Copolymerization
Nicholas Hill , Benjamin Noble , and Michelle Coote
T ime - Resolved Electr Spin Resonance Observations the Initial Stages
Conventional and Contr olled Radical Polymerization ocesses .
Atsushi Kajiwara
Elements RAFT Navigation
Joris J Haven , Matthew Hendrikx , T anja Junkers , Pieter J Leenaers , Theodora T sompanoglou , Cyrille Boyer , Jiangtao , Almar Postma , and Graeme Moad
Reducing the Hydr ogen Atom Abstraction Efficiencies Benzophenone - Based Photosensitive Alkoxyamines
Jason Morris , Jean - Louis Clément , Y ohann Guillaneuf , Steven Bottle , Kathryn Fairfull - Smith , and Didier Gigmes
105
Catalyzed Radical T ermination (CR the Metal - Mediated Polymerization Acrylates: Experimental and Computational Studies . . . . . . 135 Thomas Ribelli , W ahidur Rahaman , Krzysztof Matyjaszewski , and Rinaldo Poli
Electr ochemical ocedur T o Determine Thermodynamic and Kinetic Parameters Atom T ransfer Radical Polymerization
Francesca Lorandi , Marco Fantin , Francesco Bon , Abdirisak Isse , and Armando Gennaro
Insights into the Reactivity Epoxides Reducing Agents
Low - Catalyst - Concentration A TRP Reactions
David McLeod , Kapil Dev Sayala , and Nicolay V . T sarevsky
T oward
Butadiene - A TRP with oup (Ni, Pd, Pt) Metal Complexes .
205 V ignesh V asu , Joon - Sung Kim , Hyun - Seok Y u , W illiam Bannerman , Mark Johnson , and Alexandru Asandei
1
10. Reversible Deactivation Radical Polymerization V inyl Chloride . .
Carlos Abreu , Ana Fonseca , Nuno P Rocha , James T Guthrie , Arménio Serra , and Jor F Coelho
Photoinduced Metal Strategies for Atom T ransfer Radical Polymerization
Y ilmaz , Kutahya , Allushi , A ydogan , A ykac , and Y . Y agci
12. Recent Developments External Regulation Reversible Addition Fragmentation Chain T ransfer (RAFT) Polymerization
Sivaprakash Shanmugam , Cyrille Boyer , and Krzysztof Matyjaszewski
13. How Can Xanthates Contr the RAFT Polymerization Methacrylates?
291 Mathias Destarac , Dimitri Matioszek , Xavier V ila , Juliette Ruchmann - Sternchuss , and Samir Zard
14. Hydr oxyl Radical Activated RAFT Polymerization .
Thomas McKenzie , Amin Reyhani , Mitchell Nothling , and Greg Qiao
15. V inyl Ether / V inyl Ester Copolymerization Cationic and Radical Inter convertible Simultaneous Polymerization
Kotaro Satoh , Y uuma Fujiki , Mineto Uchiyama , and Masami Kamigaito
16. Catalytic Chain T ransfer Polymerization and Reversible Deactivation Radical Polymerization V inyl Acetate Mediated Cobalt(II) Phenoxy - imine Complexes
Y i - Hao Chen , Hung - Hsun , Jia, and Chi - How Peng
17. T ailor - Made Poly(vinylamine)s via Thermal Photochemical Organometallic Mediated Radical Polymerization
Pierre Stiernet , Mathilde Dréan , Christine Jérôme , Patrick Midoux , Philippe Guégan , Jutta Rieger , and Antoine Debuigne
18. Alkyl omide ecursor Initiating Dormant Species Organocatalyzed Living Radical Polymerization
Feifei , Longqiang Xiao , and Atsushi Goto
19. Biocatalytic A TRP
Jonas Pollard and Nico Bruns
Editors’ Biographies
eface
This book and a following volume are addressed chemists and polymer scientists interested controlled / living radical polymerization. These volumes are intended summarize recent mechanistic advances and innovation the area materials and applications.
The two volumes comprise the topical reviews and contributions presented the American Chemical Society symposium Contr olled Radical Polymerization (the IUP preferred term Reversible Deactivation Radical Polymerization and used the titles these volumes) that was held W ashington, DC, August24, 2017. This most recent meeting was a sequel several previous ACS Symposia controlled / living radical polymerization, held San Francisco, California (1997), New Orleans, Louisiana Massachusetts W Pennsylvania (2008), Denver , Colorado (201 1), and San Francisco, California The work presented those symposia was summarized the ACS Symposium Series V olume 685: Contr olled Radical Polymerization , V olume 768: Contr olled / Living Radical Polymerization: ogr ess A TRP , NMP and RAFT , V olume 854: Advances Contr olled / Living Radical V olume 944: Contr olled / Living Radical Polymerization: Synthesis V olume 1023: Contr olled / Living Radical Polymerization: ogr ess A TRP , V olume 1024: Contr olled / Living Radical Polymerization: ogr ess RAFT , , NMP and OMRP , V olume 1 100: ogr ess Contr olled Radical Polymerization: Mechanisms and T echniques, and V olume 1 101: ogr ess
Contr olled Radical Polymerization: Materials and Applications , V olume 1 187: Contr olled Radical Polymerization: Mechanisms , and V olume 1 188: Contr olled Radical Polymerization: The W 2017 meeting was very successful with lectures and a similar number posters presented. a clear sign the health the field that this level participation has continued grow over the years and only limited the available time / speaking slots the meeting.
The chapters submitted for publication the ACS Symposium Series could not fit into one volume, and therefore were asked the ACS divide the contents into two Similar the volumes originating from the most recent Denver and San Francisco meetings, these two volumes are dedicated mechanisms and techniques (the current volume consisting chapters) and materials and applications (the following volume, which consists chapters). All chapters published these two volumes show that reversible deactivation radical polymerization has made significant progress within the last two decades. New systems have been discovered, substantial progress has been achieved
understanding the mechanism and kinetics reactions involved all reversible deactivation radical polymerization systems. Significant progress has also been made towards developing a comprehensive relationship between molecular structure and macroscopic Several commercial applications reversible deactivation radical polymerization were announced the W ashington, Meeting and anticipated that new products made controlled / living radical polymerization will soon the market.
The financial support for the symposium from the following ganizations acknowledged: ACS Division Polymer Chemistry , Inc., Army Research fice, Anton Parr , Boron Molecular , the National Science Royal Society Chemistry , Sigma Aldrich, T osoh, and W ileyVCH.
Krzysztof Matyjaszewski
Center for Macromolecular Engineering
Department Chemistry
Carnegie Mellon University 4400 Fifth A venue
Pittsbur Pennsylvania 15213
Haifeng Gao
Department Chemistry
University Notre Dame South Bend, Indiana 46556
ent Sumerlin
Geor & Josephine Butler Polymer Research Laboratory
Center for Macromolecular Science & Engineering
Department Chemistry
University Florida Florida 32605 - 7200
Nicolay V . T sar evsky
Department Chemistry
Southern Methodist University 3215 Daniel A venue
Dallas, T exas 75275
Reversible
Deactivation Radical Polymerization: State - - the - Art 2017
Sivaprakash Shanmugam and Krzysztof Matyjaszewski *
Center for Macr omolecular Engineering, Department Chemistry , Carnegie Mellon University , 4400 Fifth A venue, Pittsburgh, Pennsylvania 15213, United States * E - mail: matyjaszewski@cmu.edu.
This chapter highlights the current advancements reversible - deactivation radical polymerization (RDRP) with a specific focus atom transfer radical polymerization
The chapter begins with highlighting the termination pathways for acrylates radicals that were recently explored via RDRP This led a better understanding the catalytic radical termination (CR A TRP for acrylate
The designed new ligands for A TRP also enabled the suppression T and increased chain end functionality . further mechanistic understandings SARA - A TRP with 0 activation and comproportionation were studied using model reactions with dif ferent ligands and alkyl halide
Another focus RDRP recent years has been systems that are regulated external stimuli such light, electricity , mechanical forces and chemical redox Recent advancements made RDRP the field complex polymeric ganic - inor ganic hybrid materials and bioconjugates have also been summarized.
Intr oduction
The overarching goal this chapter provide overall summary the recent achievements reversible - deactivation radical polymerization primarily atom transfer radical polymerization TRP), and also reversible addition - fragmentation chain transfer (RAFT) polymerization, tellurium mediated
polymerization iodine mediated polymerization and nitroxide mediated polymerization (NMP). these techniques, progress A TRP will covered more depth, since subsequent chapters this book will provide more insights into the development the other RDRP techniques especially RAFT polymerization. Due the lar volume literature generated over the the discussion the progress these techniques will focused recent literature spanning from 2013 2017, since the last ACS Meeting Controlled Radical Polymerization ( 1 , 2 ) readers are directed several excellent reviews for - depth discussions the dif ferent areas explored ( 3 – 7 ) Initial sections will explore the recent discoveries catalytic radical termination (CR A TRP , design a novel ligand that reduces T , and insights into A TRP the presence 0 polymer chemists aim design complex macromolecules similar protein and DNA found achieving spatial, temporal, sequence and stereochemical regulations during polymer synthesis has recently become the central theme RDRP . This book chapter will highlight the recent literature photochemical, electrochemical, and mechanochemical means achieving these These achievements externally regulated polymer synthesis are translated into advancing the synthesis complex polymeric hybrid materials (polymer and bioconjugates. The advancements these areas will also highlighted.
Advancements A TRP
Mechanism Catalytic Radical T ermination (CR A TRP
the discussion novel mechanisms and initiation pathways, paramount highlight the fundamental importance understanding the mechanism radical termination. Dispersity , which a measure polymer molecular weight relies monomer number monomer addition per activation / deactivation cycle, and amount dead chains. current literature often neglects the termination factor calculation dispersity for A TRP , a new dispersity expression blending dormant and dead chain populations was recently derived ( 8 ) Bimolecular radical termination relies two pathways – disproportionation and combination. disproportionation, two one with saturated chain end and the with unsaturated chain are formed. combination, a single chain made through C - C coupling ( 9 ) . The pathway chain termination radical polymerization relies the nature the radical species Styrene and acrylonitrile propagating radicals under chain termination primarily through combination while methacrylate radicals terminate both disproportionation and combination ( ) . Dif ferent methods study selectivity radical termination have been proposed but these methods were unable provide definitive For the ratios disproportionation combination for polymerizations methyl methacrylate (MMA) and styrene (St) vary between reports despite identical polymerization conditions ( 1 1 ) . A new method study radical termination with TERP was The use TERP allowed for activation ganotellurium dormant species photoirradiation generate polymer - end This method was
used estimate ratio disproportionation combination for poly(methyl methacrylate) and polystyrene radicals that approximately agreed with previous reports ( 1 1 ) comparison the well understood termination PSt and PMMA termination acrylate radicals was a subject intense debate.
A recent report claimed that acrylate radicals generated from ganotellurium polymerizations are terminated primarily (99%) through disproportionation room temperature ( ) . initio molecular dynamics computations suggested that the polyacrylate radicals can terminated through a direct disproportionation reaction well a new stepwise process involving initial formation C - O coupling product followed intramolecular a subsequent study , avenue control the ratio disproportionation combination for termination MMA and radicals using TERP was suggested changing reaction temperature and viscosity ( ) . Disproportionation was proposed favored over combination lower temperature and higher viscosity . The observed viscosity fect the selection mode termination was reasoned with collision model”, a derivative the collision model proposed Fischer ( ) The proposed model suggested that combination was more viscosity sensitive than disproportionation, and therefore, increase viscosity should result a more significant retardation the rate constant for combination than that observed for disproportionation ( ) . However , a subsequent investigation revealed that alkyl radicals generated through the decomposition diazo initiator dimethyl 2,2′ - azobis(isobutyrate)601), which directly generate methacrylate radicals, yielded similar Disp / Comb ratios a lar range temperatures and viscosities ( ) . The discrepancies with the previously discussed results were explained the role alkyl tellurium radical acting a catalyst for a typical V - 601 promoted bimolecular chain coupling took place within the solvent cage outside the solvent cage. the case TERP mediated cage escape was needed enable bimolecular termination possible side reaction that involved β - H abstraction from the carbon - based radical tellanyl radicals took place within the solvent cage. They formed alkene and T e - H species which generated saturated chain ends a fast reaction with Metals such iron have been successfully implemented both ganometallic - mediated radical polymerization (OMRP) and A TRP ( – ) , but copper complexes which are widely used A TRP have not been adapted for OMRP despite the proven existence copper (II) ganometallic complexes ( ) attempts have been made characterize these ganometallic intermediates. For instance, EPR measurements were carried out characterize copper (II) species generated T following the reaction [Cu I ] + formed the presence poly(butyl acrylate) radicals EPR, a new signal distinct from [Cu I I (TPMA)Br] + complex was observed ( ) ganometallic complexes generated A TRP were also investigated electrochemical reduction and - V is. These findings revealed that radicals generated from bromoacetonitrile and chloroacetonitrile (to a lesser extent ethyl α - react with [Cu I (TPMA)] + and [Cu I (Me 6 tren)] + dimethylsulfoxide (DMSO) and acetonitrile (ACN) form cupric complexes with alkyl moiety the axial coordination site ( ) .
A TRP systems contain highly active copper additional side reactions where acrylate radicals can react with L / I reversibly generate a complex / Cu I I - These species can then under catalytic radical termination (CR which proposed the primary mode termination acrylate radicals A TRP ( ) . The T process often results termination acrylates form disproportionation - like products a tris(2 - pyridylmethyl)amine (TPMA) - copper catalyzed A TRP ( ) . Thus, a study better understand the bimolecular termination and T A TRP was Poly(methyl acrylate) terminated with bromine end group was activated with copper(I) complexes with tris[2 - (dimethylamino)ethyl]amine (Me 6 tris(2 - pyridylmethyl)amine (TPMA) tris(3,5 - dimethyl - 4 - methoxy - 2 - pyridylmethyl)amine (TPMA * 3 ) the absence monomer ( ) Under conditions kinetically promoting bimolecular size exclusion chromatography (SEC) revealed a polymer product with double molecular weight relative macroinitiator distribution indicating a termination pathway via radical conditions kinetically promoting T resulted shifting macroinitiator distribution suggesting products resembling disproportionation polymer chains. PREDICI simulations for TPMA mediated termination reactions highlighted the importance midchain radical generated from acrylate radical backbiting the termination profile. Majority the terminated chains originated from CR T and cross - termination between secondary and tertiary midchain radicals ( Scheme 1 Further studies into the mechanistic pathways acrylate termination will discussed another chapter .
Scheme 1 . oposed mechanistic pathways for acrylate radical termination A TRP
Design New Ligands for A TRP
The recent exploitation reducing agents A TRP significantly decreased the amount copper complexes needed for polymerization ( ) . order
further reduce the amount copper complexes significant forts were placed towards development more active catalysts fine tuning the ligand structure tailor electronic properties copper(I) center T o achieve this electron - donating groups were systematically incorporated highly active A TRP ligands further increase the reactivity copper(I) complex. For a series novel and II) complexes with TPMA - based ligands containing 4 - methoxy3,5 - dimethylsubstituted pyridine arms were reported ( ) . Cyclic voltammetry measurements revealed that increasing substitution electron donating groups the 4 ( - OMe) and 3,5 ( - Me) positions the pyridine rings the reactivity the copper(I) complexes also increases due increased stabilization copper(II) oxidation state ( ) . addition, design new ligands allowed for better understanding the T Polymerization n - butyl acrylate (BA) with azobis(isobutyronitrile) (AIBN) the presence copper complexes with tridentate and tetradentate ligands showed higher rates T for more reducing copper complexes with higher A TRP activity . the other hand, ligand denticity had smaller fect polymerization kinetics but fected the rate determining step for T ( ) Recently , tris[(4 - dimethylaminiopyridyl) methyl]amine (TPMA N M e 2 ) was reported a novel ligand for A TRP . The TPMA N M e 2 based copper catalyst was billion times more active than seminal bipyridine - based catalyst, ~300 000 more active than TPMA, and ~30 000 more active than 6 Polymerization acrylates via ICAR and 0 A TRP were well - controlled for catalyst loadings low ppm relative monomer . The high values activation rate constants A TRP and low concentration TPMA N M e 2 / Cu I suppressed T and allowed for high - end functionality ( ) .
ogr ess SARA - A TRP
T raditional A TRP systems often required the use lar amounts copper catalyst (>1000 ppm) account for the conversion I activator I I deactivator due termination reactions. However , slow and continuous reduction controlled polymerizations can carried out using low <10 ppm copper complex. The build - I I deactivator due unavoidable termination reactions can reduced back I activator One example reducing agent that can added 0 which can acts both a supplemental activator (SA) alkyl halides and reducing agent (RA) for Cu I I , and aptly terming the process SARA - A TRP ( – ) . A recent 0 mediated polymerization provided a set universal conditions perform polymerization for methacrylates and styrene using commercially available and inexpensive reagents, such PMDET A ( N , N , N N ′′, N - isopropanol and 0 wire ( ) Isopropanol not a good solvent for some the resulting polymers and complexes and a surprising accelerated kinetics higher temperatures could associated with solubility changes activators and deactivators variable Although 0 able activate alkyl majority activation (99%) still takes place from highly active I catalysts ( , , , ) addition, comproportionation between 0 and I I generate I the dominant process comparison disproportionation I polar
ganic solvents such DMSO and also kinetically dominant even water ( , – ) . a recent attempt, 0 activation and comproportionation was studied using model reactions with three dif ferent ligands, including 6 TREN , and PMDET the presence three dif ferent alkyl including methyl 2 - bromopropionate (MBrP), ethyl α - bromoisobutyrate (EBiB), and ethyl α - bromophenylacetate (EBP using solvents such dimethylformamide (DMF) and DMSO ( ) . I / ligand with lower reduction potential linearly increased A TRP activation rate coef ficient a ) solution ( , ) A similar fect was seen activation with less active ligands and alkyl halides 0 .
The chemical step involving atom transfer reaction was the rate determining step, however , the surface activation with more active ligand reached the same value for all active
This a typical behavior heterogeneous reactions where adsorption / desorption instead chemical reaction become the rate determining step. The activation rate achieved did not surpass the comproportionation rate hinting that both reactions could have the same rate determining step, the desorption I from the surface 0 . This study was expanded understand the mechanism alkyl halide activation the surface carrying out model reactions with less active MBrP with activation rate limited surface atom transfer assisted and very active EBP A with activation rate limited desorption and adsorption I from surfaces, using 6 TREN ligand the DMSO and acetonitrile ( ) low reactant first order reaction was observed for both alkyl halide initiator and ligand, while high reactant concentrations led zero order reaction for both ligand and initiator .
The absence ligand led poor activation EBP A with activation for MBrP while increasing the amount ligand above certain concentration did not increase the activation rate competed with alkyl halides for access the 0
The surface area 0 activation rate the EBP A and MBrP followed the first order kinetics where adsorption these initiators onto 0 surface facilitated carbon - bromine bond cleavage through the inner sphere electron transfer (ISET)
important progress RDRP has been achieved polymer synthesis mediated external stimuli including applied current ( – ) . Light ( – ) , mechanical force ( – ) , and chemical redox triggers ( – ) enable temporal, and sequence control. Several reviews are available these dif ferent external stimuli techniques and interested readers are directed these works ( 6 , , , – ) . This section highlights several external regulations A TRP via and sonochemical means during the last few years. This section also covers recent progress photochemical IMP and TERP . external regulation for RAFT polymerization covered the following will not highlighted
Photochemical A TRP
Photomediated A TRP under visible LED lights (violet and blue) and sunlight was initially performed the presence copper catalyst using A TRP initiators, EBiB for acrylates and ethyl α - bromophenylacetate (EBP for methacrylates, and PMDET and tris((4 - methoxy3,5 - dimethylpyridin - 2 - yl) - methyl) amine ligands ( – ) . This approach enabled temporal control over polymerization where polymerization was promoted the presence light and suppressed the absence light. Initial polymerizations MMA and were carried out DMF while polymerization oligo(ethylene glycol) methyl ether methacrylate (OEGMA) was promoted water with TPMA using 100 ppm CuBr 2 . Further optimizations for aqueous polymerization OEGMA / TPMA / CuBr 2 system led reduction copper loadings low ppm the presence sodium bromide ( ) Based experimental and kinetic photo - A TRP was proposed take place through ARGET - A TRP pathway where I I complexes were reduced free amine ligands generate I activator species and initiating radicals ( Scheme 2 A ) ( , ) . The oxidized amine radical cation could then initiate new chains after proton transfer . Although interaction between alkyl halide and amine ligand can generate radicals via photochemical ICAR - A TRP , this was a minor pathway was one order magnitude slower than reduction I I electron donor through photo - A TRP
Scheme 2 . oposed mechanism for copper mediated polymerization under r eductive quenching cycle (A) activated electricity , sonication r edox chemistry , and oxidative quenching cycle (B) activated visible - light
another - mediated photo - A TRP was also performed with excess aliphatic tertiary amine, 6 TREN, initiate polymerization acrylates, methacrylates and styrene ( ) Polymerization under irradiation DMSO led high monomer conversions (92 - 97%) with low dispersities with a ratio [CuBr 2 ]:[Me 6 TREN] 1:6 ( , ) . A survey various solvents including ionic liquids ( , ) and water ( ) for polymerization a wide range hydrophobic and functional monomers was performed demonstrate the versatility CuBr 2 / 6 TREN catalytic system ( ) the range photocatalysts used mediate polymerization was also expanded copper(II) formate - 6 TREN ( , ) and copper gluconate ( , ) The photo - A TRP was studied using pulsed - laser polymerization (PLP) and electrospray - ionization mass spectrometry (ESI - MS) for better mechanistic understandings ( ) The main pathway initiation depended the reaction alkyl bromide CuBr /
The ligand played important a reducing agent CuBr 2 . The high extinction coef ficient CuBr 2 / 6 TREN allowed the complex enter the excited state before being quenched another 6 TREN generate CuBr / 6 TREN and cation radical 6 TREN the dominant pathway
addition using tertiary amine reductive quenchers for photo - A TRP , oxidative quenching pathways for activation polymerization were also explored ( Scheme 2 B
Thus, bis(1,10 - phenanthroline)copper(I) (Cu(phen) 2 + ) was excited under visible light enable reduction EBP A initiator for polymerization MMA which resulted the formation propagating radical and CuBr 2 ( , ) . The copper (I) complex was regenerated upon deactivation CuBr 2 a propagating Parallel the reduction cycle for copper mediated photo - A TRP described above, polymerization only took place the presence irradiation and was completely suppressed the absence Similar oxidative cycles were also implemented for photo - A TRP acrylates and methacrylates using transition metal photocatalysts such fac - Ir(ppy) 3 ( – ) and iron halides ( – 103 )
For instance, iron mediated photo - A TRP for polymerization methacrylates was demonstrated without the need for additional reducing and radical initiators ( ) . Polar solvent such DMF and ACN directly solubilized iron(III) bromide (FeBr 3 ) without Polymerization dif ferent including ethyl methacrylate benzyl methacrylate (BzMA), n - butyl methacrylate ( n BMA), trimethylsilyloxyethyl methacrylate and furfuryl methacrylate (FMA) ( , 104 ) , resulted well - defined homopolymers and block copolymers. Photolysis FeBr 3 generated FeBr 2 and bromine radical under light with the latter reacting with MMA form 2,3 - dibromoisobutyrate initiator . The situ generation initiator under light enabled the polymerization proceed without the need for external addition initiator .
Metal - A TRP - A TRP)
fort reduce residual metal A TRP , a recent focus has been centered expanding metal free catalysis promote polymerization under visible light ( 105 ) . The first example metal free A TRP was introduced Miyake where excited state perylene reduced alkyl bromide promote polymerization
methacrylates and styrene under visible light and sunlight irradiation ( 106 ) This work was further expanded include pyrene and anthracene mediate metal free photo - A TRP ( 107 ) Although these polynuclear hydrocarbons imposed spatio - temporal control over similar photo - A TRP with copper , iridium and iron complexes, these ganic catalysts showed low initiation ficiency with alkyl halides leading limited control over molecular weight the synthesized polymer . Further improvements metal - free A TRP was through the implementation - phenylphenothiazine (PTH) catalyst and variations this catalyst for polymerization methacrylates ( 105 ) and acrylonitrile ( 108 ) . Moreover , the absorption PTH primarily the region was expanded into the visible region, around 400 nm, with phenyl benzo[ b ]phenothiazine ( 109 ) .
Optimization ganocatalysis for A TRP via computational modelling led a novel class photocatalysts with - diphenyl - - dihydrophenazine core that could activate alkyl halide initiators via electron transfer under photoirradiation.
Photocatalysts with dihydrophenazine and N - aryl phenoxazine cores with various electron donating, neutral and electron withdrawing functionalities were designed based computational modelling and tested experimentally determine their ficiency promoting polymerization ( 1 – 1 ) . Mechanistic investigations revealed that phenoxazines and dihydrophenazines have a faster activation and deactivation rate comparison phenothiazines they maintain their planar state with little reor ganization the triplet and radical cation states ( 1 ) . Nevertheless, a dif ferent activation mechanism was suggested based investigation transient vibrational and electronic absorption spectroscopy with sub - picosecond time resolution: electron transfer from short - lived singlet excited state gave better control molecular weight and dispersity suppressing the formation excess radicals ( ) . visible light photocatalysts such xanthene dyes the presence reducing amines such fluorescein / trimethylamine ( 1 ) , and Eosin Y and erythrosine B (EB) with PMDET A ( 1 ) were also explored for polymerization dif ferent acrylates and methacrylates with photo - A TRP
Electr ochemical and Mechanochemical A TRP
The current advances electrochemistry and mechanochemistry A TRP and ganic synthesis were reviewed ( 1 – 120 ) . A unique advantage electrochemistry that can reduce activators deactivators but also ficiently oxidized activators deactivators and halt polymerization. can also electrodeposit Cu the electrode and recycle transition e A TRP solved important challenge A TRP pertaining polymerization acidic monomers intramolecular cyclization reaction that leads the loss carbon - halogen chain end functionality). Electrochemical A TRP (eA TRP) was successfully implemented the synthesis poly(methacrylic acid) (PMAA) promoting a fast polymerization with chlorine atom the chain - end halogen under acidic conditions ( Scheme 3 ) ( 121 ) . addition, a fundamental understanding the mechanism aqueous A TRP was possible through electrochemistry which provided a guideline for performing A TRP water ( 122 ) .
Scheme 3 . omoting fast polymerization MAA water with chlorine initiator (A) via TRP while suppr essing intramolecular cyclization observed with omo initiator Repr oduced with permission r ef. (121) . Copyright American Chemical Society
The use piezoelectric barium titanate (BaT 3 ) nanoparticles under ultrasonic agitation was successfully implemented for water splitting (potential - ( 123 ) The highly reducing nature the nanoparticle was further exploited reduce Cu(II) catalyst mechanochemically under ultrasonic agitation the presence barium titanate nanoparticles generate Cu(I) which with EBiB initiated A TRP monomer yield polymers with low dispersities ( 124 ) Initial work with BaT 3 required the use high catalyst loadings (10 000 ppm with respect monomer) leading low molecular weight PBA ( M n < 3000). Further expansion mechano - A TRP was carried out with ultrasound the presence low concentration copper catalysts (75 / which enabled the synthesis well - defined PMA ( Scheme 4 ) ( 125 ) . this various shapes (cubic tetragonal) and sizes (50 - 100 nm) BaT 3 nanoparticles under dif ferent catalytic loadings were tested optimize the reaction barium titanate tend aggregate and precipitate solution without ultrasound, was stabilized through surface modification with PMMA which enhanced the rate The concept mechano - induced electron transfer was further expanded included zinc oxide piezoelectric nanoparticles enable well - defined polymerization acrylates and methacrylates ( 126 ) .
Scheme 4
Sonochemical A TRP mediated piezoelectric barium titanate (BaT 3 ) nanoparticles interfacial r eduction I I / TPMA complex leading the formation activator for A TRP Repr oduced with permission r (125) . Copyright American Chemical Society .
Photochemical TERP and IMP
Photomediated TERP with initiating radicals generated from a dormant ganotellurium species C - T e photolysis was reported ( 127 ) . This report was followed optimization conditions for TERP polymerization under low light intensity and mild temperature conditions with overall control the progress polymerization ( 128 ) depth understanding the photophysical properties ganotellurium compounds were later explored ( 2 , 129 ) . tellurium compounds have a broad absorption range with maximum absorption 350 nm, this led the n(Tσ*(C - T transition upon irradiation and generated radicals that initiated polymerization the presence monomer . The high quantum yield the C - T e bond homolysis led initiation even under low light intensity . the process can carried out under low temperature, back - biting reaction acrylate polymerization was essentially Benzoyl phenyltelluride was later introduced a highly reactive TERP reagent capable initiating under visible light irradiation (400 - 500 nm) for well - defined polymerization acrylates and acrylamides ( 130 ) . the iodine transfer a catalyst - free iodine mediated polymerization was initiated under abroad range visible wavelengths (460 - 720 nm) ( 131 , 132 ) . solvents such dimethylacetamide (DMAc), DMSO and DMF , homolysis carbon - iodine bond was promoted through coordination with solvent molecules under irradiation. The generation carbon radicals through this method enabled the polymerization methacrylates through a reversible termination instead degenerate chain transfer mechanism. Photoinduced reversible complexation mediated polymerization (photo - RCMP) was later developed allowing for photolysis a dormant alkyl iodide the presence tributylamine enabling polymerization methacrylates ( 133 ) . This system was then expanded functional monomers with glycidol, ethylene glycol amino functional groups, such 2 - (dimethylamino)ethyl methacrylate POEGMA and glycidyl methacrylate for
direct complexation with 2 - cyanopropyl iodide (CPI) photoinitiator that led polymerization under blue light irradiation ( 134 ) . Photo - RCMP also utilized tertiary anilines photo - ganocatalysts that absorb a wide range visible light wavelengths ( 135 ) . These photocatalysts acted ener antennas for photon absorption with the light ener then transfer alkyl iodide resulting photolysis carbon - iodide The generated radicals were successfully used for polymerization methacrylates.
Advancements Polymers with Complex chitectur e
Ultra - High Molecular W eight Polymers
RDRP techniques have enabled excellent control over the molecular weights dif ferent functional monomers, and therefore, providing avenue for synthesis hyperbranched ( 136 , 137 ) , star ( 138 , 139 ) , brush ( 140 , 141 ) , and block copolymers ( , 142 – 144 ) , while also providing narrow and tunable ( 145 , 146 ) molecular weight achieving ultra - high molecular weights (UHMW) the range 8 x 6 has only been possible under high pressure ( 147 – 149 ) and heterogeneous conditions ( 150 , 151 ) . A novel approach achieve UHMW polymers was explored with polymerization - dimethylacrylamide (DMA) water using 2 - - carboxyethylsulfanylthiocarbonylsulfanyl) - 2 - methylpropionic which acted a photoiniferter under irradiation (365 nm) ( 152 ) . The high propagation rate constant DMA water and the reversible termination with photoiniferter high solution viscosity due UHMW polymer chains were key features permitting UHMW , decrease translational dif fusion was reduced due high viscosity preventing radical termination. This approach resulted PDMA chains with excess 8 UHMW synthesis was also achieved under oxygen tolerant condition using enzyme - cascade catalysis using pyranose oxidase (P2Ox) and horseradish peroxidase (HRP) where oxygen molecules were initially reduced the presence glucose P2Ox generate hydrogen peroxide ( 153 ) The hydrogen peroxide was then used HRP the presence acetyl acetonate (ACAC) generate radicals that initiated RAFT polymerization. Polymerization DMA conducted with this approach resulted UHMW polymer with molecular weight 2 × 6 .
Multiblock Copolymers
One the biggest challenges chain growth polymerization techniques sequence control over addition monomers units. Several techniques have been developed for synthesis multiblock copolymers with the aim synthesizing near - quantitative conversion for each block, avoiding purification steps, and enabling narrow molecular weight distributions with high end - group functionality ( 154 – 157 ) . Introduction multiblock sequence controlled polymerization with RAFT led the generation icosablock DMA, 4 - acryloylmorpholine N , N - diethylacrylamide (DEA) and N - isopropylacrylamide (NIP AM)
copolymer aqueous media ( 143 , 155 , 158 – 161 ) However , this system became limited due restriction monomer selection acrylamides with polymerization carried out high temperatures (~70 °C) which may not suitable for monomers with LCST such NIP Improving these and - polymerization approach combined with emulsion biomimetic segregation strategy was envisioned for sequence controlled synthesis multiblock methacrylate copolymers which can easily scaled ( 162 ) This strategy relied a vinyl - terminated PMMA macromonomer , synthesized through catalytic chain transfer polymerization that played the role a chain transfer agent for RAFT polymerization various methacrylate monomers. CCTP approach with low - spin d 6 Co(II) complexes (cobaloximes) led abstraction hydrogen from methacrylic radical yield Co(III) - H intermediate and a polymer terminated with vinyl The generated vinyl - terminated PMMA macromonomers showed chain transfer activity followed fragmentation generate macroradical that initiated a second monomer . The compartmentalization provided emulsion polymerization accelerated polymerization while maintaining slow radical This approach was used for hydrophobic methacrylates such BMA, BzMA, EHA, and MMA which led the successful synthesis heneicosablock (21 blocks) copolymer that exhibited relatively narrow molecular weight distribution. Initial investigations sulfur free RAFT which relied the use PMMA macromonomers, was later expanded synthesizing macromonomers composed BMA, BzMA, and EHA that led the successful synthesis sequence defined multiblock copolymers ( 163 ) . Multiblock synthesis A TRP was initially carried out employing Cu(0) which provided acrylic hexablock copolymer high yields with each block having least two monomer units added ( 154 , 164 , 165 ) . Upon tar geting blocks with higher degree average 100 monomer units per block, triblock and quasi - pentablock copolymers was prepared. Nevertheless, this approach becomes limiting full monomer conversions cannot achieved ( 166 ) . A TRP the presence 0 was later used generate multiblock glycopolymers with a monomer sequence control ( 167 , 168 ) well multiblock copolymers acrylamides less than 5 hours ( 169 , 170 ) . Photo - A TRP enabled a novel route for exploring polymer composition and microstructure high monomer conversions with good end group fidelity ( , 157 , 171 – 174 ) . mediated A TRP has enabled sequence controlled multiblock copolymers hexablock and pentablock copolymers) synthesized a single pot ( 157 ) . A lar number acrylate monomers were suitable for the synthesis sequence defined multiblock copolymers including tert - butyl acrylate (tBA), glycidyl acrylate (GA) and solketal acrylate (SA). For instance, under optimized conditions these acrylates have been used generate undecablock 1 blocks), tridecablock (13 blocks), and tricosablock (23 block) copolymers with narrow molecular distribution with high monomer conversions achieved each iterative monomer addition steps ( , 175 ) . A faster polymerization was also achieved synthesizing homopolymers and block copolymers a short period time while maintaining high end group fidelity adapting photo - A TRP into milli - flow and micro - flow reactors ( 176 ) .
Sequence Defined Oligomers
addition high molecular weight the development photo - RAFT enabled the precise synthesis dimers, trimers, well hexamers (combination two trimers) using single monomer insertions reactions ( 177 ) . This approach relies utilizing RAFT agents with high transfer with low propagation rate which ensures addition only a single monomer unit. The approach was later expanded include RAFT photoiniferter polymerization with thiolene and esterification reactions generate discrete pentamers ( 178 ) . using extensive acrylate monomer library , linear monodisperse - and - mer acrylates were also obtained disulfide coupling two sequence - defined acrylate 9 - and - mers which were initially purified column chromatography ( 179 ) . addition, situ generation multiblock copolymers with ABCDE, EDCBA and EDCBABCDE sequences composed hydrophobic, hydrophilic and fluorinated monomers was also made possible with photo - A TRP via SUMI ( 172 , 180 ) . Further analysis with small - angle X - ray scattering (SAXS) revealed that certain sequences fluorinated monomers enabled well - ordered structures ( 180 ) . oligomers synthesized via radical polymerization are often polydisperse a novel strategy prepare discrete oligomeric library with acrylates, styrenic, siloxanes well conjugated oligomers was developed using automated flash chromatography ( 181 , 182 ) This strategy allowed for preparation discrete oligomers with distributions close unity which provided opportunity study self - assembly triplex helix stereocomplex PMMA oligomers ( 183 ) and also dispersity fects the self - assembly oligomer composed dimethylsiloxane - block - methyl methacrylate ( 184 ) .
Postpolymerization Modification
Postpolymerization modification has advanced remarkably recent allowing polymer chemists employ ganic chemistry for generation complex functional Postpolymerization functionalization can fective technique generating a library functional polymer from a single parent precursor , and ensuring polymers with identical degree polymerization, stereochemistry , and molecular weight distributions. addition, postpolymerization functionalization also allows the attachment functional pendant groups polymer especially these functional groups are too reactive incorporated the initial polymer ( 185 ) . Excellent reviews provide comprehensive coverage past and current postpolymerization techniques ( , 185 , 186 ) . Several postpolymerization methods can serve examples recent progress this For ganocatalytic transesterification with 1,5,7triazabicyclo[4.4.0]dec - 5 - ene provides a new route towards site - selective acyl substitution unhindered esters ( 187 ) Polyacrylates can under acyl substitutions with nucleophilic alcohols and amines provide novel functional a reaction alkyl iodide - with azide anion 3 - ) reversibly generate the alkyl radical was also explored yield well defined polymers (polymer - through iodine mediated polymerization ( Scheme 5 ) ( 188 ) . This reaction was solvent selective where alkyl iodide and azide
anion generated alkyl radical nonpolar solvent while polar solvent generated polymer terminated with azide group. This unique solvent dependent property the polymerization was utilized generate initial polymer terminated with iodide nonpolar solvent using N 3 - Upon addition polar azide - functionalized polymer was generated. This approach was then extended postmodify iodide functionalized polymer brushes with azide functionality with control over the coverage this functionality .
Scheme 5 Solvent selective iodine mediated polymerization with alkyl iodide and azide anion generating alkyl radical nonpolar solvent while polar solvent led polymer terminated with azide oup. Adapted with permission r ef. (188) Copyright American Chemical Society
Polymeric Nanoparticles, Branched chitectur es, and Gels
Novel chemistries have also been introduced for synthesis polymeric nanoparticle through polymerization induced self - assembly (PISA) well branched architecture with both A TRP and RAFT several excellent reviews and articles have been published PISA, this area will not covered this section and interested readers are referred them ( 136 , 189 – 195 ) terms novel branched material synthesis, several synthetic discoveries have been made via photo and thermal Exploration porphyrins for photoRAFT polymerization led the discovery pheophorbide a (PheoA) photocatalyst that selectively activated dithiobenzoate and ZnTPP that selectively activated This discovery was used synthesize onetwo - step graft copolymer (PMMA - r - BTPEMA) - g - PMMA manipulation wavelengths activate dif ferent photocatalysts dif ferent instances ( 196 , 197 ) . addition, limitation additive manufacturing, where terminated chains unable further chain extend introduce new was also solved with A TRP and RAFT . A novel approach called Photo - Redox Catalyzed Growth (PRCG) where a parent gel was first grown via strain promoted alkyne - azide cycloaddition (SP AAC) 4 - arm polyethylene glycol (PEG) star polymer with dibenzocyclooctyne etra - DBCO - PEG) and a bis - azide TTC (bis - N 3 - TTC) the presence monomer , PTH and /
